Stabilizing Virtual Earth-Fault Currents; Increasing The Sensitivity Of The Earth-Fault Protection; Circuit-Breaker Failure Protection; Temperature Protection (Optional) - ABB REM 610 Technical Reference Manual

1MRS 752263-MUM
5.1.7.1.
5.1.7.2.
5.1.8.
5.1.9.
Motor protection Relay
Technical Reference Manual
transformation ratios may be used in cable current transformers, in KOLMA type
transformers even as low as 10/1 A. However, a transformation ratio of at least
50/1 A or 100/1 A is recommended.
The start value of the earth-fault stage is typically selected to be 5...30% I
fully developed earth-fault current and the operate time to be 0.5...2 seconds.
If a residual connection is preferred, the start value and operate time must be set
slightly higher in order to avoid possible stability problems due to unbalance in the
main transformers, as the unbalance will cause virtual earth-fault currents during
high phase current conditions. Also, an external stabilizing resistor can be used to
compensate too weak main transformers, thus preventing them from causing
earth-fault currents.

Stabilizing virtual earth-fault currents

An apparent earth-fault current caused by the difference between the phase current
transformers connected in parallel may cause nuisance trippings of the earth-fault
stage, especially during an overload. This can be avoided by using a stabilizing
resistor in the earth-fault current circuit. The continuous power withstand of the
resistor can be 30 W, for instance. The resistance value can be, for instance, 100 Ω
when the 1 A input is used and 10 Ω when the 5 A input is used. The value of the
knee-point voltage must be checked and should be >2 x U
resistor will also slightly reduce the earth-fault sensitivity.

Increasing the sensitivity of the earth-fault protection

The sensitivity of the earth-fault protection can be increased by using a relay
provided with a 1 A input instead of one with a 5 A input. This is possible in a solidly
earthed network as well, because the thermal withstand capability of the current
input is normally high enough.

Circuit-breaker failure protection

The operate time of the CBFP should be set to be longer than the circuit-breaker
opening time + the resetting time of the protection stage with the longest resetting
time, with the exception of the thermal, temperature and phase reversal protection
stages and the external trip.

Temperature protection (optional)

Tripping due to a beginning thermal overload can be avoided by reducing the load
of the motor at an alarm of stage ThA>/ThB>.
REM 610
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