Electrical Questions and Answers

1)  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:
  • 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.
  • 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.
2)  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 reason, if instrument 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.
3)  Two bulbs of 100w and 60w 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 60W bulb is greater and voltage across 60W is more (V=IR) 60W bulb will glow brighter.
4)  What happen if we give 230 VDC 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
5)  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.
6)  Difference 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.
7)  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
8)  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.
9)  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.
10)  Why frequency is 50 Hz only & why it is maintained 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.
11)  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.
12)  Why up to dia 70sq mm live conductor, the earth cable must be same size but       above dia 70 sq mm live conductor the earth conductor need to be only dia 70 sq mm?
  • 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 instantaneous current 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.
13)    Why ELCB cannot work when Neutral input of ELCB is not connected 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.
14)     What is the difference between MCB & MCCB, and its application?
  • 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 molded 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.
15)     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.
16)    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.
17)     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
18)  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
  • Vaiable Frequency drive
  • Soft Starter
19)    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

20)    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.
21)     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
22)    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.
23)   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.
24) 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.
25) 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.
26) Why impedance is 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.
27) 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.
28) 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. (There are two types Online and Offline tap changer)
28) 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.
29) 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.
30) 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.
31) 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.
32) 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,
33) What is 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.
34) 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 circuited, 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.
35) 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
  • 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.
36) 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.
37)    What value AC meters show, is it the RMS or peak voltage?
  • AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value when connected to varying DC providing the DC is varying quickly, if the frequency is less than about 10Hz you will see the meter reading fluctuating instead.
38)    What is the difference between Surge Arrester & Lightning Arrestor
  • LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.
  • Transmission Line Lightning Protection:
  • The transmission line towers would normally be higher than a substation structure, unless you have a multi-storey structure at your substation.
  • Earth Mats are essential in all substation areas, along with driven earth electrodes (unless in a dry sandy desert site).
  • It is likewise normal to run catenaries’ (aerial earth conductors) for at least 1kM out from all substation structures. Those earth wires to be properly electrically to each supporting transmission tower, and bonded back to the substation earth system.
  • It is important to have the catenaries’ earth conductors above the power conductor lines, at a sufficient distance and position that a lightning strike will not hit the power conductors.
  • In some cases it is thus an advantage to have two catenary earth conductors, one each side of the transmission tower as they protect the power lines below in a better manner.
  • In lightning-prone areas it is often necessary to have catenary earthing along the full distance of the transmission line.
  • Without specifics, (and you could not presently give tower pictures in a Post because of a CR4 Server graphics upload problem), specifics would include:
  • Structure Lightning Protection:
  • At the Substation, it is normal to have vertical electrodes bonded to the structure, and projecting up from the highest points of the structure, with the location and number of those electrodes to be sufficient that if a lightning strike arrived, it would always be a vertical earthed electrode which would be struck, rather than any electrical equipment.
  • In some older outdoor substation structures, air-break isolator switches are often at a very high point in the structure, and in those cases small structure extension towers are installed, with electrodes at the tapered peak of those extension towers.
  • The extension towers are normally 600mm square approximately until the extension tower changes shape at the tapered peak, and in some cases project upwards from the general structure 2 to 6 metres, with the electrode some 2 to 3 metres projecting upwards from the top of the extension tower.
  • The substation normally has a Lightning Counter – which registers a strike on the structure or connected  to earth conductors, and the gathering of that information (Lightning Days, number per Day/Month/Year, Amperage of each strike)

39)    How to measure Transformer Impedance?
  • Follow the steps below:
  • (1) Short the secondary side of the transformer with current measuring devices (Ammeter)
  • (2) Apply low voltage in primary side and increase the voltage so that the secondary current is the rated secondary current of the transformer. Measure the primary voltage (V1).
  • (3) Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage impedance of the transformer.
  • When we divide the primary voltage V1 with the full load voltage we will get the short circuit impedance of the transformer with refereed to primary or Z01. For getting the percentage impedance we need to use the formula = Z01*Transformer MVA /(Square of Primary line voltage).
40)    Why Bus Couplers are normally 4-Pole. Or When Neutral Isolation is required?
  • Neutral Isolation is mandatory when you have a Mains Supply Source and a Stand-by Power Supply Source. This is necessary because if you do not have neutral isolation and the neutrals of both the sources are linked, then when only one source is feeding and the other source is OFF, during an earth fault, the potential of the OFF Source’s Neutral with respect to earth will increase, which might harm any maintenance personnel working on the OFF source. It is for this reason that PCC Incomers & Bus Couplers are normally 4-Pole. (Note that only either the incomer or the bus coupler needs to be 4-pole and not both).
  • 3pole or 4pole switches are used in changing over two independant sources ,where the neutral of one source and the neutral of another source should not mix the examples are electricity board power supply and standalone generator supply etc. the neutral return current from one source should not mix with or return to another source. as a mandatory point the neutral of any transformer etc are to be earthed, similarly the neutral of a generator also has to be earthed. While paralling (under uncontrolled condition) the neutral current between the 2 sources will crises cross and create tripping of anyone source breakers.
  • also as per IEC standard the neutral of a distribution system shall not be earthed more than once. means earthing the neutral further downstream is not correct,
41)   Why Three No’s of Current transformer in 3 phase Star point is grounded.
  • For CT’s either you use for 3 phase or 2 phase or even if you use only 1 CT’s for the Over current Protection or for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as what you explain as above but actually it is for the safety of the CT’s when the current is passing thru the CT’s.
  • In generally, tripping of Earth faults and Over current Protection has nothing to do with the earthing the neutral of the CT’s. Even these CT’s are not Grounded or Earthed, these Over current and the Earth Faults Protection Relay still can operated.
  • Operating of the Over current Protection and the Earth Faults Relays are by the Kirchhoff Law Principle where the total current flowing into the points is equal to the total of current flowing out from the point.
  • Therefore, for the earth faults protection relays operating, it is that, if the total current flowing in to the CT’s is NOT equal total current flowing back out of the CT’s then with the differences of the leakage current, the Earth Faults Relays will operated.
42)   Why do transformers hum?
  • Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction.
  • A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization. The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect.
  • The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbour, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level Why not reduce the noise in the core by reducing the amount of flux? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear.
43) What is exciting current?
  • Exciting current 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 less to approximately 2% on larger sizes of 750 KVA.
  • , are not of the insulating or isolating variety.
44) Can transformers be operated at voltages other than nameplate voltages?
  • In some cases, transformers can be operated at voltages below the nameplate rated voltage.
  •  In NO case should a transformer be operated at a voltage in excess of its nameplate rating, unless taps are provided for this purpose. When operating below the rated voltage, the KVA capacity is reduced correspondingly.
  • For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts. If the transformer was originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.
45) Can a Single Phase Transformer be used on a Three Phase source?
  • Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.
46) Can Transformers develop Three Phase power from a Single Phase source?
  • No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phases.
47) 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. For example: Three 10 KVA single phase transformers will accommodate a 30 KVA three phase load
48) Where the lighting arrestor should be placed in distribution lines?
  • Near distribution transformers and out going feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.
49) Why Do not We Break Neutral in AC Circuits?
  • Neutral is connected to earth at some point, thus it has some value as a return path in the event of say and equipment earth being faulty. It’s a bit like asking ‘why don’t we break the Earth connection’
  • It was stupid and dangerous, as it was possible for the neutral fuse to blow; giving the appearance of ‘no power’ when in fact the equipment was still live.
50) When should we use Molded Case Circuit Breakers and Mini Circuit Breakers?
  • MCB is Miniature Circuit Breaker, since it is miniature it has limitation for Short Circuit Current and Amp Rating MCB:
  • MCB are available as Singe module and used for :-
  • Number of Pole :- 1,2,3,4 – 1+ N , & 3 + N
  • Usually Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63 Amp. Also available 80A, 100A, and 125 Amp.
  • SC are limited 10 KA
  • Applications are as: – Industrial, Commercial and Residential application.
  • Tripping Curve:
  • (1) B Resistive and lighting load,
  • (2) C Motor Load,
  • (3) D Highly inductive load.
  • MCCB:
  • MCCB: – Moulded Case Circuit Breaker.
  • MCCB are available as Singe module and used for:
  • Number of Pole :- 3 pole , & 4 Pole
  • Current range for A.C:
  • For 3.2 /6.3/12.5/25/50/100/125/160 Amp and Short Circuit Capacity 25/35/65 KA.
  • For  200 250 Amp and Short Circuit Capacity 25/35/65 KA
  • For 400 630/800 Amp and Short Circuit Capacity 50 KA
  • Protection release :
  • Static Trip :- Continuous adjustable overload protection range 50 to 100 % of the rated current Earth fault protection can be add on with adjustable earth fault pick up setting 15 to 80 % of the current.
  • Micro processor Based release:
  • Over load rated current 0.4 to1.0 in steps of o.1 of in trip time at 600 % Ir (sec),1, 1.5 , 2 ,3
  • Short Circuit :-2 to10 in steps of 1 lr , short time delay (sec),0.1, 0.2 ,0.3
  • Instantaneous pick up :2 to10 in steps of 1 in Ground fault pick up Disable: 0.2 to 0.8 in steps of 0.1 of in Ground fault delay (sec): 0.1 to 0.4 in steps of 0.1
  • MCB (Miniature Circuit Breaker) Trip characteristics normally not adjustable, factory set but in case of MCCB (Moulded Case Circuit Breaker) Trip current field adjustable.
51)    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.
52)    What is difference between power transformers & distribution transformers?
  • Distribution Transformers are designed for a maximum efficiency at 50% of load. Whereas power transformers are designed to deliver max efficiency ay 90% and above loads.
  • The distributions transformers have low impedance so as to have a better regulation power transformers have higher so as to limit the SC current.
  • Power transformers are used to step up voltages from 11 KV which is the generating voltage to 132 or whatever will be the transmission voltage levels. Power transformers are having Star-Delta connection. It will be located at power generating stations.
  • Distribution transformers are used to step down voltages from transformer levels to 11 KV/415 V. Will be having Delta-Star. It will be located in substations near load centers.
  • The main basic difference lies in the Design stage itself as power transformer are to operate at near full load so there sensing is such that they achieve equal. of copper losses & iron losses at full loads whereas this is achieved in the design itself at about 50% loading in dist transformer but friends there is a dilemma as our dist. transformer are almost fully loaded & beyond so they never go operate at their full eff. & also poor voltage regulation.
  • The difference between power and distribution transformers refers to size & input voltage. Distribution transformers vary between 25 kVA and 10 MVA, with input voltage between 1 and 36 kV. Power transformers are typically units from 5 to 500 MVA, with input voltage above 36 kV. Distribution transformer design to have a max efficiency at a load lower than full load. Power transformer design to have a max efficiency at full load
53)    What will be happen if the neutral isolator will be open or close during the running condition of power?
  • During normal condition the neutral isolating switch should be kept close. In case it is kept open, under balanced load conditions the current through neutral will not flow & nothing harmful will take place but in case an earth fault takes place then there will be no earth fault current flowing through the system & the generator will run as a ungrounded generator. Thus the earth fault will not be cleared.
  • If more number of generators are connected parallel. We will have a close loop and hence negative sequence current will flow. This will increase the rotor temperature. Hence if more number of generators are connected then only one is earthed and others are open.
  • In case of Two or more generators connected to a common bus without a transformer in between, basically in hydro stations, one of the Neutral Isolation Switch(NIS) is kept closed & rest are opened to prevent circulating currents to flow between generators. Hence the above explanation will not be valid for such systems.
  • Sometime we may want to test generator and may want to isolate the neutral from ground. like for example meggaring etc. In such case we would like to open ground connection cable in case we want to remove the NIS? we will certainly not like to open all the bolted connections for just a small test like checking insulation with a meggar etc. for such things we need a NIS.
  • Neutral isolator is required if we have delta transmission system and at the time to connection with the Grid Neutral isolation is required.
  • If we ungrounded the neutral then the generator is connected to the ground via Phase to earth capacitances. Hence during faults arcing grounds can take place. Which are dangerous both to human & equipment.
  • When we provide earthed neutral, for a fault, earth fault current will start flowing through the neutral, which we can sense thru a CT & relay & hence can immediately identify & clear the fault in about 100 ms by opening the associated breaker/prime mover/excitation. Quicker the fault clearance less is the damage.
54)    Why shorting type terminal required for CT?
  • During maintenance or secondary injection you will need to bypass the CT & for the same you need shorting link. During sec. injection you will short circuit the main CT & bypass it. Open circuiting the CT will saturate it & damage it.
55) Why fuse is given for only PT circuit and not CT?
  • Fuse if given for CT blows off due to a fault then rather than protecting the CT it will make it open circuited hence it will be saturated & damaged. For PT it gives overload & SC protection.
56) Why is insulating base required for LA?
  • The LA is provided with a dedicated Prper earthing which may be in the form of a buried treated electrode next to it.LA connection is securely made with the electrode via a surge counter. If we directly earth the LA through structure then the surge counter will not be able to measure the no of surges. For lesser rating the counter is not provided, hence we can bypass the insulated base. But then proper earthing has to be assured.
57) Difference between Restricted Earth Fault & Unrestricted Earth Fault protections?
  • Restricted earth fault is normally given to on star connected end of power equipment like generators, transformers etc. mostly on low voltage side. For REF protection 4 no’s CTs are using one each on phase and one in neutral. It is working on the principle of balanced currents between phases and neutral. Unrestricted E/F protection working on the principle of comparing the unbalance on the phases only. For REF protection PX class CT are using but for UREF 5P20 Cts using.
  • For Differential Protection CTs using on both side HT & LV side each phase, and comparing the unbalance current for this protection also PX class CTs are using.
58) Can transformers be operated at voltages other than nameplate voltages?
  • In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly.
59) How many types of cooling system it transformers?
  • ONAN (oil natural,air natural)
  • ONAF (oil natural,air forced)
  • OFAF (oil forced,air forced)
  • ODWF (oil direct,water forced)
  • OFAN (oil forced,air natural)
60) What is the function of anti-pumping in circuit breaker?
  • When breaker is close at one time by close push button, the anti pumping contactor prevents re close the breaker by close push button after if it already closed.
61) Can we operate 60 Hz transformers at 50 Hz?
  • Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service.
62) Can transformers be used in parallel?
  • Single phase transformers can be used in parallel only when their 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.
63) 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.
64) Why short circuit do not take place when electrode is touched to ground.
  • Basically during welding we force a short-circuit at the electrode tip. The fault condition produces large magnitude currents. Greater the Current value have greater I2R heat produced. The arcing energy elevates the temperature & hence melts the electrode material over the joint.
  • The transformer is designed to withstand such high currents. But welding is a very complex & detailed phenomenon. Besides there are many principles on which welding operates. Some may be a welding, dc welding, arc, constant voltage, constant current etc
65) What’s the difference between generator breaker and simple breaker?
  • Breaker is one which disconnects the circuit in fault condition and It is similar for all equipment. Based on the equipment voltage and maximum short circuit current the ratings will be decided. For better understanding we call generator or transformer or line etc breakers.
66) What is the accuracy Class of the instrument?
  • Generally the class indicates the accuracy with which the meter will indicate or equipment will measure with respect to its input.
  • The accuracy of different equipment will depend on number of factors.
  •  For example for a PT accuracy will depend on its leakage reactance & winding resistance. For a PT accuracy gives the voltage & phase error & it varies with the VA burden of secondary. Also better core material will give better heat dissipation & reduce error. class of accuracy will give the voltage error for a PT
  • different type of PTs available are:0.1, 0.2, 0.5, 1, 5 & error values will be: class% voltage error(+/_) phase displacement
    Similarly indicating instruments shall have accuracies & accordingly application as depicted below for testing the following values are generally used:
  • for routine tests : accuracy class 1
  • for type tests : accuracy class 0.5 or better.
  • indicating meters generally will have accuracy of 1.
  • Once the current is obtained we can then calculate the electrostatic KVA from the current multiplied with voltage.
  • reason we have seen only 23/24 discs in 400 kv line cause in that case the creep age obtained must have been enough & also the strain requirement.
  • 33kv insulators are generally used in a vertical installation & are not stacked together because that will make the suspension very rigid
67) Do taps work the same when a transformer is reverse fed?
  • Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed, the taps are on the output side and can be used to adjust the output voltage.
68)  Why do we get the wrong output voltage while installing a step up transformer?
  • Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H terminals are primary and X terminals secondary. This is true for step down transformers, but in a step up transformer the connections should be reversed. Low voltage primary would connect to X terminals while high voltage secondary would connect on the H terminals.
69)  How to select LA
  • The voltage rating of LA is selected as: Line voltage x sqrt(2)/ sqrt(3) so for 11kV line its 9kV
  • In that case also the values would not differ much if We takes the TOV factor as 1.4. However, we can take the value of 1.56 as TOV to be more precise.
70)  Which is more dangerous AC or DC
  • Low frequency (50 – 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death.
  • However any practical distribution system will use voltage levels quite sufficient to ensure a dangerous amount of current will flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard.

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