1- Why switch is used?It is used for opening and closing an electric circuit.

2- Why fuse is used?It is used for over current protection.

3- What is a circuit breaker?It is a switching and current interrupting device. Basically a circuit breaker comprises a set of fixed and movable contacts. The contacts can be separated by means of an operating mechanism. The separation of current carrying contacts produces an arc. The arc is extinguished by a suitable medium such as dielectric oil, air, vacuum, and SF6 gas.

4-What is control room?It is used for switching and controlling power-consuming devices.

5-What is isolator?It is disconnecting switch, which can be used for disconnecting a circuit under no current condition. It can be opened after circuit breaker.

6-What is Earthing switch?Earthing switch is necessary to earth the conducting parts before maintenance and also to provide deliberate short current while testing. There can be three types of earthing switches in metalclad switchgear manually operated, automatic high speed earthing switch, protective earthing switch for earthing the installation.

7-What is the indicator?It is a device indicates whether the switching device is open or closed position.

8-What is the function of auxiliary switches?

The auxiliary circuits serve several purposes such as:-Indication breaker open or closed by lamp, near circuit breaker and a remote place.-Electrical interlock: the breaker interlocked electrically with isolators. The connections to solenoids in operating mechanism are made through the auxiliary switch- Connection for relaying, auxiliary circuits of operating mechanisms.

9-What are the busbar modules?The busbar modules are either with single phase or three-phase enclosure. Three-phase enclosure and compact and have less eddy current losses. The three busbars are conveniently staggered by a distance equal to center-to-center spacing. The diameter of enclosures depends on rated voltage and internal clearance requirement. The main conductors are aluminum or copper tubes. The contact areas are silver-plated. Busbar is essential component of switchgear and defined as conductors to which several incoming and outgoing lines are connected.

10-Why single busbar is used?

Single busbar is used to provide least flexibility and immunity from total shutdown. Normally, it is used in small and medium sized substation and most economical and simple.

11-Why double busbar is used?Double busbar is more flexibility, continuity of supply and permits periodic maintenance without total shutdown. In the event of fault on one bus the other can be used. However, it is more expensive than single busbar.

12-What is the material of bus bar?

The material of busbar is copper and aluminum. Copper being scare and costly however aluminum is being increasingly used for busbars The material used for busbar should have-Low receptivity-Higher softening temperature -Good mechanical properties-Low cost

13-What is the shape of busbar?-Single flat rectangular sections-Multiple rectangular sections-Angles or channels-Double channels-Hollow tubular sections-Hollow rectangular sections

14-What is the Isolator or Load break switch?Isolator is either on load type load break switch or no current break type. It has a motor mechanism, which permits remote control. Necessary interlocks are provided between the circuit breaker isolator and earthing switch.

15-What is current transformer?The current transformer is generally with ring core. One to three cores can be housed in one module. The secondary terminals are brought out through gas tight insulating terminal plate.

16-What are voltage transformer modules?-For rated voltage upto 145Kv, inductive transformer with cast resin coil is preferred.-For rated voltages of 245Kv, inductive V.T with SF6 gas as main insulation is preferred- For 420Kv, capacitive voltages transformers are preferred

17-What are the bushings?They are provided with SF6 metal clad switchgear for connecting the live parts to overhead lines. They are SF6 filled. The connections between the conductor of bushing and conductor in switchgear are made by mean of plug in type contact supply.

18-What are cable terminations?They are provided for connecting he live parts with high voltage cables.

19-What is the components of switchgear?-Switches-Switch- fuse combinations-Circuit breakers-Isolators-Earthing switches-H.R.C fuse

20-What is components measurements?-Current transformers-Potential transformers-Measuring instrument

21-What is contactor?The contactors are mechanical switching device capable of making, carrying and breaking electric current under normal circuit conditions including operating overload conditions.

22-What is the types of contactors?-Electromagnetic contactor: the opening and closing of main contactors is achieved by means of electromagnet.-Electro pneumatic contactor: it is pneumatic device operated electrically. The electrically operated valve opens the passage of compressed air, thereby the air from auxiliary compressed air enters the cylinder of the contactor and contacts are operated.

23-What is the rated values?-Rated voltages:a- rated operational voltage for three phases contactors. It is the rated voltage between phases.b. Rated insulation voltage. It is the voltage to which the dielectric test creepage distance is referred.

24-What is the single line diagram?

The single line diagram is a diagram, which illustrates the main circuits and the main components such as circuit breakers, busbars, incomers, outgoing lines, CTs, VTs etc.

25-What is the control diagram?The control diagram is a diagram that illustrates control circuits, protection circuits, interlock circuits, measurements, metering etc.

26-Why bus coupler is used?Bus coupler is used to connect two buses to transfer power from one bus to other one. It is used in more important substation.

27-Why bus-section is used?Bus-section is used to:

-Shut down one section completely for maintenance and repair while the other continuous to supply.- Add a current limiting reactor between the sections, the fault MVA can be reduced, there by circuit breaker of lesser capacity may be permitted.

28-What is a ring bus?A ring bus provides greater flexibility. The supply can be taken from any adjacent section. The effect of the fault in one section is localized to the section alone. And the other sections continuous to operate.

29-What is a one –and a half breaker arrangement?A one and a half breaker uses three breakers for two circuits. Any circuit breaker can be switched off for purpose of maintenance without the provision of bypass. This arrangement gives high security against loss of supply. Normally, this system is used with high power that can be handled by individual circuit.

30-What is a mesh system?In a mesh system, the circuit breakers are installed in mesh formed by the buses. The circuits are tapped from the node points of the mesh.

31-What is a Load Break Switches (LBS)?LBS are used in distribution system. So using circuit breakers not economical comparing to the fault levels. It is used in conjunction with H.R.C fuses and circuit breakers. LBS are capable of making breaking currents under normal conditions.They can carry the specified current of specified values for specified time.Load break switches can:-Making rated currents-Making specific short circuit currents-Carrying specified short circuit currents-Interrupt small inductive, capacitive currents

32-What are the types of the circuit breaker according to the medium of arc extinction?- Air breaker circuit breaker / miniature circuit breaker- Oil circuit breaker (tank type of bulk oil)- Minimum oil circuit breaker- Air blast circuit breaker- SF6 circuit breaker- Vacuum circuit breaker

33-What are the modes of arc extinction?- High resistance interruption: the process of resistance of the arc is increased by

lengthening and cooling it to such an extent that the system voltage is no longer able to maintain the arc and the arc gets extinguished. The technique is employed in air break circuit breakers and DC circuit breaker.

- Low resistance or zero point interruption: the arc gets extinguished at natural current zero of the alternating current wave and is prevented from restricting again by rapid build up of dielectric strength of the contact space. This process is employed in almost all AC circuit breaker.

34-What does the structural form of circuit breakers depend on?It depends on its type, rated voltage type of design and type of operation mechanism.

35-What is the function of operating mechanism? Circuit breakers have two working position open and close. The opening should be fast, in order to reduce circuit breaker time. The operating time between instant of receiving trip signal and final contact separation is of the order of .03 sec in modern EHV circuit breakers.In slow circuit breaker used in distribution system time can be about 3 cycles.While closing, the contact closure should be fast, sure without hesitation, with adequate contact pressure at the end of contact travel.

36-What is the function of closing breakers?During normal closing normal load does not cause any difficulty. The operating mechanism has to overcome friction and accelerate the moving mass. However, when circuit breaker has to close against a short circuit, additional thermal stresses and electromagnetic stress are involved.In EHV circuit breakers, the arc is established prior to final contact touch. This is known as pre-arcing, which causes higher temperature stresses and pressure due to vaporization of oil.

37-What is the steps of circuit breakers maintenance? General inspection: observe the circuit breaker visually. Note the cleanliness, terminals, earth connections, readings of counters, levels of quenching medium (in case of oil circuit breakers), pressure of quenching, and medium in case of SF6 circuit breakers.- Cleaning and drying: use clean cloth any liquid cleaning like trichloroethylene and make sure no dust, iron particles, nutbolts washers etc inside the breaker. Avoid water, moisture or dampness during the cleaning.- Insulation surface: inspect visually, carefully for signs of cracks, tracking or any other defects.- Interrupter- Mechanism: check operation ‘open’; ‘close’ many times and make sure it is satisfactory. Check operation counter. If it is more than 1000 times it needs very close observation.

38-What is SF6 circuit breaker?SF6 is an inert, heavy gas having good dielectric and arc extinguishing properties. The dielectric strength of the gas increases with pressure. SF6 is now being very widely used in electrical equipment like high voltage metal enclosed cables, high voltage metalclad switchgear, bushing, circuit breakers, and current transformers.

39-What are the physical properties of SF6 gas?- Colourless- Odourless- Nontoxic (pure SF6 is not harmful to health)- Non- inflammable (however impure SF6 contains toxic impurities)

- State gas at normal temperature pressure- Density heavy gas density 5 times that of air at 20 ْ and atmospheric pressure

40-What are the chemical properties of SF6 gas?- Stable upto 500C ْ- The chemical inertness of this gas is advantageous in switchgear- The components does not get oxidized or deteriorated- Moisture is very harmful to properties of the gas. In the presence of moisture,

hydrogen fluoride is formed during arcing which can attack the metallic and insulating parts in the circuit breaker

- Electronegative gas- During arc extinction process SF6 is broken down to same extent into SF4 & SF2.

The products of decomposition recombine upon cooling to form the original gas. Theses products of decomposition are toxic attack certain structural materials.

41-What are the dielectric properties of SF6 gas?-Dielectric strength of SF6 at atmospheric pressure is 2.35 times than air; it is 30% les than that of dielectric oil used in oil circuit breaker.-Effect of pressure on breakdown voltage. The breakdown voltage in SF6 gas depends on several aspects such as electrode configuration, roughness of electrodes distribution of electric field, vicinity of insulating supports, moisture and wave shapes.- Effect of electrode surface. Conductor surfaces should be smooth. Rough electrode

surface reduce the breakdown voltage with rough surface the ionization starts earlier near the sharp point on conductor.

- Effect of insulating support on breakdown voltage. The conductors in SF6 insulating equipment are supported on epoxy or porcelain insulators. The insulators should be extremely clean and should have anti tracking properties. The breakdown can occur at extremely low values if the insulator supports are covered by moisture and conducting dust.

42- Explain arc extinction in SF6 circuit breaker During the arcing period, SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation. The arc diameter reduces during the decreasing node of the current wave. So, the diameter becomes small during current zero. Turbulent flow is introduced around current zero to extinguish the arc. In SF6 breaker, the gas is made to flow a high-pressure zone to a low-pressure zone through a convergent divergent nozzle.

43-How can SF6 gas be transported and handled? It is transported in liquid form in cylinder of various sizes such as 15, 40, 100 kg. The necessary heat for conversion from liquid to gas is taken from atmosphere. However, in

cold countries it becomes necessary to keep the gas cylinder in hot water to convert liquid into gas.Before filling the circuit breaker, it should be evacuated and the air and moisture must be removed from inside the breaker. During maintenance, the gas sample from SF6 circuit breaker is collected and tested for moisture and other impurities. The activated alumina absorbs the impurities like S2, F2, and SF4 moisture. The gas can be used again after regeneration.

44-What is SF6 G.I.S?SF6 Gas Insulated metalclad Switchgear is called Gas Insulated Substation (G.I.S). In such a substation, the various equipment like circuit breakers, Busbar, Isolator, Load break switch, Current transformers, Voltage transformer, Earthing switches are housed in separate metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation.

45-What is the advantages of G.I.S?- Compactness: the space occupied by SF6 installation is only about 10% of that of

conventional outdoor substation. High cost is partly compensated by saving in cost of space.

- Protection from pollution: the moisture, pollution, dust etc have littlie influence on SF6 insulated substation

- Reduced switching over voltages: the over voltages while closing and opening line, cables, motors capacitors etc are low.

- Reduced installation time: the principle of building block construction reduces the installation time to a few weeks. Conventional substations require a few months for installation.

- Superior arc interruption: SF6 gas is used in the circuit breaker unit for arc quenching. This type of breaker can interrupt current without over voltages and with minimum acing time. Contacts have long life and the breaker is maintenance free.

- The gas pressure 4 kgf/cm2 is relatively low and does not pose serious leakage problems.

- Increased safety: as the enclosures are at earth potential, there is no possibility of accidental contact by service personal to live parts.

46-What are the demerits of SF6 Insulated Switchgear?- High cost compared to conventional outdoor substation- Excessive damage in case of internal fault. Long outage periods as repair of damaged

part at side may be difficult - Requirement of cleanliness are very stringent. Dust or moisture can cause internal

flashover- Substations need a separate building not like outdoor substations

47-How the constant space in circuit breaker can be ionized?-Thermal ionization of gas-Ionization by collision of particles-Thermal emission from surface of constants- Secondary emission from surface of constants- Field emission from the surface of constant

48- There are several aspects, which the arc extinction process in circuit breaker is, influenced what are these?- Speed of contact- Material of contact- The pattern of flow of quenching medium- Magnitude of arc current and variation for arc diameter- Energy liberated during arcing, energy in system inductance- Rate of rise of transient recovery voltage - Rate of gain of dielectric of dielectric strength- Instant of contact separation with respect to voltage and current

49-What is the name of substation which does not have transformer?The name of substation that does not have transformer is switching transformer.

50-How is spacing insulators determined?A spacing of support insulators is determined on the basis of the force on a span length of busbar under short circuit conditionsF=2.04is2×L/r10*-2 kgf Is= peak momentary short circuit current (asymmetrical), KA L = span between insulator supports, cmR= spacing between neighbouring conductors in three phases bus system, cmDistance r is determined by clearance consideration and by shapes of conductors selected.

51-How can insulation requirement be determined?-Highest power frequency system voltage (continuous)-Temporary power frequency over voltages (a few milliseconds to seconds) caused by load throw off, faults, resonance-Transient over voltage surges (few hundred microseconds) caused by lighting, switching, restricts-With stand levels of equipmentBasic Impulse insulation Level (BIL) is reference level expressed in peak (crest) voltage value with standard 1.2/50 microseconds lighting impulse wave.-Protective levels of surge arrestors and available protective margin against lighting / switching surges.

Protective Margin = [without stand level of equipment]-[protective level by surge arrestor]-Co-ordination with other equipment connected to same voltage level.-Co-ordination between various voltage levels in the network -System Neutral Earthing

52-What is surge arresters?Surge arrestors coordinate spark gaps, surge suppressors, overhead ground wires, neutral earthing, shunt capacitors etc are located strategically to intercept the lighting surges or to reduce the peak and rate of rise of surges. It is used against over voltage.

53-What is a neutral grounding?A neutral grounding points (start points) of start-connected three phases winding of power transformers, generators, motors, earthing transformers are connected to low resistance ground a connection is called neutral earthing.

54-What are the types of reactors?-Current limiting reactors: saturated (serious reactors) are inserted in serious with the line, to limit the current flow in the event of a short circuit and thereby bring down the fault level. They are also called Serious Reactors.-Reactors in neutral to earth connection called arc suppression coils/Peterson coil/ground fault neutralizers. This type helps in eliminating or suppressing arcing grounds.-Shunt reactor (compensation reactors) are connected with transmission lines, for absorbing reactive power.-Reactors in harmonic filters-Smoothing reactors in HVDC system

55-Where is reactor location in the following:-Generator reactor: it is inserted between the generator and generator bus-Feeder reactors: they are connected in serious with the feeders- Busbar reactors: they are on the busbar- Smoothing reactors: they are installed in serious with rectifier sets on DC side to reduce ripple in DC current and minimize requirement of harmonic filters.

56-What are the advantages of feeder reactors?-The voltage of bus does not drop substantially in the event of fault on the feeder.-The other feeders affected less

57-What are the disadvantages of feeder reactors?-There are so many feeders. Therefore, many reactors are necessary-There is constant loss in the reactor as full feeder current flows through it

58-What is transformer?The transformer is an indispensable component of power system. It is one of the main reasons for widespread use AC power system. It makes possible electric power generation at most economical voltage, transmission and distribution at the most economical voltage levels and power utilization at the most suitable voltage. The transformer is also used in measurement of high voltages and large current. Other uses of transformers include impedance matching insulating one circuit from another or insulating DC circuit from AC circuits. A power transformer is a static piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. A transformer will have at least two windings, one primary and one secondary.

59-What is the actual transformer?It can be desired as having resistance in its winding. Not all of the flux produced by one winding will link the other winding because of flux leakage. The core of actual transformer is not perfectly permeable; it has a finite permeability. Thus, it requires a finite magnetomotive force for its magnetization. Because the flux in the core is alternating, there are hysteresis and eddy current losses, collectively called core loses or iron losses.

60-What is the autotransformer?The autotransformer serves a function similar to that the ordinary transformer to raise or lower voltage. It consists of a single continuous winding with tap brought out at some

intermediate point. Because the primary and secondary winding of the autotransformer are physically connected, the supply and output voltage are not insulated from each other. When a load is connected to the secondary of autotransformer, a current flows in the direction of load and some current flows in secondary winding.

61-What is disadvantages of autotransformers?-The secondary current flows by means of the actual physical (conductive) connection between windings instead of solely by magnetic induction. Therefore, more power can be transferred through an autotransformer than through a conventional two winding transformer of the same physical size.-This increased power flow allows for a greater efficiency, lower cost and smaller physical size.

62-What is the advantages of autotransformer?-There is no way to separate the line windings in an emergency

-They are important when two or more transformers are to be paralleled 63-What are the formed of three phases transformer?

-Three phases connect to three single-phase transformers to form a three-phase bank.-Manufacturing a three phases transformer bank with all three phases located on common multilegged coreHowever, there is no difference between the two methods.

64-What is voltage regulation?Voltage regulation is used in distribution and power transformer to supply loads that are designed to operate at essentially constant voltage. The amount of secondary (load) current drown depends on the load connected to the transformer secondary terminals.At this current changes with changing load, with the same applied primary voltage the load voltage likewise changes. This change is due to voltage drop across the internal impedance of transformer. A measure of how much the voltage will change as load is varied is voltage regulation.

65-What is the efficiency?The percent efficiency of transformer is defined as the ratio of the power output Pout to the power input Pin

66-What is the four types of three phases transformer?- Y-Y connection is seldom used because of possible voltage- Y-∆ connection is commonly used in stepping down from high voltage to a medium

or low voltage level, as in distribution transformer.- ∆-Y connection is used for stepping up to a high voltage, as in generation station

transformer- ∆-∆ connection is used because of its advantage that the one of the three single-

phases transformer can be removed for repair or maintenance. The remaining two transformers continuo to function as three phases bank, although the KVA rating of the bank is reduced to 38% of the original three phases bank rating. This mode of operation is known as open-delta connection or V-V connection. It is used when load is presently small but is expected to grow in the future.

In addition Y-Y or ∆-∆ connection corresponding phase voltage are in phase. On the other hand, for both Y-∆ and ∆-Y connection have the primary phase or line –to-line voltage lead by 30 ْ

67-What is the main requirement for occurring of electromagnetic?-There must be a magnetic filed-There must be a conductor-There must be relative motion between the magnetic filed and the conductor

68-What is the leakage flux?The leakage flux is the flux that does not pass between the two winding. This leakage will also cause some heating of the core, which in effect is a power loss.

69-Why oil is used in transformer?It is used to conduct heat away from the core by natural circulation. The oil itself is cooled in a radiator attached to the transformer.

70-What is the advantages of oil?-Insulates energized transformer parts from each other -Cool transformer-Protect transformer’s solid insulation

71-What is the factors of oil damage?-Oxygen-Moisture-HeatOxygen and moisture interact chemically with transformer oil to create decay products that affect the oil’s ability to insulate, cool, and protect. In addition, heat speeds up the decaying effects of oxygen and moisture.

72-How can oil be checked?-Basic oil quality tests-Dissolved Gas Analysis (DGA)-Free gas analysis from Bucholz relay

73-What is the basic oil quality test?-Visual examination of oil-Color comparison test-Dielectric breakdown voltage-Water content-Interfacial tension-Acid neutralization number

74-Why fans are normally fitted to the radiator?Fans are fitted to provide extra cooling during period of high load or overload.

75-Why silicagel is used? Silicagel is used to dry air

76-What are the reasons for transformer loss?-Iron losses in the core-I2R loss due to no load currents-Hysteresis loss in core laminations

-Eddy current loss in core laminations-Stray eddy current loss in core clamps, blot etc-Loss in the dielectric circuit

77-What is hysteresis loss?The magnetic flux or lines of force in the core lag behind the magnetizing force that causes them. The losses are produced by the amount of power or energy required to magnetize the coil, first in one direction and then in reverse direction as current alternates. This rapid reversal of the magnetization produces heat similar to the effect produced by the rapid hammering of iron.

78-What is the eddy current loss?The eddy current is the electric current induced in the iron by changing magnetic flux. Eddy currents produce heat in the core and tend to heat the coil, increasing their resistance.

79-How eddy current can be kept minimum?Eddy current can be kept minimum by constructing the core of laminated thin sheets of high quality, non-aging, high permeability, cold reduced, silicon and steel.

80-What is the copper loss?Copper loss is caused by I2R only because no load. This loss is very small in primary since there is no Cu loss in secondary as it is open.

81-What is the transformer efficiency?Under full load operating condition transformer efficiency ranges between 98% for a small unit in distribution system and 99.5% for a bulk power unit in transmission system.

82-What is the turn ratio?The ratio of transformer is determined by the relative number of turns in each of the winding.

83-What is the relation between frequency and transformer?V=4.44fØNAn AC voltage applied to one coil will induce a voltage on the other coil by means of the flux linking both coils. This flux is directly proportional to the applied voltage and inversely proportional to the frequency. So, for any given frequency, any increase in voltage will cause the flux to increase.84-Why transformer rating is in KVA and not in KW?As known copper loss of transformer depends on current and iron loss on voltage. Hence, total transformer loss depends on volt-amp (VA) and not on phase angle between voltages and current i.e. it is independent of load power factor.

85-Why three phases is used instead of single phase?Three single-phase units are more costly, demand more space and are heavier than three phases unit. However, single-phase units are used especially for high ratings.

86-What is the main requirement for satisfactory parallel operation of two or more transformers?- The polarity should be the same

- The voltage ratios should be the same- The percentage impedance should be equal- The phase rotation should be the same- The vector diagrams and the phase displacement should be the same

87-What is the tap changers?It is two types:-Load tap changer-No load tap changerThey may be installed in either or both the high or low side windings. The load tap changer permits the changing of taps while the transformer continuo to carry load. The no load tap changer requires the transformer be removed from service to permit the tap to be changedThe tap changer is applied to power transformer to maintain a constant secondary voltage with a variable primary voltage to control or adjust the flow of reactive KVA.

88-What is the rated power?The rated power of the transformer can be defined as the allowable power for normal life span with the ambient and temperature rise conditions imposed by standard.

89-What is the normal or permissible temperature for oil and winding?-Permissible oil temperature is approximately 65C ْ-Temperature of the winding is approximately 80C ْFor higher ambient temperatures, the load on the transformer should be reduced. The result of excessive over loads on transformer is cellulose insulation becomes mechanically weak resulting in winding failure. The oil gets rapidly oxidized during excessive over load.

90-What is Ferroresonance? Ferroresonance is a serious resonant circuit involving iron-core inductance and distributed line to ground capacitance.

91-What does Ferroresonance do?Ferroresonance causes over voltages across the inductance and capacitance elements. This means that both the line conductor or cable insulation and the transformer windings and insulation are subjected to over voltages. The amount of over voltages depending on the Xc/Xm ratio, shape of saturation curve, time of switch –closing effective resistance in the circuit and prior magnetization off the core just to name some of the factors.

92-What is the effects of partial discharge in transformer?- Ion and electron bombardment can be damaging to the insulation and shortens the life of the transformer.- The transient currents produced due to partial discharges may interfere with electrical communications

93-What is the oil temperature indicator?- It indicates the temperature of the transformer’s insulating oil.

- Switching device to operate transformer-cooling fans, oil circulating pump and alarm when the oil reaches predetermined temperatures.

94-What is the winding temperature indicator?- It measures the equivalent or simulated, hot-spot temperature of the

transformer winding. It cannot measure directly. However, it measures by using various components in winding temperature indicator system to simulate the hot-spot temperature of the power transformer’s windings.- Switching device to operate transformer cooling fans, oil pumps and

alarm.

95-What is the type of cooling?-Self-cooled power transformer: itself naturally without the use of mechanical cooling equipment such as fans, heat exchanger or pumps. A self-cooled power transformer (with or without radiators) is cooled by the natural circulation of oil and air, no mechanical mean are used to cool the transformer. It is named ONAN (Oil Natural / Air Natural)-Self-cooled / Forced air-cooled power transformer: it has more cooling capacity that a self cooled power transformer. Self cooled still means that the transformer is cooled by natural circulation of oil and air. Forced air-cooled means that the forced circulation of air aids the cooling. Also this type is named ONAF (Oil Natural/Air Forced)- Forced oil / Forced air-cooled power transformer: it has more cooling capacity than either a self-cooled or a self cooled / forced air-cooled. Forced oil means that the forced circulation of oil aids the cooling, and forced air means the cooling is aided by the forced circulation of air. A forced oil/ forced air-cooled power transformer has heat exchangers, pumps and flow gauges. This types of cooling named OFAF (Oil Forced /Air Forced)

96-What is a heat exchanger?A heat exchanger on a power transformer typically consists of fans and radiator with finned tubes. When the pump is running it circulates oil through the heat exchanger and transformer. The flow gauge indicates whether or not the pump is circulating oil through the system.

97-What are the types of maintenance?-Breakdown or corrective maintenance

-Preventive maintenanceThe breakdown or corrective maintenance activities are undertaken after failure of equipment. Such maintenance results in outage of circuit and supply. In general, it consists of locating trouble repair and re-commissioning.The preventive maintenance is undertaken to ensure smooth and efficient working of a system, equipment. Preventive maintenance is undertaken as per schedule before break down of a system or machine takes place.

98-What is the choice of activities of maintenance?-Inspection: it comprises careful observation of equipment without dismantling. It includes visual and operational checks.-Servicing such as cleaning, adjustment, and lubrication also without dismantling.-Examination: this refers to inspection with necessary dismantling, measurement and non-destructive test to obtain data regarding the condition of components.

-Overhaul: this refers to the work done with the objective of repairing/ replacing worn-out parts and detective parts.

99-What is a fault?The fault in a circuit breaker is any failure that interfaces with the normal flow of current.

100-What is the classifying of AC fault?-Simultaneous fault- Open circuit

101-What is the classifying of fault?- Single line-to- line ground fault (SLG)- Line –to-line (L-L)- Double line-to-ground fault (2LG)- Balanced three phases fault

102-What are the abnormal conditions in AC system?- Over voltage - Voltage and current unbalance - Temperature rise- Reversal power- Power swing - Under frequency- Instability etc

103-What does happened when a fault occurs in a power system? The current increases several times the normal current because of the low fault impedance. The value of the fault current depends on the voltage at the faulty point and the total impedance upto the fault.

104-What are the causes of faults?-Breaking of conductor-Failure of insulationFault impedance is low, and fault currents are generally high and the voltages of three phases become unbalancedIn addition fault currents being excessive, they can damage not only the faulty equipment, but also the installation through which the fault current is fed. Fault in certain important equipment can affect the stability of the power system.

105-How faults can be minimized?Fault can be minimized to some extent by taking the following measures:-Improvement in the quality of machines, equipment, installation etc by improvement in design, manufacturing techniques, materials, quality control, adequate testing, research and development.-Improvement in system design, correct lay out, choice of equipment.-Adequate and reliable protection system; control-Regular and detailed maintenance by trained personnel-Trained personnel for operation and management of electrical plant.

106-What are the normal faults in system

Equipment Cause of fault

1. Overhead line

- Lighting strokes-Storms, earthinquakes-Birds, trees, kites aeroplanes, snakes etc-Internal over voltages

2. Underground cables

-Damage during digging-Insulation failure due to temperature rise-Failure of joint

3. Transformers

-Insulation failure-Faults in tap changer-Faults in bushing-Faults in protection circuit-Inadequate protection-Overloading, over voltage

4.CT, PT

-Over voltages-Insulation failures-Breaking of conductors-Wrong connections

5. switchgear

-Insulation failure-Mechanical defect-Leakage of air/oil/gas-Inadequate rating-Lack of maintenance

107- What are the transient phenomena?

Electric transient occurs on a power system, which are generally caused by a sudden change in the operating condition or configuration of the system. Theses transient over voltages are caused either by lighting striking transmission lines or by switching operation.

108- How does relay work?During the abnormal or faulty conditions the relays sense the fault and close the trip circuit of circuit breaker. Therefore, the circuit breaker opens. The operation of automatic opening and closing the contacts achieved by means of the operating mechanism of the circuit breaker. When the relay contact close, the contacts of the circuit breaker open and an arc is drawn between them.

109-What is the function of protective relaying?-To sound an alarm or to close the trip circuit of circuit breaker so as to disconnect a component during an abnormal condition in true rise, unbalanced load, reverse power, under frequency, short circuit etc-To disconnect the abnormally parts so as to prevent the subsequent fault e.g. over load protection of a machine protects the machine and prevents insulation failure.-To disconnect the faulty part quickly so as minimize the damage to faulty part e.g. if a machine is disconnected immediately after a winding, only a few coil may need replacement. If the fault is sustained, entire winding may get damaged and the machine may be beyond repairs.- To localize the effect of fault by disconnecting the faulty part from the healthy part causing least disturbance to the healthy system.-To disconnect the faulty part quickly so as to improve the system stability, service continuity and system performance. Transient stability can be improved by means of improved protective relaying.Fault cannot be avoided completely. They can be minimized. Protective relaying plays as importance role in minimizing the faults and also in minimizing the damage in the event of faults.

110-What is the protective relay?The protective relay is a device that responds to signals from the transducers by quickly initiating or allowing a control action to be implemented in order to prevent damage to the faulted equipment and to restore service as soon as possible.A relay is said to pick up when it operates to open its normally closed (NC) contact or to close its normally open (NO) contact in response to a disturbance to produce a desired control action. The smallest value of the actuating quantity for the relay to operate is called its pickup value.

111-How does a relay work?A relay works or operates if the net force is positive in this equation:F=Fo-FrWhereF= net forceFo= operational forceFr= restraining force

112-Explain how relay works in electromechanical relays?

Electromagnetic attraction/electromagnetic induction/thermal effects of electric current produce the operating torque. The restraining torque is given by spring.

113-What are the types or relays units?-Attracted armature type (electromagnetic) relay-Balanced beam (electromagnetic) relay-Induction disc (electromagnetic) relay-Induction cup (electromagnetic) relay-Moving coil and moving iron (electromagnetic) relay-Gas operated (bucholz) relay (gas pressure)-Rectifier relays (rectifier plus moving coil unit)-Static relay (static electronic circuit for measurement)The electromagnetic relay operates when operating torque/force exceeds the restraining torque/force.

114-What is the electric arc?The electric arc is a type of electric dishrag between electrodes. In circuit breakers, the arc persists during the brief period after separation of current carrying contacts. The arc plays on important role in the behavior of the circuit breaker. The interruption of DC arcs is relatively more difficult than AC arcs. In AC arcs, as the current become zero during the regular wave, the arc vanishes and it is prevented from restricting.

115-What is the dissociation?The matter can be ionized. The ionized matter consists of charge particles such as ions and electrons. There is relation between temperature and velocity that all increasing together. At temperature of the order of 3000k the molecules break up into simpler forms such as simpler molecules and atoms. This is process is called dissociation.

116-What is the classifying of protection schemes?- Primary protection (main): provides rapid and selective clearing of fault within the

primary zone of protection. It is provided for each section or major pieces of equipment.

- Backup protection: provides the protection needed whenever the primary protection fails or under maintenance.

117-What are the three principles components of a protection system?There are there principle components of a protection system:-Transducer serves a sensor to detect abnormal system condition and to transform the high values of short circuit current and voltage to lower levels.-Protective relay is a device that processes the singles provided by the transducer, which may be in the form of a current, a voltage or combination of current and voltage. These signal arise as a result of a faulted condition such as short circuit defective equipment or lines, lighting strikes or surges.-Circuit breaker is a mechanical device used to energize and interrupt an electric circuit. It should be able to open and close quickly may be in the order of a few milliseconds. It should be able to carry the rated current continuously at nominal voltage, and it must be able to able to withstand the large short circuit current (called its momentary rating) that flows during the first cycle after a fault occurs.

118-What are the characteristics of protection system?-Speed refers to the opening times of the protective relays. Protective relays may be characterized as instantaneous with operating time of about .05s.-Selectivity is the ability of protection system to detect a fault identifies the point at which the fault occurred and isolates the faulted circuit element by tripping the minimum number of current breaker.-Reliability is the ability to operate upon the occurrence of any fault for which it was designed to protect. In other words, the protection system should operate when it is supposed to and not operate when it is not supposed to.-Sensitivity refer to the relay when it operates reliably, when required in response to fault that produces the minimum short circuit current flowing through the relay

119-What does a balance or symmetrical phases mean? A balanced or symmetrical set of phasors is defined as set of phasors that have equal magnitude and separated by equal angles.

120-What is corona?Corona is the generic name for electrical discharges that occur in electrical insulation as a result of high velocity ionization under influence of an electric field that exceeds the dielectric strength of the insulation.

121-What is the cause of partial discharges?-Improper processing or drying of insulation-Over stressed insulation due to a lack of proper recognition of the voltage limitation of the insulation-High stress areas on conducting parts, which can be caused by sharp edges on either the conducting part or ground plane.

122-How does protection work against temporary power frequency over voltages?Protection works against temporary power frequency over voltage by Inverse Definite Minimum Time over voltage (IDMT). Over voltage connected to secondary of voltage transformer. During power frequency over voltage beyond permissible limit, the over voltage relay acts and sends appropriate command to busbar/ line C.B and the circuit breaker opens

123-What is the time for tripping of-Temporary over voltage (over voltage relay & C.B > 70ms)-Lighting surge (surge Arrester 1.2 microseconds)-Switching surge (surge arrester 100 microseconds)

124-What is a nature of problem of leakage current (ground faults)?The charge is given by Q=CVWhere Q=charge, coulombsC = capacitance, farad

V= voltage, voltFor high voltage system, the reactive power may be hundreds of KVAr. So, it should be controlled. During ground faults, the reactive KVAr cause substantial flow of capacitance current flow with ground as return path. Neutral grounding is a simple method of reducing such as currents

125-What are advantages of neutral grounding?-Arcing grounds are reduced or eliminated. The arcing ground current flowing through the neutral to ground connections is made almost equal and opposite to the capacitive current from healthy lines to ground. Thereby IL+IR+IY=0 and arcing grounds are eliminated.-The voltages of healthy phases lines with respect to earth remain at normal value. They do not increase to 3 time normal value as in the case of underground system.-The life of insulation is long due to prevention of voltage surges caused by arcing grounds. Thereby reduced maintenance, repairs, and breakdowns. Improved continuity -Stable neutral point -The earth fault relaying is relatively simple. Useful amount of earth fault current is available to operate earth fault relay. -The over voltages due to lighting are discharged to earth.-By employing resistance or reactance in earth connection, the earth fault current can be controlled.-Improved service reliability due to limitation of arcing grounds and prevention of unnecessary tripping of circuit breakers.-Greater safety to personal and equipment due to operation of fuses or relays on earth fault and limitation of voltages.-Life of equipments, machines, installation is improved due to limitation of voltage. Hence overall is economy.

126-What is the types of grounding?-Underground system: it is used no more. The neutral not connected to earth.-Solid grounding: the neutral is directly connected to ground without any intention impedance between neutral and ground. The coefficient is less than 80%-Reactance-Resonant

127-What are advantages of earthing system?-Safety against shocks to operation and maintenance staff working in substation-Discharging fault currents and discharge current/voltage from the earthed point of lighting masts, lighting conductors.-Ensures freedom from electromagnetic interference in communication and data processing equipment in the substation

128-What is auto reclosure using?Many fault on overhead lines are transient in nature. Statically evidence shows that about 90% of faults are used by lighting, birds, vines, tree branches etc. theses conditions result in arcing faults and the arc in fault can be extinguished by de-energizing the line by simultaneous opening of circuit breakers on both ends of the line or one end of the line. The time of reclosure is about 12 cycles (.2sec). So re-closing of lines restores the supply

continuity of service is the main advantage. Also, theses auto re-closing of EHV is single shot only one re-closing is attempted. However, auto reclosure for distribution up to 33Kv is three times between 15 to 120 seconds.

129-What is the over current?Over current is designed to operate when the actuating equals or exceed its pickup value. An over current relay can be instantaneous or time delay. Both relay types are frequency provided in one relay case and are actuated by the same current however, their individual pickup values can be adjusted separately. The pickup values may be adjusted by varying the top setting in the input.The instantaneous relay element has no intentional time delay, and it operates quickly from ½ to 3 cycles depending on the value of the fault current. The difficulty in using over current relays is that they are inherently nonselective. They detect over current (fault) not only in their own zones of protection but also in adjacent zones. Over current relay are popular especially on low voltage circuits because of their low cost. They are also used in specific application on high voltage system.

130-What is the directional relay?Directional relay are able to distinguish between current flowing in one direction and current flowing in opposite direction. The relay responds to the phase angle difference between the actuating current and a reference current (or voltage) called the polarizing quantity. Directional relaying is typically used in conjunction with some other relay usually the over current relay to improve its selectivity.

131-What is the types of group of directional relays?-Directional power relays-Directional over current

-Directional earth fault

132-What is an under voltage relay? An under voltage relay is provided for AC circuit, busbar, rectifier, transformer etc. it is necessary for voltage control and reactive power control of network buses and load buses. It can have instantaneous characteristic or inverse characteristic depending upon the construction and design

133-What is the differential relay?The operation of a differential relay is based on the vector difference of two or more similar electrical actuating quantities. The most common application is current differential relaying in which the current entering and current leaving the protected element are compared. If differences exceed the pickup value of the relay, it operates to trip the breakers to isolate element. The differential current that will flow through the over current relay may be sufficient for the relay to pickup and cause false tripping of the circuit breaker.

134-What is the percentage differential relay?

The difficulty encountered in differential relaying due to CT errors is eased by the use of a percentage differential relay. This type has an operating coil and two restraining coil. The operating current is proportional to (IA-IB) and must exceed a certain percentage of the restraining current, which is proportional to ½ (IA+IB) before the relay will operate. 135-What is the distance relay?A voltage and a current are balanced against each other and the relay responds to the ratio of the voltage to the current that is the impedance of the transmission line from the relay location to the point of interest. The impedance may be used to measure a distance along transmission line.The distance relay is useful because it is able to difference between a fault and normal operating conditions and to difference between faults in specific area and a fault in different parts of system. The distance relay picks up whenever the measured impedance is less than or equal to the selected pickup value of impedance.There are several types of distance relay has an inherent directional characteristic that is it responds to or sees fault in only one direction. On the other hand, the impedance and reactance relays see in all directions. Thus, a directional relay is commonly used together which impedance relay and a mho relay is used as a starting unit for reactance relay.

136-What is the pilot relay?Pilot relaying is a means of communicating information from the end of a protected line to the protective relays at both line terminals. The relays determine whether a fault is internal or external to the protected line. The communication channel or pilot is used to transmit this information between line terminals. If the fault is internal to protected line, all circuit breakers at the terminals of line are tripped in high speed. However, if it is external the tripping of circuit breakers is prevented or blocked.

137-What are the types of pilot?The types of pilot are:-Wire pilot consist of a twisted pair of copper wires of telephone line type. It may be leased from the telephone company or it may be owned by the electric utility.-The power line carrier is the most commonly used pilot for protective relaying. In this type of pilot a low voltage, high frequency current is transmitted along one phase of high voltage power line to a receiver at the other end of the protected line. Line traps located at both line terminals serve to contain a carrier signal inside the zone of the protected line.-The microwave pilot is an ultra high frequency radio system operating above 900MHZ. In this pilot, transmitter and receivers operate the same way as in power line carrier pilot; however, line traps are replaced by a line of sight antenna.

138-What is the power line carrier channel (PLC)?High frequency signals are transmitted through the transmission line conductor for the purpose of communication, protection, signaling and monitoring.Carrier current equipment is installed at sending end and receiving end of transmission line sections.PLC equipment can be used for the following:-To send tripping signals to the other end transmission line so as to open the circuit breaker at that end (inter-tripping)-To send signal to the remote end so as to accelerate the relays at the other end of the transmission line (carrier acceleration)

-To send blocking signal to the other end of transmission line so as to prevent tripping of circuit breaker at that end (carrier blocking)-Carrier current protection of transmission line based on differential principle Carrier current signaling is used along with digital computers for network monitoring, central load control, and central back-up protection

139-What is the plug setting?The plug setting is a facility for selecting the plug setting and time setting such that the same relay can be used for a wide range of current, time and characteristics.Plug setting bride is provided with induction disc relays and it provides a wide range of current settings. The plug setting refers to the magnitude of current at which the relay starts to operate. The plug setting bridge comprises connections tapped from relay coil. By inserting the plug, in a particular gap in the bridge, a certain number of turns of the relay coil are brought into current.

140-What is the effect of time setting?In induction disc relays, the starting position of the moving contact is adjusted by means of backstop. The length of arc through which moving contact travels before touching the fixed contacts decides the time taken by relay to close.By increasing the length of travel moving contacts the relay time is increased. By reducing the length of travel the relay time is minimized. The time setting dial is marked from 1 to .1.

141-What is an under voltage relay? An under voltage relay is provided for AC circuit, busbar, rectifier, transformer etc. it is necessary for voltage control and reactive power control of network buses and load buses. It can have instantaneous characteristic or inverse characteristic depending upon the construction and design. 142-What is repeat contactors?A repeat contactor is important components of relay contact system. They repeat the operation of the measuring relay and relive the latter of duties such carrying current for longer period. Thus the contacts of measuring relay are relived of other tasks and they can be made of lightweight, delicate resulting in higher sensitivity of relay and less burden.

143-What is an alarm relays initiate alarm?In many abnormal conditions such as overload the tripping of essential service motor or equipment may not be desirable. In such causes alarm is provided. So, the operator is alerted and can take corrective action. The alarm is visual by lamp or audible by bell.

144-What is the tripping relay?

A tripping relay is fast, instantaneous relay and is generally attached armature type. It is either hand or electrically reset type. The order time of tripping is 10 ms. so it is used for high speed tripping.

145-What is a flag indictor relay?A flag indicator relay is used for obtained indications of the operation of a remote protective device. The operation of a relay indicates operation of the corresponding measuring and circuit breaker.

146-What are the standard numbers for relays?

MAIN FUNCTION OF THE DEVICEDEVICE NUMBER

Time Delay Relay is used as a prefix for other functions

2

Checking or interlocking relay 3 Distance Protection Phase to Phase 21 Distance Protection Phase / Earth 21G Distance Protection Zone 1 21-1 Distance Protection Zone 2 21-2 Distance Protection Zone 3 21-3 Distance Protection Zone 4 21-4 Room Temperature High 23 Synchro Check Relay 25 System Synchronizing Relay 25S Oil Temperature High 26D Thermal Overload 26 TOL

MAIN FUNCTION OF THE DEVICEDEVICE NUMBER

Sergi 26S Battery Faulty (Hi-v, Lo-v) 27/59B Under Voltage Relay 27 Trip Circuit Supervision Relay 27 TC Close Circuit Supervision Relay 27 CC Transformer H.V on-load Isolator 29T Annunciator Relay for Local Indication & remote Alarm 30 Directional Power Relay 32 Selector Switch 43 Smoke Detector 45 Phases Not Together 48

Winding Temperature Alarm 49 WG Winding Temperature Trip 49T Instantaneous O/C Non-directional 50 Circuit Breaker Fail Current Check 50 BF Instantaneous E/F Non-directional 50G I.D.M.T O/C Non-directional 51 A.C Circuit Breaker 52 Stuck Breaker Protection 13.8 KV Phase and E/F 51B/ 51NB Definite Time O/C Non-directional 51 DT I.D.M.T E/F Non-directional 51G Feeder O/C Non-directional 51F Feeder E/F Non-directional 51 NF Feeder O/C Instantaneous / I.D.M.T 50/51F Transformer H.V O/C I.D.M.T 51P Transformer H.V O/C Inst/ I.D.M.T 50/51P Transformer L.V O/C 51S Transformer H.V E/F 51 NP Transformer L.V E/F 51 NS Standby E/F Protection 51N Two stages, Standby E/F stage 1 51N1 Two stages, Standby E/F stage 2 51N2 Neutral Current E/F Alarm 51 NA Earth Switch 57

MAIN FUNCTION OF THE DEVICEDEVICE NUMBER

Over voltage Relay 59 Neutral Voltage Displacement Relay 59N Circuit Breaker Fail Timer 62BF Air pressure switch of C.B 63 AC Gas pressure switch of C.B 63 GA Gas pressure High in Bushing 63 GB Gas pressure switch of G.I.S 63 GL Oil Flow Relay 63 QF Oil Level Low of Transformer 63 QL-1 Oil level Low of Tap Changer 63 QL-2 Tap Changer Protective Device (trip) 63 QP Low Oil Pressure 63 QPL Pressure Switch Followed by a Suffix 63 Battery Earth Fault 64B Transformer H.V R.E.F 64 NP Transformer L.V R.E.F 64 NS Directional O/C 67 Directional E/F 67N Feeder Directional O/C 67F Feeder Directional E/F 67 NF Transformer L.V Directional O/C 67 S Transformer L.V Directional E/F 67 NS

Blocking Relay (power swing out of step) 68 Alarm Relay 74 Phase Comparison Relay 78 Auto Re-closing Relay 79 Under Frequency Relay 81UF Ferro-Resonance Detection Relay 81FR Inter trip Receive Relay 85R Inter trip Receive Follower Relay 85RX Carrier Equipment Failure 85F Inter trip Send 85S

MAIN FUNCTION OF DEVICEDEVICE NUMBER

Trip Relay Electric / Hand Rest 86 Bus bar Protection Trip Relay 86B Connections Trips Relay 86C Feeder Protection Trip Relay 86F Feeder End Protection Trip Relay 86 FE Reactor Protection Trip Relay 86R Transformer H.V Trip Relay 86T Bus bar Differential Protection 87B Bus bar Protection Relay section “A” 87B1 Bus bar Protection Relay section “B” 87B2 Bus bar Protection Relay section “C” 87B3 Bus bar Protection section “A” Front 87BA2 Bus bar Protection section “A” Rear 87BA1 Bus bar Protection section “B” Front 87BB2 Bus bar Protection section “A” Rear 87BB1 Connections Circulating Current Protection 87C Feeder End Protection 87 FE Cable Differential Protection 87 PW Transformer Overall Differential Protection 87T Transformer End Protection 87 TE Electrically Operated Isolator 89 Voltage Regulating Relay 90 Trip Relay Self-rest 94 Remote Emergency Trip Receive 94R Transformer L.V Trip Relay 94T C.T Supervision Relay 95 Buchholz Gas Alarm 96-1 Buchholz Surge Trip 96-2 Burst Plate Breaking Device 96D Line Drop Compensating Unit for Automatic Voltage

Control EquipmentLDC

Hand-Operated Earthing Switch ES Unstabilising Relay USB

147-What does SCADA mean?It means Supervisory Control And Delta Acquisition

148-What is the sinusoidal waveform?The sinusoidal waveform is an ideal one, and it is nearly impossible to realize such as a waveform in practice. All the alternating waveform deviates to a greater or lesser degree from this ideal sinusoidal shape.

149-What is the complex waveform?The complex waveform is produced due to the superposition of sinusoidal waves of different frequencies. A fundamental wave and its higher multiples form a harmonic serious.

150-What is full wave?Full wave represents a disturbance that occurs some distance from the transformer and travel along the transmission lines to the transformer.

151-What is chopped wave?Chopped wave represents a traveling wave created by a disturbance some distance from transformer that flashes near the transformer terminals after reaching its crest. The chopped just after the rest is reached.

152-What is front- of- wave?The disturbance represented by this wave is a direct or very close proximity hit of lighting stroke on the transformer terminal. The wave is chopped just before it reaches its crest.

153-What is main protection for transmission line?-Distance relay-Directional earth faultAnd the backs up protections are-Over current relay-Earth fault relay

154-What are the main protections for under ground cable? -Distance relay

- Directional earth fault -Pilot wire protection / transely and the backs up protection are -Over current relay

-Earth fault

155-What are the main protections for transformer?-Differential relay-Restricted earth fault-Over current-Earth fault-Bucholz relay-Bursting plate (safety valve)-Winding temperature (stage 1 alarm and stage 2 trip)-Oil temperature (alarm only)

156-What are the main protections for busbar?-Differential busbar protection

157-What are the main protection for bus-coupler and bus-section?-Over current relay

158-What are the types of under ground cable that used in SEC-CRB?For 33Kv-Paper cables-XLPE cables

For 132Kv-XPLE cables-Oil filled cables

For 380Kv-Oil filled cables

159-What is static var compensator?Due to varying nature of load, it is some time necessary to supply reactive power and absorb reactive power at the same location, but at different times of the day.Both functions may be accomplished by combing capacitors and reactors together with some switching and control mechanism. This arrangement is called a Static Var Compensator. At low voltages the compensator supplies Vars, at high voltages it absorb Vars.

160-What are the types of substations in SEC-CRB?

1- 33Kv substations

2- 132Kv single busbar3- 132Kv double busbar4- 132Kv-mesh corner5- 132Kv breaker and half

6- 230Kv substations

7- 380Kv substations