Substation equipment and its Function

SUB-STATION EQUIPMENTS & ITS FUNCTIONS

Lightening Arrester

Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation the main important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a cone. 

C V T

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the secondary terminals.

Wave Trap

Wave trap is an instrument using for tripping of the wave. The function of this trap is that it traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected to the main incoming feeder so that it can trap the waves which may be dangerous to the instruments here in the substation. 

 Instrument Transformer

Instrument transformers are used to step-down the current or voltage to measurable values. They provide standardized, useable levels of current or voltage in a variety of power monitoring and measurement applications. Both current and voltage instrument transformers are designed to have predictable characteristics on overloads. Proper operation of over-current protection relays requires that current transformers provide a predictable transformation ratio even during a short circuit.

These are further classified into two types which are discussed below.

a. Current Transformers

b. Potential Transformers

Current Transformer

Current transformers are basically used to take the readings of the currents entering the substation. This transformer steps down the current from 800 amps to 1 amp. This is done because we have no instrument for measuring of such a large current. The main use of this transformer is

a. Distance Protection

b. Backup Protection

c. Measurement

A current transformer is defined as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need to be insulated only for a low voltage.

The current transformer works on the principle of variable flux. In the ideal current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns are utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be transformed into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into current ratio error and the phase error

 Potential Transformer

There are two potential transformers used in the bus connected both side of the bus. The potential transformer uses a bus isolator to protect itself. The main use of this transformer is to measure the voltage through the bus. This is done so as to get the detail information of the voltage passing through the bus to the instrument. There are two main parts in it

a. Measurement

b. Protection

The standards define a voltage transformer as one in which the secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately equal to zero for an appropriate direction of the connections. This in essence means that the voltage transformer has to be as close as possible to the ideal transformer.

In an ideal transformer, the secondary voltage vector is exactly opposite and equal to the primary voltage vector when multiplied by the turn’s ratio.

In a practical transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance. One can thus talk of a voltage error which is the amount by which the voltage is less than the applied primary voltage and the phase error which is the phase angle by which the reversed secondary voltage vector is displaced from the primary voltage vector.

Bus Bar

The bus is a line in which the incoming feeders come into and get into the instruments for further step up or step down. The first bus is used for putting the incoming feeders in la single line. There may be double line in the bus so that if any fault occurs in the one the other can still have the current and the supply will not stop. The two lines in the bus are separated by a little distance by a conductor having a connector between them. This is so that one can work at a time and the other works only if the first is having any fault.

A bus bar in electrical power distribution refers to thick strips of copper or aluminum that conduct electricity within a switchboard, distribution board, substation, or other electrical apparatus. The size of the bus bar is important in determining the maximum amount of current that can be safely carried. Bus bars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross sectional area ratio. The skin effect makes 50-60 Hz AC bus bars more than about 8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod of equivalent current carrying capacity, which allows a greater span between bus bar supports in outdoor switchyards. A bus bar may either be supported on insulators or else insulation may completely surround it. Bus bars are protected from accidental contact either by a metal enclosure or by elevation out of normal reach.

Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly onto any metal chassis of their enclosure. Bus bars may be enclosed in a metal housing, in the form of bus duct or bus way, segregated-phase bus, or isolated-phase bus.

Circuit Breaker

The circuit breakers are used to break the circuit if any fault occurs in any of the instrument. These circuit breaker breaks for a fault which can damage other instrument in the station. For any unwanted fault over the station we need to break the line current. This is only done automatically by the circuit breaker. There are mainly two types of circuit breakers used for any substations. They are

a. SF6 circuit breakers

b. Spring circuit breakers.

The use of SF6 circuit breaker is mainly in the substations which are having high input kv input, say above 220kv and more. The gas is put inside the circuit breaker by force i.e. under high pressure. When if the gas gets decreases there is a motor connected to the circuit breaker. The motor starts operating if the gas went lower than 20.8 bar. There is a meter connected to the breaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gas to reduce the torque produce in it due to any fault in the line. The circuit breaker has a direct link with the instruments in the station, when any fault occur alarm bell rings.

The spring type of circuit breakers is used for small kv stations. The spring here reduces the torque produced so that the breaker can function again. The spring type is used for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution side.

Transformer

There are three transformers in the incoming feeders so that the three lines are step down at the same time. In case of a 220KV or more KV line station auto transformers are used. While in case of lower KV line such as less than 132KV line double winding transformers are used.

The transformer is transported on trailor to substation site and as far as possible directly unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally oil filled, and those of higher capacity are transported with N2 gas filled in them +ve pressure of N2 is maintained in transformer tank to avoid the ingress of moisture. This pressure should be maintained during storage, if necessary by filling N2 Bushings - generally transported in wooden cases in horizontal position and should be stored in that position. There being more of fragile material, care should be taken while handling them. Radiators – These should be stored with ends duly blanked with gaskets and end plates to avoid in gross of moisture, dust, and any foreign materials inside. The care should be taken to protect the fins of radiators while unloading and storage to avoid further oil leakages. The radiators should be stored on raised ground keeping the fins intact. 

Oil Piping. The Oil piping should also be blanked at the ends with gasket and blanking plates to avoid in gross of moisture, dust, and foreign All other accessories like temperature meters, oil flow indicators, PRVs, buchholz relay; oil surge relays; gasket ‘ O ‘ rings etc. should be properly packed and stored indoor in store shed. Oil is received in sealed oil barrels. The oil barrels should be stored in horizontal position with the lids on either side in horizontal position to maintain oil pressure on them from inside and subsequently avoiding moisture and water ingress into oil. The transformers are received on site with loose accessories hence the materials should be checked as per bills of materials.

Isolator

The use of this isolator is to protect the transformer and the other instrument in the line. The isolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line. Thus an isolator is used after the bus also for protection.

Control and Relay Panel

The control and relay panel is of cubical construction suitable for floor mounting. All protective, indicating and control elements are mounted on the front panel for ease of operation and control. The hinged rear door will provide access to all the internal components to facilitate easy inspection and maintenance. Provision is made for terminating incoming cables at the bottom of the panels by providing separate line-up terminal blocks. For cable entry provision is made both from top and bottom. The control and relay panel accepts CT, PT aux 230 AC and 220V/10V DC connections at respective designated terminal points. 220V/10V DC supply is used for control supply of all internal relays and timers and also for energizing closing and tripping coils of the breakers. 230V AC station auxiliary supply is used for internal illumination lamp of the panel and the space heater. Protective HRC fuse are provided with in the panel for P.T secondary. Aux AC and battery supplies. Each Capacitor Bank is controlled by breaker and provided with a line ammeter with selector switch for 3 phase system & over current relay (2 phases and 1 Earth fault for 3 ph system). Under voltage and over voltage relays. Neutral Current Unbalance Relays are for both Alarm and Trip facilities breaker control switch with local/remote selector switch, master trip relay and trip alarms acknowledge and reset facilities.

Protective Relaying

Protective relays are used to detect defective lines or apparatus and to initiate the operation of circuit interrupting devices to isolate the defective equipment. Relays are also used to detect abnormal or undesirable operating conditions other than those caused by defective equipment and either operate an alarm or initiate operation of circuit interrupting devices. Protective relays protect the electrical system by causing the defective apparatus or lines to be disconnected to minimize damage and maintain service continuity to the rest of the system. There are different types of relays.

i. Over current relay

ii. Distance relay

iii. Differential relay

iv. Directional over current relay

i. Over Current Relay

The over current relay responds to a magnitude of current above a specified value. There are four basic types of construction: They are plunger, rotating disc, static, and microprocessor type. In the plunger type, a plunger is moved by magnetic attraction when the current exceeds a specified value. In the rotating induction-disc type, which is a motor, the disc rotates by electromagnetic induction when the current exceeds a specified value.

Static types convert the current to a proportional D.C mill volt signal and apply it to a level detector with voltage or contact output. Such relays can be designed to have various current-versus-time operating characteristics. In a special type of rotating induction-disc relay, called the voltage restrained over current relay. The magnitude of voltage restrains the operation of the disc until the magnitude of the voltage drops below a threshold value. Static over current relays are equipped with multiple curve characteristics and can duplicate almost any shape of electromechanical relay curve. Microprocessor relays convert the current to a digital signal. The digital signal can then be compared to the setting values input into the relay. With the microprocessor relay, various curves or multiple time-delay settings can be input to set the relay operation. Some relays allow the user to define the curve with points or calculations to determine the output characteristics.

ii. Distance Relay

The distance relay responds to a combination of both voltage and current. The voltage restrains operation, and the fault current causes operation that has the overall effect of measuring impedance. The relay operates instantaneously (within a few cycles) on a 60-cycle basis for values of impedance below the set value. When time delay is required, the relays energizes a separate time-delay relay or function with the contacts or output of this time-delay relay or function performing the desired output functions. The relay operates on the magnitude of impedance measured by the combination of restraint voltage and the operating current passing through it according to the settings applied to the relay. When the impedance is such that the impedance point is within the impedance characteristic circle, the relay will trip. The relay is inherently directional. The line impedance typically corresponds to the diameter of the circle with the reach of the relay being the diameter of the circle.

iii. Differential Relay

The differential relay is a current-operated relay that responds to the difference between two or more device currents above a set value. The relay works on the basis of the differential principle that what goes into the device has to come out .If the current does not add to zero, the error current flows to cause the relay to operate and trip the circuit.

The differential relay is used to provide internal fault protection to equipment such as transformers, generators, and buses. Relays are designed to permit differences in the input currents as a result of current transformer mismatch and applications where the input currents come from different system voltages, such as transformers. A current differential relay provides restraint coils on the incoming current circuits. The restraint coils in combination with the operating coil provide an operation curve, above which the relay will operate. Differential relays are often used with a lockout relay to trip all power sources to the device and prevent the device from being automatically or remotely reenergized. These relays are very sensitive. The operation of the device usually means major problems with the protected equipment and the likely failure in re-energizing the equipment.

iv. Directional Over current Relay

A directional over current relay operates only for excessive current flow in a given direction. Directional over current relays are available in electromechanical, static, and microprocessor constructions. An electromechanical overcorrect relay is made directional by adding a directional unit that prevents the over current relay from operating until the directional unit has operated. The directional unit responds to the product of the magnitude of current, voltage, and the phase angle between them or to the product of two currents and the phase angle between them. The value of this product necessary to provide operation of the directional unit is small, so that it will not limit the sensitivity of the relay (such as an over current relay that it controls). In most cases, the directional element is mounted inside the same case as the relay it controls. For example, an over current relay and a directional element are mounted in the same case, and the combination is called a directional over current relay. Microprocessor relays often provide a choice as to the polarizing method that can be used in providing the direction of fault, such as applying residual current or voltage or negative sequence current or voltage polarizing functions to the relay.

DC Power Supply

I . DC Battery and Charger

All but the smallest substations include auxiliary power supplies. AC power is required for substation building small power, lighting, heating and ventilation, some communications equipment, switchgear operating mechanisms, anti-condensation heaters and motors. DC power is used to feed essential services such as circuit breaker trip coils and associated relays, supervisory control and data acquisition (SCADA) and communications equipment. This describes how these auxiliary supplies are derived and explains how to specify such equipment. It has Single 100% battery and 100% charger, Low capital cost, No standby DC System outage for maintenance. Need to isolate battery/charger combination from load under boost charge conditions in order to prevent high boost voltages.

I I . Battery and Charger configurations

Capital cost and reliability objectives must first be considered before defining the battery and battery charger combination to be used for a specific installation. The comparison given in Table 5.1 describes the advantages and disadvantages of three such combinations.

Capital cost and reliability objectives must first be considered before defining the battery/battery charger combination to be used for a specific installation. The comparison given describes the advantages and disadvantages of three such combinations

III . 400V DC Battery

Make: Exide                                                               

Capacity: 300 AH at 27°

No. of Cells: 110 No.

Date of installation: 06/2001

Make: Universal,

Sr. No. : BC 1020/82

Date of manufacturing: 4/2000

Input Rating: Voltage: 415 V + 10 %

Output Rating : Float: 220 V, 10 Amp

  Boost: 180 V, 30Amp          

Functions of Associated System in Substation

Functions of Associated System in Substation is as shown below in table-4.1

Table-4.1 Functions of Associated System in Substation

Sr.	System	Function
1.	Substation Earthing system - Earth mat - Earthing spikes - Earthing risers	To provide an earth mat for connecting neutral points, equipment body, support structures to earth. For safety of personnel and for enabling earth fault protection. To provide the path for discharging the earth currents from neutrals, faults, Surge Arresters, overheads shielding wires etc. with safe step-potential and touch potential.
2.	Overhead earth wire shielding or Lightning masts.	To protect the outdoor substation equipment from lightning strokes.
3.	Illumination system (lighting) - for switchyard - buildings - roads etc.	To provide proper illumination to substation yard.
4.	Protection system - protection relay panels - control cables - circuit breakers - CTs, VTs etc.	To provide alarm or automatic tripping of faulty part from healthy part and also to minimize damage to faulty equipment and associated system.
5.	Control cable	For Protective circuits, control circuits, metering circuits, communication circuits
6.	Power cable	To provide supply path to various auxiliary equipment and machines.
7.	PLCC system power line carrier communication system	For communication, telemetry, tele-control, power line carrier protection etc.
8.	Telephone, telex, microwave, OPF	For internal and external communication
9.	Auxiliary standby power system	For supplying starting power, standby power for auxiliaries.
10.	Fire Fighting system - Sensors, detection system - water spray system - fire port, panels, alarm System. - water tank and spray system	To sense the occurrence of fire by sensors and to initiate water spray, to disconnect power supply to affected region to pinpoint location of fire by indication in control room.

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