TABLE OF CONTENTS To take into account the magnetising currents and core configuration of multilimb transformers the EMTP package has developed a program based on the principle of duality [3]. The resulting duality-based equivalents involve a large number of components; for instance, 23 inductances and nine ideal transformers are required to represent the three-phase three-winding transformer. Additional components are used to isolate the true non-linear series inductors required by the duality method, as their implementation in the EMTP program is not feasible [4].
To reduce the complexity of the equivalent circuit two alternatives based on an equivalent inductance matrix have been proposed. However one of them [5] does not take into account the core non-linearity under transient conditions. In the second [6], the non-linear inductance matrix requires regular updating during the EMTP solution, thus reducing considerably the program efficiency.
Another model [7] proposes the use of a Norton equivalent representation for the transformer as a simple interface with the EMTP program. This model does not perform a direct analysis of the magnetic circuit; instead it uses a combination of the duality and leakage inductance representation.
The rest of this section describes a model also based on the Norton equivalent but derived directly from magnetic equivalent circuit analysis [8], [9]. It is called the UMEC (Unified Magnetic Equivalent Circuit) model and has been recently implemented in the EMTDC program.
The UMEC principle is first described with reference to the single-phase transformer and later extended to the multilimb case.
