Ct Tap - GE L90 Instruction Manual

9.2 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
the local current phasor is used as a reference to determine which differential equation is to be 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.
For example:
• 2-Terminal Configuration: CT
Consequently, CT TAP 1
To achieve maximum differential sensitivity, the minimum pickup is set to 0.2 pu at the terminal with a higher CT pri-
mary current, in this case 2000:5. The other terminal pickup is adjusted accordingly: PICKUP
PICKUP = 0.2
RELAY2
Choosing the RELAY1 as a reference with break point BREAK PT
chosen as BREAK PT
BREAK PT
RELAY1
and BREAK PT
RELAY2
• 3-Terminal Configuration: CT
Therefore,

CT TAP 1

CT TAP 2
where: for RELAY1, Channel 1 communicates to RELAY2 and Channel 2 to RELAY3
for RELAY2, Channel 1 communicates to RELAY1 and Channel 2 to RELAY3
for RELAY3, Channel 1 communicates to RELAY1 and Channel 2 to RELAY2
Consequently, to achieve the maximum sensitivity of 0.2 pu at the terminal with a CT = 2000/5 (400 A line primary dif-
ferential current), PICKUP
Choosing RELAY1 as a reference with a break point BREAK PT
RELAY3 are determined as follows:
BREAK PT
RELAY2
BREAK PT
RELAY3
Check;
BREAK PT
RELAY1
BREAK PT
RELAY2
BREAK PT
RELAY3
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.
9
9-4
and
CURRENT DIFF RESTRAINT 2
= 1000/5 and CT
RELAY1
= 2 and CT TAP 1
RELAY1
= BREAK PT
RELAY2 RELAY1
x CT should be equal to BREAK PT
RELAY1
= 2.5.
= 1000/5, CT
RELAY1
= 2.0, CT TAP 1
RELAY1 RELAY2
= 0.5, CT TAP 2
RELAY1 RELAY2
= 0.4, PICKUP
RELAY1
= BREAK PT x CT
RELAY1 RELAY1
= BREAK PT x CT
RELAY1 RELAY1
x CT = 5.0 x 1000/5 = 1000
RELAY1
x CT = 2.5 x 2000/5 = 1000
RELAY2
x CT = 10.0 x 500/5 = 1000
RELAY3
L90 Line Differential Relay
setting must be used to correct the ratios to
CURRENT DIFF CT TAP 1(2)
and
CURRENT DIFF BREAK PT
to the same value (e.g. 40% or 50%). This converts the
= 2000/5.
RELAY2
= 0.5.
RELAY2
= 5.0, the break point at RELAY2 must be
RELAY1
x CT / CT = 2.5. The simple check for this is as follows:
RELAY1 RELAY2
x CT
RELAY2 RELAY2
= 2000/5, and CT
RELAY2 RELAY3
= 0.5, and CT TAP 1
RELAY3
= 0.25, and CT TAP 2
RELAY3
= 0.2, and PICKUP
RELAY2 RELAY3
= 5.0 pu, the break points for RELAY2 and
RELAY1
/ CT = 2.5 pu
RELAY2
/ CT = 10.0 pu
RELAY3
9 APPLICATION OF SETTINGS
9.2.6 CT TAP
setting because
CURRENT DIFF PICKUP
setting. Next, adjust
CT TAP
= 0.4 and
RELAY1
. As such, BREAK PT
RELAY1
= 500/5.
= 2.0
= 4.0.
= 0.8.
GE Multilin
= 5.0