ABB Relion REG670, Applications Manual

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If protection for lower frequencies than 9Hz is required (for example, for pump-
storage schemes) four step overcurrent protection with RMS measurement shall be
used. This function is able to operate for current signals within frequency range from
1Hz up to 100Hz and it is not at all dependent on any voltage signal. Its pickup for very
low frequency is only determined by main CT capability to transfer low frequency
current signal to the secondary side. However it shall be noted that during such low
frequency conditions this function will react on the measured current peak values
instead of usual RMS value and that its operation shall be instantaneous (that is,
without any intentional time delay). Function can be used either as normal overcurrent
function or even as generator differential function when currents from two generator
sides are summed and connected to the four step overcurrent function. Typically for
such installations dedicated overcurrent steps are used during such low frequency
conditions while some other overcurrent steps with different setting for pickup value
and time delay are used during normal machine operation. Such logic can be easily
arranged in REG670 application configuration tool.
Four step overcurrent protection with RMS measurement shall also be used as
machine overcurrent and differential protection during electrical braking. During
such operating condition intentional three-phase short-circuit is made at the machine
terminal. This will effectively force voltages at the machine terminal to zero and
effectively disabled voltage based frequency tracking in REG670. Similar logic as
described for pump-storage scheme above can be used.
1MRK 502 071-UEN -
Section 2
Application
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 37
Application manual