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Section 7
Current protection
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
unwanted operation of the protection function. Therefore, OC4PTOC has a possibility of 2
harmonic restrain if the level of 2
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
high current in one phase only. If set
7.1.3
Setting guidelines
The parameters for the directional phase overcurrent protection, four steps OC4PTOC are set
via the local HMI or PCM600.
The following settings can be done for OC4PTOC.
Common base IED values for the primary current (
power (
GlobalBaseSel : Selects the global base value group used by the function to define IBase , UBase
SBase . Note that this function will only use IBase value.
and
MeasType : Selection of discrete Fourier filtered ( DFT ) or true RMS filtered ( RMS ) signals. RMS
is used when the harmonic contents are to be considered, for example in applications with
shunt capacitors.
Operation : The protection can be set to On or Off .
AngleRCA : Protection characteristic angle set in degrees. If the angle of the fault loop current
has the angle RCA, the direction to the fault is forward.
AngleROA : Angle value, given in degrees, to define the angle sector of the directional function,
shown in Figure 86.
StartPhSel : Number of phases, with high current, required for operation. The setting
possibilities are:
144
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.
SBase ) are set in global base values for settings function GBASVAL.
1 out of 3 , 2 out of 3 and 3 out of 3 . The default setting is 1 out of 3 .
nd
harmonic content. This can be used to avoid
nd
harmonic current reaches a value above a set percent of the
2 of 3 or 3 of 3 single-phase earth faults are not detected.
IBase ), primary voltage ( UBase ) and primary
1MRK 505 370-UEN D
1 of 3 it is sufficient to have
Busbar protection REB670
Application manual
nd
IP14982-1 v1
M12982-4 v14
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