Circuit Breaker Fail Implementation; Circuit Breaker Fail Timers; Zero Crossing Detection - GE MiCOM P40 Agile Technical Manual

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P14x
3

CIRCUIT BREAKER FAIL IMPLEMENTATION

Circuit Breaker Failure Protection is implemented in the CB FAIL column of the relevant settings group.
3.1

CIRCUIT BREAKER FAIL TIMERS

The circuit breaker failure protection incorporates two timers, CB Fail 1 Timer and CB Fail 2 Timer, allowing
configuration for the following scenarios:
Simple CBF, where only CB Fail 1 Timer is enabled. For any protection trip, the CB Fail 1 Timer is started, and
normally reset when the circuit breaker opens to isolate the fault. If breaker opening is not detected, the CB
Fail 1 Timer times out and closes an output contact assigned to breaker fail (using the programmable
scheme logic). This contact is used to back-trip upstream switchgear, generally tripping all infeeds
connected to the same busbar section.
A retripping scheme, plus delayed back-tripping. Here, CB Fail 1 Timer is used to issue a trip command to a
second trip circuit of the same circuit breaker. This requires the circuit breaker to have duplicate circuit
breaker trip coils. This mechanism is known as retripping. If retripping fails to open the circuit breaker, a
back-trip may be issued following an additional time delay. The back-trip uses CB Fail 2 Timer, which was
also started at the instant of the initial protection element trip.
You can configure the CBF elements CB Fail 1 Timer and CBF Fail 2 Timer to operate for trips triggered by
protection elements within the device. Alternatively you can use an external protection trip by allocating one of the
opto-inputs to the External Trip DDB signal in the PSL.
You can reset the CBF from a breaker open indication (from the pole dead logic) or from a protection reset. In these
cases resetting is only allowed if the undercurrent elements have also been reset. The resetting mechanism is
determined by the settings Volt Prot Reset and Ext Prot Reset.
The resetting options are summarised in the following table:
Initiation (Menu Selectable)
Current based protection
Sensitive Earth Fault element
Non-current based protection (e.g. 27/59/81/32L)
External protection
3.2

ZERO CROSSING DETECTION

When there is a fault and the circuit breaker interrupts the CT primary current, the flux in the CT core decays to a
residual level. This decaying flux introduces a decaying DC current in the CT secondary circuit known as
subsidence current. The closer the CT is to its saturation point, the higher the subsidence current.
The time constant of this subsidence current depends on the CT secondary circuit time constant and it is generally
long. If the protection clears the fault, the CB Fail function should reset fast to avoid maloperation due to the
subsidence current. To compensate for this the device includes a zero-crossing detection algorithm, which ensures
that the CB Fail re-trip and back-trip signals are not asserted while subsidence current is flowing. If all the samples
within half a cycle are greater than or smaller than 0 A (10 mS for a 50 Hz system), then zero crossing detection is
asserted, thereby blocking the operation of the CB Fail function. The zero-crossing detection algorithm is used
P14xEd1-TM-EN-1
CB Fail Timer Reset Mechanism
The resetting mechanism is fixed (e.g. 50/51/46/21/87)
IA< operates AND IB< operates AND IC< operates AND IN< operates
The resetting mechanism is fixed.
ISEF< Operates
Three options are available:
All I< and IN< elements operate
Protection element reset AND all I< and IN< elements operate
CB open (all 3 poles) AND all I< and IN< elements operate
Three options are available.
All I< and IN< elements operate
External trip reset AND all I< and IN< elements operate
CB open (all 3 poles) AND all I< and IN< elements operate
Chapter 8 - CB Fail Protection
185

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