A method for constructing a reduced order transformer model for system studies from detailed lumped parameter models

All power transformer manufacturers maintain computer programs that compute the internal transient voltage distribution when the transformer is subjected to transient voltages. This information is used to design the insulation structure for the transformer. Utility engineers also need to represent the power transformer in some detail in their system studies since significantly higher failure rates on large EHV units suggest the transient voltage that the system places on the terminals of the transformer is in some measure a function of the impedance characteristic of the transformer. A reduction technique is presented which uses the detailed lumped parameter transformer model as a starting point and allows its reduction to any size specified by the user. The method is straightforward mathematically and while retaining the physical configuration of the transformer does not require any proprietary information from the transformer suppliers. The necessary mathematics are presented, and the method is illustrated using a 500 MVA auto-transformer