Question { 13332 }

why neutral of 6.6 kv star connected sq. cage induction
motor does not get earthed, though for earth fault
protection it is must.

Answer

Hi Pratikjain,
The answer in #4 is partly correct. Your question is on the
motor and not on the feeder. Provide appropriate cts and
wire correctly using an over-current earth fault relay.
Note, this relay has three operating elements known as red,
yellow and blue-phase elements. The yellow-phase element is
for earth fault. This relay works on the principle that Ir
+ Iy + Ib = 0 for a balanced load. This summation is
algebraic. In times of an earth fault the above equation is
no longer equal to zero hence the residual current flows
through the yellow-phase element as an earth fault provided
your cts and relay are properly connected. In this way the
cb for the motor is tripped by the yellow-phase element as
an earth fault if the relay is properly set.

Question { 10575 }

i have 28/32 MVA power transformer with 11/6.6 KC voltage
rating and 12%impedance.it have star connection at both
side.but it have another thied winding tertiary
winding.what is used of this winding.

pl tell me

Answer

Hi Paliwalsureshi,
Makers of such transformers usually provide a tertiary
winding connected in delta. This winding is sometimes called
compensating winding. As others have rightly said this
winding help to eliminate the harmoics which may interfer
with communication in power system. Before the advent of
microprocessor-based differential relays the old
differential scheme with star-star on one must have a delta
winging in between.In a nutshell any star-star connection
must have a delta tertiary.

Question { Power Grid, 100094 }

What core magnetic balance test of transformer?
How to do this test in a three phase Star delta (star-LV,
Delta-HV) transformer?
How to measure and where to measure?

thanks and regards
Austin

Answer

Hi,
You may use either single-pase on the star side of the
transformer as in Answer #2 or apply two line voltages i.e.
apply 415V on A and B terminals on the HV-side of the
transformer and follow the steps of Rajukumar Sripelli in
Answer #5.

Question { 9888 }

if step down transformer step down the voltage,will current
will also be proportional step up. If yes then how
transformer handle such a large current?

Answer

Transformer works on the principle that,
I1V1 = I2V2 at all times.
where I1V1 are primary current and voltage respectively;
,, I2V2 are the corresponding secondary current and
voltage respectively. From the above equation, if you step-
down the voltage the current must increase to maintain the
equality and vice versa. Based on this principle, the
makers of transformers ensure that the gauge of copper
wire used in the LV-winding can withstand the current
together with tolerance.

Question { 9888 }

if step down transformer step down the voltage,will current
will also be proportional step up. If yes then how
transformer handle such a large current?

Answer

Answer #2 by Phanendra Babu is correct in a way but is not
related to the answer for the question. The question is all
about what happens to current when voltage is decreased by
transformation.
Using Babu's answer; MMF = IN where I is current and N is
the number of turns i.e. I1N1 = I2N2. This equation does not
satisfy the question.Therefore, the relevant equation is
I1V1 = I2V2.
From this equation, you can now understand the relationship
between voltage and current.

Question { Power Grid, 12202 }

IN bateery size of positive is biger than negative .why?

Answer

The bigger size of the battery positive terminal is to make
it easily differentiable from the negative which by design
is smaller. Definitely, the positive terminal is at a
higher potential than the negative.

Question { HAL, 16483 }

What is the difference btwn grounding and earthing?

Answer

Both are the same. It is a matter of choice between the
two. For example, on a 132/33kV star-delta T/F the zig-zag
T/F on the delta side may be referred to as earthing T/F or
grounding T/F.

Question { 4361 }

In a distribution system,having 3phases & 1 nuetral.ecah
phase to neutral is 230V,line-line voltage is 440V,but what
is the voltage of each line.but it should obey the KVL law.

Answer

Hi,
Voltage by definition is the potential difference between
two points e.g phase-neutral is 230V and line-line is 440V
according to you. You cannot measure the voltage of a line
alone. If, however, you insist on measuring the voltage of
the line alone your result will be zero. It must be between
the line and something else.

Question { Engineers India Limited, 6514 }

A 25MVA 3300/400V T/F is termed as per winding is ……
a) primary 3300V & Secondary 400V
b) primary 3300V & low voltage winding 400V
c) High voltage winding 3300V & Secondary 400V
d) High voltage winding 3300V & low voltage winding 400V

Answer

Hi,
The T/F will be termed as 25MVA 3.3/0.400kV delta-star T/F.
The interpretation is that the primary is 3.3kV while the
secondary is 0.400kV.

Question { 6130 }

why bucholtz relay placed inclined in transformer?

Answer

Hi
The bulb of the Buchholz relay contains liquid mercury which
closes the alarm or alarm/trip contacts whenever the relay
is activated. Note that the liquid mercury closes the
alarm contacts only if the gas flow through the relay is
not serious. However, if the flow is serious both alarm and
trip contacts will close to give alarm and trip signals.
From the above explanation you can understand that liquid
mercury being a good conductor must be kept away from the
contacts under normal condition. The only way to do this is
by tilting the relay.

Question { 7527 }

For polarity test on a 11/3.3kV 2.5MVA delta-star Unit
Auxiliary transformer,415V 3-phase supply was applied at
the HV. Connection between Aa was made with a 2.5mm cable.
As soon as power was switched on the cable caught fire and
burnt. This test was then abandoned. It is noteworthy to
say that a similar cable was also used in a similar
transformer for the same. The cable did not burn and test
was successfully completed with good result. What is wrong?
Please, help me out.

Answer

Thanks to the respondent.I would like to tell him that no
load was connected besides linking Aa in accordance with
the method for the test. Also, note that similar 2.5mm2
cable has been used on a similar Unit Auxiliary 2.5MVA
11/3.3kV delta-star T/F and 60MVA 132/11kV star-delta T/F
respectively. No burning occurred. Test results were
satisfactory. This piece of information should guide any
contributor. Thanks.

Question { APGenco, 114702 }

What is the major difference between P.T. & C.V.T ? Why CVT
is used in HT Transmission line in place of P.T.?

Answer

Hi Sharma,
Answer #2 is correct but let me add more to it. For
voltages like 11 and 33kV the voltage transformers may be
referred to as P.Ts unlike 132kV etc where CVTs are used.
For either 11 or 33kV the magnetic part with iron core is
not bulky hence are used directly to step down 11 or 33kV
to say 110V used for protection and metering.
In the case of CVTs series capacitors are used to drop down
(note, not stepping down) the voltage i.e.like a voltage
divider from 132kV to a manageable value that can be
transformed like the 11 or 33kV P.T. Take a careful look at
the CVT you will find out that the bottom part is the
transformer part. That part may be transforming say 13kV to
110V. If 132kV were to be transformed directly to 110V then
the voltage transformer or P.T as it were will be so bulky
that there may be little space for other devices. Both are
for the same purpose but the use of each depends on the
voltage level.

Question { 130051 }

What is the difference between a Junction Box and a
Marshalling Box?

Answer

Hi Cindy,
Junction boxes are used in domestic wirings especially when
supply is to be taken through a switch to a lamp holder or
from one room to another.
Marshalling boxes better called Marshalling Kiosks(MKs) are
used in substation switchyards and in power transformers.
For switchyard MKs they contain terminal blocks to which
control cables are connected. They may also contain mcbs
and other devices. The same is true for transformer MKs.

Question { 12166 }

Why earthing transformer used in Distribution Sub-Station?

Answer

Hi questioner,
Earthing T/F or T/Fs are not required in distubution S/S.
For example, here in Nigeria the distribution volage for
the T/F is either 11/0.415kV or 33/0.415kV detla-star.
However, in Transmission S/S where there are 132/33kV star-
delta connected T/F or T/Fs earthing T/F or T/Fs are
necessary. The earthing T/F is connected on the 33kV side
of the main T/F while the star-point of earhing T/F is
connected to ground to provide an artificial star-point for
the main T/F. This so because 33kV distribution ground-
fault current must flow back to the main T/F. Earth fault
protection for the main T/F must sense this current and
isolate the main T/F otherwise the earthing T/F will roast
i.e. burn.

Question { 12166 }

Why earthing transformer used in Distribution Sub-Station?

Answer

Further to my answer,note that a ground fault at the
distribution S/S must be sensed fast by the relay so as to
trip the disribution cb before the cb for the 132/33kV in
the Transmission S/S is trip by its relay as a back-up.