Operation and Maintenance of a Substation - Download as PDF File .pdf), Text File .txt) or read online. Operation and Maintenance of a Substation. Operation and Maintenance Manual for Single Phase & Three Phase Substation transformer August 1. Operation & Maintenance. The maintenance of the electrical equipment in substations plays a major role in have become important tasks in power system operation and maintenance.

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V.L Line & Substation Operation and Maintenance. General all equipment critical to the reliable operation of the bulk power system. 3) Electrical equipment details of GIS substation. 4) Preliminary operation before preventive maintenance. 5) Preventive maintenance operation details of AIS. Operation and Maintenance of a Gas Insulated Substation. A major difference between conventional air insulated substations (AIS) and the gas insulated.

Voltage control is used to regulate voltage profiles; however it may actually increase losses and cause instability from highly leading currents.

Substation Operation and Maintenance - Alexander Publications

Voltage control requires no current sensors. Fig: 5. Control effectiveness depends on how well load characteristics are known. Not useful in cases where those characteristics change often.

Temperature control does not require any current sensors.

Time control is based on assumptions about load characteristics. Time control does not require any current sensors. Power factor control is not the best way to control capacitor banks because power factor by itself is not a measure of reactive current. Current sensors are needed. Combination control using various above methods is usually the best choice. Busbars gave some operational flexibility Demerits: 1.

If fault occurs during bus maintenance, ring gets separated into two sections. Auto-reclosing and protection complex. Requires VTs on all circuits because there is no definite voltage reference point. These VTs may be required in all cases for synchronizing live line or voltage indication. Breaker failure: During fault on one circuit causes loss of additional circuit because of breaker failure.

Most widely used for very large power stations having large no. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground. Fig 4 i shows the basic form of a surge diverter.

It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the nonlinear resistance is that its resistance increases as the voltage or current increases and vice-versa.

In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.

As the gap sparks over due to over voltage, the arc would be a short circuit on the power system and may cause power-follow current in the arrester.

Since the characteristic of the resistor is to offer low resistance to high voltage or current , it gives the effect of short circuit. After the surge is over, the resistor offers high resistance to make the gap non conducting.

Guide for selection of LA: The LAs employed for protecting transformers should be installed as close as possible to the transformer. The electrical circuit length between LA and the transformer bushing terminal should not exceed the limits given below: Rated system BIL Max.

The impedance to ground Resistance of the earthing system should be as low as possible and should not exceed. Ms cast iron plates 25mm thick Plates are to be buried vertically in pits and surrounded by finely divided coke.

Large sub-stations -1 ohm Small sub-stations -2 ohms Power stations The earthing practice adopted at generating stations. Plates should not be less than 15 m apart and should be buried to sufficient depth to ensure that they are always surrounded by moist earth. Pipes are to be placed vertically at intervals of not less than All joints and connections in earth grid are to be brazed.

Joints in the earthing conductor between the switch gear units and the cable sheaths. 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.

All joints in steel earthing system should be made by welding except the points for separating the earthing mat for testing purposes which should be bolted. Welded surfaces should be painted with red lead and aluminium paint in turn and afterwards coated with bitumen. These points should be accessible and frequently supervised. For rust protection the welds should be treated with barium chromate.

Joints are to be kept down to the minimum number. These are high pass Filters carrier frequency 50KHZ to KHZ pass carrier frequency to carrier panels and power frequency parameters to switch yard. This results in a large voltage drop across the stack of capacitors. In its most basic form the device consists of three parts: CVTs are typically single-phase devices used for measuring voltages in excess of one hundred KV where the use of voltage transformers would be uneconomical.

In practice the first capacitor. The device has at least four terminals. Its shape is like a drum. Wave trap is an instrument using for trapping of the wave. The function of this wave trap is that it traps the unwanted waves.

CVT type: Generally it is used to exclude unwanted frequency components. Traps are usually unable to permit selection of unwanted or interfering signals.

It is connected to the main incoming feeder so that it can trap the waves which may be dangerous to the instruments in the substation. This is relevant in Power Line Carrier Communication PLCC systems for communication among various substations without dependence on the telecom company network.

Substation Operation and Maintentance

It has non control devices. If these are not present in the substation. Isolator are operated after the circuit breaker is opened. Either line equipment. With the isolators. The signals are primarily tele protection signals and in addition. Strictly speaking Isolators are operated under no current condition.

Isolator is a type of switching device.

In the following cases it is permissible to use isolator for making and breaking of the circuits. Isolators are the no load switches and used to isolate the equipment.

Line trap also is known as Wave trap. This isolator prevents the instruments to get damaged. The line isolators are used to isolate the high voltage from flow through the line into the bus. The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation bus bars. It also allows the only needed voltage and rest is earthed by itself.

While closing the circuit. These are available mainly in two types vertical break type and horizontal break type. Basic Function of Instrument Transformers: Earthing facility shall be provided wherever required.

Air break isolators or disconnecting switches are not intended to break load though these are meant for transfer of load from one bus to another and also to isolate equipment for maintenance.

However the space requirement can be reduced in the horizontal break isolators by having double break with a centre rotating pillar. The later type requires larger width. Pantograph and semi-pantograph disconnects involve vertical movements of contact arm and therefore require less separation between phases and thereby require less separation between phases and thereby help in reducing the sub-station area to a larger extent. The isolators could be operated mechanically or hydraulically or pneumatically or by electric motor.

Types of Instrument Transformers: Instrument transformers are of two types: Secondary Current. Direct measurement of current or voltage in high voltage system is not possible because of high values and insulation problems of measuring instruments they cannot be directly used for protection purposes.

Current transformer is a current measuring device used to measure the currents in high voltage lines directly by stepping down the currents to measurable values by means of electromagnetic circuit. Secondary Winding Turns Therefore an instrument transformer serves the purpose and performs the following function: Primary current Np.

Primary Winding Turns Is. Design of a current transformer depends on the frequency of excitation. The winding which supplies the current to the measuring instruments.

D I These are of two types: N G with the 1. The relay. The line diagram of a current transformer contains different components: Single turn primary winding 2. D Y I Primary Winding: Performance of any current transformer depends on its accuracy of transformation and characteristics of the core material used. Usage of core is decided by the accuracy class of the CT.

Primary to earth and primary to secondary cores are to be checked with 5KV motor operated insulation tester megger and secondary to earth values are to be checked with V insulation tester or preferably with V insulation tester. Core material decides the accuracy class Core with accuracy class 1. Primary to earth. Core 2 is for metering. Core 1 is generally for Over current and earth fault protection. In 5p At the rate of KV level we should use 1: CT with 5 cores 4 cores for special protection.

CT with 3cores protection. CT with 4 cores protection.

Substation Operation and Maintenance - Alexander Publications

Core with accuracy class 5P P denotes protection and 10 denotes accuracy limit factor. IT Frequency: This indicates that 0. In all the above cases.

At the rate of LV KV side we can use 1: The specifications of LVCT are given below: Entry of moisture in to the VCB insulator chamber cause vacuum interrupter failure and pull rod failure due to electrical break down.

Hence loose connections should not be allowed. It increases the contact resistance which in turn the rises the temperature in that area due to load current. Basic Functions of Voltage Transformers are: An instrument transformer in which the secondary voltage.

Even if we connect it directly from high voltage to earth. It must always be connected in parallel only. These values are to be checked with V insulation tester megger or preferably with V insulation tester. By applying single phase voltage across primary the voltage induced in the secondary winding is to be measure. As heavy primary voltages will be reduced to low secondary voltages. It operates at constant flux density. This is approximately equal to voltage applied in the primary winding or voltage ratio of the PT.

Its core is a set of assembled laminations. It extinguishes the arc drawn between contacts when the CB opens and it provides adequate insulation between the contacts and from each contact to earth. First core is of metering core with 1.

There are For any unwanted fault over the station we need to break the line current. This will be used metering. For successful operation of the circuit breaker. The basic construction of any circuit breaker requires the separation of contacts in an insulating fluid.


It is expressed in volt-amperes VA. It is able to make or break the normal load current as well as the fault currents. This is only done automatically by the circuit breaker. The second core is protection core with 3P accuracy class.

The circuit breakers are used to break the circuit if any fault occurs in any of the instrument. This will be used for the directional earth fault protection open delta voltage of power transformers and KV feeders.

These are load switches. Permissible load to be connected on PT secondary winding is decided by the burden of the PT secondary winding. These circuit breaker breaks for a fault which can damage other instrument in the station. Accuracy class 0. If more than rated burden is connected then error will be increased.

There are mainly two types of circuit breakers used for any substations. Spring charge mechanism. Pneumatic mechanism. SF6 gas circuit breakers are preferred. They are a SF6 circuit breakers. Vacuum integrity test is the correct test to know the healthiness of the vacuum interrupter.

VCBs are preferred and beyond 33KV. If it is reaching to SF6 gas pressure low alarm stage. And it is to be replaced based on condition of oil. Oil condition in the air compressor is to be checked periodically. SF6 is now being widely used in electrical equipment like high voltage metal enclosed cables. Some of the properties of SF6 are.

The motor starts operating if the gas went lower than The gas is put inside the circuit breaker by force i. Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas. Sulphur hexafluoride SF6 is an inert. The circuit breaker uses the SF6 gas to reduce the torque produce in it due to any fault in the line. There is a meter connected to the breaker so that it can be manually seen if the gas goes low.

The use of SF6 circuit breaker is mainly in the substations which are having high input KV. The steel box is made gas tight. The gas is liquefied at certain low temperature.

The circuit breaker has a direct link with the instruments in the station. When if the gas gets decreases there is a motor connected to the circuit breaker. Vacuum type of circuit breakers is used for small KV rated stations below 33KV. They are only used in low distribution side.

It is to be ensured at regular intervals at least once shift. The first bus is used for putting the incoming feeders in la single line. The two lines in the bus are separated by a little distance by a conductor having a connector between them. Transformers are essential for high voltage power transmission. While new technologies have eliminated the need for transformers in some electronic circuits. All operate with the same basic principles.

This is so that one can work at a time and the other works only if the first is having any fault. 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.

The bus is a line in which the incoming feeders come into and get into the instruments for further step up or step down. 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 voltage induced across the secondary coil may be calculated from Faraday's law of induction. The primary and secondary coils are wrapped around a core of very high magnetic permeability.

Changing the current in the primary coil changes the magnetic flux that is developed. Current passing through the primary coil creates a magnetic field. An ideal transformer is shown in the adjacent figure. The transformer is based on two principles: The changing magnetic flux induces a voltage in the secondary coil. The area is constant. If the secondary coil is attached to a load that allows current to flow.

Giving the ideal transformer equation Transformers are efficient so this formula is a reasonable approximation. If the voltage is increased.

If this condition is met. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer. If an impedance ZS is attached across the terminals of the secondary coil. If the turns of the coil are oriented perpendicular to the magnetic field lines. The changing magnetic field induces an electromotive force EMF across each winding. While in case of lower KV line such as less than KV line double winding transformers are used.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. The current required to create the flux is termed the magnetizing current. Since the ideal windings have no impedance.

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.

The simplified description above neglects several practical factors. There are three transformers in the incoming feeders so that the three lines are step down at the same time. The primary EMF. When voltage is applied to the primary winding. In case of a KV or more KV line station auto transformers are used.

Top oil. The capacitor banks are used across the bus so that the voltage does not get down till at the require place. A capacitor bank is used in the outgoing bus so that it can maintain the voltage level same in the outgoing feeder. Rated MVA: Rated KV at no load: Energy losses in transmission lines and transformers are of two kinds: Reactive losses come from circuit In the case of concentrated industrial loads.

Reduce KVA demand. The latter. Experience shows that switched feeder capacitors produce some of the fastest returns on equipment investment Sources of Energy Loss. Improve voltage profile. Capacitor Control is usually done to achieve the following goals: Reduce losses due to reactive load current.

The former are caused by resistive component of the load and cannot be avoided. Indirectly capacitor control also results in longer equipment lifetimes because of reduced equipment stresses. Current controls require current sensors. Since reactive current at any point along a feeder is affected by downstream capacitor banks. Voltage control requires no current sensors. Voltage control is used to regulate voltage profiles. VAR controls require current sensors.

Current control is not as efficient as VAR control because it responds to total line current. Time control is based on assumptions about load characteristics. Combination control using various above methods is usually the best choice. If enough current. Not useful in cases where those characteristics change often.For any unwanted fault over the station we need to break the line current. It has a vital influence of reliability of service.

The guard diverts current that leaks to the low voltage windings so that this leakage current is not included in the test measurement. Most of the sub-stations in the power system are of this type. It increases the battery life period. Breaker failure: Feeder Bus isolator 8.

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