Page 260 - Practical Power System and Protective Relays Commissioning
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Protection Relays Chapter | 18 257
FIGURE 18.9.8 Internal fault in high-impedance differential protection relay.
18.9.3.3.2.1 Through-Fault Stability Limit
The relay has a stability limit, which is the maximum through-fault current
under which the relay will remain stable.
18.9.3.3.3 Fault Setting Resistor
This fault setting resistor is used to increase the effective primary fault current
by creating a shunt resistance across the relay circuit, as shown in Fig. 18.9.9.
FIGURE 18.9.9 Fault setting resistor (R st ).
18.9.3.3.4 Check Zone Feature
As backup protection to confirm the tripping decision, a duplicate of the
primary protection using a second set of CT on all circuits other than the bus
section and coupler units is provided. The check system is arranged in a
similar manner to the primary protection but forms one zone only, covering the
whole of the busbars and does not discriminate between faults in busbar
sections.
