Question { 6851 }

How to make vector group test of transformer?

Answer

vector group testing: Assume that the terminals of the
primary windings are denoted as A, B, C in counter clock
wise direction and for secondary winding as a, b, c also in
counter clockwise direction. short circuit the terminals A
and a, and apply LT supply to the three terminals of HV
(Aa, B, C)
For Dy1 transformer: VbB = VcB and VbC grater than VcC.
For Dy11 transformer: VbC = VCc and VBb less than VcB.
For Yy0 transformer: VBb = VCc and VbC = VBc.

Question { 5178 }

In domestic supply, when neutral touches earth the earth
leakage trips. Does this mean that if the star (neutral)
point of an alternator is earthed that power will flow to
earth i.e. a direct short to earth?

Answer

NO. Normally for distribution purposes the utilities are
using Dy transformers. Hence the zero sequence current
produced by the consumer side fault is not transfered to
the High voltage side.
The earth fault current in the LT side is circulated from
the fault point to the Dy transformer neutral and the
faulted phase.

Question { 5335 }

the distance from my office to d.g is aprox 400mtrs and
voltage drops ibn my office near about 40 volt what should
i do p[ls suggest.

Answer

Friend
You can either shift ur DG near to ur office, or u change
the size of the cable connecting from DG terminal to switch
board, preferably with a copper cable. Present cable size
is very low compared to the distance and the load taken by
the cable, and hence u have a voltage drop of 40V.

Question { 10153 }

How can i calculate knee point voltage of 800/5A Current
transformer?please explain me with suitable calculation?

Answer

For design purposes we can calculate the required knee
point voltage of a CT as follows.
Vknee= (50/In) + (If/(800/5))*(Rct+0.081)
In = CT secondary current (5A)
If is the fault current expected to flow through the CT
Rct - Resistance of the CT secondary.

Question { Siemens, 43500 }

What is the function of restraining coil in differential
relay? as well as restraining current?

Answer

restraining coils are used in the relay for controlling the
sensitivity characteristic of the relay. Differential
relays are highly sensitive relays. restraining coils
prevents the unwanted tripping of the transformer due to
the spill current due to CT saturation effect, or due to
the mismatch current due to CT characteristic.