17
Section 4
APPLICATION NOTES
4.1
General
The type MBCH relay is a high speed biased differential relay suitable for the
protection of two or three winding power transformers, autotransformers, or
generator transformer units.
Three versions of the relay are available as follows:
Designation
Number of bias inputs
Application
MBCH 12
2
Two winding transformers or three
winding transformers where
significant fault infeed can pass
through the transformer from one
winding only. See Figure 7.
MBCH 13
3
Generally three-winding transformers
where bias is required from each of
the three groups of CTs or;
Where a two-winding transformer has
one or other of the windings
controlled by two circuit breakers as
in mesh or one-and-a-half breaker
arrangements. See Figure 6.
MBCH 16
6
For applications requiring 4, 5, or
6 bias circuits. See Figure 8.
4.2
Matched line current transformers
For optimum performance, the differential scheme should be arranged so that the
relay will see rated current when the full load current flows in the protected circuit.
Where line current transformers are matched, but secondary current with full load
current flowing is less than the relay rated current (as illustrated in Figure 9), the
effective sensitivity of the relay will be reduced.
The transformer current is 262.4A at 66kV, giving a secondary current of 4.37A
from the 300/5A current transformer. For a 20% relay setting, the relay will operate
when the differential exceeds 0.2 x 5 = 1A.
1
4.37
Thus the effective setting is 22.9% for a relay setting of 20%.
1A =
x 100% = 22.9% of transformer full load current.
