Algorithm Of The Restricted Ground Fault Protection - ABB Relion Technical Manual

1MRK 502 048-UUS A
6.2.7.5
Technical manual
Directional criterion
The directional criterion is applied in order to positively distinguish between internal and
external ground faults. This check is an additional criterion, which should prevent
malfunctions at heavy external ground faults, and during the disconnection of such faults
by other protections. Ground faults on lines connecting the power transformer occur much
more often than ground faults on a power transformer winding. It is important therefore
that the Restricted ground fault protection, low impedance (REFPDIF87N) must remain
stable during an external fault, and immediately after the fault has been cleared by some
other protection.
For an external ground faults with no CT saturation, the residual current in the lines (3I
and the neutral current (I
degrees out-of-phase. The current in the neutral (I
because it has the same direction for both internal and external ground faults. The
directional criterion in REFPDIF (87N) protection makes it a current-polarized
protection.
Second harmonic analysis
When energizing a transformer a false differential current may appear in ground fault
protection, low impedance function (REFPDIF 87N). The phase CTs may saturate due to
a high DC component with a long duration, but the current through the neutral CT does not
have either the same DC component or the same magnitude and the risk for saturation in
this CT is not as high. As a result the differential current due to the saturation may be so
high that it reaches the operate characteristic. A calculation of the content of 2
in the neutral current is made when the neutral current, residual current and bias current
are within some windows and some timing criteria are fulfilled. If the ratio between
second and fundamental harmonic exceeds 60%, REFPDIF (87N) is blocked.

Algorithm of the restricted ground fault protection

1. Check if the current in the neutral (IN) is less than 50% of the base sensitivity Idmin.
If yes, only service values are calculated, and REFPDIF (87N) algorithm is blocked.
2. If the current in the neutral (IN) is more than 50% of Idmin, the bias current (IBIAS)
is determined.
3. The differential current phasor (IDIFF) is determined.
4. Check if the point P (Ibias, Idiff) is above the operate - restrain characteristic. If so,
increment the trip request counter by 1. If the point P(Ibias, Idiff) is found to be below
the operate - restrain characteristic, then the trip request counter is reset to 0.
5. If the trip request counter is still 0, search for an eventual heavy external ground fault.
The search is only made if the neutral current is at least 50% of the Idmin current. If
an external ground fault has been detected, a flag is set which remains set until the
external fault has been cleared. The external fault flag is reset to 0 when IN falls below
50% of the base sensitivity Idmin. Any search for an external fault is aborted if the trip
request counter is more than 0.
in figure 44) are theoretically equal in magnitude and are 180
N
Differential protection
) serves as a directional reference
N
Section 6
)
o
nd
harmonic
127