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Note that during secondary injection testing of this feature, it is absolutely necessary to
also inject the voltage signals used for frequency tracking even when a simple
overcurrent protection is tested.
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-UUS A
Section 2
Application
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
41
Application manual
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