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Section 9
Current protection
9.2.3
348
delay characteristics. The selectivity between different overcurrent protections is
normally enabled by co-ordination between the function time delays of the different
protections. To enable optimal co-ordination between all overcurrent protections, they
should have the same time delay characteristic. Therefore, a wide range of standardized
inverse time characteristics are available for IEC and ANSI. It is also possible to tailor
make the inverse time characteristic.
Normally, it is required that the phase overcurrent protection shall reset as fast as
possible when the current level gets lower than the operation level. In some cases some
sort of delayed reset is required. Therefore, different kinds of reset characteristics can
be used.
For some protection applications, there can be a need to change the current pick-up
level for some time. A typical case is when the protection will measure the current to a
large motor. At the start up sequence of a motor the start current can be significantly
larger than the rated current of the motor. Therefore, there is a possibility to give a
setting of a multiplication factor to the current pick-up level. This multiplication factor
is activated from a binary input signal to the function.
Power transformers can have a large inrush current, when being energized. This
phenomenon is due to saturation of the transformer magnetic core during parts of the
period. There is a risk that inrush current will reach levels above the pick-up current of
the phase overcurrent protection. The inrush current has a large 2
This can be used to avoid unwanted operation of the protection function. Therefore,
OC4PTOC (51/67) has a possibility of 2
harmonic current reaches a value above a set percent of the fundamental current.
The phase overcurrent protection is often used as a protection for two and three phase
short circuits. In some cases, it is not wanted to detect single-phase ground faults by
the phase overcurrent protection. This fault type is detected and cleared after operation
of ground fault protection. Therefore, it is possible to make a choice how many phases,
at minimum, that have to have current above the pick-up level, to enable operation. If
set 1 of 3 it is sufficient to have high current in one phase only. If set 2 of 3 or 3 of 3
single-phase ground faults are not detected.
Setting guidelines
When inverse time overcurrent characteristic is selected, the trip time
of the stage will be the sum of the inverse time delay and the set
definite time delay. Thus, if only the inverse time delay is required, it is
important to set the definite time delay for that stage to zero.
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
nd
harmonic restrain if the level of 2
1MRK 502 071-UUS A
nd
harmonic content.
nd
IP14982-1 v1
M12982-4 v13
Application manual
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