GE L90 Instruction Manual page 649

CHAPTER 8: APPLICATION OF SETTINGS
remote currents. If the setting is not modified, the responses of individual relays, especially during an external fault, can be
asymmetrical, as one relay can be below the breakpoint and the other above the breakpoint. There are two methods to
overcome this potential problem, as follows:
1. Set
CURRENT DIFF RESTRAINT 1
converts the relay characteristics from dual slope into single slope and the breakpoint becomes immaterial. Next,
adjust differential pickup at all terminals according to CT ratios, referencing the required pickup to the line primary
current (see below).
2. Set the breakpoints in each relay individually in accordance with the local CT ratio and the CT TAP setting. Next, adjust
the differential pickup setting according to the terminal CT ratios. The slope value must be identical at all terminals.
Consider a two-terminal configuration with the following CT ratios for relays 1 and 2.
Consequently, we have the following CT tap value for relays 1 and 2.
To achieve maximum differential sensitivity, the minimum pickup is set as 0.2 pu at the terminal with the higher CT primary
current; in this case 2000:5 for relay 2. The other terminal pickup is adjusted accordingly. The pickup values are set as
follows:
Choosing relay 1 as a reference with a breakpoint of 5.0, the break point at relay 2 is chosen as follows:
Use the following equality the verify the calculated breakpoint:
Therefore, we have a breakpoint of 5.0 for relay 1 and 2.5 for relay 2.
Now, consider a three-terminal configuration with the following CT ratios for relays 1, 2, and 3.
Consequently, we have the following CT tap value for relays 1, 2, and 3.
In this case, the relay channels communicate as follows:
• For relay 1, channel 1 communicates to relay 2 and channel 2 communicates to relay 3
• For relay 2, channel 1 communicates to relay 1 and channel 2 communicates to relay 3
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
and
CURRENT DIFF RESTRAINT 2
CURRENT DIFFERENTIAL (87L) SETTINGS
to the same value (for example, 40% or 50%). This
Eq. 8-4
Eq. 8-5
Eq. 8-6
Eq. 8-7
Eq. 8-8
8
Eq. 8-9
Eq. 8-10
8-5