Experimental Study of the Dynamic Short-Circuit Withstand Capability of an 8400 kVA Power Transformer Specially Designed for Photovoltaic Applications

Abstract

This article, besides offering data of great value for any designer of high-power short-circuits of special three-winding design, illustrates the correlation with the corresponding FRA measurements, validating this type of measurement. The frequency response measurements can provide data about the transformer’s status after it is put into service, its vulnerability in incipient states, and, particularly for this type of transformer, its insulation, which is the subject of high dielectric stress due to its invertor working regime. This article presents the behavior of a three-phase 8400 kVA medium-voltage step-up transformer (corrugated hermetic tank) specially designed for photovoltaic applications during short-circuit tests. This transformer, fed by two inverters, has two secondaries with elliptical windings (non-circular aluminum foil for LV windings and an aluminum conductor for HV windings). Various experiments were performed, including measurements of winding resistance, measurements of voltage ratio, measurements of short-circuit impedance and load loss on three tappings, measurements of no-load loss and current, a frequency response analysis, and short-circuiting. These experiments were performed to study the behavior of the transformer, which, in real life, is powered by photovoltaic inverters on the LV side that feed into the MV grid on the HV side, making it the interface between the photovoltaic inverter and the MV grid. An auxiliary supply transformer may be connected to the LV side. Given these elements, concerning both the importance and the particularities of the problem studied, we can say that this article represents a niche study on the guarantee of good functioning and safety in operation given the passing of the test to withstand the dynamic effects of short-circuiting.

Revisited Concept of Three-Phase Transformers’ Short-Circuit Resistances in Light of the Institute of Electric and Electronics Engineers (IEEE) Standard C57.110-2018

“…according to the load losses (P ccN ) and secondary current (I sN ) nominal values, determined after performing a short-circuit test at the industrial frequency (50-60 Hz) [33][34][35][36] or, by any of the procedures described in references [37][38][39][40][41][42][43][44]. The nominal short-circuit resistance (R ccN ) defined by Equation ( 2) is a parameter independent of the frequency of the winding current harmonics and has the same values in each phase of the three-phase transformers.…”