Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-93
OP
1.34 Residual overvoltage (neutral displacement) protection
The NVD element within the MiCOM P54x is of two stage design, each stage having
separate voltage and time delay settings. Stage 1 may be set to operate on either an IDMT
or DT characteristic, whilst stage 2 may be set to DT only. Two stages are included for the
NVD protection to account for applications which require both alarm and trip stages.
The relay internally derives the NVD voltage from the 3 input phases which must be supplied
from either a 5-limb or three single phase VT’s. These types of VT design allow the passage
of residual flux and consequently permit the relay to derive the required residual voltage. In
addition, the primary star point of the VT must be earthed. A three limb VT has no path for
residual flux and is therefore unsuitable to supply the relay.
The IDMT characteristic available on the first stage is defined by the following formula:
t = K/( M - 1)
Where:
K = Time multiplier setting
t = Operating time in seconds
M = Derived residual voltage/relay setting voltage (VN> Voltage Set)
The functional block diagram of the first stage residual overvoltage is shown below:
VTS Block
IDMT/DT
P1635ENa
&
N
V >
&
N
V > Timer Block
N
V > Trip1st stage
N
V > Start1st stage
Figure 64 Residual overvoltage logic (single stage)
Each stage of Residual Overvoltage protection may be disabled by a DDB (475 or 476)
Inhibit VN>x (x = 1, 2).
1.35 Circuit breaker fail protection (CBF)
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, ‘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 backtrip upstream switchgear, generally tripping all infeeds connected to the
same busbar section.
A re-tripping scheme, plus delayed backtripping. Here, ‘CB Fail 1 Timer’ is used to route
a trip to a second trip circuit of the same circuit breaker. This requires duplicated circuit
breaker trip coils, and is known as re-tripping. Should re-tripping fail to open the circuit
breaker, a backtrip may be issued following an additional time delay. The backtrip uses
‘CB Fail 2 Timer’, which is also started at the instant of the initial protection element trip.
CBF elements ‘CB Fail 1 Timer’ and ‘CB Fail 2 Timer’ can be configured to operate for trips
triggered by protection elements within the relay or via an external protection trip. The latter
is achieved by allocating one of the relay opto-isolated inputs to ‘External Trip’ using the
programmable scheme logic.
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