ABB REM 610 Technical Reference Manual

Also see for REM 610: Operator's manual Product guide Installation manual Technical reference manual Product guide

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ABB REM 610 Technical Reference Manual Manual pdf 122 page image

1221MRS 752263-MUMMotor Protection RelayTechnical Reference ManualREM 610The operate time of the unbalance stage should be set to be shorter than the safe stalltime stated by the motor manufacturer in case a phase is lost.5.1.6.3. Connection with two phase current transformersIf two-phase connection is used, it is recommended that a current corresponding tothe 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 stage does not have to beset out of operation and the current measurement is more accurate compared totwo-phase measurement.However, an earth-fault current may affect the unbalance measurement. Therefore,it is recommended that the unbalance protection is used to protect the motor againstsingle-phasing only. ( ( (-+#:/0Fig. 5.1.6.3.-1 Connection with two phase current transformers5.1.7. Earth-fault protectionIn solidly or low-resistance earthed networks, the earth-fault current can be derivedfrom the line CTs, provided that the CTs have been residually connected. In thiscase, the operate time of the earth-fault stage is typically set to be short, e.g. 50 ms.To avoid damaging the contactor in a contactor controlled drive, the earth-fault stagecan be set to be inhibited when one or several phase currents exceed the FLC of themotor four, six or eightfold. This can also be done to ensure that the earth-faultprotection will not trip even though the line CTs would partially saturate during amotor start up. The start value of the earth-fault stage is typically set to 15...40% In.A core balance transformer is recommended for isolated neutral networks and highresistance earthed networks. The use of a core balance transformer makes theearth-fault protection very sensitive and the variations in the load current will notaffect the earth-fault measurement. Consequently, a relatively low start value can beselected in high resistance earthed networks.The transformation ratio of the core balance transformer can be freely selectedaccording to the earth-fault current, and consequently, the sensitivity of theearth-fault protection as well. Due to the extremely low burden of the relay, low