Phase of protection Operate current Polarizing voltage
B Phase IB VCA
C Phase IC VAB
Under system fault conditions, the fault current vector lags its nominal phase voltage by an angle depending on
the system X/R ratio. The IED must therefore operate with maximum sensitivity for currents lying in this region. This
is achieved by using the IED characteristic angle (RCA). This is the is the angle by which the current applied to the
IED must be displaced from the voltage applied to the IED to obtain maximum sensitivity.
The device provides a setting I> Char Angle, which is set globally for all overcurrent stages. It is possible to set
characteristic angles anywhere in the range –95° to +95°.
A directional check is performed based on the following criteria:
Directional forward
-90° < (angle(I) - angle(V) - RCA) < 90°
Directional reverse
-90° > (angle(I) - angle(V) - RCA) > 90°
For close up three-phase faults, all three voltages will collapse to zero and no healthy phase voltages will be
present. For this reason, the device includes a synchronous polarisation feature that stores the pre-fault voltage
information and continues to apply this to the directional overcurrent elements for a time period of a few seconds.
This ensures that either instantaneous or time-delayed directional overcurrent elements will be allowed to operate,
even with a three-phase voltage collapse.
P543i/P545i Chapter 13 - Current Protection Functions
P54x1i-TM-EN-1 367
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