Basic Electrical Interview Q&A Part-3
1) What is the reason of grounding or earthing of equipment?
with a ground path, in case of short circuit the short circuit current goes to the body of the equipment & then to the ground through the ground wire. Hence if at the moment of fault if a person touches the equipment body he will not get a shock cause his body resistance (in thousands of ohms) will offer a high resistance path in comparison to the ground wire. Hence the fault current will flow thru the ground wire & not thru human body.
Providing a ground path helps in clearing the fault. A CT in the ground connection detects the high value fault current hence the relay connected to the CT gives breaker a trip command.
Grounding helps in avoiding arcing faults. IF there would have been no ground then a fault with the outer body can cause a arcing to the ground by breaking the air. This is dangerous both for the equipment & the human beings.
2) A type-C MCB has thermo magnetic capability 5In to 10In that means a short circuit current will be interrupted as the value will reach between 5In to 10In but the MCB breaking capacity is (for example) define as 10kA.
5In to 10In is the pickup threshold for the magnetic trip element. The MCB will trip instantaneously when the current is between these limits. 10kA is the short circuit withstands capacity of the MCB.
Under normal condition, a current limiting type MCB will trip on short circuit (magnetic trip) and the current during circuit interruption will be much less than the prospective current. However, the MCBs have to have a short circuit capacity more than or equal to the fault level at the location where it is installed.
3) What is Ferrari Effect?
Ferranti Effect is due to the rise in voltage at the receiving end than that of the sending end. This occurs when the load on the system reduces suddenly.
Transmission line usually consists of line inductance, line to earth capacitance and resistance. Resistance can be neglected with respect to the line inductance .When the load on the system falls the energy stored in the capacitance gets discharged. The charging current causes inductive reactance voltage drop. This gets added vector ally to the sending end voltage and hence causes the voltage at the receiving end to raise
A Long transmission line draws significant amount of charging current. If such line is open circuited or very lightly loaded at the receiving end, the voltage at the receiving end may become greater than sending end voltage. This effect is known Ferranti effect and is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance is responsible to produce this phenomenon.
The capacitance (charging current) is negligible in short lines, but significant in medium and long transmission line. Hence, this phenomenon is applicable for medium and long transmission line.
The main impact of this phenomenon is on over voltage protection system, surge protection system, insulation level etc.
4) Can single phase transformers be used for three phase applications?
Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer.
5) What is BIL and how does it apply to transformers?
BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The BIL of a transformer is a method of expressing the voltage surge that a transformer will tolerate without breakdown.
6) Where Auto-recloser is used?
For Generator protection / Transformer Protection / Transmission Line / Bus bar protection.
Many faults on overhead transmission lines are transient in nature 90% of faults are used by birds, tree branches. These condition results in arching faults and the arc in the fault can be extinguished by de-energizing the lines by opening of CB on the both ends of the lines.
Open-0.3 second-Close-3minute-Close this is the sequence of AR. i.e.-OPEN,C-CLOSED
whenever faults occurs CB opens, then after 0.3 sec it closes automatically, if faults persists then it will open after 3 min it closes and if still fault persists. It remains in open condition.
Auto reclosure is generally used for Transmission lines where the general types of faults are transient in nature.
It can be three phase auto-reclosure or single pole auto-reclosure.
The single pole auto reclosures are generally for 400kV line below this three pole auto- reclosures are used.
The reason for a line the single pole reclosures provides a better stability of the system since some part of power is still transferred through the healthy phases.
Also 400kV breaker till date has a independent drive/ trip/ close coils for the three poles, below that all breakers have common drive/ trip / closing coils for the three poles.
7) What is difference between power transformers & distribution transformers?
Distribution Transformers are designed for a maximum efficiency at 50% of load. Whereas power transformers are designed to deliver max efficiency ay 90% and above loads.
The distributions transformers have low impedance so as to have a better regulation power transformers have higher so as to limit the SC current.
Power transformers are used to step up voltages from 11 KV which is the generating voltage to 132 or whatever will be the transmission voltage levels. Power transformers are having Star-Delta connection. It will be located at power generating stations.
Distribution transformers are used to step down voltages from transformer levels to 11 KV/415 V. Will be having Delta-Star. It will be located in substations near load centers.
The main basic difference lies in the Design stage itself as power transformer are to operate at near full load so there sensing is such that they achieve equal. of copper losses & iron losses at full loads whereas this is achieved in the design itself at about 50% loading in dist transformer but friends there is a dilemma as our dist. transformer are almost fully loaded & beyond so they never go operate at their full eff. & also poor voltage regulation.
The difference between power and distribution transformers refers to size & input voltage. Distribution transformers vary between 25 kVA and 10 MVA, with input voltage between 1 and 36 kV. Power transformers are typically units from 5 to 500 MVA, with input voltage above 36 kV. Distribution transformer design to have a max efficiency at a load lower than full load. Power transformer design to have a max efficiency at full load
8) What will be happen if the neutral isolator will be open or close during the running condition of power?
During normal condition the neutral isolating switch should be kept close. In case it is kept open, under balanced load conditions the current through neutral will not flow & nothing harmful will take place but in case an earth fault takes place then there will be no earth fault current flowing through the system & the generator will run as a ungrounded generator. Thus the earth fault will not be cleared.
If more number of generators are connected parallel. We will have a close loop and hence negative sequence current will flow. This will increase the rotor temperature. Hence if more number of generators are connected then only one is earthed and others are open.
In case of Two or more generators connected to a common bus without a transformer in between, basically in hydro stations, one of the Neutral Isolation Switch(NIS) is kept closed & rest are opened to prevent circulating currents to flow between generators. Hence the above explanation will not be valid for such systems.
Sometime we may want to test generator and may want to isolate the neutral from ground. like for example meggaring etc. In such case we would like to open ground connection cable in case we want to remove the NIS? we will certainly not like to open all the bolted connections for just a small test like checking insulation with a meggar etc. for such things we need a NIS.
Neutral isolator is required if we have delta transmission system and at the time to connection with the Grid Neutral isolation is required.
If we ungrounded the neutral then the generator is connected to the ground via Phase to earth capacitances. Hence during faults arcing grounds can take place. Which are dangerous both to human & equipment.
When we provide earthed neutral, for a fault, earth fault current will start flowing through the neutral, which we can sense thru a CT & relay & hence can immediately identify & clear the fault in about 100 ms by opening the associated breaker/prime mover/excitation. Quicker the fault clearance less is the damage.
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9) Why shorting type terminal required for CT?
During maintenance or secondary injection you will need to bypass the CT & for the same you need shorting link. During sec. injection you will short circuit the main CT & bypass it. Open circuiting the CT will saturate it & damage it.
10) Why fuse is given for only PT and not CT?
Fuse if given for CT blows off due to a fault then rather than protecting the CT it will make it open circuited hence it will be saturated & damaged. For PT it gives overload & SC protection.
11) Why is insulating base required for LA?
The LA is provided with a dedicated Prper earthing which may be in the form of a buried treated electrode next to it.LA connection is securely made with the electrode via a surge counter. If we directly earth the LA through structure then the surge counter will not be able to measure the no of surges. For lesser rating the counter is not provided, hence we can bypass the insulated base. But then proper earthing has to be assured.
12) Can 60 Hz transformers be operated at 50 Hz?
Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service.
13) Can transformers be used in parallel?
Single phase transformers can be used in parallel only when their voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers which will cause excess heating and result in a shorter life of the transformer. In addition impedance values of each transformer must be within 7.5% of each other.
14) 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.
15) Why short circuit do not take place when electrode is touched to ground.
Basically during welding we force a short-circuit at the electrode tip. The fault condition produces large magnitude currents. Greater the Current value have greater I2R heat produced. The arcing energy elevates the temperature & hence melts the electrode material over the joint.
The transformer is designed to withstand such high currents. But welding is a very complex & detailed phenomenon. Besides there are many principles on which welding operates. Some may be a welding, dc welding, arc, constant voltage, constant current etc
16) What’s the difference between generator breaker and simple breaker?
Breaker is one which disconnects the circuit in fault condition and It is similar for all equipment. Based on the equipment voltage and maximum short circuit current the ratings will be decided. For better understanding we call generator or transformer or line etc breakers.
17) What is accuracy Class of the instrument?
Generally the class indicates the accuracy with which the meter will indicate or equipment will measure with respect to its input.
The accuracy of different equipment will depend on number of factors.
For example for a PT accuracy will depend on its leakage reactance & winding resistance. For a PT accuracy gives the voltage & phase error & it varies with the VA burden of secondary. Also better core material will give better heat dissipation & reduce error. class of accuracy will give the voltage error for a PT
different type of PTs available are:0.1, 0.2, 0.5, 1, 5 & error values will be: class% voltage error(+/_) phase displacement
Similarly indicating instruments shall have accuracies & accordingly application as depicted below for testing the following values are generally used:
for routine tests : accuracy class 1
for type tests : accuracy class 0.5 or better.
indicating meters generally will have accuracy of 1.
18) First pole to clear factor-Circuit breakers
The first pole to clear factor (kpp) is depending on the earthing system of the network. The first pole to clear factor is used to calculating the transient recovery voltage for three phase faults. In general following cases apply:-
1. kpp = 1.3 corresponds to three phase faults in system with an earthed neutral.
2. kpp = 1.5 corresponds to three phase faults in isolated or resonant earthed system.
3. kpp = 1.0 corresponds to special cases e.g. railway systems.
A special case is when there is a three phase fault without involving earth in a system with earthed neutral. This case responds to kpp = 1.5 . This special case is however not normally considered in the standards.
19) Why we use a resistance to ground the neutral when we need always low resistivity for the grounding?
If we ground the generator directly then whenever a fault will take place at any phase with ground the fault current flowing throw the faulted phase-to ground-to neutral will be very high cause there will be no resistance to limit the value of fault current. Hence we insert a resistance in the neutral circuit to limit this fault current. Also we need to reduce the fault current to such a value that the protection CTs are able to identify the fault current without saturating the CTs. Communicate it to the protection relays & hence the relays can then isolate the system from the fault; so that the system is isolated from the fault before the harm is done by the fault current. That is the reason that all the equipment will be designed for fault KA values for 1 sec so that the total operation(CT sensing-relay functioning-circuit breaker operation ) time will be less than 1 sec. hence the Breakers will isolate the fault before 1 sec i.e. within the time period the equipment are designed to carry the fault current. Thus all your objectives of:
preventing the arcing.
limiting the fault current.
isolating the faulted system are achieved
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20) Why are NGR’s rated for 10sec?
NGR are placed in the neutral circuit & hence will be energized only in the fault conditions thus their continuous loading is not expected. Hence they are selected for intermittent rating. Similarly when we place a transformer in the neutral grounding circuit the KVA rating obtained after the calculation is multiplied by a diversity factor to obtain smaller rating cause the therefore It will not be continuously rated.
NIS is also provided to cut the circulating negative sequence current in 2 more generators connected in parallel.
in some grid conditions they ask to keep neutral isolated after being connected to grid.
21) How to calculate knee point voltage and significance of knee point voltage?
Knee point voltage: That point on the magnetizing curve (BH curve) where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current). Its significance lies mainly in PS class core of CTs used for diff protection
22) Design method for neutral grounding resistor?
NGR design basics:
Capacitive coupling of generator, equipment and the ground
-Generator to ground capacitance.
-Generator cable to ground capacitance (or bus duct as the case may be)
-Low voltage winding of trafo & ground capacitance.
-Surge arrestor capacitance.
The total capacitance is then obtained from the above values & then we calculate from that the capacitive reactance. The capacitive current then produced is calculated from the generator voltage & the capacitive reactance obtained above. Once the current is obtained we can then calculate the electrostatic KVA from the current multiplied with voltage.
23) Criterion is there for selection of Insulation Disc in Transmission and Distribution Line.
11kV is the phase to earth voltage for 220kV =220/ (sqrt(3)*11)=12 No’s of disc are suitable.The number can be increased to increase the creep age distance.
While selecting the disc insulators one has to keep in mind the following things:
1. EM-strength of the string. All the forces coming on to the string & the ability of the string to withstand them.
2. Sufficient Cree page distance so as not to cause a flashover .
3. Interface with the type of conductor used (moose, tarantula, zebra etc)
So we will get the value of no of discs by dividing the phase to earth voltage with 1.732. Once that is done then we need to see its suitability with respect to EM strength.
After this we need to consider the force that the stack has to bear. If we have a strain type of fitting i.e. the stack has to bear horizontal conductor tension, weight load of the conductor, wind load, ice load etc then the number of insulator discs required may be more.
But for a suspension type system which has to bear only the weight then number of discs required may be less than what we get by dividing by 11. That is the reason we have seen only 23/24 discs in 400 kv line cause in that case the creep age obtained must have been enough & also the strain requirement.
33kv insulators are generally used in a vertical installation & are not stacked together because that will make the suspension very rigid
24) Do taps work the same when a transformer is reverse fed?
Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed, the taps are on the output side and can be used to adjust the output voltage.
25) Why may I get the wrong output voltage when installing a step up transformer?
Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H terminals are primary and X terminals secondary. This is true for step down transformers, but in a step up transformer the connections should be reversed. Low voltage primary would connect to X terminals while high voltage secondary would connect on the H terminals.
26) 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.
27) Why in Double circuit wire are transposed (R – B, Y – Y, B – R)
This is done to avoid
1. Proximity effect
2. Skin effect
3. Radio interference
4. Reduction in noise in communication Signals
28) Selection of LA
The voltage rating of LA is selected as: Line voltage x sqrt(2)/ sqrt(3) so for 11kV line its 9kV
In that case also the values would not differ much if We takes the TOV factor as 1.4. However, we can take the value of 1.56 as TOV to be more precise.
29) Which is more dangerous AC or DC
Low frequency (50 – 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death.
However any practical distribution system will use voltage levels quite sufficient to ensure a dangerous amount of current will flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard.
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30) What all are the applications where high speed grounding switches are used.
Generator neutral is earthed directly or through distribution transformer. This neutral earthing is through done through a switch. This is general practice for only one generator.
For two generators in parallel to a bus the neutral earthing is different. If both the neutral earthing is closed the negative sequence current will be flowing though both the generator taking earth as path. This leads to increase in loss and increase in temperature (This may leads to false tripping also). Hence once the second generator is synchronized with the bus or grid the neutral is isolated.
Neutral grounding switch we do not need a high speed grounding switch. A normal switch with the correct rating capacity would also work.
31) What is Skin Effect and how does it happen??
According to faradays law of electromagnetic induction, a conductor placed in a changing magnetic field induces an emf. The effect of back emf is maximum at the centre because of maximum lines of field there. Hence the maximum opposition of current at inner side of conductor and minimum opposition at the surface. Hence the current tries to follow at the surface. It is due to this reason that we take hollow tube conductors in bus duct.
Taking into account the inductance effect, its simple consider the DC current. Since its constant & not varying hence no back emf but if we gradually start increasing the frequency then the flux cutting the conductor goes on increasing, hence greater the frequency greater the alternating flux cutting the conductor & hence greater the back emf & therefore greater the skin effect.
32) Why we ground the sheath of single core power cables and to avoid grounded at both the ends?
A single core cable with a sheath is nothing but a conductor carrying current surrounded by another conductor (sheath). Hence the Alternative current in the conductor induces voltages in the sheath or the armour. Hence grounding these cables at both ends will cause the potential of the armour to be same as ground potential & hence shall become safe for the personnel.
But grounding the cables at both the end will cause a problem. In that case the circulating currents will start flowing with the armor, the ground & with the two ends of the grounding completing the circuit. This will also provide path for the fault currents to flow. Hence this whole thing will cause the cable to produce some I2R losses, hence heating & hence the current carrying capacity will be de rated. This system of cable earthing is called both-end bonding. This system is suggested only when one wants to avoid the voltage development because can either go with the de rated cable or if one updates the cable in advance.
When only one end of the cable sheath is grounded then there is no path for the circulating current to flow. Hence the current carrying capacity of the cable will be good. But in this case potential will be induced between sheath & ground. This potential is proportional to the length of the cable & hence this will limit the length of the cable used. This method is called single point bonding. This is thus used only for short lengths.
There is another system called the cross bonding system in which the sheath are sectionaliosed & cross connected so that the circulating currents are minimized. Although some potential will also exist between sheath & ground, the same being maximum at the link boxes where bonding is done. This method provides maximum possible current carrying capacity with the maximum possible lengths.
33) What is EDO & MDO type breaker?
In the Breakers for the operation spring charging is must.
In EDO breaker the spring charging is done with a motor and draw out manually by hand. so EDO means Electrically spring charged Draw Out breaker
In MDO breaker the spring charging is also done by hand manually and draw out about also by hand only. so MDO means Manual spring charge Draw Out breaker
34) Why transformer rating is in KVA or KW?
Because power factor of the load is not defined in case of transformer that’s why it is not possible to rate transformer in KW.
The losses (cu loss and iron loss) of the transformer depends on current and voltage purely, not on load i.e, phase angle between the current and voltage i.e. why transformer rated in kVA
Transformer is not a load and having no effect on P.F (that’s why no change in its power factor) and it only transfer the constant power from one voltage level to another voltage level without changing frequency. since both the losses viz copper loss(depends on current) and iron loss(depends on voltage) are independent of power factor, that is why a Transformers rating is not on kW, but on KVA
35) Why the secondary of CT never open when burden is connected on the CT.?
secondary of CT is never opened as because CT is always connected to the line so opening the secondary will mean there will be no counter mmf to balance the primary current as a result of which a very high induced emf may appear in the secondary as flux is very high and no counter mmf and this will be dangerous for the personnel in the secondary side and for pt if it is shorted then with full voltage applied to the primary.
If we short the secondary then much high current will circulate in the secondary due to high induced emf much higher than the actual full load current as a result of which the transformer’s secondary winding may burn out.
36) Distance relay setting
Step1:
Get the conductor Details (i.e Positive Sequence Impedance (Z), Zero Sequence Impedance(Z0)) which is in Primary value. Convert in terms of secondary values.
Step 2 :
Based upon the calculated value divide into various zones
Zone 1 (Forward) means 80% of your protected line length.
Zone 2 (Forward) means 100% of protected line length + 20% Adjacent Shortest line
Zone 3 (Forward) means 100% of protected line length + 50% Adjacent Longest line.
Zone 4 (Reverse) means 10% of protected line.
37) Difference between CT class 0.2 and 0.2S?
0.2S & 0.5S: Special type of measurement CTs they guarantee the declared accuracy, even with
20% loading. And some definite error can be defined even with a load as low as 1%. Thus they are suitable for industries where loads are commissioned in steps or stages. Also for tariff metering purposes.
0.2S: Special class for metering. It is more accurate than 0.2 classes. Generally if we use 0.2s class CT than VA burden of core is also come down.
In 0.2 classes CT, ratio & phase angle errors must be within the specified limits at 5%, 20%, 100% & 120% of rated secondary current. Whereas in 0.2s class CT, ratio & phase angle errors must be within the specified limits at 1%, 5%, 20%, 100% & 120% of rated secondary current. Also in 0.2s class, Ratio & Phase angle errors limits are lower than 0.2 classes.
38) Why we use inductors
Inductors have the property to oppose sudden changes in Current. When connected to the primary side of transformer, if any sudden short circuit (very high) current flows due to some fault in the system, the inductor will oppose the flow of that current saving the transformer.
Secondly, for the problem of lagging current. Capacitors are connected across the inductor to improve the lagging current. So Mainly Inductor is used to (i) protected the transformer, (ii) solved the problem of lagging current.
39) Why do we need a bigger breaker when reverse feeding a transformer?
Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online.
40) How many types of Neutral grounding system?
There are primarily three types of grounding system which are:
(1)Solid grounding – The neutral point of the system is grounded without any resistance. If the ground fault occurs, high ground current passes through the fault. Its use is very common in low voltage system, where line to neutral voltage is used for single phase loads.
(2) Low Resistance grounding (LRG) – This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited. The ground fault current is limited to in the rage from 25A to 600A.
(3) High Resistance Grounding (HRG) – It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn’t trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted. The ground fault current is limited to 10A or less.
There are other two types such as Corner Grounding (for Delta system) and ungrounded system but they are not commonly used.