ABB ret650, Technical Manual

Page: 124 / 670

(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 45 ) 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.
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 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
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 60%,
REFPDIF is blocked.
6.2.7.5 Algorithm of the restricted earth-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 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 earth
fault. The search is only made if the neutral current is at least 50% of the Idmin
current. If an external earth 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.
6. For as long as the external fault persists an additional temporary trip condition is
introduced. That means that REFPDIF is temporarily desensitized.
7. If point P (Ibias, Idiff) is found to be above the operate - restrain characteristic),
a directional check can be made. The directional check is made only if (3I0) is
more than 3% of
IBase . If the result is an external fault the internal trip request is
reset. If the directional check cannot be executed, then direction is no longer a
condition for a trip.
8. When neutral current, residual current and bias current are within some windows
and some timing criteria are fulfilled, the ratio of 2
nd
to fundamental tone is
Section 6
1MRK 504 135-UEN A
Differential protection
118
Technical manual