Toshiba GRZ100 B Series, Инструкция по эксплуатации

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2.1.3 Main Protection and Backup Protection
The power system protection system generally consists of a main protection and a backup
protection to reliably remove all faults. In principle, system faults must be removed in the shortest
possible time and cause the minimum outage. This important function is served by the main
protection. In distance protection, this function is served by the zone 1 element and command
protection, etc. However, the main protection may not always function perfectly. For example, the
main protection relay may not be able to function correctly due to a power supply failure, CVT
failure, data transmission device failure, circuit breaker failure or failure of the main protection
relay itself. In such cases, power system integrity depends on the backup protection.
The backup protection provides power system protection with a set time delay, its timer value is
set in a range that allows coordination with the main protection. To achieve time coordination with
the main protection, the time delay of the backup protection is determined with a margin in
consideration of the following factors:
•
Operating time of main protection relay
•
Operating time of circuit breaker
•
Reset time of backup protection relay
There are two types of backup protection: remote backup protection that provides backup from a
remote substation at a different location to the main protection, and local backup protection
installed in the same location as that of the main protection that provides backup from that
substation.
Each of these protections has the following features:
Remote backup protection: Possible causes for main protection failures include relay faults,
power supply faults, and various other factors. It is therefore
important to provide backup protection from a remote substation to
prevent the backup protection from failing due to the same causes as
the local main protection. The zone 2 and zone 3 elements of distance
relays, etc. provide as these remote backup protection functions.
Local backup protection: Provides backup protection at the same substation as that of the main
protection and often has the purpose of providing backup when the
circuit breaker fails to operate.
2.1.4 Distance Relay - General Performance
For distance relays, the reach of the zone 1 protection is usually set to approximately 80 to 90% of
the length of the transmission line. This is to ensure that overreach tripping does not occur for
external faults that occur beyond the busbar at the remote end. For internal faults that occur
beyond the reach of zone 1, time delayed tripping by the zone 2 element is applied. High-speed
tripping can be achieved by means of a "command protection system" that exchanges relay
operation information with the remote end.
There are various causes for measuring errors in a distance relay. In the case of a fault with
resistance, the reactance component seen by the relay at the power sending terminal is smaller than
the actual value and it tends to overreach. On the contrary, the reactance component seen by the
relay at the power receiving terminal is greater than the actual value and it tends to underreach.
The line impedance has different values in different phases. When its average value is used for the
relay setting, underreach will occur in a phase with a greater impedance than the average value. In
the case of fault resistance, its impedance is greater for earth faults where the fault is grounded via
a steel tower or tree rather than a phase fault consisting of arc resistance only. Therefore,
measuring errors in the earth fault relay are generally greater than those in the phase fault relay.
The fault arc is considered to be almost equivalent to pure resistance. But if the phase of a current
that flows into a fault point from the remote end is different from the phase of the local current, the