The covariance matrix for noise, harmonics, transients, and current transformer saturation is computed as follows. First, the
sum of the squares of the errors in the data samples is computed from the sum of squares information, phaselets, and pha-
sors for each phase for each terminal at each time step n :
The covariance matrix is then computed as a function of the time index and window size using the previously defined trans-
Current differential protection is inherently dependent on adequate CT performance at all terminals of the protected line
especially during external faults. CT saturation, particularly if happens at one terminal of the line only, introduces a spurious
differential current that may cause the differential protection to misoperate.
The L90 applies a dedicated mechanism to cope with CT saturation and ensure security of the protection for external faults.
The relay dynamically increases the weight of the square of errors (so-called sigma) portion in the total restraint quantity
but for external faults only.
The following logic is applied:
First, the terminal currents are compared against a threshold of 3 pu to detect overcurrent conditions that may be
caused by a fault and may lead to CT saturation.
For all the terminal currents that are above the 3 pu level, the relative angle difference is calculated. If all three termi-
nals see significant current, then all three pairs (1, 2), (2, 3), and (1, 3) are considered and the maximum angle differ-
ence is used in further calculations.
Depending on the angle difference between the terminal currents, the value of sigma used to calculate the adaptive
restraint current is increased by the factor of 1, 3 or 5 as shown in the figure below. As it is seen from the figure, for
internal faults factor "1" is used, but for external-"3" or "5". This allows relay to be sensitive for internal faults while
robust for external faults with a possible CT saturation.
The basic premise for the operation of differential protection schemes in general, and of the L90 line differential element in
particular, is that the sum of the currents entering the protected zone is zero. In the case of a power system transmission
line, this is not entirely true because of the capacitive charging current of the line. For short transmission lines the charging
current is a small factor and can therefore be treated as an unknown error. In this application the L90 can be deployed with-
out voltage sensors and the line charging current is included as a constant term in the total variance, increasing the differ-
ential restraint current. For long transmission lines the charging current is a significant factor, and should be computed to
provide increased sensitivity to fault current.
Figure 8–7: CT SATURATION ADAPTIVE RESTRAINT MULTIPLIER
L90 Line Differential Relay
) Re Phasor
8.1.18 CT SATURATION DETECTION
8.1.19 CHARGING CURRENT COMPENSATION
8 THEORY OF OPERATION
) Im Phasor
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