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Electrical Q&A Set-1

1)     Why ELCB cannot work if Neutral input of ELCB does not connect to ground?

  • ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that same current will have to return neutral so resultant current is zero.
  • Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

2)     What is the difference between MCB & MCCB, Where it can be used?

  • MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit.
  • Normally it is used where normal current is less than 100A.
  • MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt.
  • Normally it is used where normal current is more than 100A.

3)     Why in a three pin plug the earth pin is thicker and longer than the other pins?

  • It depends upon R=ρL/A where area (A) is inversely proportional to resistance (R), so if  area (A) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

4)     Why Delta Star Transformers are used for Lighting Loads?

  • For lighting loads, neutral conductor is must and hence the secondary must be star winding and this lighting load is always unbalanced in all three phases.
  • To minimize the current unbalance in the primary we use delta winding in the primary So delta / star transformer is used for lighting loads.

5)      What are the advantages of star-delta starter with induction motor?

  • The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting  Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

6)     What is meant by regenerative braking?

  • When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as “Regenerative Breaking”.

7)     When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

  • No. We cannot sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different.
  • Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay.

8)     If one lamp connects between two phases it will glow or not?

  • If the voltage between the two phases is equal to the lamp voltage then the lamp will glow.
  • When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

9)     What are HRC fuses and where it is used?

  • HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers

10)  Mention the methods for starting an induction motor?

  • The different methods of starting an induction motor
  • DOL:direct online starter
  • Star delta starter
  • Auto transformer starter
  • Resistance starter
  • Series reactor starter

11)  What is the difference between earth resistance and earth electrode resistance?

  • Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition:
  • Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
  • The resistance of the electrode has the following components:
    (A) the resistance of the metal and that of the connection to it.
    (B) The contact resistance of the surrounding earth to the electrode.

12)  Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

  • 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation.
  • For example: A pressure instrument gives output 4mA to indicate 0 psi  up to 20 mA to indicate 100 psi or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

13)  Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?

  • Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the Bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

14)  What happen if we give 220 volts dc supply to bulb or tube light?

  • Bulbs or devices for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element

15)  What is meant by knee point voltage?

  • Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.

16)  What is reverse power relay?

  • Reverse Power flow relay are used in generating stations’ protection.
  • A generating station is supposed to feed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

17)  What will happen if DC supply is given on the primary of a transformer?

  • Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out
  • When AC current flow to primary winding it induced alternating flux which also link to secondary winding so secondary current flow in secondary winding according to primary current.Secondary current also induced emf (Back emf) in secondary winding which oppose induced emf of primary winding and thus control primary current also.
  • If DC current apply to Primary winding than alternating flux is not produced so no secondary emf induced in secondary winding  so primary current may goes high and burn transformer winding.

18)  Different between megger and contact resistance meter?

  • Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance meter used to measure low resistance like relays, contactors.

19)  When we connect the capacitor bank in series?

  • We connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is considerable voltage drop along the transmission line due to impedance of the line. so in order to bring the voltage at the load terminals within its limits i.e (+ or – %6 )of the rated terminal voltage the capacitor bank is used in series

20)  What is Diversity factor in electrical installations?

  • Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Diversity factor is usually more than one.

21)  Why humming sound occurred in HT transmission line?

  • This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

22)  Why frequency is 50 hz only & why should we maintain the frequency constant?

  • We can have the frequency at any frequency we like, but then we must also make our own motors, transformers or any other equipment we want to use.
  • We maintain the frequency at 50 Hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are made to operate at these frequency.

23)  If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

  • As we know the Voltage & current relation for transformer-V1/V2 = I2/I1
    We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.
    By putting these value on Relation-
    540/11000= I2/2334
    So,I2 = 114.5 Amps

24)  What are the points to be considered for MCB (miniature circuit breaker selection)?

  • I(L)x1.25=I(MAX) maximum current. Mcb specification is done on maximum current flow in circuit.

25)  How can we start-up the 40w tube light with 230v AC/DC without using any choke/Coil?

  • It is possible by means of Electronic choke. Otherwise it’s not possible to ionize the particles in tube. Light, with normal voltage.

26)  What is “pu” in electrical engineering?

  • Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).

27)  Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

  • Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps.
  • But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.
  • If Fuse is provided in Neutral and if it is blowout and at the same time Supply is on than due to open or break Neutral Voltage is increase and equipment may be damage.

28)  If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.

  • Each bulb when independently working will have currents (W/V= I)
  • For 200 Watt Bulb current (I200) =200/230=0.8696 A
  • For 100 Watt Bulb current (I100) =100/230=0.4348 A
  • For 60 Watt Bulb current (I60) =60/230=0.2609 A
  • Resistance of each bulb filament is (V/I = R)
  • For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms
  • For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and
  • For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively
  • Now, when in series, current flowing in all bulbs will be same. The energy released will be I2R
  • Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest.
  • The 60W lamp as it has highest resistance & minimum current requirement.
  • Highest voltage drop across it X I [which is common for all lamps] =s highest power.
  • Note to remember:
  • Lowest power-lamp has highest element resistance.
  • And highest resistance will drop highest voltage drop across it in a Series circuit
  • And highest resistance in a parallel circuit will pass minimum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation

29)  How to check Capacitor with use of Multi meter.

  • Most troubles with Capacitors either open or short.
  • An ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly show very low resistance. A open Capacitor will not show any movement on ohmmeter.
  • A good capacitor will show low resistance initially, and resistance gradually increases. This shows that Capacitor is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To know the value and other parameters, you need better instruments

30)  What is the difference between Electronic regulator and ordinary rheostat regulator for fans?

  • The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator give the power needed for that particular speed .But in case of ordinary rheostat type regulator the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.

31)  What will happen when power factor is leading in distribution of power?

  • If there is high power factor, i.e if the power factor is close to one:
  • Losses in form of heat will be reduced,
  • Cable becomes less bulky and easy to carry, and very cheap to afford.
  • It also reduces over heating of transformers.

32)  What the main difference between UPS & inverter?

  • Uninterrupted power supply is mainly use for short time. Means according to ups VA it gives backup. Ups is also two types: on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But ups start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup

33)  Which type of A.C motor is used in the fan?

  • It is Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

34)  What is the difference between synchronous generator and asynchronous generator?

  • In simple, synchronous generator supplies’ both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observe reactive power for magnetizing. This type of generators is used in windmills.

35)  What is the Polarization index value?

  • Its ratio between insulation resistance (IR)i.e meager value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 .

36)  What is Automatic Voltage regulator (AVR)?

  • AVR is an abbreviation for Automatic Voltage Regulator.
  • It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

37)  Difference between a four point starter and three point starters?

  • The shunt connection in four point starter is provided separately from the line where as in three point starter it is connected with line which is the drawback in three point starter

38)  What happens if we connect a capacitor to a generator load?

  • Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf.
  •  Don’t connect a capacitor across an alternator while it is picking up or without any other load

39)  Why the capacitors work on ac only?

  • Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.

40)  Why the up to dia 70mm² live conductor, the earth cable must be same size but above dia 70mm² live conductor the earth conductor need to be only dia 70mm²?

  • The current carrying capacity of a cable refers to it carrying a continuous load.
  • An earth cable normally carries no load, and under fault conditions will carry a significant instantaneouscurrent but only for a short time most Regulations define 0.1 to 5 sec before the fuse or breaker trips. Its size therefore is defined by different calculating parameters.
  • The magnitude of earth fault current depends on:
  • (a) the external earth loop impedance of the installation (i.e. beyond the supply terminals)
  • (b) the impedance of the active conductor in fault
  • (c) the impedance of the earth cable.
  • i.e. Fault current = voltage / a + b + c
  • Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth loop impedance (a); when It is quite large its impedance can be ignored. At this point there is no merit in increasing the earth cable size
  • i.e. Fault current = voltage / a + c
  • (c) is also very small so the fault current peaks out.
  • The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for continuous full load. In a 3-phase system,
  • If balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables are available with say half-size neutrals (remember a neutral is always necessary to provide single phase voltages). However the increasing use of non-linear loads which produce harmonics has made this practice dangerous, so for example the current in some standard require full size neutrals. Indeed, in big UPS installations I install double neutrals and earths for this reason.

Electrical Q&A Set-2

1)    Why We use of Stones/Gravel in electrical Switch Yard

  • Reducing Step and Touch potentials during Short Circuit Faults
  • Eliminates the growth of weeds and small plants in the yard
  • Improves yard working condition
  • Protects from fire which cause due to oil spillage from transformer and also protects from wild habitat.

2)    What is service factor?

  • Service factor is the load that may be applied to a motor without exceeding allowed ratings.
  • For example, if a 10-hp motor has a 1.25 service factor, it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied with rated voltage and frequency.
  • However a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions

3)     Why transmission line 11KV OR 33KV, 66KV not in 10KV 20KV?

  • The miss concept is Line voltage is in multiple of 11 due to Form Factor.  The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is 1.11.
  • The Main reason is something historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor (1.11).
  • Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.
  • Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise.  Now a days when, we have voltage correction systems, power factor improving capacitors, which can boost/correct voltage to desired level, we are using the exact voltages like 400KV in spite of 444KV

4)    What is electrical corona?

  • Corona is the ionization of the nitrogen in the air, caused by an intense electrical field.
  • Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night.
  • Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8″ at 3500 volts).

5)    What are the indications of electrical corona?

  • A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.

6)    What damage does corona do?

  • The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-over.
  • The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

7)    How long does corona require creating visible damage?

  • In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey.

8)    What voltage are corona rings typically installed at?

  • It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in the strings.

9)    How do we select transformers?

  • Determine primary voltage and frequency.
  • Determine secondary voltage required.
  • Determine the capacity required in volt-amperes. This is done by multiplying the load current (amperes) by the load voltage (volts) for single phase.
  • For example: if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required.
  • Always select Transformer Larger than Actual Load. This is done for safety purposes and allows for expansion, in case more loads is added at a later date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3) then divide by 1000.
  • Determine whether taps are required. Taps are usually specified on larger transformers.

10)   Why Small Distribution Transformers not used for Industrial Applications?

  • Industrial control equipment demands a momentary overload capacity of three to eight times’ normal capacity. This is most prevalent in solenoid or magnetic contactor applications where inrush currents can be three to eight times as high as normal sealed or holding currents but still maintain normal voltage at this momentary overloaded condition.
  • Distribution transformers are designed for good regulation up to 100 percent loading, but their output voltage will drop rapidly on momentary overloads of this type making them unsuitable for high inrush applications.
  • Industrial control transformers are designed especially for maintaining a high degree of regulation even at eight time’s normal load. This results in a larger and generally more expensive transformer.

11) Can 60 Hz transformers be used at higher frequencies?

  • Transformers can be used at frequencies above 60 Hz up through 400 Hz with no limitations provided nameplate voltages are not exceeded.
  •  However, 60 Hz transformers will have less voltage regulation at 400 Hz than 60 Hz.

12) What is meant by regulation in a transformer?

  • Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage.
  • For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the regulation would be 5%. Distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used.

13) Why is impedance important?

  • It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.
  • Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.
  • Calculate:
  • Normal Full Load Current = Nameplate Volt Amps / Line Volts = 10,000 VA / 480 V = 20.8 Amperes
  • Maximum Short Circuit Amps = Full Load Amps / 4% =20.8 Amps / 4%= 520 Amp
  • The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.
  • Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 KVA, three phase transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the secondary is short circuited (faulted), the following capacities are required:
  • Normal Full Load Current =Volt Amps / √ 3 x Line Volts= 75,000 VA / √ 3 x Line Volts √ 3 x 480 V =90 Amps
  • Maximum Short Circuit Line Current = Full Load Amps / 5%=  90 Amps /  5% =1,800 Amps
  • The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts.
  • Note: The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is the only winding being interrupted.

14) What causes flash-over?

  • Flash-over causes are not always easily explained, can be cumulative or stepping stone like, and usually result in an outage and destruction. The first flash-over components are available voltage and the configuration of the energized parts, corona may be present in many areas where the flash-over occurs, and flash-over can be excited by stepping stone defects in the insulating path.

15) What are taps and when are they used?

  • Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Taps are generally set at two and a half and five percent above and below the rated primary voltage.

16) Can Transformers be reverse connected?

  • Dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phases can be reverse connected without any adverse effects or loss in KVA capacity.
  • The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio.
  • Example: A transformer with a 480 volt input, 240 volt output— can have the output connected to a 240 volt source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or 480 volt secondary.
  • On transformers rated below 1 KVA single phase, there is a turn’s ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load.
  • For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be approximately 460 volts at no load and approximately 442 volts at full load. As the KVA becomes smaller, the compensation is greater—resulting in lower output voltages.
  • When one attempts to use these transformers in reverse, the transformer will not be harmed; however, the output voltage will be lower than is indicated by the nameplate.

17) What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers?

  • Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary windings to attenuate (lessen) transient noise.

18) How many BTU’s of heat does a transformer generate?

  • The heat a transformer generates is dependent upon the transformer losses. To determine air conditioning requirements multiply the sum of the full load losses (obtained from factory or test report) of all transformers in the room by 3.41 to obtain the BTUs/hour.
    For example: A transformer with losses of 2000 watts will generate 6820 BTUs/hour.

19) What is a transformer and how does it work?

  • A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle.
  • A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core.
  • When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

20) Factors Affecting Corona Discharge Effect:

  • Corona Discharge Effect occurs because of ionization if the atmospheric air surrounding the voltage conductors, so Corona Discharge Effect is affected by the physical state of the atmosphere as well as by the condition of the lines.
  • (1) Conductor: Corona Discharge Effect is considerably affected by the shape, size and surface conditions of the conductor .Corona Discharge Effect decreases with increases in the size (diameter) of the conductor, this effect is less for the conductors having round conductors compared to flat conductors and Corona Discharge Effect is concentrated on that places more where the conductor surface is not smooth.
  • (2) Line Voltage: Corona Discharge effect is not present when the applied line voltages are less. When the Voltage of the system increases (In EHV system) corona Effect will be more.
  • (3) Atmosphere: Breakdown voltage directly proportional to the density of the atmosphere present in between the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather condition So Corona Breakdown voltage occurs at low voltages in the stormy weather condition compared to normal conditions
  • (4)Spacing between the Conductors: Electro static stresses are reduced with increase in the spacing between the conductors. Corona Discharge Effect takes place at much higher voltage when the distance between the power conductors increases.

21) Will a transformer change Three Phases to Single Phase?

  • A transformer will not act as a phase changing device when attempting to change three phase to single phase.
  • There is no way that a transformer will take three phase in and deliver single phase out while at the same time presenting a balanced load to the three phase supply system.
  • There are, however, circuits available to change three phase to two phase or vice versa using standard dual wound transformers. Please contact the factory for two phase applications.

22) Can 60 Hz transformers be operated at 50 Hz?

  • Transformers rated below 1 KVA can be used on 50 Hz service.
  • Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due to the higher losses and resultant heat rise. Special designs are required for this service. However, any 50 Hz transformer will operate on a 60 Hz service.

23) Can transformers be used in parallel?

  • Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other.
  • For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.

24) What are causes of insulator failure?

  • Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona rings, … This corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid expansion causing fracture or rupture of the material.

25)  Causes of Corona

  • Corona is causes by the following reasons:
  • The natural electric field caused by the flow of electrons in the conductor. Interaction with surrounding air.
    Poor or no insulation is not a major cause but increases corona.
  • The use of D.C (Direct Current) for transmission.(Reason why most transmission is done in form of AC)

26) Effects of Corona

1)     Line Loss – Loss of energy because some energy is used up to cause vibration of the air particles.

2)     Long term exposure to these radiations may not be good to health (yet to be proven).

3)     Audible Noise

4)     Electromagnetic Interference to telecommunication systems

5)     Ozone Gas production

6)     Damage to insulation of conductor.

27) What is polarity, when associated with a transformer?

  • Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding.
  • Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below. This leaves one high voltage and one low voltage terminal unconnected.
  • When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings.
  • This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

28) What is exciting current?

  • Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR.

29) What is Boucholz relay and the significance of it in to the transformer?

  • Boucholz relay is a device which is used for the protection of transformer from its internal faults,
  • it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

30) Why we do two types of earthing on transformer (Body earthing & neutral earthing)

  • The two types of earthing are Familiar as Equipment earthing and system earthing.
  • In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human beings.
  • The System Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted. So that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment ( transformer or alternator)and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

31) Conductor corona is caused by?

  • Corona on a conductor can be due to conductor configuration (design) such as diameter too small for the applied voltage will have corona year-around and extreme losses during wet weather, the opposite occurs during dry weather as the corona produces nitric acid which accumulates and destroys the steel reinforcing cable (ACSR) resulting in the line dropping. Road salts and contaminants can also contribute to starting this deterioration.

32) What is flash-over and arcing?

  • Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc, an arc can have the grid fault current behind it and sustain until the voltage decreases below 50% or until a protective device opens.
  • Flash-over can also occur due to induced voltages in unbounded (loose bolts, washers, etc) power pole or substation hardware, this can create RFI/TVI or radio/TV interference. Arcing can begin at 5 volts on a printed circuit board or as the insulation increases it may require 80kVAC to create flash-over on a good cap and pin insulator.

33) How to Minimizing Corona Effects

  • Installing corona rings at the end of transmission lines.
  • A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in the vicinity of a terminal of a high voltage device. It is electrically insulated.
  • Stacks of more spaced rings are often used. The role of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing the corona discharge.

34) What is BIL and how does it apply to transformers?

  • BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown.
  • All transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV.
  • This assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar surge protection devices.

35) The difference between Ground and Neutral?

  • NEUTRAL is the origin of all current flow. In a poly-phase system, as its phase relationship with all the three phases is the same, (i.e.) as it is not biased towards any one phase, thus remaining neutral, that’s why it is called neutral.
  • Whereas, GROUND is the EARTH on which we stand. It was perceived to utilize this vast, omnipresent conductor of electricity, in case of fault, so that the fault current returns to the source neutral through this conductor given by nature which is available free of cost. If earth is not used for this purpose, then one has to lay a long. long metallic conductor for the purpose, thus increasing the cost.
  • Ground should never be used as neutral. The protection devices (eg ELCB, RCD etc) work basically on principle that the phase currects are balanced with neutral current. In case you use ground wire as the neutral, these are bound to trip if they are there – and they must be there. at least at substations. And these are kept very sensitive i.e. even minute currents are supposed to trip these.
  • One aspect is safety – when someone touches a neutral, you don’t want him to be electrocuted – do you? Usually if you see the switches at home are on the phase and not neutral (except at the MCB stage). Any one assumes the once the switch is off, it is safe (the safety is taken care of in 3 wire system, but again most of the fixtures are on 2 wire) – he will be shocked at the accidental touching of wire in case the floating neutral is floating too much.

36) What is impedance of a transformer?

  • If you mean the percentage impedance of the transformed it means the ratio of the voltage( that if you applied it to one side of the transformer while the other side of the transformer is short cuitcuted, a full load current shall flow in the short circuits side), to the full load current.
  • More the %Z of transformer, more Copper used for winding, increasing cost of the unit. But short circuit levels will reduce, mechanical damages to windings during short circuit shall also reduce. However, cost increases significantly with increase in %Z.
  • Lower %Z means economical designs. But short circuit fault levels shall increase tremendously, damaging the winding & core.
  • The high value of %Z helps to reduce short circuit current but it causes more voltage dip for motor starting and more voltage regulation (% change of voltage variation) from no load to full load.

37) How are transformers sized to operate Three Phase induction type squirrel cage motors?

  • The minimum transformer KVA rating required to operate a motor is calculated as follows:
  • Minimum Transformer KVA =Running Load Amperes x 1.73x Motor Operating Voltage / 1000
  • NOTE: If motor is to be started more than once per hour add 20% additional KVA. Care should be exercised in sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is approximately 5 to 7 times the running load amperage. This severe starting overload will result in a drop of the transformer output voltage.
  • When the voltage is low the torque and the horsepower of the motor will drop proportionately to the square of the voltage.
  • For example: If the voltage  were to drop to 70% of nominal, then motor horsepower and torque would drop to 70 % squared or 49% of the motor nameplate rating.
  • If the motor is used for starting a high torque load, the motor may stay at approximately 50% of normal running speed The underlying problem is low voltage at the motor terminals. If the ampere rating of the motor and transformer over current device falls within the motor’s 50% RPM draw requirements, a problem is likely to develop. The over current device may not open under intermediate motor ampere loading conditions.
  • Overheating of the motor and/or transformer would occur, possibly causing failure of either component.
  • This condition is more pronounced when one transformer is used to power one motor and the running amperes of the motor is in the vicinity of the full load ampere rating of the transformer. The following precautions should be followed:
  • (1)When one transformer is used to operate one motor, the running amperes of the motor should not exceed 65% of the transformer’s full load ampere rating.
  • (2) If several motors are being operated from one transformer, avoid having all motors start at the same time. If this is impractical, then size the transformer so that the total running current does not exceed 65% of the transformer’s full load ampere rating.

38) Which Point need to be consider while Neutral Earthing of Transformer?

  • The following points need to check before going for Neutral Grounding Resistance.
  • Fault current passing through ground, step and touch potential.
  • Capacity of transformer to sustain ground fault current, w.r.t winding, core burning.
  • Relay co-ordination and fault clearing time.
  • Standard practice of limiting earth fault current. In case no data or calculation is possible, go for limiting E/F current to 300A or 500A, depending on sensivity of relay.

39) Why a neutral grounding contactor is needed in diesel generator?

  • There would not be any current flow in neutral if DG is loaded equally in 3 phases , if there any fault(earth fault or over load) in any one of the phase ,then there will be un balanced load in DG . at that time heavy current flow through the neutral ,it is sensed by CT and trips the DG. so neutral in grounded to give low resistance path to fault current.
  • An electrical system consisting of more than two low voltage Diesel Generator sets intended for parallel operation shall meet the following conditions:
  • (i) Neutral of only one generator needs to be earthed to avoid the flow of zero sequence current.
  • (ii) During independent operation, neutrals of both generators are required in low voltage switchboard to obtain three phases, 4 wire system including phase to neutral voltage.
  • (iii) required to achieve restricted earth fault protection (REF) for both the generators whilst in operation.
  • Solution:
  • Considering the requirement of earthing neutral of only one generator, a contactor of suitable rating shall be provided in neutral to earth circuit of each generator. This contactor can be termed as “neutral contactor”.
  • Neutral contactors shall be interlocked in such a way that only one contactor shall remain closed during parallel operation of generators. During independent operation of any generator its neutral contactor shall be closed.
  • Operation of neutral contactors shall be preferably made automatic using breaker auxiliary contacts.

40) Neutral grounded system vs solidly grounded system

  • In India, at low voltage level (433V) we MUST do only Solid Earthing of the system neutral.
  • This is by IE Rules 1956, Rule No. 61 (1) (a). Because, if we option for impedance earthing, during an earth fault, there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being determined by the fault current magnitude and the impedance value.
  • This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment, as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too. In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral potential and the high fault current would invariably cause the Over current or short circuit protection device to operate in sufficiently quick time before any harm could be done

Electrical Q&A Set-3

1)    What is the reason of grounding or earthing of equipment?

  • with a ground path, in case of short circuit the short circuit current goes to the body of the equipment & then to the ground through the ground wire. Hence if at the moment of fault if a person touches the equipment body he will not get a shock cause his body resistance (in thousands of ohms) will offer a high resistance path in comparison to the ground wire. Hence the fault current will flow thru the ground wire & not thru human body.
  • Providing a ground path helps in clearing the fault. A CT in the ground connection detects the high value fault current hence the relay connected to the CT gives breaker a trip command.
  • Grounding helps in avoiding arcing faults. IF there would have been no ground then a fault with the outer body can cause a arcing to the ground by breaking the air. This is dangerous both for the equipment & the human beings.

2)    A type-C MCB has thermo magnetic capability 5In to 10In that means a short circuit current will be interrupted as the value will reach between 5In to 10In but the MCB breaking capacity is (for example) define as 10kA.

  • 5In to 10In is the pickup threshold for the magnetic trip element. The MCB will trip instantaneously when the current is between these limits. 10kA is the short circuit withstands capacity of the MCB.
  • Under normal condition, a current limiting type MCB will trip on short circuit (magnetic trip) and the current during circuit interruption will be much less than the prospective current. However, the MCBs have to have a short circuit capacity more than or equal to the fault level at the location where it is installed.

3)    What is Ferrari Effect?

  • Ferranti Effect is due to the rise in voltage at the receiving end than that of the sending end. This occurs when the load on the system reduces suddenly.
  • Transmission line usually consists of line inductance, line to earth capacitance and resistance. Resistance can be neglected with respect to the line inductance .When the load on the system falls the energy stored in the capacitance gets discharged. The charging current causes inductive reactance voltage drop. This gets added vector ally to the sending end voltage and hence causes the voltage at the receiving end to raise
  • A Long transmission line draws significant amount of charging current. If such line is open circuited or very lightly loaded at the receiving end, the voltage at the receiving end may become greater than sending end voltage. This effect is known Ferranti effect and is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance is responsible to produce this phenomenon.
  • The capacitance (charging current) is negligible in short lines, but significant in medium and long transmission line. Hence, this phenomenon is applicable for medium and long transmission line.
    The main impact of this phenomenon is on over voltage protection system, surge protection system, insulation level etc.

4)    Can single phase transformers be used for three phase applications?

  • Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer.

5)    What is BIL and how does it apply to transformers?

  • BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The BIL of a transformer is a method of expressing the voltage surge that a transformer will tolerate without breakdown.

6)    Where Auto-recloser is used?

  • For Generator protection / Transformer Protection / Transmission Line / Bus bar protection.
  • Many faults on overhead transmission lines are transient in nature 90% of faults are used by birds, tree branches. These condition results in arching faults and the arc in the fault can be extinguished by de-energizing the lines by opening of CB on the both ends of the lines.
  • Open-0.3 second-Close-3minute-Close this is the sequence of AR. i.e.-OPEN,C-CLOSED
  • whenever faults occurs CB opens, then after 0.3 sec&
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