ABB Relion 670 series, Техническое руководство

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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
o
) and the
neutral current (I
N
in Figure 64 ) are theoretically equal in magnitude and are 180 degrees out-of-
phase. The current in the neutral (I
N
) serves as a directional reference 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.
However, if one or more CTs saturate under external fault conditions, then the measured currents
3I
o
and I
N
may no longer be equal, nor will their positions in the complex plane be exactly 180
degrees apart. There is a risk that the resulting false differential current Idiff enters the operate
area of the operate-restrain characteristic under external fault conditions. If this happens, a
directional test may prevent a malfunction.
A directional check is only executed if:
1. a trip request signal has been issued (REFPDIF (87N) function PICKUP signal set to 1)
2. the residual current in lines (3I
o
) is at least 3% of the
IBase current.
If a directional check is either unreliable or not possible to do, due to too small currents, then the
direction is cancelled as a condition for an eventual trip.
If a directional check is executed, the REFPDIF (87N) protection operation is only allowed if
currents 3I
o
and I
N
(as seen in Figure 64 and Figure 65 ) are both within the operating region
determined by the set value of ROA, in degrees.
ROA = 60 to 90 degrees; where ROA stands for Relay Operate Angle.
Second harmonic analysis
M5447-106 v12
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 of 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
nd
harmonic 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 40%, REFPDIF
(87N) is blocked.
6.4.7.5 Algorithm of the restricted ground fault protection
M5447-95 v13
1. Check if current in the neutral Ineutral (IN) is less than 50% of the base sensitivity Idmin. If
yes, only service values are calculated, and rest of the REFPDIF (87N) algorithm is not
executed.
2. If current in the Ineutral (IN) is more than 50% of Idmin, then determine the bias current Ibias.
3. Determine the differential (operate) current Idiff as a phasor, and calculate its magnitude.
4. Check if the point P(Ibias, Idiff) is above the operate-bias characteristic. If yes, increment the
trip request counter by 1. If the point P(Ibias, Idiff) is found to be below the operate-bias
characteristic, then the trip request counter is reset to zero.
5. If the trip request counter is still zero, 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 zero when Ineutral falls below 50% of the base
Section 6 1MRK 502 066-UUS B
Differential protection
174
Technical manual