CHAPTER 10: THEORY OF OPERATION
OVERVIEW
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
10-5
10
10.1.7 Ground differential element
The line ground differential function allows sensitive ground protection for single-line-to-ground faults, allowing the phase
differential element to be set higher (above load) to provide protection for multi-phase faults. The L90 ground differential
function calculates ground differential current from all terminal phase currents. The maximum phase current is used for
the restraint. The L90 is applied in dual-breaker applications to cope with significant through current at remote terminals
that can cause CT errors or saturation.
The line ground differential function uses the same CT matched and time-aligned phasors as the phase-segregated
current differential function. The operate signal is calculated for both real and imaginary parts as follows:
Eq. 10-12
Eq. 10-13
The terms for the second remote terminal are omitted in two-terminal applications.
The maximum through current is available locally and re-constructed from the received remote restraint based on the
maximum remote restraint current shown in the previous section and as indicated as follows.
For two-terminal applications:
Eq. 10-14
For three-terminal applications:
Eq. 10-15
The 87G restraining signal is calculated as follows:
Eq. 10-16
The terms for the second remote terminal are omitted in two-terminal applications.
The operate signal for the ground differential function (I
OP_87G
)
2
is then calculated as:
Eq. 10-17
The restraint signal (I
87G
)
2
is calculated as follows for two-terminal applications:
Eq. 10-18
