Table of Contents
Advertisement
Breaker failure protection (BFP)
•
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 max-
imum suppression of the above dc components.
The intention of the adaptive signal detection (ASD) concept is to achieve indepen-
dence 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 sta-
bilizing signal to compare the current with.
The ASD works continuously, regardless of if the BFP was started. Its result is however
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 cur-
rent and when it does not, it decays. This adaptive behaviour makes it possible to rap-
idly 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 cur-
rent level, the breaker-failure function is momentarily reset.
At normal operation of the circuit breaker, the stabilizing signal exceeds the post-fil-
tered signal for a consecutive period of maximum 10 ms before it is reset. Resetting oc-
curs before the back-up trip timer t2 has timed out.
At a breaker failure situation, the post-filtered current exceeds the stabilizing signal, re-
sulting in a trip from the breaker-failure function within 10 ms after the trip timer t2 has
elapsed.
The breaker-failure protection works totally separated when comes to current measure-
ment and timers. The back-up trip is always non-segregated.
Current
detector
Current
Start
ASD
RMS
t
&
t
t
Back-up trip
&
t1
&KDSWHU
&XUUHQW
99000104.vsd
127
Advertisement
Table of Contents
