ABB REL 356

ABB REL 356 Current Differential Protection
5-21
Testing
15. Reverse External Faults
For reverse Zone 3 Phase-Phase External faults use the Table 5-15 of voltages and currents. In each case apply the 3-
phase voltage to the relay system first then suddenly apply the currents listed. In each case the relay system should
not trip as these faults are reverse direction from the PANG setting.
Table 5-15. Zone 3 Phase-Phase Reverse External Faults: Z3P = 7.0 Ohms
Volts
∠
Angle
I
Amps
∠
Angle
Fault Z
Ohms
∠
Angle
Relay System
Operation
AB at MTA -180˚ 50% of reach fault
Va = 17.3
∠
0 Ia = 4.28
∠
-225 (135) 3.5
∠
-105
Vb = 17.3 ∠ -120 (240) Ib = 4.28 ∠ -45 (315)
Vc = 69 ∠ -240 (120) Ic = 0
No Trips
BC at MTA -180˚ 50% of reach fault
Va = 69
∠
0 Ia = 0 3.5
∠
-105
Vb = 1.73 ∠ -120 (240) Ib = 4.28 ∠ -345 (15)
Vc = 1.73 ∠ -240 (120) Ic = 4.28 ∠ -165 (195)
No Trips
CA at MTA -180˚ 50% of reach fault
Va = 17.3
∠
0 Ia = 4.28
∠
-285 (75) 3.55
∠
-105
Vb = 69 ∠ -120 (240) Ib = 0
Vc = 17.3 ∠ -240 (120) Ic = 4.28 ∠ -105 (255)
No Trips
3-Phase Units
To calculate the fault impedance seen by the relay system the following formula applies:
Z fault =
Vxg
Ixg
-------------
-
where x is either phase a, b, and c phases.
The above formula is rigorous and general. However, if a quick approximation of the minimum trip current required at
different angles (q) is desired, the following formula applies:
V
XG
Z2P (cos (PANG - q))
---------------------------------------------------------
-
lxg =
where x is either phase a, b, and c phases.