Question { 4153 }

in a three pin socket y the pin of earthng is longer n
bigger...?

Answer

It helps in two ways:
1. Longer earth pin will first earth the equipment body before the phase is connected to the equipment.
2. Its cross section is larger. As resistance is inversely proportional to area, Lesser is the area of cross section , less is the resistance which helps fault current to adopt this path instead of human body.

Question { 12218 }

Which ciass CT can be use in diff. protection. Can be use
5p20?

Answer

Conventionally only PS class CT shall be used for unit
protections. But now a days, with the use of miro processor
based relays having facility of low impedance or biased
differential protection, 5P20 may be used as biasing in the
relay takes care of the unmatched characteristics of the
both side Cts.

Question { NHPC, 10291 }

protection used for alternator , feeder , transmission line

Answer

GENERATOR:
Directional O/C & E/F,
Differential
Voltage dependant O/C
NPS
Reverse Power
Low fwd power
O/C
E/F
Standy By E/F
V/F
100 % Stator EF
Over Freq
Under Freq
Over Vtg
Under Freq
Rotor EF

TRANSMISSION SYSTEM
Distance Prot.
Local Breaker Bkp
Pole discrepancy
O/C
E/F

FEEDER PROT.

O/C
E/F
Directinal O/C & E/F

Question { 13240 }

what is the difference between potential Transformers and
capacitive voltage transfoemers and where it is used?

Answer

The purpose of VT and CVT in EHV system is same ie. to step
down the EHV to low voltage for measurement and protection
purpose. The extra function which CVT performs is to trap
the high voltage signals from the line which are used for
PLCC communication.

Question { DLF, 52474 }

why Batteries & Battery chargers are used in substation?
what is the different between UPS & battery charger?

Answer

In lieu of a more reliable power supply source, DC supply
is used in place of normal AC supply. If there is a
blackout, AC power will not be available n the electrical
protection devices will not operate. To avoid any such
situation, DC supply is used. DC supply is provided by
batteries and charger is used to keep the batteries healthy
by trickle charging the batteries in Float / Boost mode as
required. UPS is an uninteruptable power supply which uses
converter inverter set. AC input is first converted to DC
and again converted to AC. The battery bank installed for
UPS is connected in between converter inverter set so that
if AC supply to converter fails, battery comes on load and
supplies DC to inverter for maintaining the AC power supply
at output end.

Question { 9331 }

The capacity of the transformer is 20/25 MVA, Ratio is
132/11 KV, Vactor group YNyno. and impedance is 10.07%. As
per the calculation the secondary current is 1312.2Amps.
The maximum fault current the transformer should sustain is
13.030KA. How many times the transformer should sustain the
maximum short circuit current of 13.030KA over a period up
1 Year. The transformer is repaired one & 10 Years old.

Answer

Failure / service life of the transformer depends on various factors like continuous loading of the transformer, the health of the insulation, type of oil etc. Moreover, the shortcircuits in the electrical system is not a daily phenomenon hence there is no mathematical formula to calculate transformer life.

Question { 3526 }

How does the sensitive earth fault protection provided by cbct
for motor can help in reducing the size of cable used to supply the motor?

Answer

In my opinion, using CBCT do not help in reducing the cable size.

Question { 6989 }

Two alternators say 1 & 2 are poised to parallel operation.
But at the time of paralleing alternator 2 gets out of the
system by tripping Earth fault relay. Please send me reply
why this is happening?

Answer

In addition to the above answer, what type of earthing is
provided for both the generators also matters. Secondly, if
it is NGR, which NGR is closed, or both are closed.

Question { 7207 }

What is the advantage of Neutral graunding transformer
insted of Neutral graunding Resistance in generating
station explain briefly.

Answer

NGR and NGT serves the same purpose of limting the earth
fault current to the rated values which basically depends
upon the core damage curve of generator. Differece is NGT
gives earth fault protection of 100 % winding whereas NGR
protects only 95% of the generator winding. Potential
difference of the 5% winding (towards neutral) with respect
to neutral is very less so the earth fault in that region
of the winding is not sensed. This is avoided in NGT using
sensing of 3rd harmonic under voltage based principle in
secondary of the NGT.

Question { Lucas TVS, 31383 }

what is the advantages of Lighting transformer?.. is it
possible to reduce the power consumption ?.

Answer

Lighting transformer is a small transformer with taps to
adjust the output voltage. If we reduce the voltage, it
gives a big amt of power saving till some amt of vtg
reduction. In this way lighting tr helps in power saving.

Question { 94919 }

What is the difference between XLPE & FRLS cables? Is FRLS
cables is a type of XLPE cable or both r different?

Answer

XLPE means Cross Linked Poly ethylene cables which may be
FRLS or ordinary. FRLS are fire resistant cables which are
used at the locations which are more prone to fire.

Question { Reliance, 19480 }

What is the basic difference between 90% stator earth fault
and 100% stator earth fault, and its phenmenona?

Answer

As neutral of the generator is approched, the driving
voltage available between the fault point and the neutral
is very low & hence not detectable. So last 5% of the
generator winding remains nprotected with the conventional
schemes of E/F protection. that is why it is called 95%
stator earthfault protection. For covering the rest of 5%
winding other techniques like 3rd harmonic principle ets
are available.

Question { NBCC, 3078 }

What is Partial Discharge test .Please specify that this
test can be test on 11/0.433 KV,630 KVA Transformer.

Answer

PD is localised discharge which happens within the insulation because of small air pockets formed during winding insulation is being done in workshop. A Ultra high frequency signal is injected at the terminals which detects the frequeny and magnitude of electrical dischages. These values halps us in dentifying healthiness of insulation of the equipment.