GE L30 Instruction Manual page 476

9.2 CURRENT DIFFERENTIAL (87L) SETTINGS
• 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:
Choosing relay as a reference with a breakpoint value of 5.0 pu, breakpoints for relays 2 and 3 are calculated as follows:
To verify the calculated values, we have:
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.
9
9-6
Pickup relay 1 (
Pickup relay 2 (
Pickup relay 3 (
( )
Breakpoint relay 2 Breakpoint relay 1
=
5.0 ×
=
( )
Breakpoint relay 3 Breakpoint relay 1
=
×
5.0
=
Breakpoint relay 1 ( ) CT × (
ratio
( ) CT × (
Breakpoint relay 2
ratio
( ) CT × (
Breakpoint relay 3
ratio
L30 Line Current Differential System
) 0.4
=
) 0.2
=
) 0.8
=
(
CT relay 1
ratio
( ) × --------------------------------------- -
(
CT relay 2
ratio
1000 5 ⁄
------------------- - 2.5
=
2000 5 ⁄
CT ( relay 1
ratio
( ) × --------------------------------------- -
(
CT relay 3
ratio
1000 5 ⁄
------------------- -
10.0
=
500 5 ⁄
relay 1 ) 5.0 1000 5 ⁄ × 1000
= =
) 2.5 2000 5 ⁄ ×
relay 2 1000
= =
) 10.0 5000 5 ⁄ ×
relay 3
= =
9 APPLICATION OF SETTINGS
)
)
)
)
1000
GE Multilin
(EQ 9.11)
(EQ 9.12)
(EQ 9.13)
(EQ 9.14)