ABB RELION 620 Series Technical Manual page 436

Section 4
Protection functions
430
consecutive fulfillments of the condition. When the condition is not fulfilled, the counter
is decreased (if >0).
Waveform blocking
The biased low stage can always be blocked with waveform blocking. It cannot be
disabled with Restraint mode. This algorithm has two parts. The first part is intended for
external faults while the second is intended for inrush situations. The algorithm has
criteria for the low-current periods of differential current during the inrush and external
fault.
Reset of the blocking
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) of the compared currents is close to
zero (in normal situation, 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
Differential current caused by CT errors and tap changer position increases at the same
percent ratio as the load current increase.
In the protection of generators, the false differential current can be caused by:
• CT errors
• CT saturation at high currents flowing through the generator
1MAC504801-IB E
620 series ANSI
Technical Manual