ABB Relion Technical Manual page 112

Generator protection

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Differential protection

"steady state"

neg. seq. phasor

Contribution to neg. seq. differential current from HV side

Contribution to neg. seq. differential current from LV side

IEC05000189 V1 EN

Trajectories of Negative Sequence Current Contributions from HV and

LV sides of Yd5 power transformer during external fault

Under external fault conditions, the relative angle between the phasors is theoretically

equal to 180 degrees. During internal faults, the angle shall ideally be 0 degrees, but due

to possible different negative sequence source impedance angles on the W1 and W2 sides

of the protected power transformer, it may differ somewhat from the ideal zero value.

However, during heavy faults, CT saturation might cause the measured phase angle to

differ from 180 degrees for an external, and from 0 degrees for an internal fault. See

for an example of a heavy internal fault with transient CT saturation.

"steady state"

neg. seq. phasor

1MRK 502 048-UUS A

en05000189.vsd

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