The negative-sequence component current I
2
, drawn by an asynchronous or a
synchronous machine, is linearly proportional to the negative-sequence component
voltage U
2
. When U
2
is P% of U
n
, I
2
is typically about 5 x P% x I
n
.
The negative-sequence overcurrent NSPTOC blocks are used to accomplish a
selective protection against the voltage and current unbalance for each machine
separately. Alternatively, the protection can be implemented with the NSPTOV
function, monitoring the voltage unbalance of the busbar.
If the machines have an unbalance protection of their own, the NSPTOV operation
can be applied as a backup protection or it can be used as an alarm. The latter can
be applied when it is not required to trip loads tolerating voltage unbalance better
than the rotating machines.
If there is a considerable degree of voltage unbalance in the network, the rotating
machines should not be connected to the network at all. This logic can be
implemented by inhibiting the closure of the circuit breaker if the NSPTOV operation
has started. This scheme also prevents connecting the machine to the network if
the phase sequence of the network is not correct.
An appropriate value for the setting parameter
Voltage start value
is approximately
3 percent of U
n
. A suitable value for the setting parameter
Operate delay time
depends on the application. If the NSPTOV operation is used as backup protection,
the operate time should be set in accordance with the operate time of NSPTOC used
as main protection. If the NSPTOV operation is used as main protection, the operate
time should be approximately one second.
4.4.4.6 Signals
Table 459: NSPTOV Input signals
Name Type Default Description
U
2
SIGNAL 0 Negative phase se-
quence voltage
BLOCK BOOLEAN 0=False Block signal for acti-
vating the blocking
mode
Table 460: NSPTOV Output signals
Name Type Description
OPERATE BOOLEAN Operate
START BOOLEAN Start
4.4.4.7 Settings
NSPTOV Group settings
Table 461: NSPTOV Group settings (Basic)
Parameter
Values (Range) Unit Step Default Description
Start value 0.010...1.000 xUn 0.001 0.030 Start value
Operate delay time 40...120000 ms 1 40 Operate delay time
Protection functions 1MRS758755 C
442 REC615 & RER615
Technical Manual
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