(REFPDIF) must remain stable during an external fault, and immediately after the
fault has been cleared by some other protection.
For an external earth faults with no CT saturation, the residual current in the lines
(3I
o
) and the neutral current (I
N
in Figure
) 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 earth faults.
The directional criterion in REFPDIF 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 function START 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 protection operation is only allowed
if currents 3I
o
and I
N
(as seen in Figure
and Figure
) 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
When energizing a transformer a false differential current may appear in earth fault
protection, low impedance function (REFPDIF). 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 amplitude 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 is blocked.
Section 6
1MRK502052-UEN B
Differential protection
174
Technical manual
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