220 KV Grid Station Gakkhar Pakistan

NTDCL 220/132 kV Grid Station (Gakkhar) Internship Report (Summer internship 2014)

Submitted by: Naseeb Ali Shahvaiz Ali Ibrar Hussain Ihtasham Ali Zeesham Afzal Ans Saljook M.Zahid M.Kashif Junaid

2011-EE-502 2011-EE-511 2011-EE-517 2011-EE-550 2011-EE-574 2011-EE-576 2011-EE-587 2011-EE-588

Rachna College of Engineering and Technology, Gujranwala (A Constituent College of UET Lahore)

Starting Date: 15-JUN-2014 End Date: -JUL-2014

Preface

The grooming of an engineering student is incomplete without proper exposure to the industry.we are completing our Bachelor’s degree in Electrical Engineering at Rachna College of Engineering and Technology,

Gujranwala. We are students of final year in specialization of Power engineering therefore, we have to conduct an internship for learning purposes. This report documents the work done and learning during the summer internship at National Dispatch and Transmission Company Ltd. 220/132 kV grid station Gakkhar. The internship report contains and overview of the internship company and the activities, tasks that we have worked on during our internship. This report shall give and overview of tasks completed and technical details we have learnt during the period of internship and the discussions and overview of lectures taken.

We have tried our best to write the complete knowledge we gain during this internship in report.

ACKNOWLEDGMENTS

The work culture in grid station really motivates.we could not have done this work without the lots of help we received cheerfully from whole NTDCL. Everybody is such a friendly and cheerful companion here that work stress is never comes in way.Special thanks to Sir Akbar Ali and Sir Mansor for providing the nice ideas to work upon. Their lectures were very informative and made clear a lot of our concepts about field work and related to our Electrical Engineering perspective.

Contents:                     

Why there is a need of Grid Stations? An introduction to NTDCL Gakkhar Grid Station Single line diagram Explanation of Equipment used Transmission line Isolators Terminal Tower Wave Trap CCVT Earthing Switch Current Transformer (CT) Potential Transformer (PT) Circuit Breaker (CB) Auto Transformer Operation Limitations Parallel Operation of transformer Vector group of transformer PROTECTION OF GRID Transformer DC Protection

Why there is a need of Grid Stations? An electrical grid (also referred to as an electricity grid or electric grid) is an interconnected network for delivering electricity from suppliers to consumers. It consists of generating stations that produce electrical power, high-voltage transmission lines that carry power from distant sources to demand centers, and distribution lines that connect individual customers. Power stations may be located near a fuel source, at a dam site, or to take advantage of renewable energy sources, and are often located away from heavily populated areas. They are usually quite large to take advantage of the economies of scale. The electric power which is generated is stepped up to a higher voltage-at which it connects to the transmission network. When voltage level of a power is increased, the electric current of the power is reduced which causes reduction in ohmic or 𝐼 2 𝑅losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system(as low I results in low voltage drop in the line). Because of these, low level power must be stepped up for efficient electrical power transmission. The transmission network will move the power long distances, sometimes across international boundaries, until it reaches its wholesale customer (usually the company that owns the local distribution network). On arrival at a substation, the power will be stepped down from a transmission level voltage to a distribution level voltage. As it exits the substation, it enters the distribution wiring. Finally, upon arrival at the service location, the power is stepped down again from the distribution voltage to the required service voltage(s).

Candle light dinners are most enjoyable when they are not forced. In today’s modern world of electrical home appliances we realize that poor maintenance of the electricity supply may mean a loss of a few expensive appliances alongside added inconvenience. Thus in order to keep your passionately light decorated houses glimmering there are grid stations to ensure high reliability power supply. There are different kinds of Grid stations such as:  66 KV Grid Station  132KV Grid Station  220 KV Grid Station  500 KV Grid Station

In this report only 220 KV Grid Station is discussed.

GRID STATION GAKHAR 132kv Ghakhar G/Stn. which was commissioned during 01/77 was upgraded to 220KV level with commissioning on 11.06.1982. 220KV Ghakhar G/Stn. has 04Nos. 160MV, 220/132KV Power T/Fs and 02 Nos. 26 MVA, 132/11KV Power T/Fs installed at the G/Stn. The G/Stn. is being fed from Mangla Power House through 02 Nos. 220KV D/C T/Lines. This G/S is also connected with 220KV G/S Sialkot through Single Circuit and 500KV Nokhar and Mangla Power House through IN Out arrangement.

An introduction to 220/132 kV NTDCL Grid Station (Gakkhar) This is 220/132 kV grid station, located at Gakkhar (near Gujranwala), under NTDCL (National Transmission and Dispatch Company Ltd.). The lines coming from MANGLA-I, MANGLA-II and NOKHAR (500/220 kV grid) and outgoing to SIALKOT. (Although, any of lines can be used to send or receive power so, don’t confuse yourself with the direction of arrows in single line diagramas shown in fig (1) below). It uses double bus scheme on both of its 220 and 132 kV sides with sectionalizing scheme on 132 kV side. Which will be discussed below in single line diagram. Single line diagram:In power engineering, a one-line diagram or single-line diagram (SLD) is a simplified notation for representing a three-phase power system. Electrical elements, such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols. Instead of representing each of three phases with a separate line or terminal, only one conductor is represented. It is a form of block diagram graphically depicting the paths for power flow between entities of the system. Elements on the diagram do not represent the physical size or location of the electrical equipment.

Fig (1) single line diagram of grid station

There are some conventions and symbols for single line diagram, such as: the colour of different potential lines are different. Which are given below: 500kV

Brown

220kV

Green

132kV

Red

11kV

Blue

Same like, there are symbols for equipment to show on single line diagram for complete understanding. Some of them are shown below: The equipment in single line diagram are denoted by some of symbols which are given in LEGEND with it. Here, the circuit breakers are represented by “Q” and for understanding its potential and position, there are also keywords, like: 132kV

EnQn

220kV

DnQn

500kV

BnQn

Where, n=1, 2, 3…. Which represent the location and exact name of the circuit breaker and make ease in communication. Some other key words are: L.A

Lightning Arresters

W.T

Wave Trap

C.T

Current Transformer

P.T

Potential Transformer

Others are represented by their symbols. Shown in fig. The single line diagram of this grid station is shown below. According to which there are total four lines, coming form MANGLA-I, MANGLA-II and NOKHAR and going to SIALKOT. On each of line (entering or leaving) there are some common equipment installed such as: Instrument Transformers (C.T & P.T), Wave Trap, CCVT, Isolators, Grounding Switches, Lighting Arresters. Bus bars: There is double bus bar scheme in grid station. The advantage of this is that, when there is any fault or any maintenance issue, we can convert whole load on any one bus bar without any interruption in supply of power. Also there is a bus coupler, which couples both of the buses. In normal conditions, the load is distributed on both of the buses, using this bus coupler. Although, each of bus has capacity to handle the complete load. During maintenance or any fault on any of the bus, the isolator of bus coupler is set such that the load is transferred on the healthy bus. Sectionalizer:On 132kV side, the two buses are sectionalized in two section each by using the SECTIONALIZER. These sections are made to increase the reliability of the buses and the transformers. This is also used for parallel operation of transformer (which will be discussed later) and coupling the transformers for the power to transformer, of same rating, percentage impedance and ratio. The buses are divided in many sections, if there is fault in any of the sections, this section can be isolated from the system without interruption in other sections.

Explanation of Equipment used: Transmission line: There are three types of transmission line:   

Short Transmission Line (up to 80 Km) Medium Transmission Line (80-240 Km) Long Transmission Line (above 240 Km)

Effects on the transmission line: The transmission lines have following three major effects:Ferranti effect: When the voltage at the receiving end is increased as compared to sending end voltage at normal load, called as Ferranti Effect. This is due to the capacitance between line to line and line to ground. As with increase in length of transmission line, the capacitance of line increases and inductance reduces, so due to this capacitance the Ferranti effect is more in Long and transmission lines as compared to Short transmission line as capacitance is negligible in short transmission lines. Skin Effect: Due to frequency in alternating current, current starts to flow on the surface of conductor instead of using whole conductor area. Due to this used area of conductor is reduced and resistance faced by current is increased. So due to this effect our conductor is wasted as it is not fully utilized. To eliminate this effect we use stranded conductors. Corona Effect: Electric power transmission practically deals in the bulk transfer of electrical energy, from generating stations situated many kilometers away from the main consumption centers or the cities. For this reason the long distance transmission cables are of utmost necessity for effective power transfer, which in-evidently results in huge losses across the system. Minimizing those has been a major challenge for power engineers of late and to do that one should have a clear understanding of the type and nature of losses. One of them being the corona effect in power system. For corona effect to occur effectively, two factors here are of prime importance as mentioned below:1) Alternating electrical potential difference must be supplied across the line.

2) The spacing of the conductors, must be large enough compared to the line diameter.

Insulators: The overhead line conductors should be supported on the poles or towers in such a way that currents from conductors do not flow to earth through supports i.e. line conductors must be properly insulated from supports. The insulator provides necessary insulation between line conductors and supports and thus prevent any leakage current form conductor to earth. The insulators are made up of Porcelain, Glass, Steatite and special types of materials. Types:    

Pin type (up to 33kV) Suspension type (above 33kV) Strain Type insulator Shackle type insulator

Terminal Tower: The tower at the end of transmission line i.e. at the start of the grid station is known as the terminal tower. This tower carry the power line entering the grid station having ‘strain insulators’ on it. It’s a double circuit tower having two parallel power lines on it with a ‘SKY WIRE’ or ‘EARTHED WIRE’ or ‘OPGW’ (optical ground wire) which is used for the protection of the transmission system from lightning strokes and any of lightning falls on the transmission system, is grounded through this wire as this wire is on the top of the tower. The insulators used for insulation of line with ground (tower) and with each other, depend upon the potential in the line. The “string” of insulator discs (made of porcelain, glass or such type of insulating material) are made for this purpose, according to the potential of the lines. For a rough estimate of the voltage of line passing can be made by counting the number of plates of the insulator on the tower, as they are designed and installed according to voltage level. The rough estimate is like- if you multiply the number of discs with number 15, the answer will provide the voltage in kV.

Wave Trap: For the communication between two grid station and between the main head office and the grid station, NTDCL has its own communication system on the transmission line. As the communication requires very high frequency signals (>500kHz) so, to separate these signals from the power signals (low frequency) we need an equipment known as Wave Trap, which is a high pass filter and allows to pass high frequency signals only to the communication equipment. The equipment or the system used for it is called as POWER LINE CARRIER (PLC).

CCVT: This is Capacitor Coupling Transformer. As the power signal is of low frequency (50 Hz) so to pass this CCVT is used which is a low pass filter and always gives output 110 volts. Its working is same as the Potential Transformer with addition of low pass filter circuit.

Isolator: Isolators are provided for isolation from live parts for the purpose of maintenance. Isolators are located at either side of the circuit breaker. Isolators are operated under no load. If it is operated under load, there will be arc between the contacts of the isolator. Isolator does not have any rating for current breaking or current making. Isolators are interlocked with circuit breakers Types of Isolators are 1. Central rotating, horizontal swing 2. Centre-Break 3. Vertical swing 4. Pantograph type

Earthing Switch: Earthing switches are mounted on the base of mainly line side isolator. Earthing switches are normally vertically break switches. Earthing arms (contact arm of earthing switch) are normally aligned horizontally at off condition. During switching on operation, these earthing arms rotate and move to vertical position and make contact with earth female contacts fitted at the top of the post insulator stack of isolator at its outgoing side. The earthing arms are so interlocked with main isolator moving contacts that it can be closed only when the main contacts of isolator are in open position. Similarly the main isolator contacts can be closed only when the earthing arms are in open position.

Current Transformer: For the measurement of the current in the line, the current transformer (C.T) is used. It is an Instrument transformer which brings current in the range to be measured. It comes in different primary to secondary ratio (800/1 or 800/5). For the safety precaution, the secondary of C.T must not be opened as on the secondary side there are a large amount of voltage as current is very low, so due to high potential arc will produce and CT will burst out. Care must be taken that the secondary of a current transformer is not disconnected from its load while current is in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance up to its core saturation voltage. This may produce a high voltage across the open secondary into the range of several kilovolts, causing arcing, compromising operator and equipment safety, or permanently affect the accuracy of the transformer. The accuracy of a CT is directly related to a number of factors including:



Burden



Burden class/saturation class



Rating factor



Load



External electromagnetic fields



Temperature and Physical configuration.



The selected tap, for multi-ratio CTs



Phase change

Maintenance Test: For the maintenance of the current transformer, two tests are performed,



Capacitance test The capacitance between HV and LV side of transformer is checked. From the reading of the instrument we multiply it with the place of knob reading to get the capacitance. Then percentage depreciation factor is measured according to the temperature. If the temperature is different form the ambient (20 degree C) then multiplied by the correction factor provide in table with the instrument manual.

The DF must be below 1 (or may be up to 1.2) for the best operation.



Insulation test: Insulation of transformer is checked using MEGGAR (mega ohm-meter). The MEGGAR is connected between HV side and ground and the resistance is measured. Generally, for one kV, 1Mohm is suitable resistance. i.e. for 132kV the insulation must be about 132 mega ohm. The calculations were taken at atm temperature and after correction factor for 20 degree Celsius were applied and result was 5000 MOHM. Which is suitable for working.

Potential transformer (PT):Voltage transformers are used to step down the voltage for measurement, protection and control. Voltage transformers are of two types. 1. Electromagnetic type 2. Capacitive VT located on the feeder side of the Circuit Breaker. The primary of potential transformer must not be short as it is connected in parallel and there will be a burst and PT could be damaged. A voltage transformer theory or potential transformer theory is just like a theory of general purpose step down transformer. Primary of this transformer is connected across the phase and ground. Just like the transformer used for stepping down purpose, potential transformer i.e. PT has lower turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the PT.

The secondary voltage of the PT is generally 110 V. In an ideal potential transformer or voltage transformer, when rated burden gets connected across the secondary; the ratio of primary and secondary voltages of transformer is equal to the turn ratio and furthermore, the two terminal voltages are in precise phase opposite to each other. But in actual transformer, there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.

Circuit Breaker: Circuit breaker is device used to break the circuit in case of excess amount of current passing (due to any fault or other). It is protective device for the protection of all equipment in case of fault that can damage the other devices. Here, the circuit breakers used are SF6 circuit breakers. Sulfur Hexafluoride is an excellent gaseous dielectric for high voltage power applications. It has been used extensively in high voltage circuit breakers and other switchgears employed by the power industry. Application for SF6 include gas insulated transmission lines and gas insulated power distributions. The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgears. Some of the outstanding properties of this are below:   

Very high dielectric strength. Very unique arc-quenching ability. Very excellent thermal stability. Very good thermal conductivity.

There is a tank of SF6 having specific pressure for the operation of circuit breaker below which CB is unable to operate and there is pressure gauge for it showing and monitoring the pressure in tank. When the pressure goes down, it causes a specific relay to operate for alarm purposes.

MAINTAINANCE TEST: For the checking the health of circuit breakers, three following tests are performed annually:1. SF6 purity test For the best working of circuit breaker the gas should be pure. For this purpose, test is performed at regular intervals to check the purity. 2. Contact resistance test : Due to operation of circuit breaker, contacts of breaker become rough due to arc between them. So, the resistance of contacts increases above limit resulting very increase in I2R losses. So, the resistance of contacts are checked annually and it must be in micro ohms and if exceed, these must be replaced. This is off load test. 3. Timing test. The opening and closing the contacts of circuit breaker matters a lot. So, there must be a little time for circuit breaker to sense and break the circuit. For this timing test is performed using a test set model number TM1600. This test is off load test i.e. the circuit breaker is disconnected from the circuit and then test is performed. For the best operation of circuit breaker, following must be full filled:  Opening time must be between 28-30 msec.  Closing time must be about 60 msec.

AUTO-TRANSFORMER:An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased KVA rating.

Autotransformers are often used to step up or step down voltages in the 110115120 V range and voltages in the 220-230-240 volt range—for example. Providing 110 V or 120 V (with taps) from 230 V input, allowing equipment designed for 100 or 120 volts to be used with a 230 volt supply.

OPERATION:An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points, or a transformer in which the primary and secondary coils have part or all of their turns in common. 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 therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding. However the voltage and current ratio of autotransformers can be formulated the same as other two-winding transformers.

As in a two-winding transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting the load between the middle and bottom of the autotransformer will reduce the voltage by 50%. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected. LIMITAION:An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output). These are important safety considerations when deciding to use an autotransformer in a given application. Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is usually more economical. In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of ground fault currents. A large three-phase autotransformer may have a "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents. In practice, losses mean that both standard transformers and autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if it is used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical. Like multiple-winding transformers, autotransformers use time-varying magnetic fields to transfer power. They require alternating currents to operate properly and will not function on direct current.

Parallel Operation of transformer To connect two or more transformers with each other, the operation is followed is called as the parallel operation of transformer. Parallel operation is carried out to increase the reliability of the system in such a way that the total load on both is equal to the maximum rating of one transformer. That is, in case one transformer goes down for some reason, the one can handle the load without interruption. In normal

conditions, both transformers are sharing the load and hence the reliability increases and the life of transformer. For the parallel operation of transformers, three following conditions must be satisfied: All transformers should have 1. Same impedance drops. 2. Same turn ratios. 3. Same ratings. Transformers connected in parallel have the same voltage on each primary and the same voltage on each secondary. The difference in the voltage between the primary and secondary windings is the turn ratios. For these terminal voltages to be the same for the paralleled transformers, their impedance drops must be identical. Therefore, under any condition of load, the current will be divided such that the product of impedance and current in one transformer is equal to the product of impedance and current in the other. Also, if the turn ratios of the transformers are different, but the primary and secondary terminal voltages are the same in both transformers, then circulating currents must flow between the transformers, even at no load.

Typically, transformers should not be operated in parallel when: • The division of load is such that, with the total load current equal to the combined kVA rating of the transformers, one of the transformers is overloaded. • The no-load circulating currents in any transformer exceed 10% of the full load rating • The combination of the circulating currents and full load current exceed the full load rating of either transformer. Following table (1) shows conditions for transformers under which we can use transformers in parallel operation:-

Vector group of transformer:In electrical engineering, a vector group is the International Electro technical Commission (IEC) method of categorizing the high voltage (HV) windings and low voltage (LV) winding configurations of three-phase transformers. The vector group designation also indicates the windings configurations and the difference in phase angle between them.



Symbol designation:-

Y = HV side connected in star y= LV side connected in star D= HV side connected in delta d= LV side connected in delta N= neutral connected to HV side n= neural connected to LV side

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At the end there may be integers like 1, 2, 3… 12 for phase displacement between LV and HV side. E.g. 1=300 , 2=600

,3=900,…… 12=3600or 00

For example the vector group of auto transformer is Yyan0 This shows that this transformer is connected in star in HV and LV side and ‘a’ for auto transformer and ‘n’ shows that neutral grounded to LV side and ‘0’ shows that phase difference between HV and LV is zero degree.

PROTECTION OF GRID:Transformer protection:Transformer protection mainly divided into two major groups: 1. Mechanical Protection 2. Electrical Protection Mechanical Protection: Mechanically operated protection is called as mechanical protection which do not have any concern with electric signal to operate. Some are following: A) MAIN BUCHHOLZ RELAY: It is gas operated, gas actuated relay. When there is any fault with the windings of transformer. i.e. it is short from any part, spark will be produced and it will decompose the oil in the transformer due to which gases will produced and goes upward to the conservator tank where, there is buchholz relay main and conservator tank and due to pressure of gases, relay operated. B) Pressure relief relay: This relay is operated due to sudden change of pressure of gases within the tank. When winding is short from more than one place and more arc is produced and large pressure is created in the tank. C) ON LOAD TAP CHANGER RELAY: This is for the protection of the tap changer. If there is any short circuit or any fault in tap changer this relay operated.

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D) WINDING TEMPERATURE RELAY: This relay is used to maintain the temperature of winding of transformer at safe level. When more current passes from the winding, it causes to increase the temperature of winding hence, this current is used to maintain the temperature of winding. From the current, we can find the voltage drop of it: V=IR This voltage is applied to the relay and used to operate it. When the voltage drop increase form set level it will operate. This relay has four taps during working. At first step when temperature rises to about 60 percent of total it activate FAN group 1 if temperature further rises, it activate FAN GROUP 2 on further increase in temperature, alarm will be activated and at last relay is operated to trip circuit breakers of both sides. E) OIL TEMERATURE AND OIL LEVEL RELAY: These relays are used to check and maintain the temperature and level of in transformer. These relays use some kind of sensors to sense the temperature and oil level of main tank of transformer and operate after a set level. Electrical Protection: A) Main Differential Relay: Differential relay takes the difference of currents between primary and secondary side of transformer- through the CT of both sides. If there is some kind of difference between them, it operates the circuit breaker taking it as fault. As when fault occur on either side of transformer, current increases and hence the difference of current increases so relay will operate the circuit breakers.

Ispill = IHV - ILV To make zero the difference between them, Matching CTs are also placed after them which make both currents equal for difference. When about 0.1A difference occurs, main differential relay trip the circuit. B) HV and LV Over Current Relay: These relays are used to protect the HV and LV side of transformer in case of high current. By the use of CT on both sides, it senses the current and operates accordingly.

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C) Tertiary Over Current Relay: This relay is for the protection of the tertiary windings of transformerwhich is used to remove the harmonics in windings. When there is over current flows in tertiary winding of transformer, this operates the circuit breakers of transformer. D) Breaker Failure Relay: This relay operates when there is fault in breaker or when breaker is not working properly. This checks the connection of circuit breaker and in case of fault there, it operates. E) Pole Discrepancy Relay: For circuit breaker to be operated properly, it is necessary condition that all the three poles must be operated at the same time. If all the three poles fail to cut off or come to circuit at same time, fault occurs. This relay ensures that, all the poles are operating, during circuit breaker operation. It operates with the time delay of less than 300msec. This relay avoid that only one or two phases are open during steady state operation. F) Over Excitation Relay: Over excitation of transformer can occur whenever ratio of per unit voltage to per unit frequency at secondary terminals exceeds its rating. That is, as we know that the core of transformer is excited and de-excited on each cycle and due to the voltage applied. This excitation is given by formula E =4.44 f N Φ From this, it can be seen that Φ is directly related to the ratio of (V/f). So, to avoid to exceed this ratio for specific transformer, a relay is set at point above which it operates and trips the circuit.

DC Protection:The protection of grid is done by the DC source. The 220kV protection is done at 220 volts while 132kV protection is carried out at 110 volts. For these voltages, there is a battery room where batteries are placed. Each cell is of 2volts and each plate of cell has capacity of 300Ah. So, for 220 volts, 110 such cells are placed in series and for 110 volts, 55 such cells are placed. These are lead-acid batteries and use electrolyte in them.

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For the maintenance of these batteries, the specific gravity of electrolyte is checked regularly by the instrument called as ‘Hydrometer’. Another thing for their maintenance is that they are charged properly from a panel providing suitable voltage for charging. Normally, float charging is done in normal days but once a week, boast charging also applied on these batteries for their good health.

Fig (battery room)

Fig (hydrometer)

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