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 (L₀^oc) and short-circuit, that is, leakage, zero-sequence inductance (L₀^sc). 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 L₀^oc and L₀^sc 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.