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Now a day’s electricity has played a vital role for the economic development of the country as well as makes people’s life easy and comfortable, reliable, uninterrupted, safe and adequate power supply is a pre-requisite for the development of the country.
1.2 Broad Objective
The main objective of the report has been done to show the total working procedure of power transmission, distribution, substation operation, controlling and various types’ protection systems on Indoor Type Substation.
1.3 Specific Objective
The specific objective of this report includes:
● To study of a substation
● To study the major components of an indoor type substation
● To study the major components of indoor type substation
● To study the protection system of an indoor type substation
● To study the bus-bar arrangement system
● To make an analysis of total power consumption, various losses and power factor improvement.
● To specify the fault and their Troubleshooting
● To study consideration of a substation designing
The guidance and data in this report paper are intended to be used by operating and maintenance, personnel. It includes operating instructions, standard inspections, safety precautions and maintenance instructions.
The research of this paper has been done with the help of the different sources. During the preparation of the paper several times it was cheeked by the authorized person of the SS Steel (Pvt) Ltd. The data was chosen accurately throughout the entire period of the session. Although there were several sources but here some are mentioned as for the proper references. The information of this report has been collected from the following sources:
● Management Manual
● Operation Manual
● Maintenance Manual
● Product Brochure and Catalog
● Quality Management Manual
● Environment, Health & Safety Manual
This report has been prepared on the period of my practicum. Total paper was covered nicely through the time duration what was allocated for me. I show the practical implementation of everything. Although it was not expected but several times there were problems with most major equipments. Right here I had the opportunity came to check the entire maintenance, operation of the system. Along with the procedure my supervisor used to check up the documents by the in charge experts of the company.
For economic development of a country electricity is the main important part. We can’t thing without electricity in our daily life. However, electric power generating in power plant and transferred to the consumers into a substation. Substations are integral parts of a power system and form important links between the generating stations, transmission systems, distribution systems and the load points. Various power substations located in generating stations, transmission and distribution systems have similar layout and similar electrical components.
2.1 About SS Steel
SS Steel (Pvt) Ltd., located in the heart of mill-gate road in Tongi, also known to be the oldest industrial region in Dhaka, Bangladesh; re-established the steel manufacturing business by its second generation family.
Nearly 26 years ago Kazi Shafi father of the 2 sons started the business that was dedicated solely to producing reinforcement-bars from small rolling mills. At the time rolling mills were fed with sheered and cut plates derived out of abandoned ships. Rods were produced to meet the needs for affordable steel and iron for the growing territories and basic infrastructure in the newly formed Bangladesh then.
Since the inception of the company SS Steel continued to expand its capability in every aspects of the production spectrum. Today we narrowed our product line to focus and become a Specialty Production Mill seeking to service a much more select group of demanding, quality-conscious customers.
Fig. 2.1 Electric Induction Furnace
2.2 The Vision:
The Rod/Bar mill at SS Steel is unique in nature capable of producing rod products of fine quality compared to any mills of its kind and style. Product quality is ensured throughout the manufacturing processes in both Steelmaking and the Rod/Bar mill through strict adherence to process control requirements. Every SS Steel Mill employee is devoted to quality processes, quality products, and quality service. Each individual is focused on his or her responsibility within the process from hauling scrap to in-the-field customer service and understands how the power of synergy and the value of each facet of the process results in an exceptional outcome. Because SS Steel rod and bars and finished products meet the customer's need, individual focus within a team environment creates an exceptional end product.
The knowledgeable and experienced personnel of SS Steel are committed to customer service through the development, manufacturing, delivery and installation processes. We know our success is dependent upon your satisfaction.
2.3 Finished Good:
Specializing in 500 TMT and 60 grade Re-bars/ Rods, we also produce 40 grade Re-Bars/ Rods from time to time to meet our clients requirement.
Fig. 2.2 Rolling mill
2.4 Reinforced bar:
Various types of Re-Bar/ MS Rod producing in SS steel plant. There are
| 300 W / 40
|400 W / 60
2.5 Factory Setup:
SS of testing equipment are available in-house are available for our steel and steel is a composite A to Z solution for construction contractors and builders. The entire process from melting Mild Steel Scrap in induction furnaces to producing the end product, reinforced bars (Re-Bar/ MS Rod) at the rolling mills, is conducted in our own facility. An assortment rolling mill to produce deformed bar and plain bar using TMT technology to our client’s requirement.
SS Steel (Pvt) Ltd. one of the largest steel manufacturing companies in Bangladesh. They are producing reinforced bars (Re-Bar/ MS Rod) at the rolling mills by their own facilities. Product quality is ensured throughout the manufacturing processes in both Steelmaking and the Rod/Bar mill through strict adherence to process control requirements.
The present-day electrical power system is ac i.e. electric power is generated, transmitted and distributed in the form of alternating current. The electric power is produced at the power stations which are located at favorable places, generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the line of the power system, it may be desirable and necessary to change some characteristic (e.g. voltage, current, ac to dc, frequency, p.f. etc) of electric supply. This is accomplished by suitable apparatus called sub-station. For example, generation voltage (11KV or 6.325KV) at the power station is stepped up to high voltage (say 230KV or 132KV) for transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly, near the consumers’ localities, the voltage may have to be stepped down to utilization level. This job is again accomplished by a suitable apparatus called sub-station. Yet at some places in the line of the power system, it may be desired to convert large quantities of ac. Power to dc power e.g. for traction, electroplating, dc motors etc. This job is again performed by suitable apparatus (e.g. ignition) called sub-station.
The assembly of apparatus used to change some characteristic (e.g. Voltage, Current, a.c to d.c, frequency Hz, p.f etc) of electric supply is called a substation.
Fig. 3.1 Sub-station
An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps. A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution.
3.3 Importance of Substation
Substation is an important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations. It is therefore essential to exercise utmost care while designing and building substations. The following part is important point which must be kept in view while laying out a substation.
(i) It should be located at a proper site as far as possible it should be at the center of load.
(ii) It should provide safe and reliable arrangement. For safety consideration must be given to the maintenance abnormal occurrence such as possibility of explosion or fire etc. Consideration must be given for design and constriction. The provision of suitable protection gear etc.
(iii) It should be easily operated and maintenance.
(iv) It should involve minimum capital cost.
3.4 Functions of a Substation
Substations are designed to accomplish the following functions, although not all substations have all these functions:
- Change voltage from one level to another.
- Regulate voltage to compensate for system voltage changes.
- Switch transmission and distribution circuits into and out of the grid system.
- Measure electric power qualities flowing in the circuits.
- Connect communication signals to the circuits.
- Eliminate lightning and other electrical surges from the system.
- Connect electric generation plants to the system.
- Make interconnections between the electric systems of more than one utility.
- Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt- amperes in the circuits.
- Supply electric power to the consumers continuously.
- Supply of electric power within specified voltage limits and frequency limits shortest possible fault duration.
- Optimum efficiency of plants and the network
- Supply of electrical energy to the consumers at lowest cost
3.5 Classification of substation
According to Service Requirement
1. Transformer Substation.
2. Switching substation.
3. Power factor correction substations.
4. Frequency changer substations.
5. Converting substation.
6. Industrial substation.
According to Constructional Feature the Substation are Classified as
1. Indoor substation.
2. out door substation.
According to Based on Operation Voltage
Ultra High Voltage Electrical Power Substation.
2. Extra High Voltage Electrical Power Substation.
3. High Voltage Electrical Power Substation.
According to Based on Design Configuration
1. Air Insulated Electrical Power Substation.
2. Gas Insulated Electrical Power Substation.
3. Hybrid Electrical Power Substation.
3.5.1 Transformer substation
Those substations which change the voltage level of electric supply are called transformer substation. These substations receive power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such substation. Most of the substations in the power system are of this type.
3.5.2 Switching Substation
These substations do not change the voltage level i.e. incoming and outgoing lines have the same the voltage. However, they simple perform the switching operations of power lines.
3.5.3 Power Factor Correction Substation
Those substations which improve the power factor of the system are called power factor correction substations. Such substation is generally located at the receiving end of transmission lines. These substations generally are use synchronous condensers as the power factor improvement equipment.
3.5.4 Frequency Changer Substation
Those substations which change the supply frequency are known as frequency changer substation. Such a frequency change may be required for industrial utilization.
3.5.5 Converting Substation
Those substations which change a.c power into d.c power are called converting substations. These substations receive a.c power converting into d.c power with suitable apparatus (e.g. Ignitron) to supply for such purpose as traction electroplating electric welding etc.
3.5.6 Industrial Substation
These types of substations which supply power to individual industrial concerns are known as industrial substations.
3.5.7 Indoor Substation
In indoor power substations the apparatus is installed within the substation building. Such substations are usually for the rating of 66kv. Indoor substations are preferred in heavily polluted areas and power substations situated near the seas (saline atmosphere causes insulator failures results in flashovers).
Fig. 3.2 Indoor Substation
3.5.8 Outdoor Substation
In outdoor power substations, the various electrical equipments are installed in the switchyard below the sky. For voltage beyond 66kv equipment is invariably installed out door. It is because for such voltage the clearance between conductor and the space required for switch, circuit breaker and equipment becomes so great it is not economical to install the equipment indoor.
3.5.9 High Voltage Electrical Power Substation
This type of substation associated with operating voltages between 11kv and 66kv.
3.5.10 Extra High Voltage Electrical Power Substation
This type of substation is associated where the operating voltage is between 132kv and 400kv.
3.5.11 Ultra High Voltage Electrical Power Substation
Substations where operating voltages are above 400kv are called ultra high voltage substation.
3.5.12 Air Insulated Electrical Power Substation
In air insulated power substations bus bars and connectors are visible. In this power substations circuit breakers and isolators, transformers, current transformers, potential transformers etc are installed in the outdoor. Bus bars are supported on the post insulators or strain.
Insulators substations have galvanized steel structures for supporting the equipment, insulators and incoming and outgoing lines. Clearances are the primary criteria for these substations and occupy a large area for installation.
3.5.13 Gas Insulated Electrical Power Substation
Fig. 3.3 Gas Insulated Substation
In gas insulated substation has various power substation equipments like circuit breakers, current transformers, voltage transformers, bus bars, earth switches, surge arresters isolators etc are in the form of metal enclosed SF6 gas modules. The modules are assembled in accordance with the required configuration. The various live parts are enclosed in the metal enclosures (modules) containing SF6 gas at high pressure. Thus the size of power substation reduces to 8% to 10% of the air insulated power substation.
3.5.14 Hybrid Electrical Power Substation
Hybrid substations are the combination of both conventional substation and gas insulated substation. Some bays in a power substation are gas insulated type and some are air insulated type. The design is based on convenience, local conditions available, area available and cost.
3.6 Substation Types
Generally five types of substations that are
3.6.1 Distribution Substation
These are located near to the end-users. Distribution substation transformers change the transmission or sub-transmission voltage to lower levels for use by end-users. Typically distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts. 34,500Y/19,920 volts are interpreted as a three-phase circuit with a grounded neutral source. This would have three high-voltage conductors or wires and one grounded neutral conductor, a total of four wires. The voltage between the three phase conductors or wires would be 34,500 volts and the voltage between one phase conductor and the neutral ground would be 19,920 volts. From here the power is distributed to industrial, commercial, and residential customers.
Fig. 3.4 Distribution substations
3.6.2 Step-up Transmission Substation
A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations. A transmission bus is used to distribute electric power to one or more transmission lines. There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant.
Fig. 3.5 Step–up Transmission Substations
3.6.3 Step-down Transmission Substation
Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for sub-transmission lines or distribution lines. The step-down substation can change the transmission voltage to a sub-transmission voltage, usually 69 kV. The sub-transmission voltage lines can then serve as a source to distribution substations. Sometimes, power is tapped from the sub-transmission line for use in an industrial facility along the way. Otherwise, the power goes to a distribution substation.
Fig. 3.6 Step-down transmission substation
3.6.4 Underground Distribution Substation
Underground distribution substations are also located near to the end-users. Distribution substation fig transformers change the sub-transmission voltage to lower levels for use by end users. Typical distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.
Fig. 3.7 Underground Distribution Substation
An underground system may consist of these parts:
From here the power is distributed to industrial, commercial, and residential
The assembly of apparatus use to change some characteristic (e.g. voltage, ac. to dc., frequency, p.f. etc ) of eclectic supply is called a substation.
In this cheater I have to describe various type of substation and it’s characteristic. A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Electric power may flow through several substations between generating plant and consumer, and its voltage may change in several steps.
Electricity is generated in a thermal power plant, hydroelectric power plant, and nuclear power plant, etc. This electricity is then supplied by a substation near the generating plant. In the substation the voltage is increased substantially using step up transformers. The voltage is increased to reduce the transmission losses over long distances. This electricity then is supplied to a power substation where it is stepped down using step down transformers and then supplied to a distribution grid. In the distribution grid there are additional transformers and voltage is further reduced for distributing further down the grid. From here the electricity is supplied to step down transformers near residential quarters that step down the voltage as per each requirement.
THE MAJOR COMPONENTS OF INDOOR TYPE SUBSTATION
In every electrical substation, there is various type of indoor and outdoor equipment. Each equipment has a certain functional requirements. The choice of the equipment depends on technical considerations, rated voltage, rated MVA and the type of substation. The indoor type substations equipment includes busbars, Circuit Breakers, and Isolators with earthing switches, Power transformers, Current transformers, Instrument transformers, surge arresters etc.
Substations generally contain one or more transformers and have switching, protection and control equipment. In a substation, transformers is the main component employed to change the voltage level, circuit breakers are used to interrupt any short circuits
or overload currents that may occur on the network. Smaller distribution stations may use re-closer circuit breakers or fuses for protection of branch circuits. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, grounding (earthing) system, and metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.
A transformer is a static (or stationary) piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. A transformers transfers electrical energy from one circuit to another through magnetically coupled electrical conductors. A changing current in the first circuit (the primary) creates a changing magnetic field in turn; this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
Fig. 4.1 (a) Transformer
Fig.4.1 (b) Transformer showing the primary and secondary windings
4.2.1 11/ 0.415 kV Distribution Transformer
The secondary induced voltage VS, of an ideal transformer, is scaled from the primary VP by a factor equal to the ratio of the number of turns of wire in their respective windings. By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up – by making NS more than NP – or stepped down, by making it less. Transformers are some of the most efficient electrical 'machines', with some large units able to transfer 99.75% of their input power to their output. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide.
Fig. 4.2 Distribution Transformer
4.2.2 Working Principle of Transformer
The transformer is based on two principles; first, that an electric current can produce a magnetic field (electromagnetism), and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.
Fig. 4.3 An ideal transformer
An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability such as iron so that most of the magnetic flux passes through both the primary and secondary coils.
4.2.3 Induction law
The voltage induced across the secondary coil may be calculated from Faraday's law of induction which states that,
Where Vs is the instantaneous voltage Ns is the number of turns in the secondary coil and Φ is the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals,
Taking the ratio of the two equations for Vs and Vp gives the basic equation for stepping up or stepping down the voltage,
4.2.4 Ideal power equation
Fig. 4.4 The ideal transformer as a circuit element
If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power.
Giving the ideal transformer equation,
Transformers normally have high efficiency, so this formula is a reasonable approximation.
If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turn’s ratio For example, if impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2 Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2 Zp.
4.2.5 Detailed operation
The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.
Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance when a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core. The current required to create the flux is termed the magnetizing current since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.
The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMF’s. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.
4.2.6 Effect of frequency
If the flux in the core is purely sinusoidal the relationship for either winding between its rms voltage Erms of the winding , and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation,
If the flux does not contain even harmonics the following equation can be used for half-cycle average voltage Eavg of any wave shape,
The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to time of the applied voltage. Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time. In practice, the flux would rise to the point where magnetic saturation of the core occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current.
The EMF of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact because a given core. Conversely, frequencies used for some railway electrification systems were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50 – 60 Hz) for historical reasons concerned mainly with the limitations of early electric traction motors.
Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current; at lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from over voltage at higher than rated frequency.
One example of state-of-the-art design is those transformers used for electric multiple unit high speed trains particularly those required to operate across the borders of countries using different standards of electrification. The position of such transformers is restricted to being hung below the passenger compartment. They have to function at different frequencies (down to 16.7 Hz) and voltages (up to 25 kV) whilst handling the enhanced power requirements needed for operating the trains at high speed. Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient response of the windings to impulse and switching surge voltages.
4.2.7 Energy losses
An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.
Experimental transformers using superconducting windings achieve efficiencies of 99.85%. The increase in efficiency from about 98 to 99.85% can save considerable energy, and hence money, in a large heavily-loaded transformer; the trade-off is in the additional initial and running cost of the superconducting design.
Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteretic and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost; designing transformers for lower loss requires a larger core, good-quality silicon steel or even amorphous steel for the core, and thicker wire, increasing initial cost, so that there is a trade-off between initial cost and running cost. Transformer losses are divided into losses in the windings, termed copper loss and those in the magnetic circuit, termed iron loss. Losses in the transformer rise from:
a) Winding resistance
Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.
b) Hysteretic losses
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteretic within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.
c) Eddy currents
Ferromagnetic materials are also good conductors and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and Inverse Square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.
d) Mechanical losses
In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise and consuming a small amount of power.
e) Stray losses
Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat. There are also radioactive losses due to the oscillating magnetic field, but these are usually small.
4.3 Power Transformer
Power Transformer is connected to generating house & step up voltage from 11kV to 33kV level as requirement of line voltage. They are used in the high-power generating stations for voltage step up and in the transmission substations for voltage step up or step down.
Fig. 4.5 Power Transformer used in SS Steel Plant
4.3.1 Power Transformer Specifications for SS Steel Plant
Application Standard ANSI 5713
Type 3 Phase
Type of Cooling ONAN/ONAF
Rated MVA 20/35
Voltage Ratio 33/11
Rated KV HV(33), LV(11)
Rated Line Current HV(349.91/437.39), LV(1049.73/1312.16)
Vector Group YNd1
% Impedance on 25MVA 13.22
Total Mass 37000 Kg
Temp. Rise (At 75cel) Top Oil 55, AVG WDG 60
L1 200 AC 70
L2 200 AC 70
L3 110 AC 34
4.4 Instrument transformers
Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. Instrument transformers are used for measuring voltage and current in electrical power systems, and for power system protection and control.
Fig. 4.6 Instrument Transformers
Where a voltage or current is too large to be conveniently used by an instrument, it can be scaled down to a standardized low value. Instrument transformers isolate measurement, protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled.
4.5 Current transformer (CT)
A current transformer is a transformer designed to provide a current in its secondary coil proportional to the current flowing in its primary coil. Current transformer (CT) is used for measurement of electric currents. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.
Fig. 4.7 Current Transformers, designed for placing around conductors
Current transformers are used for protection, measurement and control in high voltage electrical substations and the electrical grid. Current transformers may be installed inside switchgear or in apparatus bushings, but very often free-standing outdoor current transformers are used. In a switchyard, live tank current transformers have a substantial part of their enclosure energized at the line voltage and must be mounted on insulators. Dead tank current transformers isolate the measured circuit from the enclosure. Live tank CT’s are useful because the primary conductor is short, which gives better stability and a higher short-circuit current withstand rating. The primary of the winding can be evenly distributed around the magnetic core, which gives better performance for overloads and transients. Since the major insulation of a live-tank current transformer is not exposed to the heat of the primary conductors, insulation life and thermal stability is improved. A high-voltage current transformer may contain several cores with multiple secondary windings for different purposes (such as metering circuits, control, or protection).
4.6 Potential transformers
Potential transformers are devices that reduce line voltage to a proportionally lower
and safer voltage for metering and relaying. A potential transformer, like the one shown in Figure, normally has a large porcelain bushing that insulates the higher voltage conductor going into the transformer. The transformer itself is usually enclosed in a metal housing. The output wires of the transformers are enclosed in conduit to protect them. These wires connect to meters or relaying equipment in a control house.
Fig. 4.8 Potential Transformer
Potential transformers come in many shapes and sizes. They are sometimes difficult to distinguish from other devices such as some current transformers and surge arrestors. For this reason, potential transformers are often identified in substations by signs like the one shown in Figure.
4.7 Voltage Transformers (VT)
Voltage transformers (VT) also referred to as "potential transformers" (PT) are designed to have an accurately known transformation ratio in both magnitude and phase, over a range of measuring circuit impedances. A voltage transformer is intended to present a negligible load to the supply being measured. The low secondary voltage allows protective relay equipment and measuring instruments to be operated at lower voltages.
Both current and voltage instrument transformers are designed to have predictable characteristics on overloads.
Fig. 4.9 Voltage Transformers (VT)
Proper operation of over-current protective relays requires that current transformers provide a predictable transformation ratio even during a short circuit.
4.8 H.T Switchgear with (VCB)
In such quenching, Vacuum is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. When the contacts of the breaker are opened in vacuum (10-7 to 10-5 tore), an arc is produced between the contacts by the ionization of metal vapors. However, the arc rapidly condenses on the surface of the circuit breaker contacts, resulting in quick recovery of dielectric strength. The reader may note the salient feature of vacuum as an arc quenching medium. As soon as the arc is produced in vacuum, it quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.
4.8.1 Technical Specification of (VCB)
Sheet steel clad power coated (14 SWG), dust and vermin proof, free standing, Floor mounting indoor HT Switchgear 11kv, 50 Hz, three phase , 630 A hard drawn electrolytic copper bus-bars equipped with 1 No. 630 A, 11kv, breaking current 20 KA(3sec),making current 50KA, 50Hz, TP Vacuum circuit breaker (Fixed type) with motor operated mechanism with closing solenoid shunt releases, auxiliary contacts 5NO + 5NC and limit switch (1 NO + 1 NC) for indication “Closing spring charged” mechanical on/off/trip indicator.
Fig. 4.10 (a) V H.T Switchgear with (VCB)
Fig. 4.10 (b) Vacuum Circuit Breaker of 11 kV line (VCB)
The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply.
The very earliest central power stations used simple open knife switches mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.
4.9 Power Factor Improvement (PFI) Unit
The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit and is a dimensionless number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor. Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system.
Fig. 4.11 PFI Panel with Capacitor Bank
In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor. The devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment.
4.10 Lightning Arrester
A lightning arrester (in Europe surge arrester) is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.
A lightning arrestor is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike.
Fig. 4.12 Lightning Arrester
If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.
4.11 H.T Metering Panels
It is used for Instantaneous/Stored Measuring and Recording Currents, Voltages and Energy accumulators (kHz, KVA lag, KVA rah lead and KVA in forward and reverse directions) in 11KV installations.
Fig. 4.13 H.T Meter 11 KV sides
4.11.1 Advantages of HT Metering Panel
i) Live Parts are not directly accessible, more safety to Electrical Maintenance persons and other personals.
ii) Incoming and Outgoing by means of UG Cables. (Suitable Detachable gland plates are provided to fit different sizes of cables).
iii) Double side Ear thing is provided in the panel for effective Means of earthing.
iv) Panels are painted with suitable Epoxy and Enamel (standard Shades of IS & IEC standards) to suit indoor and outdoor applications.
When a number of generators or feeders operating at the same voltage have to be directly connected electrically, bus-bars are used as the common electrical component. Bus-bars are copper rods or thin walled tubes and operate at constant voltage.
Fig. 4.14 Single bus-bar system
There are different types of bus bars
i) Single bus-bar system
ii) Single bus-bar system with Sectionalisation
iii) Duplicate bus-bar system
Mimic bus materials shall be brass, bronze or copper with backed enamel finished or anodized aluminum or plastic. The mimic bus shall be attached to the panel by mechanical devices not with adhesive.
The apparatus by use for supply electric power from generating station to consumers is discussed in this chapter. The electrical energy produced in the generating station is conveyed to the consumer through a network of transmission and distribution system. So at first the generating voltage needs to transfer from generating station to consumers. For this purpose it needs some apparatus to continue its operation. An electrical grid is a vast, interconnected network for delivering electricity from suppliers to consumers.
Substations generally have switching, protection and control equipment, and transformers. In a substation transformers is employed to change the voltage level, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits. Substations themselves do not usually have generators, although a power plant may have a substation nearby. Other devices such as capacitors and voltage regulators may also be located at a substation. Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions. Where a substation has a metallic fence, it must be properly grounded or earthed to protect people from high voltages that may occur during a fault in the network. Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet this touch potential presents a hazard of electrocution.
PROTECTION SYSTEM OF INDOOR TYPE SUBSTATION
Form reliable point of view, this protection devices used in power system takes on important role in the system. There are several types of faults occur in a power system. Such as short circuit current, over voltage, earth fault, phase-to-phase fault, surge voltage etc. When a fault occur, if the protective devices are not used to protect the system then a huge amount of current flow through the circuit damage it. There are several types of protective devices are used to protect the equipments of substation and the consumers electrical load.
5.2 Protection of Indoor Type Substation
Substations generally contain one or more transformers, and have switching, protection and control equipment. In a large substation, circuits breakers are used to interrupt any short-circuit or overload currents that may occur on the network. Smaller distribution stations may use auto re-closer circuit breakers or fuses for protection of branch circuits. Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, grounding (earthling) system, and metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation. Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.
The apparatus used for switching controlling and protecting the electrical circuits and equipments is known as Switchgear. The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment.
Typically, the switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear on the low voltage side of the transformers may be located in a building, with medium-voltage circuit breakers for distribution circuits, along with metering, control, and protection equipment. For industrial applications, a transformer and switchgear line-up may be combined in-one-housing, called a unitized substation or USS.
5.4 Switchgear Functions
One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear also is used to enhance system availability by allowing more than one source to feed a load.
5.5 Essential Feature of Switchgear
The essential features of switchgear are:
a) Complete reliability: With the continued trend of interconnection and the increasing capacity of generating stations, the need for reliable switchgear has become of paramount importance. This is not surprising because switchgear is added to the power system to improve the reliability.
b) Absolute certain discrimination: When fault occurs on any section of the power system, the switchgear must be able to discriminate between the faulty section and healthy section.
c) Quick operation: When fault occurs on any part of the power system, the switchgear must operate quickly so that no damage is done to generators, transformers and other equipment by the short-circuit currents.
d) Provision for manual control: Switchgear must have provision for manual control. In case the electrical (or electronics) control fails, the necessary operation can be carried out through manual control.
e) Provision for instruments: There must be provision for instruments which may be required. These may be in the form of ammeter or voltmeter on the unit itself or the necessary current and voltage transformers for connecting to the main switchboard or a separate instrument panel.
5.6 Types of Switchgear
There are two three type of switchgear depended on voltage
01. Low Voltage Switchgear
02. Medium Voltage Switchgear
03. High voltage switchgear
5.6.1 Low Voltage Switch gear (LV Switch gear)
Low Voltage Switchgear is designed for switching and protection of electrical equipments. The selection of switching devices is based on the specific switching task e.g. isolation, short circuit current breaking, motor switching, protection against over current and personnel hazards. Depending on the type, switching device can be used for single or multiple switching tasks. Switching task can also be conducted by a combination of several switchgear units. Low voltage switchgear used to less than 1000 volts AC.
5.6.2 Medium voltage Switchgear (MV Switch gear)
Medium voltage Switchgear is used to connect or disconnect a part of a Medium voltage power system. This Switchgear is essential elements for the protection and safe operation, without interruption, of a Medium voltage power system. Medium voltage switchgear used to 1000 volts to 35,000 volts.
5.6.3 High voltage switchgear (HV Switch gear)
High voltage switchgear is used to connect or disconnect a part of a high-voltage power system. This Switchgear is essential elements for the protection and safe operation, without interruption, of a high voltage power system. This type of equipment is important because it is directly linked to the quality of the electricity supply. High voltage switchgear used to more than 35,000 volts.
5.7 Equipments of Switchgear
a) Switches: A switch is a device which is used to open or close an electrical circuit in a convenient way. It can be used under full-load conditions but it cannot interrupt
the fault currents. The switches may be classified into
i. Air-break switch
ii. Isolator or disconnecting switch .
iii. Oil switches
b) Fuses: A fuse is a short piece of wire or thin strip which melts when excessive current flows through it for sufficient time. It is inserted in series with the circuit to be protected.
c) Circuit breakers: A breaker is equipment which can open or close a circuit under all conditions viz. no load, full load conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault condition.
d) Relays: A relay is a device which detects the fault and supplies information to the breaker for circuit interruption.
5.8 HT Switchgear
Fig. 5.1 HT switchgear
The term HT switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. Switchgear is already a plural, much like the software term code/codes, and is never used as switchgears.
The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.
A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.
Fig. 5.2 Internal Arrangement of Circuit breaker
A circuit breaker is a piece of equipment, which can
(i) Make or break a circuit either manually or by remote control under normal condition.
(ii) To break a circuit automatically under fault condition.
(iii) Make a circuit either manually or remote control under fault conditions.
The 10 ampere DIN rail mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe.
5.10 Classification of Circuit Breaker
Classification of Circuit Breaker as follows
(i) Air Beak circuit breaker
(ii) Oil Circuit breaker
(iii) Minimum oil circuit breaker (M.O.C.B)
(iv) Sulphur hexafluoride Circuit Breaker (SF6)
(v) Vacuum Circuit Breaker (VCB)
• Oil Circuit breaker
a) Bulk oil circuit breaker
b) Plain Breaker Oil circuit Breakers (POCB)
c) Arc control Oil circuit Breakers
• Self-blast Oil circuit Breakers
(i) Plain explosion pot
(ii) Cross jet explosion pot
(iii) Semi compensated explosion pot
• Self blast oil circuit breakers
(i) Low Oil circuit Breakers
5.10.1 Air-circuit Breaker (ACB)
In which high pressure air blast is used for extinguishing the arc. Air break circuit breaker is used dc circuit’s ac circuit up to 12 KV. These breakers employ a high pressure air-blast as an arc quenching medium. The contacts are opened in a flow of air-blast established by the opening of blast valve. The air-blast cools the arc and sweeps away the arcing products to the atmosphere.
Fig. 5.3 Air Blast Circuit Breaker (ACB) & Flow of Air around Contacts, In Air Blast Circuit Breakers
This rapidly increases the decreases the dielectric strength of the medium between contacts and prevents from re-establishing the arc. Consequently, the arc is extinguished and flow of current is interrupted.
5.10.2 Oil Circuit Breakers (OCB)
Oil Circuit Breakers are used to switch circuits and equipment in and out of a system in a substation. They are oil filled to provide cooling and to prevent arcing when the switch is activated.
Fig. 5.4 Oil circuit breaker
5.10.3 Plain Break Oil circuit Breakers (POCB)
A plain breaker circuit breaker involves the simple process of separating the contacts under the whole of the oil in the tank. There is no special system for are control other than the increase in length caused by the separation of contacts. The arc extinction occurs when a certain critical gap between the contacts is reached.
Under normal operating conditions, the fixed and moving contacts remain closed and the breaker carries the normal circuit current. When a fault occurs, the moving contacts are pulled down by the protective system and an arc is struck which vaporizes the oil mainly into hydrogen gas.
Fig. 5.5 Plain Break Oil circuit Breakers (POCB)
5.10.4 Sulphur Hexafluoride Circuit Breaker (SF6)
In which sulphur-hexa-Fluoride, (SF6) gas is used for arc extinction. In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a pressure of about 2.8 Kg/cm2. When the breaker operates, he moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the opening of a valve which permits SF6 gas at 14Kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The Simplicity of the interrupting chamber which does not need an auxiliary chamber for breaking autonomy provided by the buffer technique.
Fig. 5.6 Sulphur Hexafluoride Circuit Breaker (SF6)
Several characteristics of SF6 circuit-breakers can explain their success:
(i) The possibility to obtain the highest performances, up to 63 KA, with a reduced number of interrupting chambers.
(ii) Short break time of 2 to 2.5 cycles.
(iii) High electrical endurance, allowing at least 25 years of operation without reconditioning.
(iv) Possible compact solutions when used for GIS or hybrid switchgear.
(v) Integrated closing resistors or synchronized operations to reduce switching over voltages.
(vi) Reliability and availability.
(vii) Low noise level.
(viii) Sulphur Hexafluoride CB is used in high voltage up to 245 KV.
5.10.5 Vacuum Circuit Breaker (VCB) Operation
When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of the arc due to the ionization
Fig. 5.7 Vacuum circuit breaker
of metal ions and depends very much upon the materials of the contacts. The arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields. Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in vacuum breaker occurs which a short contact separation (say 0.625 cm).
5.11 Lightning Arrester
A lightning arrester is a device used on electrical power systems to protect the insulation the system from the damaging effect of lightning.
Fig: 5.8.1 Arrester, Fig. 5.8.2 Surge arrester, lightning arrester, Metal Oxide Surge Arrester, Fig. 5.8.3 Substation Lightning Arrester 198kV , Fig. 5.8.4 Lightning Arrester for Transmission Line 220kV, Fig. 5.8.5 Surge Arrester (Distribution Type)
Metal oxide varsities (MOVs) have been used for power system protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth.
5.12 Isolator Switch
In electrical engineering an isolator switch is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair.
Fig. 5.9 Isolator switch
High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance. Isolating switches are commonly fitted to domestic extractor fans when used in bathrooms in the UK. Often the isolation switch is not intended for normal control of the circuit and is only used for isolation. Isolator switches have provisions for a padlock so that inadvertent operation is not possible (see: Lock and tag). In high voltage or complex systems, these padlocks may be part of a trapped-key interlock system to ensure proper sequence of operation. In some designs the isolator switch has the additional ability to earth the isolated circuit thereby. Providing additional safety such an arrangement would apply to circuits which inter-connect power distribution systems where both end of the circuit need to be isolated.
The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the connector while it supplies a circuit.
5.13 Fuse (electrical)
In electronics and electrical engineering a fuse (short for fusible link) is a type of over current protection device. Its essential component is a metal wire or strip that melts when too much current flows, which breaks the circuit in which it is connected, thus protecting the circuit's other components from damage due to excessive current.
Fig. 5.10 Fuse (electrical)
A practical fuse was one of the essential features of Thomas Edison's electrical power distribution system. Fuses (and other over current devices) are an essential part of a power distribution system to prevent fire or damage. When too much current flows through a wire, it may overheat and be damaged or even start a fire. Wiring regulations give the maximum rating of a fuse for protection of a particular circuit. Local authorities will incorporate national wiring regulations as part of law. Fuses are selected to allow passage of normal currents, but to quickly interrupt a short circuit or overload condition.
A relay is an electrically operated switch. Many relays use an electromagnet to
operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal.
Fig. 5.11 Relay
The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly drive an electric motor is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protection relays".
5.15 Substation Grounding/ Ear thing
The sole purpose of substation grounding/earthling is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthling system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the non current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Ear thing of surge arresters is through the ear thing system. The function of substation ear thing system is,
● To provide discharge path for lightning over voltages coming via rod-gaps, surge arresters, and shielding wires etc.
● To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation.
5.16 Earth Resistance
Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth potential with respect to a remote earth fault. Persons touching any of the non current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each structure, transformer tank, body of equipment, etc, should be connected to ear thing mat by their own earth connection. Generally lower earth resistance is preferable but for certain applications following earth resistance are satisfactory
• Large Power Station s– 0.5 Ohm
• Major Power Stations – 1.0 Ohm
• Small Substation – 2.0 Ohm
• In all Other Cases – 8.0 Ohm.
5.17 Step Potential and Touch Potential
Grounding system in an electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’.
a) Step Potential
Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault current through the grounding system.
b) Touch Potential
Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e. The Touch Potential should be very small.
Fig. 5.12 Step Potential and Touch Potential
5.18 Types of Grounding
a) Un earthed Systems:
It is used no more. The neutral is not connected to the earth, also called as insulated neutral system.
b) Solid grounding or effective grounding:
The neutral is directly connected to the earth without any impedance between neutral and ground.
c) Resistance grounding:
Resistance is connected between the neutral and the ground.
d) Reactance grounding:
Reactance is connected between the neutral and ground.
e) Resonant Grounding:
An adjustable reactor of correctly selected value to compensate the capacitive earth current is connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth Fault Neutralizer.
The protection which use for supply electric power from generating station to consumers is discussed in this chapter.
The type of protection used in a substation is often determined by the size and importance of the substation. Normally higher voltage substations with larger transformer sizes require more intricate protection schemes whereas smaller substations may require only minimal protection.
Substation protection schemes are designed to protect the equipment in the substation, the lines supplying the substation’s power, and the lines leaving the substation. In most cases, a breaker or circuit switcher is used as the main protective device on the high voltage side of the substation transformer.
Substations generally contain one or more transformers and have switching, protection and control equipment. In a substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may use re-closer circuit breakers or fuses for protection of branch circuits. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, grounding or earthing system and metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.
Bus bars are the important components in a sub-station .When a number of lines operating at the same voltage have to be directly connected electrically, bus bar are used , it is made up of copper or aluminum bars and operate at constant voltage . The incoming and outgoing lines can be connected to either bus bar with help of a bus bar coupler which consist of a circuit breaker and isolators. There are several bus bar arrangements that can be used in a sub station. The choice of a particular arrangement depends upon various factors such as system voltage, position of a sub-station, degree of reliability, cost etc.
6.2 Bus Bar
A bus bar in electrical power systems refers to thick strips of copper or aluminum that conduct electricity within a switchboard, distribution board, substation, or other electrical apparatus. These are made up of copper and aluminum to which the terminal of generators, transformers, transmission lines, distribution lines, loads etc is connected. These bus bars are insulated from each other and also from the earth. The size of the bus bar is important in determining the maximum amount of current that can be safely carried. Bus bars can have a cross-sectional area of as little as 10 mm² but electrical substations may use metal tubes of 50 mm in diameter (1,963 mm²) or more as bus bars.
Fig. 6.1 11 KV Bus bar Arrangement
6.3 Protection of Bus Bar
Bus bars are vital parts of a power system and so a fault should be cleared as fast as possible. A bus bar must have its own protection, although they have high degrees of reliability. Bearing in mind the risk of unnecessary trips, the protection should be dependable, selective and should be stable for external faults, called 'through faults'. The most common fault is phase to ground, which usually results from human error. There are many types of relaying principles used in bus bar. A special attention should be made to current transformer selection since measuring errors need to be considered.
6.4 Different type of Bus bar System
(i) Single Bus bar arrangement
(ii) Main and transfer Bus bar system
(iii) Sectionalized Bus bar system
(iv) Double Bus bar single breaker system
(v) Double Bus bar with Double breaker system
(vi) Double main and transfer system
(vii) One and half breaker System
(viii) Mesh Ring Bus bar system
6.4.1 Single Bus bar Arrangement
The single bus bar has only one three-phase bus to which the various incoming and outgoing circuits are connected as shown in fig 3.2.In the event of a bus fault or a breaker failure, the entire bus has to be de-energized and a major outage occurs. Dependence on only one main bus gives lack of operational flexibility. Low cost, Simplicity in operation and protection are the major advantages of the scheme.
There are some demerits of this scheme such as fault of bus or any circuit breaker results in shut-down of entire substation; difficult to do any maintenance; bus cannot be extended without completely de-energizing substation.
Fig. 6.2 Single Bus bar Arrangements
6.4.2 Main and Transfer Bus
In this scheme between two bus bars, there are two circuit breakers and four isolators as shown in fig.3.3.Two circuit breakers are required per circuit. Isolators cannot be switched on or off while carrying full load current. Load current switching in or switching off is by circuit breakers.
Fig. 6.3 Main and Transfer Bus bar Arrangement
6.4.3 Sectionalized Bus bar Arrangement
In a Sectionalized Bus bar arrangement shown in fig 7.4, the main bus is divided into two or more sections with a circuit breaker and isolator’s in-between the adjoining sections. One section can be completely shutdown for the purpose of bus maintenance, repairs or extension without disturbing the continuity of other bus section. The numbers of sections depend unto the importance of the station and local switching requirements. The fault level of each bus can be reduced by installing a current limiting reactor (series reactor) in between the two adjoining sections.
Fig. 6.4 Sectionalized Bus bar Arrangement
6.4.4 Double Bus bar with Double Breaker System
The connection between each incoming and outgoing circuits and the duplicate bus is through either through two circuit-breakers and two insulators, one circuit-breaker, one isolator for each bus bar called Double Bus bar double breaker system.
Fig. 6.5 Double Bus bar with Double Breaker system
6.4.5 Mesh or Ring Bus bar System
In the ring bus scheme as shown in fig. the bus bar and the breakers are in series to form a ring. The circuits are connected between the breakers. The number of breakers is equal to the number of circuits. During the normal operation, all the circuit breakers are closed. During a circuit fault, two breakers in the associated bus bar are tripped. If one of these breakers fails to clear the fault, an additional circuit will be tripped by breaker–stuck-up back up relay. During the breaker maintenance, the ring is opened but all the circuits continue to serve.
In the ring scheme sources and circuits are connected alternately. During breaker maintenance no charge in protective relay is required. In case of bus bar fault, the immediate result is similar to that of single bus bar scheme; that all the circuits are lost. However, the fault can be isolated by opening the bus bar isolator on either sides and most of the circuits can be re-energized.
Fig. 6.6 Mesh or Ring Bus bar System
18.104.22.168 Advantages of Mesh Bus bar System
(i) Bus bars gave some operational flexibility.
(ii) Either Main bus may be isolated for maintenance.
(iii) Circuits can be transferred from one bus to the other by use of other bus coupler and the bus selector isolators.
(iv) Most widely used for very large power stations having large number of incoming and outgoing lines and high power transfer.
22.214.171.124 Disadvantages of Mesh Bus bar System
(i) If fault occurs during bus maintenance, ring gets separated into two sections.
(ii) Auto-re-closing and protection complex.
The apparatus bus bars are used to carry very large currents, or to distribute current to multiple devices within switchgear or equipment.
In electrical power distribution, a bus bar (sometimes incorrectly spelled as bus bar, buss bar or bus bar) is a strip or bar of copper, brass or aluminium that conducts electricity within a switchboard, distribution board, substation, battery bank or other electrical apparatus. Its main purpose is to conduct electricity, not to function as a structural member.
MAINTENANCE AND TROUBLE SHOOTING OF INDOOR TYPE SUBSTATION
The lack of maintenance may result in failure in operation. The switch gear manufacturer supplies instruction manual of station, operation & maintenance. This manual should be carefully studied by trained maintenance staff.
Maintenance is work that is carried out to preserve equipment, in order to enable its continued use and function, above a minimum acceptable level of performance, over its design service life, without unforeseen renewal or major repair activities.
7.3 Classification of Maintenance
Maintenance is classified in two categories as follows,
(i) Breakdown or corrective maintenance
The breakdown or corrective maintenance activities are undertaken after failure of equipment.
(ii) Preventive maintenance
For switch gear and protective equipment, preventive maintenance is recommended because failure of a switch gear cannot be permitted.
7.4 Routine Operation
The substation operations for 24 hours accept the interruption due to any internal/external faults or failure of generating station. So it requires continuous operation.
7.5 System Maintenance
Maintenance covers a wide range at activities aimed at keeping the equipment in
perfect working condition for performing its function as per assigned duties. The following considerations are required for successful implementation of maintenance.
(i) The pre-requisite for any maintenance program in the system is well planned properly created with good quality material.
(ii) The manor defects noticed during inspection should be removing at the time of the occasion after chalking out a program in advance.
(iii) A correct record of all inspections and test result should be maintained.
(iv) Required safety precaution must be observed while carrying out any maintenance work.
(v) Manufacturer’s instruction should always be given due to consideration while carrying out the maintenance of particular equipment.
(vi) Suitable inspection & maintenance register & chart should be maintained.
7.6 Maintenance of Transformers
When a transformer winding is suddenly subject to a short current, a rapid size in winding temperature can be expected. This temperature rise can cause conductor annulling and insulation decomposition which produces gas. A mechanical weakening of the winding can occur because to thermal aging. Insulation aging as a result of short circuit however is of normal concern.
Fault on a distribution system are characterized by relatively long duration of relation reducing in to the fault. The continuous operations of transformer cause a gradual relaxation of their strength. There are two types of physical changes in service. When a compression it is not capable of retaining its original dimension & loading pressure over a long period of time. The second change is the imbrittlement of the insulation.
The net effect of all this is a reduction in the mechanical capability of the transformers and consequently an increase in the probability of transformers failure due to short circuits.
7.7 Maintenance of Circuit Breaker
Circuit breaker needs periodic checkup and maintenance for reliable, smooth of trouble free operation. The following recommendation should be followed.
(i) To check main contacts, arcing contacts auxiliary & interlocking contacts, to look for any carbon deposits, metal beads on surface or excessive roughness, to dress up contacts with a file or to replace as necessary.
(ii) To check oil level of its several conditions to check dielectric strength of oil. to replace if oil is carbonized or is not of requisite strength.
7.8 Recommendation Maintenance
(i) Check charge voltage settings
(ii) Check cell voltage (30mv from average is acceptable)
(iii) High voltages charge if agreed for application
•Every two years
(i) Clean cell lids and battery area
(ii) Check torque values, grease
(iii) Terminals and connectors
•Every five years
(i) Capacity check as required
(ii) Top up with weather according to defined period (depend on float voltage, cycles and temperature).
7.9 Maintenance of Indoor Type Substation
Effective performance of a sub-station is mainly dependent on proper maintenance.
i) Routine checking, servicing, cleaning of H.T switchgear and its components.
ii) Routine checking, servicing, cleaning of VCB.
iii) Routine checking, testing and adjustment of protective relays.
iv) Routine checking of space heater.
v) Routine checking, testing and inspection of transformer.
vi) Quarterly insulation testing of transformer oil and daily checking of oil level. Breakdown capacity of transformer oil must be at the range of 20-28 KV. If it falls under the specified range it can be increased by centrifuging method. Transformer oil should be maintained at a constant level.
7.9.1 Maintenance Chart of Indoor Type Substation
||Transformer Oil Level
||Leakage of oil in the oil pit
||Breakdown test of transformer oil
||Temperature of oil at rated power
||Cooler of silica gel
||P.F Correction device
||H.T & L.T panel check
||Functional check of all indicating instruments (Ammeter, Voltmeter, P.F meter etc.)
||Earth resistance check (Continuity and ohm value)
||Check for fire extinguisher
||Insulation test of power cables
7.10 Supervisory Control and Data Acquisition (SCADA) System
The functions and configuration of Supervisory control and Data Acquisition Systems (SCADA) have been described. SCADA systems are indispensable in the operation and control of interconnected power systems which is shown in fig-
Fig. 7.1 Three levels of control of SCADA system and control system
7.11 Automatic Substation Control by SCADA
The electrical energy is transferred from large generating stations to distant load centers via various substations. In every substation certain supervision, control and protection functions are necessary. Every substation has a control room shown in fig-
Fig. 7.2 SCADA system in Substation
7.12 SCADA Configuration
This figure represents the simplest SCADA configuration employing a single computer. Computer receives data from RTU via the communication interface shown in figure. Operators control base one or more CRT terminals for display with this terminal it is possible to execute supervisory control commands and request and display of data in alphanumerical formats arranged by geographical location and type.
Fig. 7.3 Remote Terminal Unit and interface with computer Simple SCADA
system with single computer
The programming input/output is used for modifying the supervisory software. The basic SCADA system, all the programs and the data is stored in the main memory. The more sophisticated version of SCADA has additional auxiliary memories in the form of magnetic disk unit.
7.13 Trouble Shooting
Troubleshooting is a form of problem solving, often applied to repair failed products or processes. It is a logical, systematic search for the source of a problem so that it can be solved, and so the product or process can be made operational again. Troubleshooting is needed to develop and maintain complex systems where the symptoms of a problem can have many possible causes.
7.13.1 Trouble Shooting Cause & Remedy.
||i. Transformer winding short current.
ii. Insulation decomposition which produces gas
iii. High Temperature of Transformer oil
iv. Silica jell capacity damaged & change is color.
|i. We have to Change transformer winding
ii. We have to Change insulation
iii. We have to Change Temperature oil.
iv. Simple to replace as necessary.
|Circuit Breaker Failure
||i. Main Contacts failure
ii. Carbon deposits.
iii. Metal beads on surface or excessive roughness.
iv. Arcing contacts auxiliary & interlocking contacts fault
We have to change circuit breaker.
|Ground current protection
||i. Ground fault has occurs.
||We have to change cable or overload.
||i. Relay coil failure.
ii. Auxiliary supply failure.
|We have to change Relay.
||i. Low capacity bus-bur.
ii. Over load current supply.
iii. One unit bas-bur to another Section supply.
iii. Long time use for bas-bur.
|i. High capacity bus-bar Setting.
ii. Section by Section bas-bur Setting.
|Current limit Protection
||i. Output short or ground fault has occurred.
||We have to change cable.
Substations needed various type of maintenance (such as preventive & breakdown) to continue its operation. Preventive maintenance is to increase its service life and when a fault occurs then needed to break down maintenance.
REDUCTION SYSTEM LOSSES & POWER FACTOR IMPROVEMENT
The utility industry today has placed a high level of importance on improving efficiency. A proper review of losses experienced on a utility’s system can provide valuable insight into ways to manage these losses and improve efficiency while reducing wholesale power costs, improving voltage levels, and freeing up system capacity, potentially reducing costly investment in system improvements.
Electrical losses are a reality due to the physics associated with various system components that make up the power system. The effect of losses can be compared to a pipe that is being constricted as demand and ambient air temperature increase, thus limiting the amount of power and energy available at the end-use meter for the same amount of net generation put into the system.
The electrical energy is almost exclusively generated, transmitted and distributed the form of alternating current. Therefore, the question of power factor immediately comes into picture. Most of loads are (e.g. induction motors arc lamps) are inductive in nature and hence have low lagging power factor. The low power is highly undesirable as it causes an increase in current, resulting in additional loses of active power in all the elements of power system from power station generator down to the utilization devices. In order to ensure most favorable conditions for supply system from engineering and economical standpoint, it is important to have power factor as close to unity as possible.
8.2 Line Losses
Whole produced effective power is not useable for any system. The loss which is occurred to transmit the power from the generation end to grid sub-station end and the use of power for the system own is called transmission loss or line loss.
8.3 Types of Line Losses
There are several types of losses are introduce on transmission line.
In this section transmission lines losses actually occur in all lines. Line losses may be any of three types: (a) Copper Losses, b) System Loss
8.3.1 Copper Losses
One type of copper loss is I²R loss. In transmission lines the resistance of the conductors is never equal to zero. Whenever current flows through one of these conductors, some energy is dissipated in the form of heat. This heat loss is a power loss. With copper braid, which has a resistance higher than solid tubing, this power loss is higher. Another type of copper loss is due to skin effect. Since resistance is inversely proportional to the cross-sectional area, the resistance will increase as the frequency is increased. Also, since power loss increases as resistance increases, power losses increase with an increase in frequency because of skin effect. These losses can be minimized and conductivity increased in an transmission line by plating the line with silver. Since silver is a better conductor than copper, most of the current will flow through the silver layer. The tubing then serves primarily as a mechanical support.
8.3.2 System Loss
Transmitting electricity at high voltage reduces the fraction of energy lost to resistance. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. In an alternating current circuit, the inductance and capacitance of the phase conductors can be significant. The currents that flow in these components of the circuit impedance constitute reactive power, which transmits no energy to the load. Reactive current causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. Utilities add capacitor banks and other components (such as phase-shifting transformers; static VAR compensators; physical transposition of the phase conductors; and flexible AC transmission systems (FACTS) throughout the system to control reactive power flow for reduction of losses and stabilization of system voltage. Technical loss is electricity that escapes as heat from the distribution lines and transformers when electricity is brought to the customer s premises at the voltage that the customer needs. Non-technical system loss is the electricity lost through theft or pilferage in the process of distributing power. Technical losses are inherent in any electric system and are, therefore, legitimate costs recoverable in power rates, while pilferage has become a perennial problem among utilities in the Bangladesh and has increased the overall system loss of these utilities.
8.4 Power Factor
The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit or Power factor is defined as the ratio of active power W to the apparent power VA.
Thus, Power Factor (p.f) = W/VA = Cosf.
8.5 Importance of Power Factor Improvement
The improvement of power factor is very important for both consumers and generating stations as discussed below.
8.5.1 For Consumer
A consumer has to pay electricity charges for his maximum demand in KVA plus the units consumed. If the consumer improves the power factor, then there is a reduction in his maximum KVA demand and consequently there will be annual saving due to maximum demand charges. Although power factor improvement involves extra annual expenditure on account of p.f. correction equipment, yet improvement of power factor to a proper value results in the net annual savings for the consumer.
8.5.2 For Generating Station
A generating station is as much connected with power factor improvement as the consumer. The generators in a power station are rated in KVA but the useful output depends upon KW output. As station output in KW=KVA cosφ, therefore numbers of units supplied by it depends upon the power factor. The greater the power factor of the generating station, the higher the KWh is delivered to the system. This leads to the conclusion that improved power factor increases the earning capacity of the power station.
8.6 Methods of Power Factor Improvement
Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower in such cases it is generally desirable to take special steps to improve the power factor. This can be achieved by the following equipments.
a. Static capacitor.
b. Synchronous condenser.
c. Phase advancers.
8.6.1 Static Capacitor:
The power factor can be improved by connecting capacitor in parallel with the equipment operating at lagging power factor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load. For three phase loads, the capacitor can be connected in delta or star as shown in figure. Static capacitors are invariably used for power factor improvement in factories.
Fig. 8.1 Static Capacitor Arrangement
i) They have low losses.
ii) They require little maintenance.
iii) They can work under ordinary temperature condition
i) They have short service life ranging from 8 to 10 years
ii) They are easily damaged if the voltage exceeds the rated value
iii) Once the capacitors are damaged, their repair is uneconomical.
8.6.2 Synchronous Condenser
A synchronous motor takes a leading current when over excited and therefore, behaves as a capacitor. And over excited synchronous motor running on no load is called synchronous condenser. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.
Fig. 8.2 Synchronous condenser arrangement
i) By varying the field excitation the magnitude of current drawn by the motor can be changed by any amount.
ii) The motor windings have high thermal stability to short circuit current.
iii) The fault can be removed easily.
i) There are considerable losses in the motor
ii) The maintenance cost is high
iii) Except in sizes above 500 KVA, the cost is greater than that of static capacitors in same ratings.
8.6.3 Phase Advancers
Phase advancers are used to improve the power factor of induction motors. The low power factor of an induction motor is due to the fact that its stator winding draws exciting current which lags behind the supply voltage by 90°. If the exciting ampere turns can be provided from some other a.c. source, then the stator winding will be relieved of exciting current and the power factor of the motor can be improved. This job is accomplished by the phase advancer which is simply an a.c. exciter. The phase advancer is mounted on the same shaped as the main motor and is connected in the rotor circuit of the motor. It provides exciting ampere turn to the rotor circuit at slip frequency. By providing more ampere turns then required, the induction motor can be make to operate on leading power factor like an over excited synchronous motor.
i) Firstly, as the exciting ampere turns are supplied at slip frequency, therefore, lagging KVAR drawn by the motor are considerably reduced.
ii) Secondly, phase advancer can be conveniently used where the synchronous motors is inadmissible.
i) The major disadvantage of phase advancer is that they are not economical for motors below 200 H.P
8.7 Shunt Capacitors and Power Factor Improvement
The function of shunt capacitors applied in the form of a single unit or bank is to supply capacitive volt-ampere to the system at the point of connection. The shunt capacitors compensate the lagging KVAR absorbed by the inductive loads. The shunt capacitors improve the power factor and thereby reduce the total KVA demand. The capacitor Bank figure shown in,
Fig. 8.3 Capacitor Bank
Shunt reactors are used to improve substation efficiency by adding inductive load to counterbalance capacitive loads. A shunt reactor, like the one shown in Figure, looks like a power transformer, except that the bushings on a shunt reactor are connected with the source circuit; there are no major connections leading out of a shunt reactor (such as the secondary connections on a power transformer).
8.8 Advantage of Shunt Capacitor Banks Connected at load/receiving end
i) Reduce lagging current through supply circuit.
ii) Reduced I²R losses supply line.
iii) Savings energy and economy.
iv) Increased voltage at load end during full load. Reduced Voltage fluctuations at load end.
v) Improved voltage regulation if capacitor units are properly switched.
vi) Reduced KVA demand, hence same transformer and distribution circuit having certain rated KVA can deliver higher KW.
8.9 Power Factor, KVA, KVAR, KW
Power factor is defined as the ratio of active power W to the apparent power VA.
Power Factor (p.f) = W/VA = Cosf
KW = KVA´ P.F
In a 3-phase circuit, KVA=Ö3VI/1000
KW = Ö3VIcosf/1000
Tanf = KVAR/KW
Cosf = KW/ KVA
1. Shunt capacitor bank off position:
Power factor, Cosf1= .90
Load, MW= 40
Voltage, V1 = 32000 V
Current, I1= 200 A
KVAR= Ö3´V´I ´Sinf/1000, VAR1= Ö3´32000
= 4.83 MVAR
2. Shunt capacitor bank ON position:
Power factor, Cosf2= .95
Load, MW= 40.85
Voltage, V2 = 33000 V
Current, I1= 180 A
KVAR= Ö3´V´I ´Sinf/1000
KVAR2= Ö3´33000´180 ´Sinf
= 3.21 MVAR
Finally, when capacitor bank on,
Voltage Increase = 33000-32000=1000 V
Active Power Increase = 40.85 -40= 0.85 MW
Reactive Power Decrease = 4.83- 3.21= 1.62 MVAR
System loss = (40.85-40)/40= 0.02125= 2.125% which are reduce.
I see this calculation when capacitor bank on position the system voltage Increase and the KVAR and KVA are reduce. So Active Power Increase and the system loss are decrease shown in fig,
Fig. 8.4 Calculating Value of shunt capacitor bank
8.10 Power Factor Improvement
The low power factor is mainly due to the fact that most of the power loads are inductive and therefore take lagging currents. In order to improve the power factor, some device taking leading power factor should be connected in parallel with the load. One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load.
Fig. 8.5 Lagging & leading power factor and their vector diagram.
The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor. Linear loads with low power factor can be corrected with a passive network of capacitors or inductors. Non-linear loads distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor. The devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment.
The main issues facing a power engineer are reliability and cost to design a substation. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost. The design should also allow expansion of the station, when required.
1.2 Consideration of substation designing
Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers. Where land is costly, such as in urban areas, gas insulated switchgear may save money. The site must have room for expansion due to load growth or planned transmission additions. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects. A grounding (earthing) system must be designed. The total ground potential rise, and the gradients in potential during a fault (called "touch" and "step" potentials), must be calculated to protect passers-by during a short circuit in the transmission system. The substation site must be reasonably central to the distribution area to be served. The site must be secure from intrusion by passers-by, both to protect people from injury by electric shock or arcs, and to protect the electrical system from disoperation due to vandalism.
The first step in planning a substation layout is the preparation of a one-line diagram which shows in simplified form the switching and protection arrangement required, as well as the incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by many electrical utilities to prepare one-line diagrams with principal elements (lines, switches, circuit breakers, and transformers) arranged on the page similarly to the way the apparatus would be laid out in the actual station.
In a common design, incoming lines have a disconnect switch and a circuit breaker. In some cases, the lines will not have both, with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off, or to cut off a line when power is flowing in the 'wrong' direction. When a large fault current flows through the circuit breaker, this is detected through the use of current transformers. The magnitude of the current transformer outputs may be used to trip the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the rest of the system to continue operating with minimal impact. Both switches and circuit breakers may be operated locally (within the substation) or remotely from a supervisory control center.
Once past the switching components, the lines of a given voltage connect to one or more buses. These are sets of busbars, usually in multiples of three, since three-phase electrical power distribution is largely universal around the world.
The arrangement of switches, circuit breakers and buses used affects the cost and reliability of the substation. For important substations a ring bus, double bus, or so-called "breaker and a half" setup can be used, so that the failure of any one circuit breaker does not interrupt power to other circuits, and so that parts of the substation may be de-energized for maintenance and repairs. Substations feeding only a single industrial load may have minimal switching provisions, especially for small installations.
Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a "short circuit").
Along with this, a substation always has control circuitry needed to command the various circuit breakers to open in case of the failure of some component.
1.3 Switching function
An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".
A transmission line or other component may need to be reenergized for maintenance or for new construction, for example, adding or removing a transmission line or a transformer.
To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.
Perhaps more important, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.
There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down; similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.
Early electrical substations required manual switching or adjustment of equipment, and manual collection of data for load, energy consumption, and abnormal events. As the complexity of distribution networks grew, it became economically necessary to automate supervision and control of substations from a centrally attended point, to allow overall coordination in case of emergencies and to reduce operating costs. Early efforts to remote control substations used dedicated communication wires, often run along side power circuits. Power-line carrier, microwave radio, fiber optic cables as well as dedicated wired remote control circuits have all been applied to Supervisory Control and Data Acquisition (SCADA) for substations. The development of the microprocessor made for an exponential increase in the number of points that could be economically controlled and monitored. Today, standardized communication protocols such as DNP3, IEC 61850 and Modbus, to list a few, are used to allow multiple intelligent electronic devices to communicate with each other and supervisory control centers. Distributed automatic control at substations is one element of the so-called smart grid.
In a substation designing various type of things consideration such as reliability, cost, sufficient land area is required for installation of equipment, Environmental effects, grounding (earthing) system, switching functions, automation and also protection system of substation.
FUTURE WORK AND CONCLUSION
Electricity is the basic necessity for the economic development of a country. For the Industrial, commercial and domestic use reliable power supply is required. For this purpose the sub-station system of PDB / PGCB/ DPDC/ REB has been studied in this dissertation work.
Sub-Station is an important and integral part of power distribution system. In this project we have studied different essential elements of sub-station, such as H.T and L.T switchgear, circuit breaker, transformer, relays, lightning arresters, isolators, ear thing, C.T & P.T and other protective devices.
From this study, it is clear that adequate numbers of distribution sub-station should be installed in the distribution network for greater reliability of supply and increased system stability.
Sub-Stations have to be designed and installed according to the prevailing and future demands of the consumers.
One of the common problems is that incoming voltage is not always available. This impedes the supply & distribution system. By manipulating tap changer voltage level can be maintained. We have found this study very useful and helpful for better understanding of design and installation of indoor type distribution sub-station.
Sub-Station is an important installation at the consumer end. This is very essential for reliable and stable supply of electric power to the consumer. Sub-stations provide the electricity need of different Consumers by different feeders. Each feeder carries a particular amount of load of a certain area. Feeders are used for load management, as we see load shedding program especially in the evening peak load period. If reliable and stable power supply is available to the consumers, industrial, commercial, and agricultural sectors will definitely be developed. The economic growth will be increased, and the country will be developed as well. Our Internship topic is Study & Analysis of Electrical Design, Construction & Maintenance of Distribution Indoor Type Sub-Station. From our analysis we notice following points.
10.2 Future Work
In the modern world, no country can achieve their long term economic goals without having a sustainable power sector. Though the generation capacity of our power sector is limited but an efficient distribution system can mitigate the problem a great extent. A customer focused distribution system can relieve the miseries of the common people. Attaining the maximum consumer satisfaction level should be the primary concern of DPDC.
On the basis of the study it can be said that following the improvement initiatives mentioned bellow DPDC will be able to serve its consumers in a better way.
10.3 Effective Load Management
• Area wise even distribution of electricity according to demand.
• Updated load shedding schedule published in the web should be updated frequently.
• Distributing load shedding evenly throughout different segments of the day rather than continuing it at a stretch for hours.
• Increasing vigilances to shutting down markets, shopping malls, etc. after 8:00 PM.
• Increasing customer awareness to reduce misuse of electricity through electronic and print media.
10.4 Direct Connectivity with the Banks
DESCO IT needs to establish direct connectivity with all the banks. It can reduce the bill updating time to less than a day where as the current updating time is 15 to 30 days.
10.5 Implementing E-Governance
In order to bring transparency in the whole process, e-Governance may be introduced broadly. A full fledged e-Governance will help eliminating traditional slow movement practice of files. In this way customers will get service easily and quickly.
10.6 Enhancing Prepaid Metering System
Prepaid metering system helps reducing system loss and wastage of electricity. Customers also get benefit in two ways; by controlling their usage and getting discounts from DPDC. DPDC should take initiative to replace its entire domestic tariff post-paid meters by pre-paid meters phase by phase.
10.7 Making Data Available to Consumers and frequently updating the Website
The feeder database of consumers, meter reading sheets, ledger and monthly payment bills should be kept and checked regularly and should be made available to consumers. The DPDC website should be updated daily which will facilitate the consumer to see their bill payment status.
10.8 Generating Electricity
With the huge financial strength DPDC can easily set up its own 500 MW peaking power plant near Dhaka City and by this it can eliminate the peak hour load shedding.
Now a day’s electricity has played a vital role for the economic development of the country as well as makes people’s life easy and comfortable. Reliable, uninterrupted, safe and adequate power supply is a pre-requisite for the development of the country. In this report, I have described the total procedure about power transmission, distribution, operation, controlling and various protection systems of Indoor Type Substation/ Substation.
PFI Power factor improvement.
P.F Power factor
f Power factor angle
KVAR Kilo-volt ampere reactive
C.T Current Transformer
P.T Potential Transformer
H.T High Tension
L.T Low Tension
ACR Automatic Circuit Reclosure
OCR Oil Circuit Reclosure
C.B Circuit Breaker
MCB Miniature Circuit Breaker
MCCB Molt ate case Circuit Breaker
SF6 Sulphur Hexafluoride
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