81O/U, 81R, 78V - Frequency Protection - Eaton EDR-5000 Nstallation, Operation And Maintenance Manual

81O/U, 81R, 78V - Frequency Protection

Available elements:
81[1] ,81[2] ,81[3] ,81[4] ,81[5] ,81[6]
Frequency - Measuring Principle
The measuring principle of the frequency supervision is based in general on the time measurement of complete cycles,
whereby a new measurement is started at each zero passage. The influence of harmonics on the measuring result is
thus minimized.
Frequency tripping is sometimes not desired by low measured voltages which, for instance. occur during alternator
acceleration. All frequency supervision functions are blocked if the voltage is lower 0.15 times Vn.
All frequency protective elements are identically structured.
The frequency is calculated as the average of the measured values of the
three phase frequencies. Only valid measured frequency values are taken
into account. If a phase voltage is no longer measurable, this phase will be
excluded from the calculation of the average value.
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