Page 275 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 269

FIGURE 18.11.3 Breaker failure scheme.
current detector contact (OC) closes if the current is still passing, which
means that when breaker 1 fails to trip after time T expires, the BF relay
also sends a retrip to breaker 1 and a trip signal to trip breaker 2 and breaker
3 to stop the fault current flow and send a trip signal to the remote end of
the line if this end feeds the fault.
BF relay time setting is set in the range of 110 180 ms, some utilities
are at set 125 ms and others at 140 ms. This will be explained in the BF set-
ting chapter of this book (Sub-chapter 19.7.3).
The total fault clearing time of BF relay is about 260 300 ms, as
explained in the BF setting chapter of this book (Sub-chapter 19.7.3).

Subchapter 18.12

Transformer Protection

18.12.1 INTRODUCTION
Transformer faults can be classified as follows:
Winding and terminal faults;
Core faults;