Decoupling Model and Parameter Estimation of High-Frequency Cascaded Transformers Used for Multilevel Inverters

Cascaded transformers are the core component of multilevel inverters. Conventional studies tend to assume transformers are ideal. However, when inverters operate at high frequencies, the excitation and winding parameters of cascaded transformers significantly affect the inverter characteristics. This article proposes a decoupling model and parameters estimation method for the cascaded transformers. The cascaded transformer is simplified from a complex structure with multiple inputs and two outputs to a two-port network. The equivalent model does not change the output characteristics of the cascaded transformer and greatly simplifies the calculation. Furthermore, since cascaded transformers used in multilevel inverters are excited by square wave voltage, the parameter estimation of conventional methods is inaccurate. With the proposed method, the excitation parameters are estimated using no-load transient waveforms, and the winding parameters are estimated by load voltage fitting. The effectiveness of the proposed model and method is examined through simulation in a 17-level cascaded transformer multilevel inverter with a 30 kHz fundamental frequency. The proposed method has a higher accuracy of parameter estimation compared to the conventional method.