GE Multilin L30 Line Current Differential System 10-510 APPLICATION OF SETTINGS 10.2 CURRENT DIFFERENTIAL (87L) SETTINGS10• For relay 3, channel 1 communicates to relay 1 and channel 2 communicates to relay 2Consequently, to achieve the maximum sensitivity of 0.2 pu at the terminal with a CT ratio of 2000/5 (400 amps line primarydifferential current), the following pickup values are chosen:(EQ 10.17)Choosing relay as a reference with a breakpoint value of 5.0 pu, breakpoints for relays 2 and 3 are calculated as follows:(EQ 10.18)(EQ 10.19)To verify the calculated values, we have:(EQ 10.20)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 currentif the tap and CT ratio settings are chosen correctly.Pickup relay 1( ) 0.4=Pickup relay 2( ) 0.2=Pickup relay 3( ) 0.8=Breakpoint relay 2( ) Breakpoint relay 1( ) CT ratio relay 1( )CT ratio relay 2( )----------------------------------------×=5.0 1000 5⁄2000 5⁄--------------------×= 2.5=Breakpoint relay 3( ) Breakpoint relay 1( ) CT ratio relay 1( )CT ratio relay 3( )----------------------------------------×=5.0 1000 5⁄500 5⁄--------------------×= 10.0=Breakpoint relay 1( ) CT ratio relay 1( )× 5.0 1000 5⁄× 1000= =Breakpoint relay 2( ) CT ratio relay 2( )× 2.5 2000 5⁄× 1000= =Breakpoint relay 3( ) CT ratio relay 3( )× 10.0 5000 5⁄× 1000= =
