Page 278 - Practical Power System and Protective Relays Commissioning

272 Practical Power System and Protective Relays Commissioning

18.12.4 TRANSFORMER EARTH FAULT PROTECTION
Restricted earth fault protection is used to provide instantaneous earth fault
protection to the transformer, utilizing high-impedance principal. As shown
in Fig. 18.12.2 below, balanced earth fault for a delta (or unearthed star)
winding can be provided by connecting three line CTs in parallel (residual
connection). The relay will only operate for internal earth faults since the
transformer itself cannot supply zero-sequence current to the system. The
transformer must obviously be connected to an earth source, as shown in
Fig. 18.12.2.

FIGURE 18.12.2 REF protection for transformers in star connection and balanced EF protec-
tion for transformers with delta connection.

On resistance-earthed power systems, unrestricted earth fault protection is
referred to a standby earth fault protection of two stages of type inverse time
relay, which match the thermal characteristic of the earthing resistor.
In practice, two EF relays with stage one set at 15% of full load current
of transformer with a 5 seconds time delay and the send stage set to 20% of
full load current of the transformer with an 8 seconds time delay, the first
stage trips the LVCB and the send stage trips the HVCB.
Typical settings for REF protection are:

Solidly earthed 10% 60% of winding rated current
systems
Resistance-earthed 10% 25% of minimum earth fault current for fault at
systems transformer terminals

18.12.5 BUCHHOLZ PROTECTION

All types of fault within a transformer will produce heat, which will cause
decomposition of the transformer oil. The resulting gases that are formed
rise to the top of the tank and then to the conservator. A Buchholz relay