ABB Relion RES670, Applications Manual

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Choice of time delay characteristics: There are several types of time delay
characteristics available such as definite time delay and different types of inverse
time 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
nd
harmonic content. This can be used to avoid unwanted operation of the protection
function. Therefore, OC4PTOC has a possibility of 2
nd
harmonic restrain if the
level of 2
nd
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 earth
faults by the phase overcurrent protection. This fault type is detected and cleared
after operation of earth 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 earth faults are not detected.
8.1.3 Setting guidelines
IP14982-1 v1
M12982-4 v14
When inverse time overcurrent characteristic is selected, the
operate 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.
Section 8
1MRK 511 407-UEN C
Current protection
124 Phasor measurement unit RES670 2.2 IEC
Application manual