Page 237 - Practical Power System and Protective Relays Commissioning

236 Practical Power System and Protective Relays Commissioning

the network, they generally have time to become stabilized before a reclosure.
There are somewhat greater chances of a successful reclosure with three-phase
DAR than with only high-speed autoreclosing. When a line has been tripped,
the voltage in the network increases or decreases owing to the change in the
load. Transformer tap changers may then start to operate with the risk of being
damaged, if reclosing on to a short circuit occurs during the switching process.
The delay in the voltage regulation equipment should therefore be checked.
This delay should be longer than the system disturbance time. Several reclos-
ing shots may be justified in the case of, for example, radial lines. However,
the advantages of the increased probability of a successful reclosure must be
weighed against the disadvantages in the form of greater damage at the site of
the fault and higher breaker maintenance costs.

Subchapter 18.7

Generator Protections

18.7.1 DIFFERENTIAL PROTECTION

This main protection of stator winding compares current values at the end
of each phase winding and operates when a predetermined percentage dif-
ference is detected. This protects the winding against earth and phase to
phase faults. This protection can be of a high-impedance type or low-
impedance biased type, as shown below in Fig. 18.7.1.
There is a scheme called overall differential protection, which can protect
the generator unit, main transformer, and also the unit transformer, as shown
in Fig. 18.7.2.

18.7.2 STATOR EARTH FAULT PROTECTION (90%)

This system protects 90% of generator winding, as shown in Fig. 18.7.3.
For unearthed generators:
51 N time delayed;
50 N instantaneous only for large units;
Relays must not operate for residual current (I Resd ) caused by CT output
summation error and internal capacitive current of external earth faults.
For earthed generators:
51 N must be graded with other earth fault relays on the power system;
51 N time delayed set to 5% of generator resistor rating;
50 N instantaneous only for large units set to 10% of generator resistor rating.