GSS Sikar Report

GSS Sikar Report2012

2012Dharmendra Singh Rathore

[GSS Sikar Report]A report on the GSS Sikar, where I completed my 24 working days training. This Report includes information about all the accessories and mountings used there.

CHAPTER No. 1

Introduction to An Electrical substation and its types

1.1 Electrical substationAn 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 the days before the distribution system became a grid. The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station.1.2 Contents Elements of a substation Transmission substation Distribution substation Collector substation Stations with change of current type Switching substation Design Layout

1.2.1 Elements of a substationSubstations generally have switching, protection and control equipment and one or more transformers. 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 reclose circuit breakers or fuses for protection of distribution circuits. Substations do not usually have generators, although a power plant may have a substation nearby. 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 several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the network. Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution.1.2.2 Transmission substationA transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control devices such as capacitors, reactors or static VAr compensator and equipment such as phase shifting transformers to control power flow between two adjacent power systems.Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current transformers, relays and SCADA systems). Modern substations may be implemented using International Standards such as IEC61850.1.2.3 Distribution substationA distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the high-voltage main transmission network, unless they use large amounts of power, so the distribution 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, 115kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33kV 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.Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. 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 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.

1.2.4 Collector substationIn distributed generation projects such as a wind farm, a collector substation may be required. It somewhat resembles a distribution substation although power flow is in the opposite direction, from many wind turbines up into the transmission grid. Usually for economy of construction the collector system operates around 35kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide power factor correction if it is needed, metering and control of the wind farm. In some special cases a collector substation can also contain an HVDC static inverter plant.Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity. Examples for such substations are Brauweiler in Germany and Hradec in the Czech Republic, where power of lignite fired power plants nearby is collected. If no transformers are installed for increase of voltage to transmission level, the substation is a switching station.1.2.5 Stations with change of current typeSubstations may be found in association with HVDC converter plants or, formerly, where rotary converters changed frequency or interconnected non-synchronous networks.1.2.6 Switching substationA switching substation is a substation which does not contain transformers and operates only at a single voltage level. Switching substations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for parallelizing circuits in case of failure.1.2.7 DesignThe main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost. The design should also allow easy expansion of the station, if required.Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers. Where land is costly, such as in urban areas, gas insulated switchgear may save money overall. The site must have room for expansion due to load growth or planned transmission additions. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects. Grounding (earthing) and ground potential rise must be calculated to protect passers-by during a short-circuit in the transmission system. And of course, the substation site must be reasonably central to the distribution area to be served.1.2.8 LayoutThe first step in planning a substation layout is the preparation of a one-line diagram which shows in simplified form the switching and protection arrangement required, as well as the incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by many electrical utilities to prepare one-line diagrams with principal elements (lines, switches, circuit breakers, and transformers) arranged on the page similarly to the way the apparatus would be laid out in the actual station.Incoming lines will almost always have a disconnect switch and a circuit breaker. In some cases, the lines will not have both; with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off. When a large fault current flows through the circuit breaker, this may be detected through the use of current transformers. The magnitude of the current transformer outputs may be used to 'trip' the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the rest of the system to continue operating with minimal impact. Both switches and circuit breakers may be operated locally (within the substation) or remotely from a supervisory control center.Once past the switching components, the lines of a given voltage connect to one or more buses. These are sets of bus bars, usually in multiples of three, since three-phase electrical power distribution is largely universal around the world.The arrangement of switches, circuit breakers and buses used affects the cost and reliability of the substation. For important substations a ring bus, double bus, or so-called "breaker and a half" setup can be used, so that the failure of any one circuit breaker does not interrupt power to branch circuits for more than a brief time, and so that parts of the substation may be de-energized for maintenance and repairs. Substations feeding only a single industrial load may have minimal switching provisions, especially for small installations.Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a 'short circuit').Along with this, a substation always has control circuitry needed to command the various breakers to open in case of the failure of some component.1.3 An A.C. Sub Station consists following components (A) Outdoor Switchyard1. Incoming & Outgoing Lines2. Bus-Bars3. Substation Equipment such as circuit breaker, insulators, isolators, lightning arrestor, Wave Trap, CTs, PTs, & neutral grounding equipments. 4. Transformers5. Galvanized steel structures for towers, gantries, supports.6. Control cables for protection and control. 7. Capacitor Bank 8. Station lightning system. (B) Main office building. (C) Switch yard. (D) Battery room D.C. distribution system1. D.C. lead acid batteries and charging equipment.2. D.C. Distribution system. (E) PLCC (Power Line Career Communication) & Wireless network (F) Mechanical, Electrical & other auxiliaries1. Lightning system2. Oil purification system3. Cooling water system4. Telephone system5. Store, workshop etc. (G) Protection system1. Lightning arrestors2. Capacitance Voltage Transformer3. Wave Trap4. Isolator5. Circuit Breaker6. Current transformer.

Electricity distribution from Generating Stations to Customer

1.4 Switching functionAn important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer.To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.

CHAPTER No. 2

Main Componentsofan Electrical Substation

2.1 TRANSFORMERA transformer is a device that transfers electrical energy from one circuit to another through a shared magnetic field. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a 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. The secondary induced voltage VS is scaled from the primary VP by a factor ideally 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 Application of transformer is to reduce the current before transmitting electrical energy over long distances through wires. By transforming electrical power to a high-voltage, low-current form for transmission and back again afterwards, the transformer allows electricity to be transmitted more efficiently, enabling the economic transmission over long distances. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer. 2.2 INSTRUMENT TRANSFORMER Instrument Transformer are small transformers intended to supply low value of current or voltage to measuring instruments, protective relays and other similar devices. It may become dangerous to the operator if measuring instruments are directly connected to the high voltage system by means of instrument transformer, viz. CT & PT, instruments can be completely isolated from the high voltage system and power of high voltage system. 2.3 CIRCUIT BREAKERA circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.2.4 LIGHTNING ARRESTERSA lightning Arrester (also known as surge diverter or surge arrester) is device connected between line and earth, i.e., in parallel with the equipment to be protected at the substation. It is a safety valve which limits the magnitude of lightning and switching over voltages at the substation and provides a low resistance path for the surge current to flow to the ground. It is installed adjacent to the equipment to be protected. 2.5 INSULATORS In order to prevent the flow of current from support the transmission lines or distribution lines are all secured to the supporting towers or poles with the help of insulators. Thus the insulators play an important part in the successful operation the lines. The chief operation or requirements for the insulators are: They must be mechanically very strong. Their dielectric strength must be very high. They must be free from the internal impurities. They must provide very high insulation resistance to the leakage currents. They should not be porous. They must be imperious to the entrance of gases or liquids. They should not be affected with change in temperature. They must have high ratio of puncture strength of flesh over voltage. 2.6 ISOLATORS Isolators are used to make or break the circuit in no load conditions.

2.7 BUS BAR AND CONDUCTORFollowing conductors are used in 220KV GSS in bus bar and line.2.7.1. Dog conductor: It is a standard conductor; it has seven steel wires and six Al wires. Al conductors are at outer side of Conductors, current carrying of these conductor carrying 300 Amp and used for 66 KV Bus.2.7.2. Vigal conductor: In this conductor there are seven conductors of aluminum. There are used in 33KV Bus bar.2.7.3. Zebra conductor: It is standardized ACSR conductor. There are 54 Conductor of AI and 7 conductor of steel. It is used for 220KV bus. Current carrying capacity is 736Amp at 45C ambient temperature. 2.7.4. Panther conductor: It has 32 conductors of AI and 7 of steel its current capacity is 485 Amp and it is used in 132KV Bus. 2.7.5. Recon conductor: In this type of conductor there are six conductors of Al and only one conductor of steel. This type of conductor is use in 33KV Bus.2.7.6. Conductors: This type of conductor is used for 11KV Bus. 2.8 PROTECTIVE RELAYS

Relays are devices intended to protect electrical systems and equipments against damages caused due to abnormal operating conditions. In more technical terms, relays are devices designed to produce sudden pre-determined changes in one or more physical systems on the appearance of certain abnormal conditions in the physical systems controlled by them. Protective relays act as sensors of abnormalities and actuate control gears when required. Relays may be suitably set to operate with the required discrimination between sections in order to isolate only the faulty section /sections or equipment / equipments.

CHAPTER No. 3

TRANSFORMER

3.1 TRANSFORMER A transformer is a device that transfers electrical energy from one circuit to another through a shared magnetic field. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a 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. The secondary induced voltage VS is scaled from the primary VP by a factor ideally 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 Application of transformer is to reduce the current before transmitting electrical energy over long distances through wires. By transforming electrical power to a high-voltage, low-current form for transmission and back again afterwards, the transformer allows electricity to be transmitted more efficiently, enabling the economic transmission over long distances. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer. 3.1.1 Basic principlesThe 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). By changing the current in the primary coil, one changes the strength of its magnetic field; since the secondary coil is wrapped around the same magnetic field, a voltage is induced across the secondary.

An ideal step-down transformer showing magnetic flux in the coreA simplified transformer design is shown in Figure. A 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; this ensures that most of the magnetic field lines produced by the primary current are within the iron and pass through the secondary coil as well as the primary coil.3.1.2 Induction lawFaraday's the voltage induced across the secondary coil may be calculated from law of induction, which states that Where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and equals the total magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength 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 3.1.3 Ideal power equation 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 thence to the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power Pincoming = IPVP = Poutgoing = ISVSgiving the ideal transformer equation Thus, if the voltage is stepped up (VS > VP), then the current is stepped down (IS < IP) by the same factor. In practice, most transformers are very efficient (see below), so that this formula is a good approximation.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. This relationship is reciprocal, so that the impedance ZP of the primary circuit appears to the secondary to be.

3.1.4 Practical considerations

Flux leakage in a two-winding transformer 3.1.5 Effect of frequencyThe time-derivative term in Faradays Law shows that the flux in the core is the integral of the applied voltage. An ideal transformer would, at least hypothetically, work under direct-current excitation, with the core flux increasing linearly with time. In practice, the flux would rise very rapidly to the point where magnetic saturation of the core occurred, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate under alternating (or pulsed) current conditions.Transformer universal EMF equationIf the flux in the core is sinusoidal, the relationship for either winding between its rms EMF E, 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: The EMF of a transformer at a given flux density increases with frequency, an effect predicted by the universal transformer EMF equation. 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 traditionally employ 400Hz power supplies which are less efficient but this is more than offset by the reduction in core and winding weight. 3.1.6 Equivalent circuit The physical limitations of the practical transformer may be brought together as an equivalent circuit model built around an ideal lossless transformer. Power loss in the windings is current-dependent and is easily 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 self-inductances 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 tend to be 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.

Transformer equivalent circuit, with secondary impedances referred to the primary side A core with finite permeability requires 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 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 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.

3.2 Different parts of a transformerIn a power transformer, these are the following main parts:(A) Main parts i) Core ii) Winding iii) Tank cover iv) Base channel v) L.V. & H.V. bushing vi) Tap changer vii) Conservator tank viii) Earth terminals ix) Rating plat x) Lifting lugs(B) Auxiliary parts i) Radiators ii) Cooling fans iii) Pressure Relief Valve iv) Breather v) Buchholzs relay vi) Drain valve vii) Arching horns viii) Oil level Indicator ix) Oil Temperature Indicator x) Winding temperature Indicator

3.3 CORE The magnetic core is three limbs or three limbs with two auxiliary limb types. Each limb being miter with top and bottom yokes. The lamination is made from high grade non aging cold rolled grain oriented silicon alloy steel. The insulation of lamination is carbide coating. The core has stepped core section. The gapes are clamped with end frames by yokes bolts or by fiber tube over clamped plates. For lighting the core with winding assembly. Four no. of lifting legs are provided on and frame cover. 3.4 WINDING Winding are arranged in concentric formation with lowest voltage winding next to the core in case, tertiary winding is arranged then this winding is placed next to the core over L.V. winding H.V. winding and tapping are placed some time, tapping is placed after H.V. main winding depending upon requirement of impedance between various typed of winding used for making coil are as following: Tertiary winding : Spiral/helied Low winding : Helied/disc High voltage winding : partially interlinked winding Tapping winding: inter around spiral/helied coil. Windings are usually arranged concentrically to minimize flux leakage 3.5 Drain Valve Drain Valve is used when we want to empty the transformer. With the help of drain valve we can release the total oil filled in the transformer. 3.6 BUSHING Bushings are of condenser type of protection depending upon the voltage class. Connection from the transformer winding is brought out by means of bushing ordinary protection bushing can be used up to 33Kv & above 33KV. Capacitor and oil fixed bushing is used. Bushings are fixed on the top of tank.

3.7 AIR CELLThe air cell is the flexible rubber bag and is placed inside the conservator. It floats on the oil surface. The air cell inflates or deflates depending upon the expansion or contraction of oil. The dry air sucked into the cell doesnt come in direct contact with oil, and then eliminates the probability of contamination. 3.8 TANK The tank is of welded mild steel plate construction sand/short blasted on inside and outside to remove scale formation. Tanks are designed to with stand a vacuum in line with CBIP recommended on transformer. The cover is either belt type of flat and remains mounted on the top of the tank rim. In tank insulating oil is fitted and it provides house or oil. 3.9 TAP CHANGER Adjustment of voltage is done by changing the effective turns ratio of the system transformer by proper selections of tapping of the winding. There are two types of tap changing: Off load tap changing. On load tap changing.

In first form as the name implies it is essential to switch off the transformer before changing the tap. On load tap changers are employed to regulate voltage while transformer is delivering normal load. Tap changer is provided on the outer winding or H.V. more ever the HV side as more no. of turns during transition, to adjacent taps are moment ably connected and the short circuit current is limited by automatic insertion of impedance in between the corresponding tapping other end. A value is fitted at the lowest point of the tap for draining and sampling of oil. On the feed pipe buchholzs relay is mounted.

3.10 ARCHING HORNS Arching horns protects a transformer over voltage faults. Whenever the voltage exceeds from its rated voltage. Supplying from generating station, this high voltage comes from generating station, this voltage comes on the arching horns and thus there is a sparking between the providing safety to the transformer from over voltage. Arching horns are mounted on the transformer bushing on every bushing there is a couple of arching horns, one is on upper side of the bushing and the second one is on lower side. The distance of two horns are being standardizing as per rule for various voltages.

3.11 OIL TEMPERATURE INDICATOR This desistance thermometer operating on the principle of liquid expansion. It provides local indication of the top of the oil temperature at them marshalling box. The connection between thermometer bulb and the dial indicator is made by flexible steel capillary tube is enclosed in a pocket and the pocket is fixed on the transformer at the hottest oil region. The pocket has to be filled with transformer the oil temperature is provided with a maximum pointer and the two mercury switches one for alarm and other for trip. Switches are adjustable to make contact between 50C and 120C. 3.12 OIL LEVEL INDICATOR This indicator shows the level of oil filled in transformer. It is attached with conservator tank. Indicator with complete description is shown below.

3.13 WINDING TEMPERATURE INDICATOR This indicator operating on the principle of liquid expansion provides local indications at the marshalling box of hot spot temperature of windings. The winding hot spot to top oil temperature differential is simulated by means of CT current fed to a coil around the operating bellows. Thus winding temperature indicates tem reading are proposal to load current pulse top oil tem the indicator is heated with maximum pointer and for mercury switch.3.14 CONSERVATOR TANK As the temperature of oil increase or decrease during operation there is corresponding rise or fall in the volume of oil. The account for this and expansion uses in connected to the transformer tank. The tank capacity of oil level equal to 75 % of total oil in the transformer. This is provided with magnetic level gauge on one of the end covers. Which has a low oil level alarm. The low oil level alarm is suitable for 240 D.C. or A.C. a prismatic oil gauge is also fitted at the other end. A valve is fitted at the lowest point of the tank for draining and sampling of oil. On the feed pipe buchholzs relay is mounted. 3.15 BREATHER

BREATHING PHENOMENON When the transformer is loaded or unloaded the oil temperature inside the transformer tank ruses of falls. Accordingly the air volume inside the tank changes by either sucking in or pushing out the air. This phenomenon is called Breathing of the transformer. The air, which is sucked in, contents either foreign Impurities and or humidity, which change dielectric strength of transformer oil. Hence it is necessary that the air entering into the transformer is free from moisture and foreign impurities.

3.15.1 OPERATION AND WORKING The breather is connect to an output pipe of the conservator vessel and the air which is being sucked by transformer is made to pass through to silica gel breather to de-humidity the air and to remove foreign impurities. The silica gel, which is fitted in the breather, is hard blue crystal, which has considerable absorption power for moisture. When it gets saturated with moisture it changes its colors to pinkish. White or proper dehumidification of air, it is absolutely necessary that this changes of silica gel its re-conditioned from pinkish white to deep blue by heating it to 100 C. 3.16 BUCHHOLZS REALAY The transformer if fitted double float Buchholzs relay. It is fitted in the feed pipe from conservator to tank and is provided with two set of mercury contacts (Connected between main tank and conservator tank). The devices comprises of cast iron housing containing the hinged floats. One is upper part and other part is lower part. Each float is fitted with a mercury switch, which are connected to a terminal box. This alarm detects minor or major faults in transformer. The alarm element with operates after a specified volume of gas has collected to five an indication. Such faults are: 1. Broken drown core-bolt insulation 2. Sorted lamination 3. Bad contacts 4. Over heating of part of winding

The alarm element will also operates in the even of the coil leakage or if air gets into the oil system the trip element will be operated by an oil surge in the event of more serious faults such as:- 1. Earth fault 2. Winding short circuit 3. Puncture of Bushing 4. Short circuit between phases.

The trip element will also be operated if a rapid loss of oil occurs

Fig. Buchholzs Relay

3.17 PRESSURE RELIVE VALVE The pressure relief valve is designed to use on power transformer. When pressure in the tank rises above predetermined safe limit this valve operates and perform following functions:-1. Allow the pressure to the drop by instantaneously opening aPart of about 150 mm diameters.2. Given valve operating by rising a flog.

3. Operates a micro switch.

This pressure relief valve has integral flange with six halves for mounting. The valve can be mounted vertically and horizontally on tank. The PRV has got a part of about 150 mm diameter. A stainless steel diaphragm seals this part. Whenever the pressure in the tank rises above pre determined stage limit the diaphragm gets lifted from its seal this lifting in instantaneous and allow vapors gases or liquid to come out of tank depending upon the position of valve on tank. The diaphragm restores its position as soon as pressure in the tank drops below set limit.

3.18 EARTH TERMINALS The earth terminals for the Transformers are shown with a diagram:

3.19 EARTHING ARRANG EMENTS1. Core earthing connecting leads from core and end frame are being laminated at the top of cover. By connecting those to tank cover core and core frame are earthed.

2. Tank to tank cover earthing: - It is done by connecting copper straps between tank rim and tank cover. 3. Earthing of Tank: - For farthing of tank, to earthing pads have been provided on tank.

3.19.1 RATING PLATE Type 3-

Rated Capacity 100 MVA

Type of Cooling ONAN / ONAF / OFAF

Rating HV & IV(MVA) 50 70 100

Rating LV(MVA) 16.67 23.33 33.33

No Load Voltage HV(KV) 220

No Load Voltage IV(KV) 132

No Load Voltage LV(KV) 11

Line Current HV(A) 131.37 183.92 262.74

Line Current IV(A) 218.95 306.53 437.90

Line Current LV(A) 874.97 1225.96 1751.44

Temp. Rise Oil(c) 40 40 40

Temp. Rise Winding(c) 55 55 55

Core & Coil Mass(Kg.) 59000

Mass of Oil (kg.) 38000

Tank & Fitting Mass(Kg.) 32000

Total Mass(Kg.) 129000

Transport Mass(Kg.) 75000

Volume of Oil (l.) 42510

3.20 INSTRUMENT TRANSFORMER Instrument Transformer are small transformers intended to supply low value of current or voltage to measuring instruments, protective relays and other similar devices. It may become dangerous to the operator if measuring instruments are directly connected to the high voltage system by means of instrument transformer, viz. CT & PT, instruments can be completely isolated from the high voltage system and power of high voltage system.

Instrument Transformers are of following types: Current Transformer (CT) Potential Transformer (PT)3.20.1 Current Transformer A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the current flowing in its primary. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured. A CT for operation on a 220kV gridDesign The most common design of CT consists of a length of wire wrapped many times around an annular silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many hundreds of turns. The CT acts as a constant-current series device with an apparent power burden a fraction of that of the high voltage primary circuit. Hence the primary circuit is largely unaffected by the insertion of the CT. Common secondaries are 1 or 5amperes. For example, a 4000:5 CT would provide an output current of 5amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. UsageCurrent transformers are used extensively for measuring current and monitoring the operation of the power grid. The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses (for example, protection devices and revenue metering may use separate CTs).

Safety precautions Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary, and may permanently affect the accuracy of the transformer.Physical configuration Physical CT configuration is another important factor in reliable CT accuracy. While all electrical engineers are quite comfortable with Gauss' Law, there are some issues when attempting to apply theory to the real world. When conductors passing through a CT are not centered in the circular (or oval) void, slight inaccuracies may occur. It is important to center primary conductors as they pass through CTs to promote the greatest level of CT accuracy. In an electric metering circuit, the most inaccurate component is the CT.

SPECIFICATION OF CURRENT TRANSFORMER

Rated Primary Current 60Amp to 2400Amp.

Rated Frequency 50 Hz

Secondary Current 1 Amp/5 Amp (general) 0.5772/2 Amp (special case)

Dynamic peak with stand current 1000 kA-pk

Short Time Current 16 kA/1sec. max. rms

Primary Reconnection In ratio 1:2:4 up to 1200 Amp

Primary Conductor Copper/Al pipe

Class of metering 0.5 (in general) 0.2 (special case)

Max. no. of secondary winding 5 number fix

Class of Protection 5P10/5P20

Insulator Alternate ahead profile

Creep age 25mm/kv for heavy pollution level

Insulation class A

Insulation level 275/650 kvp

Highest System Voltage 145 V

Continuous Thermal Current 480 kA

Weight of Oil 105 kg.

Total Weight of CT 590 kg.

3.20.2 POTENTIAL TRANFORMER Potential transformers are similar to the power transformer except that there power rating is considerably low. The primary of PT has more number of turns as compare to secondary .The primary is connected across the high voltage system whose voltage is to be measured the secondary is generally rated for 110 volts irrespective of primary voltage rating. The transformer is generally stepped down and shell type. In the secondary windings one point is earthed to avoid hazards. The secondary winding terminal is connected to voltmeter and the pressure coil of voltmeter. SPECIFICATION OF POTENTIAL TRANSFORMERMake Crompton Greaves

Ratio 33/110 V

Frequency 50 Hz

Insulation level 80/200 kV

H.S.V.T. 36 kV

S.R. No. 97630,97606,97629

Normal Voltage 33 kV

Commissioning 1995

Maintenance schedule for a Power TransformerS.No. Items to be inspectedFrequencyAction required

(A)

1.

2.

(B)

1.

2.

(C)

1.

(D)

1.

2.

3.

4.

(E)

1.

(F)

1.

2. 3.

4.

5.

6. 7.

(G)

1.

2.

3.

Ambient temp.Winding temp.Oil temp.Load(Amperes)Voltage(Volts)

Oil level in Xmer & tapChangerOil in Buchholzs Relay

Dehydrating Breather

Bushing

Cooler fans & pump bearing,Motor and operating mechanismOn load top changer driving

On load top changer automatic control

Oil samples from main tank and OLTC

Insulation resistance

Gasped jointsCable boxes

Relays, alarm ckt. (etc)

Temperature indicator

Paint workEarth resistance

Thermosyphon filter

Conservator

Buchholzs Relay Hourly

Daily

1. If air2. If gas

Monthly

Quarterly

Half Yearly

Yearly

Two years

Check that temp. rise is reasonable. Check against rated figure.

Check against oil level.

Release to atmosphere. Do to gas analysis.

Check against color of active agent. Check oil level in Breather cup.

Examine for cracks & dirt deposits. Oil level in bushings.

Check bearing & gear box. Check manual control & inter locks. Check gear box, oil level & examine other contacts. Check ckts. Independently Check step by step switch operation.

Check for all moving parts contacts, break mechanism top & bottom oil sample.

Compare with volume at the commissioning. Tightened the bolts. Replace gaskets if leaking. Check for sealing arrangement for filling holes. Examine relay & alarm contact and other operation. Pocket holding thermometers should be checked. Calibration Check pointer for free movement. Should be inspected. Check earth resistance by earth tester.

Check ppm of quality of oil sample from the filter Internal inspection. Air cell inspection. Adjust floats, switches etc. as required.

CHAPTER No. 4

CIRCUIT BREAKER

4.1 CIRCUIT BREAKER A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Types of Circuit BreakerThe types of Circuit Breakers are Oil Circuit Breaker (OCB) Vacuum Circuit Breaker (VCB) SF6 Circuit Breaker (SF6CB) Air Blast Circuit Breaker (ABCB)4.2 OIL CIRCUIT BREAKER These are one of the oldest types of circuit breakers. The separating contacts of the breakers are made to separate within insulating oil. The bubbles of the gas formed prevent re-string of the arc after the current reaches zero point of the three cycles. These are types of oil circuit breakers:a) Bulk oil circuit breakersb) M.O. circuit breakers. The following are advantages of oil being used as arc quenching medium: The oil used is very good insulator and allow smaller clearances between live conductors and earth components. The chord coil is capable of flowing in to arc current goes to zero. The oil high dielectric strength. It absorbs the heat energy of the arc as the oil has great heat dissipating properties. The gases so formed by the decomposition if oil has good cooling properties.

SPECIFICATION OF A M.O. CIRCUIT BREAKER

Make BHEL

Type HLC 36/1006

Rated Service Voltage 33KV

Rated Normal Current 1000Amp.

Rated Symmetrical Breaking Capacity 750MVA

Rated Making Current 33.4KAmp

Short Time Current for Three Second 13.1KAmp

Frequency 50Hz

No. of Poles 3

No. of Break Poles 1

4.3 AIR BLAST CIRCUIT BREAKER

Air Blast Circuit Breakers are used frequently now days. This type of Circuit Breaker work on the air pressure which is filled in them. They work on the principle of Arc Quenching. The pressure of air in this type of C.B. is nearly 132p. In 220kV GSS, Sikar some of the C.B.s are ABCB. 4.4 VACUUM CIRCUIT BREAKER Vacuum circuit breaker is a gang operated, triple pole circuit breaker fitted with vacuum interrupters it suitable for direct breakers the arc-quenching medium is vacuum instead of oil. The vacuum to interrupters is mounted in a protection insulator to from an interrupter assembly. There such assemblies are mounted on a frame, which has a common operating shaft. This assembly is mounted on a steel structure instead to locate the line terminals at a safe distance above the ground the structure also encloses the operating mechanism. The breakers can be electrically operated from control room or by hand locally. The shell is spring changer to change the spring after very one operation of breaker. The spring can be charged either by manually or by means of an electric motor.

SPECIFICATION OF VACUUM CIRCUIT BREAKERMake SIEMENS

Type 3AFO1

Rated Voltage 36KV

Rated Short Circuit Current 25 Kamp.

Breaking Capacity 2200 Watts.

Rated Normal Current 1600 Amp.

Operating Mechanism Motor Operated

Frequency 50 Hz

Making Current 30Amp.

Persibl Continuous Current 10 Amps.

ON Induction load at 220volt D.C. 440 Watts.

4.5 SULFUR HEXAFLUORIDE CIRCUIT BREAKER Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as sulfur hexafluoride (SF6), having excellent dielectrically and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity. Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulfur hexafluoride is generally used in present high-voltage circuit-breakers. Several characteristics of SF6 circuit breakers can explain their success: Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber. Autonomy provided by the puffer technique. The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers. Short break time of 2 to 2.5 cycles. High electrical endurance, allowing at least 25 years of operation without reconditioning. Possible compact solutions when used for GIS or hybrid switchgear. Integrated closing resistors or synchronized operations to reduce switching over voltages. Reliability and availability. Low noise levels. Thermal blast chambers New types of SF6 breaking chambers, which implement innovative interrupting principles, with the objective of reducing the operating energy of the circuit-breaker. These developments have been facilitated by the progress made in digital simulations that were widely used to optimize the geometry of the interrupting chamber and the linkage between the poles and the mechanism. This technique has proved to be very efficient and has been widely applied for high voltage circuit breakers up to 550 kV. It has allowed the development of new ranges of circuit breakers operated by low energy spring-operated mechanisms. The latest design for A.C. circuit breaker is SF6 gas type. In which SF6 gas works as quenching medium as well as insulating medium. Advantage of SF6 The advantage of SF6 as the quenching and insulating medium is following: High voltage with stand capacity because of high dielectric strength. Arc-quenching capacity is superior over all other medium. Chemical inert, non-toxic and very clean. Pure SF6 does not hydrolyze. Any chemically active impurity formed by arching can be immediately removed by in expansive observers like activated Alumina. The constructional features of all the C.B. are same, except some minor and especial differences. SPECIFICATIONS OF SF6 CIRCUIT BREAKERType ELE-SL-6-2

Make ABB

Rated Voltage 220 KV

Impulse withstand Voltage 1.2/50S 4000 A

First pole to clear factor 1.3

Rated breaking Current 40 KA

Rated making Current 100 KA

Peak capacitor breaking 600 A

Current asymmetry 55%

Operating Time 23 msec.

Rated pressure of SF6 3.4 bar

Trip/close coil voltage 220V D.C.

Motor Voltage Supply 220V D.C.

CHAPTER No. 5

PROTECTIVE RELAYING

5.1 TYPES OF PROTECTION

When fault occurs in any part of system, it must be cleared quickly to avoid damage or interference with rest of the system. Protection scheme is divided into two classes- primary protection and back up protection.5.1.1 Primary protection Its used for protection of component parts of power system. In fig 1 each line has an over current relay that protects the line. If a fault occurs on any line, it will be cleared by its relay and circuit breaker. This forms the primary or the main protection and serves as the first line of defense. Sometimes faults are not cleared by primary relaying because of trouble within the relay, wiring system, or breaker. Under such conditions back up protection is used.

5.1.2 Back up protection

Its the second line of defense in case of failure of primary protection. Its designed to operate with sufficient time delay so that primary relaying is given enough time to function if it is able to. In fig 1, A provides back up protection for each of the four lines. If a line fault is not cleared by its relay and breaker, relay a will operate after a definite time delay and clear the entire group of lines. When back up protection functions a larger part is disconnected than when primary relaying functions correctly. Therefore greater emphasis is given on better maintenance of primary relaying.5.2Introduction

Relays are devices intended to protect electrical systems and equipements against damages caused due to abnormal operating conditions. In more technical terms, relays are devices designed to produce sudden pre-determined changes in one or more physical systems on the appearance of certain abnormal conditions in the physical systems controlled by them. Protective relays act as sensors of abnormalities and actuate control gears when required. Relays may be suitably set to operate with the required discrimination between sections in order to isolate only the faulty section /sections or equipment / equipments. A relay will have one or more energising quantities and one or more characteristic quantities in terms of which the relay is calibrated (eg. voltage for over voltage relays, time for definite time lag relays, time and current for inverse time lag current relays, angle for directional relays, power for reverse power relays etc.) A relay should watch the system changes and operate when it is called for. 5.3Classification of RelaysRelays are broadly classified into two, based on the principle of operation.They are1. Conventional electro magnetic relays2. Static relaysMicroprocessor based PLC (Programmable Logic Controlled) multifunctional relays are nowavailable as a replacement for a large number of independent unifunctional relays. These areintelligent relays programmed to the requirements.Relays can also be classified based on the time of operation of the relay as

1. Instantaneous relays2. Time lag relaysAlmost all relays fall under the categories of either instantaneous relays or time lag relays. Instantaneous relays operate and reset without any intentional time delay. But instantaneous relays have inherent time delay and based on this inherent time delay (operating time) they are sub classified as given in table 8.1.

Accuracy of relaysThe manufacturer of the relay shall specify the accuracy of the relay at specified settingvalues. The standard accuracy classes which correspond to the maximum percentage error is given in table .

Characteristics of Time lag relays

Time lag relays are relays, the operation or resetting of which are intentionally time delayed. The time delay of the relay may be fixed or adjustable. Time lag relays are intended to operate after a specified time on the appearance of the energising quantity. The time lag of operation depends on the designed characteristics of the relay. Usually standard inverse, very inverse, extremely inverse and long time delay relays are used in practice. Many of the relays have definite minimum time of operation which will help to attain proper time grading between sections.

Current operated relaysThe time-current characteristics of current operated Inverse Definite Minimum Time Lag (IDMTL) relays without directional feature is given in tables 8.3.a to 8.3.e and in figures 8.1 to 8.5.5.4 Requirements of Protective RelayingThe principle function of protective relaying is to cause the prompt removal from service of any element of the power system when it starts to operate in an abnormal manner or interfere with the effective operation of the rest of the system. In order that protective relay system may perform this function satisfactorily, it should have the following qualities: 1. selectivity 2. speed 3. sensitivity 4. reliability 5. simplicity 6. economy

5.4.1 Selectivity It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system A well-designed and efficient relay system should be selective i.e. it should be able to detect the point at which fault occurs and cause the opening of the circuit breakers closest to the fault with minimum or no damage to the system. In order to provide selectivity to the system, it is a usual practice to divide entire system into several protection zones. When a fault occurs in a given zone, then only the circuit breakers within that zone will be opened. This will isolate only the faulty circuit or apparatus, leaving the healthy circuits intact. The system can be divided into the following protectionzones:(a) generators (b) low-tension switchgear (c) transformers (d) high-tension switchgear(e)transmission lines 5.4.2 Speed The relay system should disconnect the faulty section as fast as possible for the following reasons:(a) Electrical apparatus may be damaged if they are made to carry the fault currents for a long time.(b) A failure on the system leads to a great reduction in the system voltage. If the faulty section is not disconnected quickly, then the low voltage created by the fault may shut down consumers motors and the generators on the system may become unstable.(c) The high speed relay system decreases the possibility of development of one type of fault into the other more severe type.5.4.3 Sensitivity - It is the ability of the relay system to operate with low value of actuating quantity. Sensitivity of a relay is a function of the volt-ampere input to the coil of the relay necessary to cause its operation. The smaller the volt-ampere input required to cause relay operation, the more sensitive is the relay. Thus, a I VA relay is more sensitive than a 3 VA relay. It is desirable that relay system should be sensitive so that it operates with low values of volt ampere input5.4.4 Reliability - It is the ability of the relay system to operate under the pre determined conditions. Without reliability, the protection would be rendered largely ineffective and could even become a liability.5.4.5 Simplicity - The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability.5.4.6 Economy The most important factor in the choice of a particular protection scheme is the economic aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise scheme has to be adopted. The protective gear should not cost more than 5% of the total cost. However, when the apparatus to be protected is of utmost importance (eg. generator, main transmission line etc.), economic considerations are often subordinated to reliability

CHAPTER No. 6 LIGHTNING ARRESTERS,INSULATORS,&CONDUCTORS

6.1 LIGHTNING ARRESTERS A lightning Arrester (also known as surge diverter or surge arrester) is device connected between line and earth, i.e., in parallel with the equipment to be protected at the substation. It is a safety valve which limits the magnitude of lightning and switching over voltages at the substation and provides a low resistance path for the surge current to flow to the ground. It is installed adjacent to the equipment to be protected. An ideal lightning arrester should have the following characteristics:

It should not take any current under normal conditions, i.e., its spark over voltage must be higher than the normal or abnormal power frequency voltage that may occur in the system. Any abnormal transient voltage above the breakdown valve must cause it to breakdown as quickly as possible in order to provide an alternating path to earth. Breakdown having taken place it must be able to carry the resultant discharge current without causing damage to itself and without the voltage across it exceeding the breakdown value. Immediately after the surge voltage falls below the breakdown value, it must interrupt the following power frequency current instantaneously so that the circuit outage is avoided.

The simple types of lightning arresters like rod gap, horn gap, etc, are very cheap devices but fulfill only the first three of the above mentioned requirements. The fourth requirement is, however, not fulfilled and every gap operation results an outage. 6.2 INSULATORS In order to prevent the flow of current from support the transmission lines or distribution lines are all secured to the supporting towers or poles with the help of insulators. Thus the insulators play an important part in the successful operation the lines. The chief operation or requirements for the insulators are: They must be mechanically very strong. Their dielectric strength must be very high. They must be free from the internal impurities. They must provide very high insulation resistance to the leakage currents. They should not be porous. They must be imperious to the entrance of gases or liquids. They should not be affected with change in temperature. They must have high ratio of puncture strength of flesh over voltage. 6.3 ISOLATORS Isolators are used to make or break the circuit in no load conditions.

6.4 BUS BAR AND CONDUCTORFollowing conductors are used in 220KV GSS in bus bar and line.1. Dog conductor: It is a standard conductor; it has seven steel wires and six Al wires. Al conductors are at outer side of Conductors, current carrying of these conductor carrying 300 Amp and used for 66 KV Bus.2. Vigal conductor: In this conductor there are seven conductors of aluminum. There are used in 33KV Bus bar.3. Zebra conductor: It is standardized ACSR conductor. There are 54 Conductor of AI and 7 conductor of steel. It is used for 220KV bus. Current carrying capacity is 736Amp at 45C ambient temperature. .4. Panther conductor: It has 32 conductors of AI and 7 of steel its current capacity is 485 Amp and it is used in 132KV Bus. 5. Recon conductor: In this type of conductor there are six conductors of Al and only one conductor of steel. This type of conductor is use in 33KV Bus.6. Conductors: This type of conductor is used for 11KV Bus.

CHAPTER No. 7

PROTECTIVE EARTHING,TRANSFORMER PROTECTION

7.1 Protective earthing7.1.1 IntroductionEarthing is a general term broadly representing grounding of power systems and bonding of equipment bodies to grounded electrodes. Earthing associated with current carrying power conductors, usually neutral conductor, is normally essential for the stability of the system and is generally known as system earthing. Earthing of non-current carrying metal works of equipment bodies is essential for the safety of life and property and is generally known as safety equipment earthing. The basic requirements of any earthing system are(i) It should consist of equipotential bonding conductors capable of carrying the prospective earth fault current and a group of pipe/rod/plate earth electrodes for dissipating the current to the general mass of the earth without exceeding the allowable temperature limits in order to maintain all non-current carrying metal works reasonably at earth potential and to avoid dangerous contact potentials being developed on such metal works.(ii) It should limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.(iii) It should be mechanically strong, withstand corrosion and retain electrical continuity duringthe life of the installation. Earth electrodes, which form part of the earthing system, are provided to dissipate fault current during earth fault and to maintain the earth resistance to a reasonable value so as to avoid rise of potential of the earthing grid. The resistance to earth of an electrode of given dimensions is dependent on the electrical resistivity of the soil in which it is installed. In addition to the measurement of soil resistivity at the design stage, it is essential to repeat the measurement at the pre-commission stage also, as the effectiveness of the earthing system depends on the value of soil resistivity . Hence before energising electric supply lines and apparatus it is necessary that all components of the earthing system including the soil are inspected and tested to ensure efficient functioning of the system.7.2 Pre commission inspection and checks(i) General Layout Check whether the layout of earthing is as per the scheme approved by the department of Electrical Inspectorate. Check whether the number of plate electrodes and pipe electrodes are as per the approved scheme. Check whether the spacing between the electrodes are as per the approved scheme- 5 metres for pipe electrodes and 8 metres for plate electrodes.(ii) Earth electrodes Check whether all the earth electrode terminals are visible and numbered. The numbering shall be done both on the top of trough cover and inside the trough. Check the size of earth electrodes used 1200 x 1200 x 12.6 mm for cast iron plate and 600 x 600 x 6.3 mm for copper plate One cast iron plate is equivalent to 4 copper plates of standard size. Check the dimension of earth electrode trough 1000 x 500 x 600 mm for plate electrodes and 500 x 400 x 350 mm for pipe electrodes This is for easiness of connections and convenience of testing Check the class of pipe used for pipe electrodes at least class B pipes shall be used. Check whether permanent watering arrangement is provided at sub-stations and where earth resistivity is relatively high. Check whether funnels are provided for watering the electrodes. Check the size of the earth mat of EHT stations with the designed values (spread of earth mat, mesh size, conductor size, size of risers, depth of laying etc.) In the case of earth mats, check the size of blue granite jelly, its depth and area of spread. The area of spread shall extend beyond fencing atleast by 1.5 metres. Check whether all the earth electrodes are interconnected to form a closed mesh.(iii) Earth continuity strips and earthing conductors. Check whether two continuity strips have been taken from the plate electrodes to the top connector link. Where GI is used for earthing, check whether hot dip galvanized GI strips and conductors are used. GI is allowed where corrosion factor is within permissible limits and earth resistivity is more than 100 ohm-metre. Check the size of main earth bus for conformity with the approved size. Check the size of the sub earth buses and their interconnections. Check the size and effectiveness of connections of horizontal and vertical earth buses of cubicle type switch board sections. Check the interconnection of earth bus sections in switch boards. Check whether duplicate earthing of adequate size is provided for switches, isolators and control gears. Check whether duplicate earthing is provided for body of transformers, motors and other equipments. The two connections shall be taken from opposite sides. Check whether duplicate earthing is provided for the neutrals of transformers and generators . There shall be one direct connection from each neutral to a separate earth electrode but interconnected with the earthing system. Check whether continuous earth strip is run from the top of lattice type towers and structures of EHT stations and lines. Check whether the bottom of each High voltage bushing is earthed using earthing strip. Check whether outdoor CT, PT, breaker units, isolators, lightning arrestors etc. are directly earthed to the risers of the earth mat / earthing grid.

(iv) Connections and joints Check whether connections in the earthing system are made properly . The contact surfaces shall be properly tinned and contacts perfectly bonded and seated Riveting, bracing or bolting shall be done effectively. Inspect the welded joints of GI earth strips and conductors The welded surfaces shall be covered with zinc dichromate painting / bituminous coating. Check the quality of galvanisation of bolts and nuts used for earth lead connections Hot dip galvanised rust free bolts and nuts shall be used. In the case of earth mats, check the perfection of welding of mesh joints.

7.3 Precommission testsEarth Testers and principle of measurementThe most commonly used earth tester is the four terminal tester. The tester comprises of a current source and a meter in a single instrument. The resistance is directly read in the tester from which the earth resistivity is computed. Wenners four electrode method is followed for earth resistivity measurement. When four electrodes are driven along a straight line at equal intervals and a current is passed through the two outer electrodes, the current flowing into the earth produces an electric field proportional to the current density and the resistivity of the soil. The voltage measured between the two inner electrodes is therefore proportional to the field. Consequently the resistivity will be proportional to the ratio of thevoltage to current. The earth resistivity of the soil is given by

Earth Resistivity Test ProcedureThe resistivity of soil varies over a wide range depending on the composition and moisture content of the soil. It is therefore advisable to conduct earth resistivity tests during dry season in order to get conservative results. In the case of sub stations and generating stations, at least eight test directions shall be chosen from the centre of the station to cover the entire site. For very large station sites this number may be increased. In the case of transmission lines, the measurements shall be taken along the direction of the line throughout the length, at least once in every 4 kms. The connections for the test are given in fig 7.1

The four electrodes are driven into the earth along a straight line at equal intervals . The depth of driving the electrodes in the ground shall be of the order 10 to 15 cms. The earth megger is placed on a steady and approximately level base. The links between the terminals are opened and the four electrodes connected to the instrument terminals. Appropriate range in the instrument is selected to obtain accurate readings. The readings are taken while turning the crank at around 135 revolutions per minute. The resistivity is calculated by substituting the value of R obtained from the test in the equation in para 7.3.1.If the resistance of the electrodes (two inner potential electrodes) is comparatively high, a correction of the test result is necessary depending on its value. For this purpose, the resistance of the voltage circuit of the instrument Rp is measured by connecting the instrument as shown in fig.

Average earth resistivity at the site

The resistivity of the soil at many sites have been found varying with the depth of the soil and also with horizontal distances. Variation of the resistivity with depth is mainly due to stratification of earth layers and is found predominant when compared to the variation with horizontal distances. For the correct computation of earth resistivity, it is desirable to get information about the horizontal and vertical variations of earth resistivity at the site under consideration. The vertical variations may be detected by repeating the measurements at a given location in a chosen direction with different electrode spacing. The spacings may be increased in steps of 2, 5, 10, 15, 25 and 50 metres or more. The horizontal variations are studied by taking measurements in various directions from the centre of the station. If the variation in the earth resistivity readings for different electrode spacings in a direction is within 20 to 30 percent, the soil is considered to be uniform. When the spacing is increased gradually from low values, a stage will be reached at which the resistivity readings become more or less constant irrespective of the increase in the electrode spacing. This value of the resistivity is noted as the resistivity in that direction. Similarly, the resistivity for at least eight equally spaced directions from the centre of the site are measured. These resistivities are plotted on a graph sheet. A closed curve is plotted on the graph sheet joining the resistivity points to get a polar resistivity curve (see fig. 7.3). The area inside the polar curve is measured and the circle of the equivalent area is found out. The radius of the equivalent circle is the average earth resistivity of the site under consideration. The value will be reasonably accurate when the soil is homogeneous. If the soil is nothomogeneous, a curve of resistivity versus electrode spacing shall be plotted and this curve further analysed to decide stratification of the soil into two or more layers of appropriate thickness or a soil of gradual resistivity variation. Computation of earth resistivity of heterogeneous soil is highly involved and reference to text books may be made.

7.4 Measurement of earth electrode resistance

The same four terminal earth tester described under para 7.3.1 can be used for measurement of earth electrode resistance. One of the current and potential terminals are shorted to form a common terminal which is connected to the test electrode and the other current and potential terminals connected to two auxilliary electrodes. Alternately, 3 terminal earth testers with common terminal to be connected to the test electrode and independent current and potential terminals for connections to auxiliary electrodes are available for measurement of earth electrode resistance. Two standard auxiliary electrodes supplied with the instrument are used for the measurement. The depth of driving of auxiliary electrodes shall be low compared to the spacing between the electrodes. Generally, the auxilliary electrodes are driven at 15 metres and 30 metres from the test electrode. The connections may be checked before taking the measurement .Resistance of individual electrodes.Resistance of individual electrodes is measured after disconnecting all interconnections to the electrode. Earth leads from the earth bus, neutral of transformers/generators and interconnections from other earth electrodes are disconnected before taking the measurement. Connections to the earth tester are made as described above. The cranking lever of the earth tester is rotated at the specified speed. The reading of the earth tester gives the earth resistance of the particular electrode. Resistance of all earth electrodes shall be measured in the above manner and the values recorded in a register for future reference.

7.5 Effective earth resistance of the stationAfter measuring the resistance of individual electrodes, reconnect all earth leads including interconnection of earth electrodes. Now measure the earth resistance at the outer most electrode, driving the auxiliary electrodes in the outward direction. The value so measured gives the effective earth resistance of the stationPoints to note The test electrode and the auxiliary electrodes shall be in a straight line. The spacing between the electrodes shall be approximately equal. The auxiliary electrodes shall be driven to approximately the same depth and the depth shall be very low compared to the spacing between electrodes. The tester shall be cranked at the specified and uniform speed.Acceptable limits of earth resistanceThe acceptable limits of earth resistance values for various systems are given below:

7.6 Earth Continuity TestNon-current carrying metal parts of equipments, control gears and devices are provided with duplicate earth connections to ensure effective equipotential bonding with the earth bus and thereby to the earth electrodes. Duplicate earth leads are provided to ensure that failure of one lead does not result in the disconnection of the equipment from the earthing system. In order to ensure the effectiveness of the protective earthing system, it is necessary to test the continuity of various earthing conductors in the system. The test procedure for earth continuity test is the same as that for the measurement of earth electrode resistance. The earth tester is set ready with the auxilliary electrodes driven at 15 m. and 30 m. from the outer most earth electrode of the earthing grid. The common terminal of the earth tester is connected to a long flexible copper cable. The other end of the cable is connected or held tight to the body of the equipment under test. The tester is now cranked to the specified speed and the reading noted. The reading shall be very low and near to the combined earth resistance of the system. Repeat the test for all equipments and devices connected to the system.A high value of earth resistance in the earth continuity test is an indication of loose contact in the terminations/joints or a break in the earth leads/conductors. A thorough check shall be carried out to locate the fault and corrective action taken.7.7 Measurement of Earth Loop ImpedanceWhen a line to earth fault occurs, the fault current shall have a value sufficient to discriminately operate the protective devices. The value of the fault current is determined by the impedance of the closed loop available for the fault current to circulate. The earth loop impedance includes the impedance of the line conductors, fault, earth continuity conductors, earth leads, earth electrodes etc.

Measurement of earth loop impedance is of greater importance in the case of HT and EHT installations where system earthing and equipment earthing are connected to the same grid or bus. Connections for measurement the earth loop impedance is shown in fig. 7.5. When HRC fuses are used to protect the circuit, approximately five times the rated current of the fuse is taken as the minimum required current for fast clearing of the fault. When protective relays are used, a fault current of around two times the setting of the relay is considered the minimum required current. The measured value of earth loop impedance shall be low enough to produce the above fault currents. The earth loop impedance may be measured at different levels of distribution i.e. at the fag end, DB level, SSB, MCC, MSB etc.7.8 TRASFORMER PROTECTION If the electric transmission and distribution system is like a human body, then a transformer is as a backbone in human body. So protection for a transformer is much necessary.The following protections are provided to power transformer mounted at GSS:-1. Over current protection2. Differential protection3. Over pressure protection 4. Temperature over rise protection 5. Earth fault protection 6. Buchholzs protection7. Winding temperature rise protection8. Oil temperature rise protection9. Over voltage protection10. Impedance or Distance protection

7.8.1 OVER CURRENT PROTECTION When as the load increases on the transformer, current taken by transformer also increases in steps. In this situation transformer is said to be in over current position and if this position is maintained for a long period, then there can be dangerous hazard to transformer and obviously there can be damage to line. So to protect transformer from over current, there is a relay called (over current relay) is connected to the transformer, which operates when the transformer gets in the control panel and automatically the circuit breaker opens and cut-off the transformer from mains. The relay is energized by 220V DC coming from battery room.

7.8.2 DIFFERENTIAL PROTECTION The operation of relay is depends on the difference in magnitude or phase of current or voltage. For the purpose two current transformers are used at both ends of the system to be protected .These transformer have same ratio of transformation and their secondary are interconnected. For this protection there is also a relay used which is connected in for the feeder between the substations. Whenever there is a difference in the magnitude of transformer. relay operates and gives a signal to the C.B. to be operated, providing protection to the transformer.

7.8.3 OVER PRESSURE PROTECTION When we know that a transformer is filled up with transformer oil, then oil is filled in a particular pressure, when as there is any heating in winding or oil, gas formation develops which increase the pressure in the tank. Tank is made for definite pressure to be tolerated. If the pressure increase from this definite value, there can be a danger to transformer from over pressure situation there is a pressure relief value operates or opens providing free exist of a formatted gases so that the over pressure can be converted into normal pressure and escape the tank from over pressure.

7.8.4TEMPERATURE