Ground-Fault Protection - ABB REM 610 Technical Reference Manual

1MRS755537
6.1.7.
Motor Protection Relay
Technical Reference Manual - ANSI Version
However, an ground-fault current may affect the unbalance measurement.
Therefore, it is recommended that the unbalance protection is used to protect the
motor against single-phasing only.
Fig. 6.1.6.3.-1 Connection with two phase current transformers

Ground-fault protection

In solidly or low-resistance grounded networks, the ground-fault current can be
derived from the line CTs, provided that the CTs have been residually connected. In
this case, the operate time of the ground-fault element is typically set to be short, e.
g. 50 ms.
To avoid damaging the contactor in a contactor controlled drive, the ground-fault
element can be set to be disabled when one or several phase currents exceed the
FLA of the motor four, six or eightfold. This can also be done to ensure that the
ground-fault protection will not trip even though the line CTs would partially
saturate during a motor start up. The pickup value of the ground-fault element is
typically set to 15...40% FLA.
A core balance transformer is recommended for isolated neutral networks and high
resistance grounded networks. The use of a core balance transformer makes the
ground-fault protection very sensitive and the variations in the load current will not
affect the ground-fault measurement. Consequently, a relatively low pickup value
can be selected in high resistance grounded networks.
The transformation ratio of the core balance transformer can be freely selected
according to the ground-fault current, and consequently, the sensitivity of the
ground-fault protection as well. Due to the extremely low burden of the relay, low
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.
REM 610 REM 610
A070089
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