Question { Siemens, 44377 }

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

Answer

T the bias force produced due to the restraining coil greater than the operating force produced by the operating coil is relay is inoperative. When the internal fault occurs, the operating force becomes more than the bias force. Due to this, Beam moves and the trip contact sir closer to open a circuit breaker

Question { Welspun, 41953 }

How can we calculate ngr resistance for a generator and
Transformer ?
What type of transformer requires ngr ?

Answer

It is similar to the ground resistor calculation for transformer

The typical example was answered earlier for transformer dt.19-06-2009

Neutral of transformer can be grounded solidly earthed OR with Neutral Grounded with Resistance.

Typical shunt calculations fro 5 MAV 11/6.6 kv transformer neutral are as :_

Transformer rating = 5 MVA

Voltage ratio = 11/6.6 KV

Vector Group = Dyn11 (6.6 KV ground through Resistor)

During Earth fault voltage between Neutral

Question { 10750 }

What is basic impulse insulation level? And how it is calculated for sub-
station?

Answer

The Basic Impulse Insulation Levels refers to the ability of electric equipment such as transformers to withstand lightning surges. When lightning strikes a transmission line, a traveling wave is created. This traveling wave travels along the line and damages the transformer winding.

Surge arresters mounted on the line can mitigate the surge, however, they cannot totally eliminate it. Voltage surges can also be created by the switching of circuit breakers and switches.

The BIL or the Basic Impulse Insulation level indicates the ability of the transformer to withstand these heavy surges. Transformers with a rating of 600 volts and below are designed to have a BIL of 10 kV.