New Equivalent Circuit Model Of Uhvdc Converter Transformer Winding For Surge Propagation Investigation

An accurate computational model of transformer winding for potential distribution analysis under impulse voltage is very important for the design of transformer inter-turn insulation, especially for large capacity transformers such as ultrahigh-voltage DC (UHVDC) converter transformer. Quite a lot of equivalent circuit models for transformer winding have been proposed assuming that the influence of magnetic core is negligible at frequencies >10kHz, which is not always valid in practice. Lightning impulse or very fast transient overvoltage waveforms usually contain abundant frequency components >10kHz. To obtain a more accurate model, here, a new equivalent circuit representation of UHVDC converter transformer winding is given taking into consideration of comprehensive frequency characteristic of core lamination stack. Taking account of the skin effect, the equivalent relative permeability of lamination stack are investigated and implemented in the calculation of frequency-dependent parameters such as resistance, self-inductance, and mutual inductance. The equivalent circuit model is realised in ATP and the surge propagation process in the winding; the winding potential distribution under transient overvoltage is studied carefully. The representation of the iron core loss in the equivalent circuit is discussed. The new proposed model may provide more accurate winding potential distribution prediction for transformer inter-turn insulation design.