Electrical Power Systems

236 Electrical Power Systems
ig. 9.5: Zero-sequence equivalent circuit of transformers.
equivalent series leakage impedance. Transformer is a static device and if the phase sequence
is changed, leakage impedance will not change. Therefore, positive, negative and zero-sequence
impedances are same and equal to leakage impedance of the transformer. i.e.,
Z 1 = Z 2 = Z 0 = Z l
...(9.58)
The equivalent circuit for the zero-sequence impedance depends on the winding connection
and also upon whether or not the neutrals are grounded. igure 9.5 shows some of the common
transformer configurations and their zero sequence equivalent circuits.
(a) - connection: Both neutrals are grounded and there is a path for the zero sequence
current to flow in the primary and secondary. igure 9.5(a) gives the equivalent zerosequence
circuit connection.