Blocking reset
All three blocking signals, that is, waveform and the second and fifth harmonic, have
a counter or time limit which holds the blocking on for a certain time after the
blocking conditions have ceased to be fulfilled. The deblocking takes place when the
counters or time have elapsed. This is a normal case of deblocking.
The blocking signals can be reset immediately if a very high differential current is
measured or if the phase difference (angle) between the compared currents is close
to zero (in normal situations, the phase difference is 180 degrees).
Biased low stage
Biasing of the current differential protection is needed since a possible appearance
of a differential current can also be due to something else than an actual fault in the
transformer (or generator).
In the case of transformer protection, there can be several reasons for the false
differential current.
• CT errors
• Varying tap changer positions (if not automatically compensated)
• Transformer no-load current
• Transformer inrush currents
• Transformer overexcitation during overvoltage situations
• Transformer overexcitation during underfrequency situations
• CT saturation at high currents passing through the transformer
The differential current caused by CT errors and tap changer position increases at
the same percentage ratio as the load current.
In the protection of generators, the false differential current can be caused by
several reasons.
• CT errors
• CT saturation at high currents flowing through the generator
Protection functions
1MRS759142 F
724 REX640
Technical Manual
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