8-6 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CURRENT DIFFERENTIAL (87L) SETTINGS CHAPTER 8: APPLICATION OF SETTINGS
8
• For relay 3, channel 1 communicates to relay 1 and channel 2 communicates to relay 2
Consequently, to achieve the maximum sensitivity of 0.2 pu at the terminal with a CT ratio of 2000/5 (400 amps line
primary differential current), the following pickup values are chosen:
Eq. 8-11
Choosing relay as a reference with a breakpoint value of 5.0 pu, breakpoints for relays 2 and 3 are calculated as follows:
Eq. 8-12
Eq. 8-13
To verify the calculated values, we have:
Eq. 8-14
This satisfies the equality condition indicated earlier.
During on-load tests, the differential current at all terminals should be the same and generally equal to the charging
current if the tap and CT ratio settings are chosen correctly.
8.2.7 Breaker-and-a-half
Assume a breaker-and-the-half configuration shown in the following figure. This section provides guidance on configuring
the L90 relay for this application. The L90 is equipped with two CT/VT modules: F8F and L8F.
1. CTs and VTs are connected to L90 CT/VT modules as follows:
– CT1 circuitry is connected to the F1 to F3 terminals of the F8F module (three-phase CT inputs, CT bank “F”).
– CT2 circuitry is connected to the F1 to F3 terminals of the L8F module (three-phase CT inputs, CT bank “L”).
– VT1 circuitry is connected to the F8 terminals of the F8F module (single-phase VT for synchrocheck 1, VT bank “F”).
– VT2 circuitry is connected to the F8 terminals of the L8F module (single-phase VT for synchrocheck 2, VT bank “L”).
– VT3 circuitry is connected to the F5 to F7 terminals of the F8F modules (three-phase VT for distance, metering,
synchrocheck, charging current compensation, etc.; VT bank “F”).
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