ABB Relion 611 Series, Technical Manual

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4.5.4.5 Application
A continuous or temporary voltage unbalance can appear in the network for various
reasons. The voltage unbalance mainly occurs due to broken conductors or
asymmetrical loads and is characterized by the appearance of a negative-sequence
component of the voltage. In rotating machines, the voltage unbalance results in a
current unbalance, which heats the rotors of the machines. The rotating machines,
therefore, do not tolerate a continuous negative-sequence voltage higher than
typically 1-2 percent x U
n
.
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.
4.5.4.6 Signals
Table 295: NSPTOV Input signals
Name Type Default Description
U
2
SIGNAL 0 Negative phase sequence voltage
BLOCK BOOLEAN 0=False Block signal for activating the blocking mode
Section 4
1MRS757454 D
Protection functions
274 611 series
Technical Manual