Basic Electrical Interview Q&A Part-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.
Also Read:
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.
Also Read:
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.
Also Read:
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.
Also Read:
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.
Also Read: