Page 279 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 273

connected between the tank and conservator collects the gas and gives an
alarm when a certain volume of gas has been collected. A severe fault causes
so much gas to be produced that pressure is built up in the tank and causes a
surge of oil. The Buchholz relay will also detect these oil surges and under
these conditions is arranged to trip the transformer CBs. The main advantage
of the Buchholz relay is that it will detect incipient faults, which would not
otherwise be detected by conventional protection arrangements. The relay is
often the only way of detecting interturn faults, which cause a large current
to flow in the shorted turns but due to the large ratio between the shorted
turns and the rest of the winding, the change in terminal currents is very
small.

18.12.6 DIFFERENTIAL PROTECTION
The function of differential protection is to provide faster and more discrimi-
native phase fault protection than that obtainable from simple overcurrent
relays. CTs on the HV side are balanced against CTs on the LV side. There
are a number of different connections but there are some important points
that are applicable to all schemes.

18.12.6.1 Transformer Connections
Consider a delta/star transformer. An external earth fault on the star side will
result in zero-sequence current flowing in the line but due to the effect of the
delta winding there will be no zero-sequence current in the line associated
with the delta winding. In order to ensure stability of the protection this
zero-sequence current must be eliminated from the secondary connections on
the star side of the transformer, that is, the CTs on the star side of the trans-
former should be connected in delta. With the CTs on the delta side of the
transformer connected in star, the 30 phase shift across the transformer is
also catered for. Since the majority of faults are caused by flashovers at the
transformer bushings, it is advantageous to locate the CTs in the adjacent
switchgear.
There are a general rules for CT connections:
1. CT connections opposite to main power transformer connections, that is,
star CTs on delta side and delta CTs on star side;
2. If similar primary terminals, that is, P1 or P2, are toward the transformer,
then delta and star connection for the CTs should be the same as for the
transformer (or 180 degrees opposite);
3. It is usual to assume that if current flows from R1 toward R2 then the
secondary current will flow from a2 towards a1 (refer to Fig. 18.12.3).