Page 277 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 271

18.12.2 TRANSFORMER OVERLOAD PROTECTION
Excessive overloading will result in deterioration of insulation and after that

lead to failure. Transformer normal operating temperature is 98 C and is

equal to the transformer winding and cooling medium temperature of 78 C

plus an ambient temperature of 20 C. This is the normal operating tempera-
ture, which does not affect the transformer service life. By using a thermal
replica impedance relay incorporated with CT input the transformer, which
has the same time constant as the transformer winding, can be protected
against overloading condition.

18.12.3 TRANSFORMER OVERCURRENT PROTECTION

Applied to small transformers alone, and used as backup protection for large
transformers, both instantaneous and time-delayed OC relays can be applied.
Inverse time OC relays are applied on the high voltage (HV) side of
transformers and graded with those on the low-voltage (LV) side, which in
turn must be graded with LV outgoing circuits.
High-set overcurrent relays can be used and set at 120% 130% of the
through-fault level of the transformer.
Two stages of OC are used, the first stage trips the LVCB and the second
stage trips the HVCB, as shown in Fig. 18.12.1.
An instantaneous relay (short circuit element) is used to provide a fast
trip for the HV side.

FIGURE 18.12.1 Overcurrent relays for transformers.