Measurement-based frequency-dependent model of a HVDC transformer for electromagnetic transient studies

A wide-band, frequency-dependent five-terminal model is developed that represents one HVDC transformer unit in the French-English IFA2000 HVDC interconnection. Three such interconnected 1-ph units constitute one 3-ph transformer bank needed in 12-pulse conversion. The model is obtained via admittance frequency sweep measurements on the transformer's terminals, including common-mode measurements to capture the high-impedance coupling to earth at lower frequencies. The data set is modified to reduce the magnetizing current to a realistic level by a novel eigenvalue scaling procedure. The final data set is subjected to modeling by a stable and passive rational model while utilizing a mode-revealing transformation to retain the accuracy of the small eigenvalues that are related to the high-impedance coupling to earth. The paper describes details related to the measurements and modeling steps as well as the many intermediate accuracy validations that were done. Also is described challenges in the measurements that resulted due to interference of the measurements by a nearby 400 kV overhead line. The model is applied in a time domain simulation of the complete HVDC link in normal operation where voltage waveforms resulting from line commutations are compared against those by a classical model with added stray capacitances.