Measuring Principles - ABB REC 561 User Manual

ABB Network Partner AB
Version 1.0-00

2.3 Measuring principles

Breaker-failure protection
The current is filtered through a specially designed high-pass filter to
obtain the required suppression of the dc components.
High-pass filtering is performed basically for two reasons; to remove the:
• dc component caused by saturated current transformers with a
decaying current due to de-energizing of the secondary circuit. This
is done to achieve a more correct representation of the real current in
the line.
• dc component that is a part of the fault current. This is done to
achieve a correct base for both ASD and RMS calculations.
The frequency limit of the filter is very close to the service frequency, to
obtain a maximum suppression of the above dc components.
The intention of the adaptive signal detection (ASD) concept is to achieve
independence from the absolute filtering requirement, when dealing with
extremely high fault currents in combination with low preset values. This
is obtained by creating a new stabilizing signal to compare the current
with.
The ASD works continuously, regardless of if the BFP was started. Its result
is but considered only when the BFP has started and the pre-set time has
elapsed.
As the current exceeds the previously stabilized sample, it adapts the value
of the current and when it does not, it decays. This adaptive behaviour
makes it possible to rapidly and securely detect a breaker failure situation
after the pre-set time has elapsed.
Continuously and in parallel, the RMS value of the post-filtered signal is
calculated and compared with a preset current level. As the RMS value
decreases below the preset current level, the breaker-failure function is
momentarily reset.
At normal operation of the circuit breaker, the stabilizing signal exceeds the
post-filtered signal for a consecutive period of maximum 10 ms before it is
reset. Resetting occurs before the back-up trip timer t2 has timed out.
At a breaker failure situation, the post-filtered current exceeds the stabiliz-
ing signal, resulting in a trip of the breaker-failure function within 10 ms
after the trip timer t2 has elapsed.
The breaker-failure protection works with all three phases totally separated.
But a possibility exists to start all three phases simultaneously.
The back-up trip is always three-phase.
1MRK 580 171-XEN
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