ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. B
4-42
4.2.7. Breaker failure protection
4.2.7.1. Introduction
This function provides backup protection to clear a fault after
being enabled by the unit protection for the case that the circuit-
breaker (CB) should fail. It has to operate as quickly and reliably
as possible especially on EHV systems where stability is crucial.
To this end, current detectors continuously monitor the line cur-
rents and if they do not reset after a preset time, which allows for
the operating times of the unit protection and the circuit-breaker,
a tripping command is issued to either attempt to trip the same
circuit-breaker again or trip the surrounding circuit-breakers.
Resetting of current detectors is influenced by the following fac-
tors:
· Even after the main CB contacts open, the current does not
immediately drop to zero, but to a level determined by the
fault resistance and the resistance of the arc across the open
CB contacts. The current only becomes zero after the de-
ionisation time of the CB arc.
· The pick-up setting of the detector.
· The fault level prior to operation of the CB.
· Whether the main c.t’s saturate. If a c.t. saturates, its secon-
dary current may not pass through zero at the same time as
its primary current and if the primary current is interrupted at
zero, the c.t. flux may be at some positive or negative value.
The secondary current therefore decays through the burdens
of the relays thus increasing the reset time.
The resetting time varies typically between 20 and 30 ms.
Since for the above application, the current detectors should re-
set as quickly as possible, Fourier filter algorithms are included
to minimise the affect of c.t. saturation and eliminate completely
or substantially any DC offset.
The block diagram below shows the basic functions, which are
explained in the following Sections.
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