Toshiba GRZ100 B Series, Instruction Manual, Page: 13 / 447

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2. Application Notes
2.1 Power System Protection - Basic Concepts
2.1.1 The Function of The Protection Relay
The protection relay, which protects the power system from various faults, plays an extremely
important role in power system stability. Its main functions are as follows:
Prevention of power supply interruption:
Fault clearance and resumption of healthy power transmission as soon as
possible.
Prevention of damage to equipment:
Consecutive system faults will eventually lead to damage to primary
plant, for example destruction of insulators, rupture of lines, burning of
transformers, etc. The protection relay can help prevent such damage to
equipment.
Prevention of system instability:
is necessary to remove Power system faults at high speed by using
protection relays as the existence of a system fault for an extended period
of time may initiate a generator out-of-step condition.
2.1.2 Protection Relay Requirements
The protection relay, which plays the important role of protecting the power system from faults,
must meet several requirements. These requirements can be summarized as follows:
a) Selectivity: All faults that occur on the power system should be removed but at the same time
it must be ensured that only the minimum section of the power system must be
isolated in order to clear the fault. Figure 2.1.2.1 shows typical different
protection zones on the power system. In order to provide complete coverage by
the protection, the neighboring protection zones are set to overlap. Figure 2.1.2.2
shows the relationship between the circuit breaker and CT locations. In Figure
(a), the CTs are installed on both sides of the circuit breaker, one for line
protection and the other for busbar protection, enabling the protection coverage
to overlap. Figure (b) shows the case where the same CT is used for both the line
protection and busbar protection. In this case, the line protection would operate
for a fault which occurred midway between the CT and circuit breaker, but the
busbar protection would not operate, thus failing to remove the fault. It is
important to prevent blind spots in power system protection design.
b) High speed: In order to avoid damage to equipment or power system instability, it is
important to shorten the duration of faults by applying high-speed protection
relays. The GRZ100 has a minimum operating time of 18 ms. However, the
operating time of the circuit breaker and transmission delay in the case of carrier
protection, etc. must also be taken into consideration.