Page 261 - Practical Power System and Protective Relays Commissioning

258 Practical Power System and Protective Relays Commissioning

18.9.3.3.5 Nonlinear Resistance (Metrosils Resistance)
During internal faults of busbars, the high-impedance relay circuit represents
a high burden to CTs, which leads to high voltage on the relay circuit which
can damage the relay, so we use a nonlinear resistance called Metrosil
resistance connected in parallel with the relay circuit as shown in
Fig. 18.9.10, to limit this voltage.

FIGURE 18.9.10 Metrosil (nonlinear resistor) of voltage relay in high-impedance differential
protection scheme.

18.9.3.3.6 Busbar Protection Supervision Relay
When a CT is open-circuited the resultant unbalanced current in the busbar
scheme will flow through the parallel combination of relay, metrosil, fault
setting resistor, and CT magnetizing impedance—this causes the busbar
protection to operate for load or through-fault conditions depending on the
effective primary setting. We use a sensitive voltage operated relay to
operate when there is unbalanced current equal to 10% of the least loaded
feeder connected to the busbars, as shown in Fig. 18.9.11.

FIGURE 18.9.11 Supervision relay in high-impedance differential relay busbar protection scheme.