P54x/EN AP/La4 Application Notes
(AP) 6-38
MiCOM P543, P544, P545 & P546
AP
2.8 Earth fault (ground overcurrent) and sensitive earth fault (SEF) protection
Caution: The IEEE C.37.112 standard for IDMT curves permits some
freedom to manufacturers at which time dial (TD) value the
reference curve applies. Rather than pick a mid-range value, for
the MiCOM P54x the reference curve norm applies at a time dial
of 1. The time dial is merely a multiplier on the reference curve,
in order to achieve the desired tripping time. Take care when
grading with other suppliers' relays which may take TD = 5,
or TD = 7 as a mid-range value to define the IDMT curve. The
equivalent MiCOM P54x setting to match those relays is achieved
by dividing the imported setting by 5 or 7.
2.8.1 Directional earth fault protection
2.8.1.1 Residual voltage polarization
It is possi
ble that small levels of residual voltage will be present under normal system
conditions due to system imbalances, VT inaccuracies, relay tolerances etc. Hence, the
relay includes a user settable threshold (N>VNPol Set) which must be exceeded in order for
the DEF function to be operational. In practice, the typical zero sequence voltage on a
healthy system can be as high as 1% (i.e.: 3% residual), and the VT error could be 1% per
phase. A setting between 1% and 4% is typical. The residual voltage measurement
provided in the Measurements column of the menu may assist in determining the required
threshold setting during commissioning, as this will indicate the level of standing residual
voltage present.
2.8.2 General setting guidelines for directional earth fault (g
round overcurrent) protection
When setting the relay characteristic angle (RCA) for the directional earth fault element, a
positive angle setting was specified. This was due to the fact that the quadrature polarizing
voltage lagged the nominal phase current by 90°; i.e. the position of the current under fault
conditions was leading he polarizing voltage and hence a positive RCA was required. With
directional earth fault DEF, the residual current under fault conditions lies at an angle lagging
the polarizing voltage. Hence, negative RCA settings are required for DEF applications.
This is set in cell 'I>N’ in the relevant earth fault menu.
The following angle settings are recommended for a residual voltage polarized relay:-
Distribution systems (solidly earthed) -45°
Transmissions systems (solidly earthed) -60°
For negative sequence polarization, the RCA settings must be based on the angle of the
upstream negative phase sequence source impedance.
2.8.3 Sensitive earth fault protection element (SEF)
SEF woul
d normally be fed from a core balance current transformer (CBCT) mounted around
the three phases of the feeder cable. However, care must be taken in the positioning of the
CT with respect to the earthing of the cable sheath. See Figure 10 below.
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