TRANSFORMER ZERO SEQUENCE IMPEDANCE BASICS AND TUTORIALS

ZERO SEQUENCE OF TRANSFORMERS BASIC INFORMATION
What Is The Zero Sequence of Transformers?


It is usual in performing system design calculations, particularly those involving unbalanced loadings and for system earth fault conditions, to use the principle of symmetrical components. This system is described and and ascribes positive, negative and zero-sequence impedance values to the components of the electrical system.

For a three-phase transformer, the positive and negative sequence impedance values are identical to that value described above, but the zero-sequence impedance varies considerably according to the construction of the transformer and the presence, or otherwise, of a delta winding.

The zero-sequence impedance of a star winding will be very high if no delta winding is present. The actual value will depend on whether there is a low reluctance return path for the third-harmonic flux.

For three-limb designs without a delta, where the return-flux path is through the air, the determining feature is usually the tank, and possibly the core support framework, where this flux creates a circulating current around the tank and/or core framework.

The impedance of such winding arrangements is likely to be in the order of 75 to 200% of the positive-sequence impedance between primary and secondary windings. For five-limb cores and three-phase banks of single-phase units, the zero-sequence impedance will be the magnetising impedance for the core configuration.

Should a delta winding exist, then the third harmonic flux will create a circulating current around the delta, and the zero-sequence impedance is determined by the leakage field between the star and the delta windings. Again the type of core will influence the magnitude of the impedance because of the effect it has on the leakage field between the windings.

Typical values for threelimb transformers having a winding configuration of core/tertiary/star LV/star HV are:

[Z0]LV approximately equal to 80 to 90% of positive-sequence impedance LV/tertiary

[Z0]HV approximately equal to 85 to 95% of positive-sequence impedance HV/tertiary

where Z0 = zero-sequence impedance.

Five-limb transformers have their zero-sequence impedances substantially equal to their positive-sequence impedance between the relative star and delta windings.