Analytical description for zero-sequence characteristics of five-legged core construction in transformers

The zero-sequence characteristics of a transformer are predominantly decided by its core construction. An analytical description for these characteristics of a five-legged transformer is presented in this study. The transformer is analysed by developing a simple reluctance model and by using the theory of sequence components. Two types of zero-sequence inductances are discussed, viz. open-circuit, that is, magnetising, zero-sequence inductance (L0oc) and short-circuit, that is, leakage, zero-sequence inductance (L0sc). The magnetising zero-sequence inductance is expressed in terms of the inductances contributed by flow of magnetic flux through the end-legs and its analytical expression is developed. Zero-sequence characteristics of a three-legged transformer are compared analytically with that of a five-legged one, thus highlighting the effect of end-legs. A practical five-legged transformer used in a 12 V, 5000 A rectifier system is considered for the purpose of experimental verification. The magnetising zero-sequence inductance for this transformer is computed by developing its reluctance model. The short-circuit zero-sequence inductance is shown to be equal to the corresponding positive-sequence inductance and its value is computed after conducting standard open-circuit and short-circuit tests on the transformer. The rectifier system is modelled using coupled circuit-field formulation by developing a non-linear-transient finite element code in ANSYS Parametric Design Language (APDL) and values of L0oc and L0sc are computed. The results obtained from the reluctance model and the APDL are validated experimentally by performing zero-sequence tests on the five-legged transformer.