Toshiba GRZ100 B Series, Instruction Manual

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Figure 2.1.5.2 Impedance Locus during Out-of-Step Condition
In the case of a full out-of-step condition (as opposed to a transient power swing) it is desirable to
separate the system in the vicinity of the centre of the out-of-step condition. GRZ100 provides an
out-of-step detection element (OST) which can provide tripping in these circumstances.
Although the power swing and out-of-step conditions are very closely related (in fact one is
simply the most severe form of the other), completely different actions are required from the
protection relay. The PSB function must ensure stability of the distance protection during transient
power system conditions, while the OST element initiates system separation by tripping in the
event that a severe power swing results in potentially irrecoverable loss of stability in the power
system. The PSB and OST elements are therefore completely separate functions within the
GRZ100 relay, with different characteristics, separate scheme logic and different settings.
2.1.6 Redundant Configuration of Protection Relay and Improvement of Reliability
The protection relay is expected to operate correctly without fail when a system fault occurs and is
required to have a high reliability. As long as high quality components are used and quality
assurance followed during manufacture, the probability of a defect is low. However, as an option
even further security can be provided to avoid the consequences of a hardware failure by
providing redundancy in the protection relay configuration.
Undesirable phenomena in a protection relay include "mal-operation mode" whereby the relay
operates erroneously when the power system is healthy or in the event of an external fault, and
"failure-to-operate mode" whereby the relay fails to operate for a power system fault. To guard
against the first mal-operation mode, a system that provides redundancy for hardware and issues a
trip command under conditions of ANDing of two or more results is effective. In the GRZ100
relay a fault detection relay can be provided for this purpose as an option. The fault detection relay
consists of a combination of an undervoltage relay and overcurrent relay based on a simple
principle. It also has a simple hardware configuration. Since the trip command of the circuit
breaker is executed under conditions of ANDing of the outputs of the main detection relay that can
exactly identify the faulted section and the fault detection relay that checks the occurrence of a
fault only, even if a hardware defect occurs in either element, the other element prevents tripping.
Furthermore, since the output is made from only one side of the main relay or fault detection relay
for a set time, this system makes it easy to detect a hardware defect and issue an alarm. The fault
detection relay has a simple hardware configuration, and thus for only a small additional cost it is
possible to dramatically improve the reliability in preventing mal-operations.
In the case of the latter failure-to-operate mode, the aforementioned backup protection functions.
Furthermore, a duplicated protection system is also available to ensure reliable operation.
X
R
Load Area
Distance protection
characteristic (Mho)
Impedance locus during
out-of-step condition