Connection With Two Phase Current Transformers; Ground-Fault Protection - ABB REM 610 Technical Reference Manual

1MRS 755537
5.1.6.3.
5.1.7.
Motor protection Relay
Technical Reference Manual
The operate time of the unbalance element should be set to be shorter than the safe
stall time stated by the motor manufacturer in case a phase is lost.

Connection with two phase current transformers

If two-phase connection is used, it is recommended that a current corresponding to
the sum of these two phases is connected to the input circuit of the missing phase;
see Fig. 5.1.6.3.-1. This has two advantages: the unbalance element does not have to
be set out of operation and the current measurement is more accurate compared to
two-phase measurement.
However, a 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.
I a I b I c
Fig. 5.1.6.3.-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% x 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.
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