Ct Tap - GE UR Series Instruction Manual

Line differential relay.

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9.2 CURRENT DIFFERENTIAL (87L) SETTINGS
If the CT ratios at the line terminals are different, the CURRENT DIFF CT TAP 1 (2) setting must be used to correct the
ratios to a common base. In this case, a user should modify the setting CURRENT DIFF BREAK PT because the local cur-
rent phasor is used as a reference to determine which differential equation is to be used. If the setting is not modified, the
responses of individual relays 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 RESTRAINT 1 and RESTRAINT 2 to the same value (say 40% or 50%). This converts the relay characteristics
from dual slope into single slope and the breakpoint becomes immaterial.
2.
Individually set the breakpoint in each relay in accordance with the local CT ratio and the CT TAP setting.
For example:
2-Terminal Configuration
CT
= 1000/5, CT
RELAY1
RELAY1 as a reference with break point BREAK PT
BREAK PT
RELAY2
PT
x CT
RELAY1
RELAY1
3-Terminal Configuration
RELAY 1:
CT
= 1000/5
RELAY1
Therefore;

CT TAP 1

= 2.0
RELAY1
CT TAP 2
= 0.5
RELAY1
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
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
= 2000/5, consequently CT TAP 1
RELAY2
= BREAK PT
x CT
RELAY1
RELAY1
should be equal to BREAK PT
RELAY 2:
CT
= 2000/5
RELAY2
CT TAP 1
RELAY2
CT TAP 2
RELAY2
= 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
= 2 and CT TAP 1
RELAY1
= 5.0, the break point at RELAY2 must be chosen as
RELAY1
/ CT
= 2.5. The simple check for this is as follows: BREAK
RELAY2
x CT
.
RELAY2
RELAY2
RELAY 3:
CT
RELAY3
= 0.5
CT TAP 1
= 0.25
CT TAP 2
= 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
= 0.5. Choosing the
RELAY2
= 500/5
= 2.0
RELAY3
= 4.0
RELAY3
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