Study of the Design, Construction & Maintenance of Distributed Indoor Type Sub-Station
CHAPTER-1
INTRODUCTION
1.1 An Electrical 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. The word substation comes from days before the distribution system became a grid. The first substations were connected to only one power station where the generator was housed, were subsidiaries of that power station.
Fig 1.1: Diagram of an electrical system.
An Electrical Power Substation receives electric power from generating station via transmission lines and delivers power via the outgoing transmission lines. 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. Electrical power substation basically consists of number of incoming circuit connections and number of outgoing circuit connections connected to the bus bars. Bus bars are conducting bars to which number of circuit connections is connected. Each circuit has certain number of electrical components such as circuit breakers, Isolators, earth switches, current transformers, voltage transformers, etc.
In a Power Substation there are various indoor and outdoor switchgear and equipment. Transformers are necessary in a substation for stepping up and stepping down of a.c voltage. Besides the transformers, the several other equipment include bus bars, circuit breakers, isolators, surge arresters, Substation Earthling System, Shunt reactors, Shunt Capacitors etc . Each equipment has certain functional requirement. The equipment are either indoor or outdoor depending upon the voltage rating and local conditions In a large power System large number of Generating stations, Electrical Power Substations and load centers are interconnected. This large internet work is controlled from load dispatch center. Digital and voice signals are transmitted over the transmission lines via the Power substations. The substations are interlinked with the load control centers via Power Line Carrier Systems (PLCC). Modern Power System is controlled with the help of several automatic, semi – automatic equipment. Digital Computers and microprocessors are installed in the control rooms of large substations, generating stations and load control centers for data collection, data monitoring, automatic protection and control.
1.2 Functions of Electrical Power Substations
# 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
1.3 Types of Electrical Power Substations
a) Step up or primary Electrical Power substation:
Primary substations are associated with the power generating plants where the voltage is stepped up from low voltage (3.3, 6.6, 11, 33kV ) to 220kV or 400kV for transmitting the power so that huge amount of power can be transmitted over a large distance to load centers.
b) Primary Grid Electrical Power Substation:
Such substations are located at suitable load centers along with the primary transmission lines. At primary Grid Power Substations the primary transmission voltage (220kV or 400kV) is stepped down to secondary transmission voltages (110kV) . This Secondary transmission lines are carried over to Secondary Power Substations situated at the load centers where the voltage is further stepped down to Sub transmission Voltage or Primary Distribution Voltages (11kV or 33kV).
c) Step Down or Distribution Electrical Power Substations:
Such Power Substations are located at the load centers. Here the Sub transmission Voltages of Distribution Voltages (11kV or 33kV) are stepped down to Secondary Distribution Voltages (400kV or 230kV). From these Substations power will be fed to the consumers to their terminals.
Fig 1.3: Basic Substation For Transmission & Distrubtation.
1.4 Basis of Service Rendered
a) Transformer Substation:
Transformers are installed on such Substations to transform the power from one voltage level to other voltage level.
b) Switching Substation:
Switching substations are meant for switching operation of power lines without transforming the voltages. At these Substations different connections are made between various transmission lines. Different Switching Schemes are employed depends on the application to transmit the power in more reliable manner in a network.
c) Converting Substation:
Such Substations are located where AC to DC conversion is required. In HVDC transmission Converting Substations are employed on both sides of HVDC link for converting AC to DC and again converting back from DC to AC. Converting Power Substations are also employed where frequency is to be converted from higher to lower and lower to higher. This type of frequency conversion is required in connecting to Grid Systems.
1.5 Based on Operation Voltage
a) High Voltage Electrical Power Substation:
This type of Substation associated with operating voltages between 11kV and 66kV.
b) Extra High Voltage Electrical Power Substation:
This type of Substation is associated where the operating voltage is between 132kV and 400 KV.
c) Ultra High Voltage Electrical Power Substation:
Substations where Operating Voltages are above 400kV is called Ultra High Voltage Substation
1.6 Based On Substation Design
a) Outdoor Electrical Power Substations:
In Outdoor Power Substations , the various electrical equipments are installed in the switchyard below the sky. Electrical equipment are mounted on support structures to obtain sufficient ground clearance.
b) Indoor Electrical Power 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 1.6: Indoor Substation
1.7 Based on Design Configuration
a) 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.
b) Gas Insulated Electrical Power Substation:
In Gas Insulated Substation 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.
c) 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.
Fig 1.7: Gas Insulated Substation
Different Components Of Substation
Complete Explanation of all the Substation Components Such as Circuit breakers, isolators, Earth Switch, Bus bars, Substation Earthling, CVT, Current Transformer, Voltage Transformer etc with Pictures.
1.8 Gas Insulated Substation
Indoor Gas Insulated Substation : Gas Insulated Substation uses sulfur hex fluoride (SF6) gas which has a superior dielectric properties used at moderate pressure for phase to phase and phase to ground.
1.9 substation Grounding or Earthling
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.
1.11 Distribution substation
A distribution substation receives power from the transmission system and distributes it to an area. It is uneconomical to directly connect electricity consumers to the high-voltage main transmission network, unless they use large amounts of energy, so the distribution sub-station reduces voltage to a value suitable for local distribution.
The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 132 kV, or whatever is common in the area. The output consists of a number of feeders. Distribution voltages are typically medium voltage, between 11 and 33 kV depending on the size of the area served and the practices of the local utility.
The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.
Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line. Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.
1.12 Classification of Sub-Stations
There are several ways of classifying sub-station. The most
Important ways of classifying them are according to
(1) Service requirement and
(2) Constructional features
Ø According to service requirement– A sub-station may be called upon to change voltage level or improving power factor or convert A.C power etc. According to the service requirement, sub stations may be classified into.
i) Transformer sub-stations
ii ) Switching sub-stations
iii) Power factor correction substations
iv) Frequency changer sub-stations
v) Converting sub-stations
vi) Industrial sub-stations
Ø According to constructional features- A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-station are classified as-
i) Indoor sub-stations
ii) Outdoor sub-stations
iii) Underground sub-stations
Fig 1.8: Single Line Diagram Layout Plan of 400KVA Sub-Station
1.13 Elements of a Sub-Station
Substations generally contain one or more transformers, and have switching, protection and control equipment. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use recover 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.
Where a substation has a metallic fence, it must be properly grounded to protect people from high voltages that may occur during a fault in the transmission system. Earth faults at a substation can cause ground potential rise at the fault location. 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.
1.14 Site Selection for Sub-Stations
Sub-stations are important part of power system. The continuity of supply depends to a considerable extent upon the successfully operation of sub-stations. while selecting the site for sub-stations following factors should be considered.
1) It should be located at a proper site. As per as possible, it should be located at the centre of gravity of load. This will minimize the cost of distribution lines, the maintenance and power losses through them.
2) It should provide safe and reliable arrangement. for safety, consideration must be given to the maintenance of regulation clearances, facilities for carrying out repairs and maintenance, abnormally occurrences such as possibility of explosion or fire.
3) It should involve minimum capital cost. Sub-station conditions should be foundation a reasonable depth should be capable providing a strong support equipment.
4) The site should be selected where easy access road is available so that the operations and maintenance could be easy and less expensive.
5) Climate Conditions (Ambient air temperature).
Extremities 5 to 40oC, Normal range 20 to 35oC, Ambient average annual temp 25oC, Average in any one day does not exceed 35oC, Rainfall-average annual
3000mm, Average relative humidity 50-100%, Maximum wind velocity 160 Km/hour.
1.15 General Technical Requirements of a Sub-Station
The general technical requirements of a sub station are as follows
i) Economy of expenditure (i.e.) minimum capital cost & operation and maintenance cost.
ii) Safety of sub-station and personnel.
iii) Reliability.
iv) High efficiency.
v) Good working conditions.
vi) Minimum losses.
vii) Standards- All equipment supplied under this specification
shall conform to the latest editions to the International Electromechanical Commission (I.E.C) or BS specifications.
CHAPTER-2
2.1 Substation Grounding/ Earthling
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. Earthling of surge arresters is through the earthling system. The function of substation earthling system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and lower earth resistance to ensure that
Ø To provide discharge path for lightning over voltages coming via rod-gaps, surge arresters, and shielding wires etc. .
Ø To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation
Ø To provide low resistance path to the earthling switch earthed terminals, so as to discharge the trapped charge (Due to charging currents even the line is dead still charge remains which causes dangerous shocks) to earth prior to maintenance and repairs.
2.2 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 earthling 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
2.3 Step Potential and Touch Potential
Grounding system in a 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.
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2.4 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.
2.5 Different Grounding Equipment in Electrical Substation
Ø Earthling Electrodes
Ø Earthling Mat
Ø Risers
Ø Overhead shielding wire (Earthed)
2.6 H.T Metering Panels
It is used for Instantaneous/Stored Measuring and Recording Currents, Voltages and Energy accumulators (kwh, KVArh lag, KVArh lead and KVAh in forward and reverse directions) in 11KV installations.
Fig 2.6: H.T METER 11 KV SIDE
2.7 Advantages of H.T. metering panel
· Live Parts are not directly accessible, More safety to Electrical Maintenance persons and other personals.
· Incoming and Outgoing by means of UG Cables. (Suitable Detachable gland plates are provided to fit different sizes of cables)
· Double side Earthling is provided in the panel for effective Means of earthling
· Panels are painted with suitable Epoxy and Enamel (standard Shades of IS & IEC standards) to suit indoor and outdoor applications
· Free standing, Is simple and rapid to install
· Having life time more than 25 years
· Maintenance-free live parts
· In conforming with IS & IEC
· Pad locking facility is provided with Tamper proof as per EB Standards
· Benefits from the experience from 1000 functional units Installed nation-wide
2.8 Bus-bar Arrangement in Sub-Stations
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. There are different types of bus bars
i) Single bus-bar system
ii) Single bus-bar system with Sectionalizing
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 ad
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2.9 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 torr), 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.
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 2.9.1: V H.T Switchgear with (VCB)
Fig 2.9.2; 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.
High voltage switchgear was invented at the end of the 19th century for operating motors and others electric machines. The technology has been improved over time and can be used with voltages up to 1,100 kV.
2.10 Substations
Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.
For industrial applications, a transformer and switchgear (Load Breaking Switch Fuse Unit) line-up may be combined in one housing, called a unitzed substation or USS.
Fig 2.10.1: High voltage switchgear
Fig 2.10.2 : A section of a large switchgear panel, in this case, used to control on-board casino boat power generation.
Fig 2.10.3: Tram switchgear
Fig 2.10.4: This circuit breaker uses both SF6 and air as insulating gases; such devices are sometimes called “hybrid switchgear
2.11 Several different classifications of switchgear can be made
Ø By the current rating.
Ø By interrupting rating (maximum short circuit current that the device can safely interrupt)
- Circuit breakers can open and close on fault currents
- Load-break/Load-make switches can switch normal system load currents
- Isolators may only be operated while the circuit is dead, or the load current is very small.
Ø By voltage class:
- Low voltage (less than 1,000 volts AC)
- Medium voltage (1,000–35,000 volts AC)
- High voltage (more than 35,000 volts AC)
Ø By insulating medium:
- Air
- Gas (SF6 or mixtures)
- Oil
- Vacuum
Ø By construction type:
- Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
- Outdoor
- Industrial
- Utility
- Marine
- Draw-out elements (removable without many tools)
- Fixed elements (bolted fasteners)
- Live-front
- Dead-front
- Open
- Metal-enclosed
- Metal-clad
- Metal enclose & Metal clad
- Arc-resistant
- By IEC degree of internal separation
Ø No Separation (Form 1)
Ø Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
Ø Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
Ø Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
Ø Functional units separated from each other (Form 3a, 3b, 4a, 4b)
Ø Terminals for external conductors separated from each other (Form 4a, 4b)
Ø Terminals for external conductors separate from their associated functional unit (Form 4b)
Ø By interrupting device:
- Fuses
- Air Blast Circuit Breaker
- Minimum Oil Circuit Breaker
- Oil Circuit Breaker
- Vacuum Circuit Breaker
- Gas (SF6) Circuit breaker
Ø By operating method:
- Manually-operated
- Motor-operated
- Solenoid/stored energy operated
Ø By type of current:
- Alternating current
- Direct current
Ø By application:
- Transmission system
- Distribution
- By purpose
- Isolating switches (disconnectors )
- Load-break switches
- Grounding (earthing) switches
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.
Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.
2.12 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 is also used to enhance system availability by allowing more than one source to feed a load.
2.13 Safety
To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. For example, if only one of two sources of supply are permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.
Indoor switchgear can also be type tested for internal arc containment. This test is important for user safety as modern switchgear is capable of switching large currents. Switchgear is often inspected using thermal imaging to assess the state of the system and predict failures before they occur.
2.14 Current Transformer & Potential Transformer
a) Current Transformer
A current transformer is the measuring device for metering and protection system. The primary current of the CT is transformed in the required secondary current of 5A or 1A.
Measuring CT’s have different class of accuracy i.e 0.1, 0.2S, 0.2, 0.5S, 0.5 & 1.0 and protection CT’s have 5P10, 5P20 and 10P10. In the case of special protection CT’s (PS class), the knee point voltage (VK) of the CT is specified along with secondary resistance and magnetizing current
The construction of the CT not only depends upon the ratio but also on the short time current rating, burden and class of accuracy.
Fig 2.14: C.T & P.T
Current transformers shall be cast resin insulated, 11kv dry type double core CT with Ratio 50/5, 1st core for metering, 2nd core for protection.
Primary current rating = 50 amps
Secondary current rating = 5 amps
Standard frequency = 50 Hz
Continuous thermal current
rated output = 10 VA
Secondary burden = 30 VA for protection
Quantity of CT = 3 Nos. 15 VA for measuring.
b) Potential Transformer
Potential transformer shall be cast resin insulated, double pole, Potential Transformer, Ratio 11/ .11kv, class 0.5. 50 VA,
(in open delta connection)
Type = Magnetic type
Rated secondary voltage = 110 v
Rated outputs (per phase) = 25VA
Power frequency voltage = 28 Kv (rms)
2.15 Panel Wiring
The manufacture shall provide internal panel wiring and connections, in accordance with the requirements.
All wiring used with in the panel shall conform to the requirements of those specifications and shall be installed and tested at the factory. All wiring shall be neatly and carefully installed. Instruments, meters, control switches & protective relays shall be mounted on the front panel only. Panel output, mounting studs and support brackets shall be accurately located.
2.16 Protective Relays
Protective relays, as specified shall be semi flush-mounted draw-out type designed for use with 5A, 50Hz, current circuits, shall be indicated type equipped with operation indicator.
The protective relays should be sufficient for over current and earth protection. Protective relays shall comply ICE-255 standards.
CHAPTER-3
3.1 TRANSFORMER
A transformer is a device that 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.
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Fig 3.1: Transformer
Fig 3.1.2: Transformer showing the primary and secondary windings
3.2 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.
3.3 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 3.3: 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.
3.4 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 ? 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 magneti 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
3.5 Ideal power equation
Fig 3.5: 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 turns ratio For example, if an impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.
3.6 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 EMFs. 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.
3.7 Practical considerations
a) Leakage flux
Fig 3.7: Leakage flux of a transformer:
The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings. Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss (see “Stray losses” below), but results in inferior voltage regulation causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Transformers are therefore normally designed to have very low leakage inductance.
However, in some applications, leakage can be a desirable property, and long magnetic paths, air gaps, or magnetic bypass shunts may be deliberately introduced to a transformer’s design to limit the short-circuit current it will supply.Leaky transformers may be used to supply loads that exhibit negative resistance such as electric arcs mercury vapor lamps and neon signs or for safely handling loads that become periodically short-circuited such as electric arc welders.
Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a direct current flowing through the windings.
Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if the “per-unit” inductance of two transformers is the same (a typical value is 5%), they will automatically split power “correctly” (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger one will carry twice the current).
3.8 Effect of frequency
Transformer universal EMF equation
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 waveshape:
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 is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. 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. As such, the transformers used to step down the high over-head line voltages (e.g. 15 kV) are much heavier for the same power rating than those designed only for the higher frequencies.
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.
3.9 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 hysteresis 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. (Also see energy efficient transformer.
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 arise 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) Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis 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
Ferromagnete 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.
Magnetostriction
Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and can cause losses due to frictional heating.
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.
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 radiative losses due to the oscillating magnetic field, but these are usually small.
Dot convention
It is common in transformer schematic symbols for there to be a dot at the end of each coil within a transformer, particularly for transformers with multiple primary and secondary windings. The dots indicate the direction of each winding relative to the others. Voltages at the dot end of each winding are in phase; current flowing into the dot end of a primary coil will result in current flowing out of the dot end of a secondary coil.
3.10 Equivalent circuit
The physical limitations of the practical transformer may be brought together as an equivalent circuit model (shown below) built around an ideal lossless transformer. Power loss in the windings is current-dependent and is represented as in-series resistances Rp and Rs. Flux leakage results in a fraction of the applied voltage dropped without contributing to the mutual coupling, and thus can be modeled as reactances of each leakage inductance Xp and Xs in series with the perfectly coupled region.
Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional to the square of the core flux for operation at a given frequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance RC in parallel with the ideal transformer.
A core with finite permeabilityrequires a magnetizing current Im to maintain the mutual flux in the core. The magnetizing current is in phase with the flux; saturation effects cause the relationship between the two to be non-linear, but for simplicity this effect tends to be ignored in most circuit equivalents. With a sinusoidal supply, the core flux lags the induced EMF by 90° and this effect can be modeled as a magnetizing reactance (reactance of an effective inductance) Xm in parallel with the core loss component. Rc and Xm are sometimes together termed the magnetizing branch of the model. If the secondary winding is made open-circuit, the current I0 taken by the magnetizing branch represents the transformer’s no-load current.
The secondary impedance Rs and Xs is frequently moved (or “referred”) to the primary side after multiplying the components by the impedance scaling factor (Np/Ns)2.
Fig 3.10: Equivalent Circuit
Transformer equivalent circuit, with secondary impedances referred to the primary side The resulting model is sometimes termed the “exact equivalent circuit”, though it retains a number of approximations, such as an assumption of linearity. Analysis may be simplified by moving the magnetizing branch to the left of the primary impedance, an implicit assumption that the magnetizing current is low, and then summing primary and referred secondary impedances, resulting in so-called equivalent impedance. The parameters of equivalent circuit of a transformer can be calculated from the results of two transformer tests: open-circuit test and short-circuit test.
3.11 Types of Transformer
A wide variety of transformer designs are used for different applications, though they share several common features. Important common transformer types include:
a) Autotransformer
Fig 3.11.1: An autotransformer with a sliding brush contact
An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points. The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits ther