Protection against over voltage Q & A- PART-A

Protection against over voltage 

1. Why protection of transmission line important?

It is essential for electrical power engineers to reduce the number of outages and preserve the
continuity of service and electric supply.

2. What are the causes of over voltages in electric system?

 External Cause – Lightning
 Internal Cause – Switching

3. What are the causes of power frequency over voltages?

Synchronous generator interview questions with answer

 Interview questions with answer 

Q:Why syn. generators are used for the production of electricity?

Interview question with answer on circuit breaker and relay

CIRCUIT BREAKER AND RELAY

Q. Explain different types of faults?

Electrical Interview questions

Electrical Interview questions

Q: What is inrush current?

A: Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

What is a Filter?

Low-pass filters 

High-pass filters

Band-pass filters

Band-stop filters

Resonant filters

It is sometimes desirable to have circuits capable of selectively filtering one frequency or range

101 Control system Question with Answer

Control system Question with Answer

1.What is frequency respons? 

PERSONAL- INTERVIEW Q&A

 PERSONAL- INTERVIEW 

For all professional 

1. Tell me about yourself.

Start with the present and tell why you are well qualified for the position. Remember that the key to all successful interviewing is to match your qualifications to what the interviewer is looking for. In

Electrical Question with Answer- Section-D

Electrical Question with Answer-Improve your Electrical

1)    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.

2)    Why in the transmission tower construction Middle arm is longer than the upper and lower Arm.

• Conductor of Upper Arm and Lower Arm will stay apart.
• To prevent big birds (Ostriches etc) from bumping their heads against the conductor above when they sit on the wire below.
• Designed to maintain the mechanical requirement to prevent arching between conductors while maintaining a tower height that is manageable, and of course preventing head injuries to birds
• The arms are of different links to prevent a broken upper line from falling on one or more of the phase lines below.
• The clearance from other phase.
• Mutual inductance minimization.
• Preventing droplet of water/ice to fall on bottom conductor.

3)    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)

4)    How Corona Discharge Effect Occur in Transmission Line?

• In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters.
• In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision.
• The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet color appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.

5)    Methods to reduce Corona Discharge Effect:

• Critical Breakdown voltage can be increased by following factors
• By increasing the spacing between the conductors:
• Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases.
• By increasing the diameter of the conductor:
• Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.
• By using Bundled Conductors:
• By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors.
• By Using Corona Rings or Grading Rings:
• This is of having no greater significance but i presented here to understand the Corona Ring in the Power system. Corona Rings or Grading Rings are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.

6)    How to test insulators?

• Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to contain an insulator that may shatter, due to steam build-up from moisture in a cavity, arcing produces intense heat, an AM radio is a good RFI/arcing detection device, a bucket truck AC dielectric test set (130KV) is a good test set for most pin and cap type insulators. A recent article said the DC voltage required to “search out defects can be 1.9 times the AC voltage.
• Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-over that are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or other transients happen, stop when the line is de-energized or dropped below 50% of arc ignition voltage. Using a meg-ohm-meter can eliminate defective insulators that will immediately arc-over tripping the test set current overload.

7)    How to identify the starting and ending leads of winding in a motor which is having 6 leads in the      terminal box

• If it is a single speed motor then we have to identify 6 leads.
• Use IR tester to identify 3 windings and their 6 leads. Then connect any two leads of two winding and apply small voltage across it and measure the current.
• Then again connect alternate windings of same two windings and apply small amount of voltage (same as before) and measure current.
• Check in which mode you get the max current and then mark it as a1-a2 & b1-b2. You get max current when a2-b1 will be connected and voltage applied between a1-b2.
• Follow the same process to identify a1-a2, b1-b2, c1-c2.now we will be able to connect it in delta or star.

8)    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).

9)    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,

10) 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.

11) What is tertiary winding of Transformer?

• Providing a tertiary winding for a transformer may be a costly affair. However, there are certain constraints in a system which calls for a tertiary transformer winding especially in the case of considerable harmonic levels in the distribution system. Following is an excerpt from the book “The J&P Transformer Book”.
• Tertiary winding is may be used for any of the following purposes:
• (A)To limit the fault level on the LV system by subdividing the infeed that is, double secondary transformers.
• (B)The interconnection of several power systems operating at different supply voltages.
• (C) The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to the terminals of one winding.
• It is desirable that a three-phase transformer should have one set of three-phase windings connected in delta thus providing a low-impedance path for third-harmonic currents. The presence of a delta connected winding also allows current to circulate around the delta in the event of unbalance in the loading between phases, so that this unbalance is reduced and not so greatly fed back through the system.
• Since the third-order harmonic components in each phase of a three-phase system are in phase, there can be no third-order harmonic voltages between lines. The third-order harmonic component of the magnetising current must thus flow through the neutral of a star-connected winding, where the neutral of the supply and the star-connected winding are both earthed, or around any delta-connected winding. If there is no delta winding on a star/star transformer, or the neutral of the transformer and the supply are not both connected to earth, then line to earth capacitance currents in the supply system lines can supply the necessary harmonic component. If the harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion.
• Even if the neutral of the supply and the star-connected winding are both earthed, then although the transformer output waveform will be undistorted, the circulating third-order harmonic currents flowing in the neutral can cause interference with telecommunications circuits and other electronic equipment as well as unacceptable heating in any liquid neutral earthing resistors, so this provides an added reason for the use of a delta connected tertiary winding.
• If the neutral of the star-connected winding is unearthed then, without the use of a delta tertiary, this neutral point can oscillate above and below earth at a voltage equal in magnitude to the third-order harmonic component. Because the use of a delta tertiary prevents this it is sometimes referred to as a stabilizing winding.
• When specifying a transformer which is to have a tertiary the intending purchaser should ideally provide sufficient information to enable the transformer designer to determine the worst possible external fault currents that may flow in service. This information (which should include the system characteristics and details of the earthing arrangements) together with a knowledge of the impedance values between the various windings, will permit an accurate assessment to be made of the fault currents and of the magnitude of currents that will flow in the tertiary winding. This is far preferable to the purchaser arbitrarily specifying a rating of, say, 33.3%, of that of the main windings.

12) 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.

13) How can we reduce airborne noise?

• Put the transformer in a room in which the walls and floor are massive enough to reduce the noise to a person listening on the other side. Noise is usually reduced (attenuated) as it tries to pass through a massive wall. Walls can be of brick, steel, concrete, lead, or most other dense building materials.
• Put the object inside an enclosure which uses a limp wall technique. This is a method which uses two thin plates separated by viscous (rubbery) material. As the noise hits the inner sheet some of its energy is used up inside the viscous material. The outer sheet should not vibrate.
• Build a screen wall around the unit. This is cheaper than a full room. It will reduce the noise to those near the wall, but the noise will get over the screen and fall elsewhere (at a lower level). Screens have been made from wood, concrete, brick and with dense bushes (although the latter becomes psychological)
• Do not make any reflecting surface coincident with half the wave length of the frequency. What does this mean? Well, every frequency has a wave length. To find the wave length in air, for instance, you divide the speed of sound, in air (generally understood as 1130 feet per second) by the frequency. If a noise hits a reflecting surface at these dimensions it will produce what is called a standing wave. Standing waves will cause reverberations (echoes) and an increase in the sound level. If you hit these dimensions and get echoes you should apply absorbent materials to the offending walls (fibreglass, wool, etc.)

14) 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. 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.

15) 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.

16) Can a three phase transformer be loaded as a single phase transformer?

• Yes, but the load cannot exceed the rating per phase and the load must be balanced. (KVA/3 per phase)
• For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.

17) 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.
• Standard tap arrangements are at two-and-one-half and five percent of the rated primary voltage for both high and low voltage conditions.
• For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating.

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

• Insulating and isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two.
•  A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise.
• This is especially important in critical applications such as computers, process controllers and many other microprocessor controlled devices.
•  All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.

19) 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.

20) 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.

21) 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.

22) 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

23) 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.

24) 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.

25) 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)

26) 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 prevent re close the breaker by close push button after if it already close.

27) There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum?

• The motor has max load current compare to that of transformer because the motor consumes real power.. and the transformer is only producing the working flux and it’s not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

28) 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.

29) 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.

30) NGR 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 have opt 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.

31) 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.

32) What is Minimum Value of Insulation Resistance / Polarization Index?

• Motor Insulation Resistance:
• The acceptable meg-ohm value = motor KV rating value + 1 (For LV and MV Motor).
• Example, for a 5 KV motor, the minimum phase to ground (motor body) insulation is 5 + 1 = 6 meg-ohm.
• Panel Bus Insulation Resistance:
• The acceptable meg-ohm value = 2 x KV rating of the panel.
• Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 meg-ohm
• IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX (min IR at 40⁰C in MΩ)

Minimum Insulation Resistance	TEST SPECIMEN
R1 min = kV+1 R1 min = 100	For most windings made before about 1970, all field windings, and others not described below For most dc armature and ac windings built after about 1970 (form wound coils)
R1 min = 5	For most machines with random -wound stator coils and form-wound coils rated below 1kV

33) 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.
• Keep in mind, however, that 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.

34) Calculate the size the CT on the neutral point of the secondary side of 11/0.415 kV Transformer

• For high impedance relays (differential or restricted earth fault relays), ‘Class X’ current transformers are recommended to be used.
• Please note that both CTs (neutral & phase) shall have the same characteristics. The following is an example to size the CT:
• Input data: 11/0.415 kV ,2500 KVA Power transformer ,Transformer impedance is 6% ,Length of cable from neutral CT to the relay is 200 m ,Cross section of CT cable to be used is 6 mm² -copper and resistance is 0.0032 Ω/m
• Step  1: Calculation of CT Rated Primary Current
• I = kVA/ (0.415×1.732) = 2500/ (0.415×1.732) = 3478.11 A, CT with primary current of 4000 A to be selected.
• Select the secondary current of the CT 1 or 5 A. selecting 1 A secondary current, as the cross section and length of pilot wires can have a significant effect on the required knee voltage of the CT and therefore the size and cost of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the pilot wires.
• Step 2: Calculation of maximum Fault Current
• Ift = kVA/ (0.415×1.732x Z)
• Ift = 2500/ (0.415×1.732×0.06) = 57968.59 A (say 58000 A)
• Step 3: Calculation of the Knee Voltage of the CT (Vkp)
• Vkp = (2x Iftx (Rct+Rw)/CT transformation ratio)
• Where: Rct  is the CT resistance (to be given by the manufacturer), Here Rct is1.02 Ω. 
•  Rw: total CT cable resistance= 2x cable length (200 m) x wire resistance= 2x200x0.0032= 1.28 Ω
• CT transformation ratio = CT Primary Current/CT Secondary Current
• CT transformation ratio = 4000/5= 800 A, for CT with 5 A secondary current; or,
• CT transformation ratio = 4000/1= 4000 A, for CT with 1 A secondary current. We will use 1 A in this example.
• Vkp = (2x58000x (1.02+1.28)/4000)= 66.7 V.
• The Vkp of the CT should be higher than the setting of relay stability voltage (Vs), to ensure stability of the protection during maximum Through fault current.
• To calculate the stability voltage,we should follow the related formula given by the relay manufacturer, as each relay manufacturer has its own formula.
• we may calculate the Vkp as above using a CT with secondary current of 5 A, and you will notice the difference in the Vkp.

35) 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) 0.2.0.5,1, 1.5 , 2 ,3
• Short Circuit :-2 to10 in steps of 1 lr , short time delay (sec) 0.02.0.05,0.1, 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.

Electrical Question with Answer- Section-C

Electrical Question with Answer-Improve your Electrical IQ Section-C

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.

Electrical Question with Answer- Section-B

Electrical Question with Answer-Improve your Electrical IQ Section-B

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