A New Maximum Step Voltage Calculation Method of On-Load Tap-Changer for Symmetrical Two-Core Phase-Shifting Transformer

In order to select a reliable and economical type of transformer on-load tap-changer (OLTC), it is necessary to accurately determine the maximum step voltage in the process of engineering design. Because both configuration and operating conditions of a phase-shifting transformer (PST) are more complicated than those of conventional transformers, it is far harder to accurately calculate the maximum step voltage. The existing calculation methods have the shortcomings of inadequate accuracy or weak applicability. To solve these problems, based on the equivalent circuit of a PST, this paper at first derives the quantitative relations between operating conditions and the step voltage. Subsequently, the effect of different load conditions, including the load current and the phase angle difference between the load voltage and current, on the step voltage is fully investigated. This investigation reveals a law about the changes in step voltage under different load conditions. Next, a new calculation formula for the maximum step voltage is proposed, and its accuracy is verified with two practical examples. Finally, an error analysis is carried out to demonstrate the generality of this approach. The calculation method offers appealing accuracy and is very suitable for the economical and reliable selection of OLTC for symmetrical two-core PSTs in the process of engineering design.