ABB RELION REX640 Technical Manual page 724

Protection and control

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Table of Contents

Protection functions

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.

• 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 saturation at high currents flowing through the generator

1MRS759142 F

Technical Manual