Electrical Q&A Part-2

 Electrical Q&A Part-2

(1)What’s 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.
• CT with 0.2s class is more accurate than with 0.2 class. because in 0.2 class 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.
• Earthing an equipment doesn’t mean it requires a ground or actual earth … its just enough to have a sink which can dissipate all the fault current. so in the case of aero plane as well as ship the body of the same is enough to dissipate all the fault current. (ship can be grounded to water also). it mean to say the fault current will be dissipated through the body.
• A lightening strike will opt for the shortest path to reach the earth.an aeroplane will in no way connected to earth so a lightening strike will never take its path through aero plane. It mean to say an aero plane will never see a lightening strike in its path.

(6)  Why earth point in 3 pin socket is twice in size

• why earth point in 3 pin socket is twice in size compared to phase & neutral whereas conductor we used for both end of socket for earth point having same cross-section as we used for phase & neutral
• R = k *(L/A) k=resistivity      A=Area             L=Length
• so large is the area, the less will be resistance.
• Hence, if any equipment have current in the body/cover, then it will follow the less resistance path. And, body current will be earthed.
• This is the hard way coz we have to consider to the derating factor, current up rating factor, effect of short circuit current , electrodynamics effect, strength of material, But it will take 2 days to calculate the size in such a manner. So i usually opt for the easy go
• Size of Copper bus bar = Continuous Current Rating / 0.8
  Size of Aluminum bus bar = Continuous Current Rating / 1.2
  eg: if the continous current rating is 400Amperes , then if you choosing Al bus bar ,
  size should be greater than 400/ 1.2= 500sqmm
  size of Cu busbar should be greater than 400/ 0.8= 333sqmm

(8 )Where is Auto-recloser is used?

• Gen. protection / Trans. Protection / T/L / Bus bar protection
• Auto reclosure is generally used for Transmission lines where the general type 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 if employed are three pole auto- reclosures.
• The reason I know is for a line the single pole reclosure 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.
• The usual size used of copper bus bar is 25×6 Cu for panel..
• The size is calculated as per the fault current. refer IEEE-80 for SC ampacity calculations.
• The general thumb rule is 200A/sq mm for Cu and 100A/sq mm for Al for 1 sec SC rating…

(10) 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 DEL-DEL connection. 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 DEL-Y. 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.

(11)Why it that the voltage levels in India is is in Multiples of 11?

• Voltage levels in India are the multiples of 1.1. coz ripple factor of a perfect sine wave is 1.11. Hence to transfer an equivalent of 100 volts, we need to transfer 1.11*100 i.e. 110 volts. But not in dc voltage.
• In India, transmission is mostly done at 33kv. for longer transfer of power, we use 440kv at the most. the highest voltage at which power is transferred is 765kv, that’s in north America..
• There has been no change in IS and IEC considering digital meters, moreover, burden of instrument transformer depends on the lead wire used in the secondary of CT or PT.
• Generally highly accurate CT or PT of 0.1 or 0.2 are used for measurement purpose, beyond this, accuracy class of 0.05 or 0.01 is used by standard laboratories like ERDA OR CPRI for calibration purpose of ct or pt.
• “Selection of burden depends completely on the use of leads on secondary side of CT or PT .”
• When CT or PT is to be calibrated, it has to be calibrated using the same lead burden with which it has to be connected at site for connecting it to meters.
• CT Burden = I x I x( Rct + 2Rl + Rm)
  where Rct = CT Internal resistance
  Rl= Resistance of lead/cable
  Rm= Resistance of meter(s) connected in series
• PT Burden = V x V / (( Rpt + Rm), for a conservative design you can ignore the lead resistance since it will reduce the burden on PT

(13)What will be happen if the neutral isolator will be open or close during the running condition of power system??

• During normal condition the neutral isolating switch should be kept close. In case it is kept open, under balanced load conditions the current thru 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 thru the system & the generator will run as a ungrounded generator. Thus the earth fault will not be cleared.
• In case of 2 or more generators connected to a common bus without a transformer in between, basically in hydro stations, one of the 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 you may want to test you generator & may want to isolate the neutral from ground. like for example meggaring etc. In such case would you like to open your ground connection cable in case you want to remove the NIS? You will certainly not like to open all the bolted connections for just a small test like checking your insulation with a meggar etc. for such things you need a NIS.
• 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.
• Neutral isolator is not must. Smaller ratings are directly earthed through register. Two reasons are already mentioned ones. one more can is it is required if we have delta transmission system where transmission personnel ask you to isolate neutral after connected to grid.
• If you 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 you provide earthed neutral, for a fault, earth fault current will start flowing thru the neutral, which you can sense thru a CT & relay & hence can immediately identify & clear the fault in abut 100 ms by opening the associated breaker/prime mover/excitation. Quicker the fault clearance less is the damage.
• In unearthed system , the voltage of healthy phases can go up 1.732 times the P-E voltage. Hence you have to design the system for that voltage which makes it costlier.
• Only in case of very important systems like medical systems etc you will not ground the system cause you can operate such systems for single ground fault.

(15)Why it is required to operate the radio stations at high frequencies.

• It’s not regarding power coz power has no relationship with frequency. The thing is the Antenna for transmitting & receiving should bo of smaller height. The formula for which is ( C/4f) where C = velocity of light & f = frequency so if the frequency is 10 KHz then minimum antenna height require is 7.5 k.m. for C= 3000000000 but if frequency is 1 Mhz then height require is 75 Metre so thats why high freq is used. Take example of mobile & its antenna which is opearting on GHz . Also High frequency increases the quality of reception , increases the range of communication , high frequency offers high bandwidth so for large range of freq ur gain will remain constant. Much more is there but I hope ur view is cleared now.

(16)Why is 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.

(17) Why is fuse 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.

(18) Why is insulating base required for LA? Can it be used without insulator as well(coz it is possible as suggested by 1 of the LA manufacturers, especially at 6kV)

• The LA is provided with a dedicated PROPER 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 you 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 you can bypass the insulated base. But then proper earthing has to be assured.
• Is motorized earth switch allowed for isolator as per IS and IEC?
  I think it is allowed cause even if it is motorized the interlocking will prevent from eathing during the closed condition

(20) What is main zone and check zone in case of 21 protections?

• Main zone is the zone in which the distance relay will monitor the impedance & during the fault if the impedance falls under its characteristics (circle/quadrilateral) then the relay will operate. Check zone is a back up for the main zone.

(21) IS THE WELDING TRANSFORMER IS A STEP UP OR STEP DOWN TRANSFORMER?
WHY DOES SHORT CIRCUIT DONT TAKES 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 I value..greater is the I2R heat produced. The arcing energy elevates the temperature & hence melts the electrode material over the joint. Of course 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. Its a complex subject of study & for deeper knowledge detailed study is important.

(22)What’s the difference between generator breaker and simple breaker..??

• Breaker is one which disconnect the circuit in fault condition. and 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 .

(23) Class of the instrument?

• Generally the class indicates the accuracy with which the meter will indicate or an equipment will measure with respect to its input.
• The accuracy of different equipment will depend on number of factors. For example for a VT accuracy will depend on its leakage reactance & winding resistance. For a VT 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 VT
• different type of VTs available are:
  0.1, 0.2, 0.5, 1, 5 & error values will be:
  class—-% voltage error(+/_)—-phase displacement
  0.1—-0.1—-5
  0.2—-0.2—-10
  0.5—-0.5—-20
  1—-1—-40
  3—-3—-to be specified.
• 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.

(25) 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.

(26) What is the roll of NGR in a power system and why we use a resistance to ground the neutral when we need always low resistivity for the grounding??

• We need to understand why grounding is needed. well if You don’t ground the neutral then there is a possibility of production of what is called is an arcing when the fault will take place. The generator is actually connected with the ground through the unseen capacitances with the ground. so u see that always there is a connection between the generator & the ground thru such capacitances(capacitances between generator & ground, capacitance between cables/bus duct & ground etc) hence when a fault will take place an arc will produce which will deteriorate the insulation & is also dangerous to humans. NOW If you 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 vaules 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
  Of course you can have different types of earthing systems based on your requirement & system configuration.
  i hope the explanation was understandable.
• Neutral isolating switch Is required for:
  measuring / testing / repair etc.
• NIS Should be avoided as one can open the neutral switch & make the system dangerous with ungrounded condition. In that case arcing fault can take place & ur protection may become ineffective.
• NIS Should be replaced by a link I think so that it can be operated only in disconnected condition.
• however:
• Where multiple generators are solidly grounded but have switches in the neutral, there has sometimes been the practice of grounding only one of the several generators in parallel to limit ground-fault current duty or circulating third harmonic current.

(28) NGR Ratings / why are NGR’s rated for 10sec?

• These 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 you 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 generator connected in parallel.
  in some grid conditions they ask to keep neutral isolated after being connected to grid.

(29) How to calculate knee point voltage and significance of knee point voltag?

• 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

(30) Design method for neutral grounding resistor???

• NGR design basics:
• capacitive coupling of your 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 u 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 you can then calculate the electrostatic KVA from the current multiplied with voltage.

(31) 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 find 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 you will come to the value of no of discs by dividing the phase to earth volatge with 1.732. Once that is done then u need to see its suitability with respect to EM strength. & then later you can order it for a specific conductor type with all the string/suspension insulator hardware.

(32) In a 220 kV system, there are two possible LA ratings 198 kV and 216 kV. Obviously the two have different MCOV’s, with 216 kV LA having higher MCOV. WHY IS THERE TWO BIL LEVEL AS PER IS IN 220 kV and 400 kV LEVELS?

• Highest System Voltage Impulse With Stand Level are different BIL’s for various voltage systems as per IS 10118kV (rms)            kV (peak)
  12                     60
  12                     75
  36                     145
  36                     170
  72.5                  325
  123                   450
  123                   550
  145                   450
  145                   550
  145                   650
  245                   6.5
  245                   750
  245                   850
  245                   950
  245                   1050

(33) Phase double circuit three wires on the either sides but there one thing to be noticed the wires are transposed i.e.
R – B
Y – Y
B – R

• This is done for various reasons few are to avoid
  1. proximity effect
  2. skin effect
  3. radio interference
  4. reduction in noise in comm. Signals

(34) 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 wont differ much if u take the TOV factor as 1.4. However, you can take the value of 1.56 as TOV to be more precise.
• Frankly speaking one will have to do an intensive power system study after that one as to calculate the overvoltage taking place at different locations and at the location of fault. Once that is done we need to obtain the temporary over voltage factor which is then calculated from some IEC curves. But such a study is done only when an intestine power system study is carried out.
• The selection criterion of the insulator discs depend on the installation method & the system voltage, rightly said we can have 11 kv & 33 kv disc.
• Generally 11kv discs are stacked together to provide enough creep age so as to prevent any electrical fault to ground & also means for suspension. Hence system voltage divided by 11 shall give you a rough idea of the number of discs.
• After this one needs to see the force that the stack has to bear. If you have a strain type of fitting ie 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. You will have to calculate the forces occurring & what the discs can not bear.
• But for a suspension type system which has to bear only the weight then number of discs required may be less than what you get by dividing by 11. That is the reason you 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 is generally used in a vertical installation & are not stacked together cause that will make the suspension very rigid.
• 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.

(37) What will happen if dc supply will given to 100 w bulb.

• Of course the bulb will glow.
• Note that the current in case of AC flowing through the bulb will vary from zero to peak value then to zero again & then to peak value in the -ve side & then again to 0. Hence the bulb actually flickers with a 50 Hz frequency. Of course your eye is not that quick enough to notice that flickering & hence you see a continous light coming out of the bulb with AC.(I guess some stroboscopic method may enable you to see it flickering…somebody may post the method)
• Considering a sine wave AC current, A DC current with value equal to peak AC value divided by sq.rt2, wil provide exactly the same power consumption. A 100 watt bulb operating at 220 V AC will draw rms current Irms=P/V=100/230=0.43 Amps. The current hence actually varies from 0 to 0.6 (peak value, = 0.43 * sq.rt2) then to 0 then to 0.6 in the other direction & then again to 0. And of course this happens 50 times a second. Since the power consumption is calculated from the rms value hence we need to keep in mind the rms value of current when going to design a switch (or breaker for larger loads) for such a load.

(38) 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 fine if there is only one generator.
• For two generators in parallel to a bus the neutral earthing differs. Some says both the neutral are to earthed. Some say only one neutral has to be earthed.
• The explanation given was, if both the neutral earthing are 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 don’t need a high speed grounding switch. a normal switch with the correct rating capacity would work.

(39) What is Skin Effect!!! & more importantly How does it happen??

• Remember faradays law of electromagnetic induction??? a conductor placed in a changing magnetic field induces an emf well the same is happening here
• The effect of back emf is max at the centre cause of maximum lines of field there. Hence the maximum opposition nthere & 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 u 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!!!

(40) Why there is a need to ground the sheath of single core power cables. Why the grounding avoided 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 ALTERNATING 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.

(41) 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

(42) Why transformer rating is in KVA or MVA?

• Because power factor of the load is not defined in case of transformer…thats 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

(43) Complete use & operation on auto-reclosure?

• 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 simo opening of CB on the both ends of the lines.
  O-0.3-CO-3min-CO
  this is the sequence of AR.
  i.e.-OPEN,C-CLOSED
• when ever 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 remain in open condition.

TYPE OF A/R (based on phase)

• 3 phase a/r – this type of a/r causes an immediate drift apart of the two sys and hence no interchange of syn.power can take place during the dead time.
• single phase a/r- only the faulty phase is reclosed without causing interruption in interchange of syn.power between two sys through other two healthy phases.

TYPE OF A/R (based on attempts of reclosure)

• single shot – in this scheme cb is reclosed only once on a given fault before lockout of cb occurs .high speed a/r for EHT system is invariably single shot.
• Multi shot – in this scheme more than one reclosing attempt is made for a given fault before lockout of cb occurs.
  Repeated closure attempts with high fault level would seriously affect cb.
• The factor that must b taken into account.
  a) cb limitation.
  b) system condition.

TYPE OF A/R (based speed)

• high speed
• low/delayed a/r

(44) Why the secondary of CT never open when burden is connected on the CT.?

• secondary of a 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.

(45) Distance relay setting
Step1 :
Get the conductor Details (i.e Positive Sequence Impedance (Z), Zero Seqeunce Impedance(Z0)) which is in Primary value. Convert in terms of secondary values.
Step 2 :
Based upon the calculated value divide into various zones
i.e.
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.
If you need more kindly Search Hand book of Power System( ABB, AREVA Manafacctures).
(46) Difference between CT class 0.2 and 0.2S?

• 0.2S is Special class for metering. it is more accurate than 0.2 class. generally if u use 0.2s class ct than ur VA burden of core is also come down. But, PGCIL & NTPC use 0.2 class ct.
• 0.2S & 0.5S are 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.

(47) Why we use inductors

• Inductors have the property to oppose SUDDEN CHANGE 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 we have (i) protected the transformer, (ii) solved the problem of lagging current.