Micom P721, Техническое руководство

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P72x/EN AP/C21 Application Guide
Page 20/52 MiCOM P721/P723
P1524ENc
FIGURE 17: RESTRICTED EARTH FAULT PROTECTION OF A 3 PHASE, 4 WIRE
SYSTEM APPLICABLE TO STAR CONNECTED GENERATORS OR POWER
TRANSFORMER WINDINGS WITH NEUTRAL EARTHED AT THE
GENERATOR/TRANSFORMER STARPOINT.
3.2 Three phase applications
For busbar protection, it is considered good practice by some utilities to set the minimum
primary operating current in excess of the rated load such that if one of the current
transformers becomes open circuit, the high impedance relay does not maloperate. Other
utilities prefer to set the relay as sensitive as possible and use a second check relay
connected to a separate set of current transformers to provide security against maloperation.
It is desirable that the primary operating current does not exceed 30% of the prospective
minimum fault current. The primary operating current should also be 110 to 130% the
maximum feeder circuit loading.
For generator and motor protection, the relay is generally set as sensitive as possible to
maximize fault coverage. In these applications it is important to recognize that the external
fault level contribution from the generator or motor may be significantly lower than the
internal fault level. The external fault level should be used for the calculation of stability
voltage and resistor requirements whereas the internal fault level (that includes system
contribution) should be used for non-linear resistor calculations.
FIGURE 18 shows the general diagram when the P72x is used in three phase applications: