Ct Tap - GE L90 Instruction Manual

10.2 CURRENT DIFFERENTIAL (87L) SETTINGS
If the CT ratios at the line terminals are different, the
a common base. In this case, a user should modify the
because the local current phasor is used as a reference to determine which differential equation is used, based on the
value of local and 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:
1. Set
CURRENT DIFF RESTRAINT 1
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 desired 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
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 fol-
lows:
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:
Breakpoint relay 1
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:
10
• 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
10-4
CURRENT DIFF CT TAP 1(2)
and
CURRENT DIFF RESTRAINT 2
CT relay 1
ratio
CT relay 2
ratio
CT relay 1
tap
CT relay 2
tap
Pickup relay 1
Pickup relay 2
Breakpoint relay 2 Breakpoint relay 1
1000 5
------------------- -
5.0
2000 5
CT relay 1
ratio
CT relay 1
ratio
CT relay 2
ratio
CT relay 3
ratio
CT relay 1 2.00
tap1
CT relay 2 0.50
tap1
CT relay 3 2.00
tap1
L90 Line Current Differential System
10 APPLICATION OF SETTINGS
setting must be used to correct the ratios to
and
CURRENT DIFF BREAK PT
to the same value (e.g. 40% or 50%). This converts the
1000 5
2000 5
2.0
0.5
0.4
0.2
CT relay 1
ratio
--------------------------------------- -
CT relay 2
ratio
2.5
Breakpoint relay 2 CT
ratio
1000 5
2000 5
500 5
CT relay 1 0.50
tap2
CT relay 2 0.25
tap2
CT relay 3 4.00
tap2

10.2.6 CT TAP

settings
CURRENT DIFF PICKUP
setting. Next, adjust
CT TAP
(EQ 10.10)
(EQ 10.11)
(EQ 10.12)
(EQ 10.13)
relay 2
(EQ 10.14)
(EQ 10.15)
(EQ 10.16)
GE Multilin