ABB REM 610 Technical Reference Manual

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

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ABB REM 610 Technical Reference Manual Manual pdf 137 page image

137However, an ground-fault current may affect the unbalance measurement.Therefore, it is recommended that the unbalance protection is used to protect themotor against single-phasing only.A070089Fig. 6.1.6.3.-1 Connection with two phase current transformers6.1.7. Ground-fault protectionIn solidly or low-resistance grounded networks, the ground-fault current can bederived from the line CTs, provided that the CTs have been residually connected. Inthis 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-faultelement can be set to be disabled when one or several phase currents exceed theFLA of the motor four, six or eightfold. This can also be done to ensure that theground-fault protection will not trip even though the line CTs would partiallysaturate during a motor start up. The pickup value of the ground-fault element istypically set to 15...40% FLA.A core balance transformer is recommended for isolated neutral networks and highresistance grounded networks. The use of a core balance transformer makes theground-fault protection very sensitive and the variations in the load current will notaffect the ground-fault measurement. Consequently, a relatively low pickup valuecan be selected in high resistance grounded networks.The transformation ratio of the core balance transformer can be freely selectedaccording to the ground-fault current, and consequently, the sensitivity of theground-fault protection as well. Due to the extremely low burden of the relay, lowtransformation ratios may be used in cable current transformers, in KOLMA typetransformers even as low as 10/1 A. However, a transformation ratio of at least 50/1A or 100/1 A is recommended.Motor Protection RelayTechnical Reference Manual - ANSI VersionREM 610REM 6101MRS755537