Hitachi Relion 670 Series Applications Manual page 116

Section 8
Current protection
• Short circuit protection of feeders in distribution and subtransmission systems. Normally these
feeders have a radial structure.
• Back-up short circuit protection of transmission lines.
• Back-up short circuit protection of power transformers.
• Short circuit protection of different kinds of equipment connected to the power system such as; shunt
capacitor banks, shunt reactors, motors and others.
• Back-up short circuit protection of power generators.
In many applications several steps with different current pickup levels and time delays are needed.
OC4PTOC (51_67) can have up to four different, individually settable steps. The following options are
possible:
Non-directional / Directional function: In most applications the non-directional functionality is used. This is
mostly the case when no fault current can be fed from the protected object itself. In order to achieve both
selectivity and fast fault clearance, the directional function can be necessary.
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
nd
large 2
Therefore, OC4PTOC (51/67) has a possibility of 2
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.
110
If VT inputs are not available or not connected, the setting parameter DirModeSelx (x = step
1, 2, 3 or 4) shall be left to the default value Non-directional.
harmonic content. This can be used to avoid unwanted operation of the protection function.
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nd
harmonic restrain if the level of 2
Phasor measurement unit RES670
1MRK511407-UUS Rev. N
nd
harmonic
Application manual