Page 230 - Practical Power System and Protective Relays Commissioning

230 Practical Power System and Protective Relays Commissioning

18.6.9 SWITCH ON TO FAULT FEATURE IN DISTANCE RELAY
This feature in distance protection is activated after manual closure of a CB
and is designed to bypass the distance relay measuring element and switched
to a high-speed overcurrent function in case there is a close-up three-phase
fault near the relay in the local busbar of the relay, as in practice
maintenance people forgot the local three-phase earthing on the line under
maintenance inside the substation, as in this case the relay voltage will be
near zero and the fault is a three-phase close-up fault.
The criteria for detecting the switch to fault (SOTF) condition are by
voltage-level detector and current-level detector, for example, setting these
two detectors is as follows:
V ph , 75% V n and I ph . 5% In, as V n is the nominal voltage and I n is the
nominal current of the system the SOTF will be energized for about 500 ms
and then reset after this time. If the conditions above are detected during
manual closure of the breaker during this time the SOTF will trip the line
instantaneously.

18.6.10 STUB PROTECTION FUNCTION

There are short sections of the current path within a substation that are not
properly protected by the main protection system. These sections are called
stubs or blind spots protection or minimum zone protection, and they are
usually between the CB and the CT. The main protection system measures
the current of the CT and, if a fault is detected, a command is generated to
open the CB. If, however, the fault is between the CB and the CT, then
opening the CB cannot clear the fault; it is fed via the CT from the other
side of the protected object. This location is within the back-up zone of the
other side protection and, accordingly, it is cleared by a considerable time
delay. The task of the stub protection function is to detect the fault current in
the open state of the CB and to generate a quick trip command to the
other side CB. Another usual application is in the one-and-a-half CB
arrangement. Here the CTs are located either before or after the CBs.
Additionally, the VT is either on the bus side or on the line side of the
isolator. In the last case, the stub is also the section between the CBs and
the open line isolator, since if a fault occurs in this section, the detected
voltage is independent of the fault; it is unchanged and cannot be applied
for the distance protection. The stub protection function is basically a
high-speed overcurrent protection function that is enabled by the open
state of a CB or may be an isolator. If any of the phase currents is above
the start current and the binary status signal activates the operation, then
after a user-defined time delay the function generates a trip command.
The function can be disabled by programming the blocking signal in new