Amplitude-Frequency Characteristics-Based Parameter Estimation of Cascaded Transformers for High-Frequency Multilevel Inverters

Accurate transformer parameters are critical for predicting the behavior of the high-frequency cascaded transformer multilevel inverter (CTMI). Conventional root-mean-square-based parameter estimation methods work accurately for sinusoidal excitation but fail for square-wave-driven cascaded transformers due to unaccounted harmonic-induced frequency-dependent reactance variations. Therefore, this article proposes a novel parameter estimation method for cascaded transformers based on the amplitude-frequency characteristics of CTMIs. A decoupling model for the cascaded transformer considering magnetizing and winding parameters is developed. The relationship between transformer parameters and output voltage is analytically derived, considering dead-time effects and switch voltage drops. Magnetizing parameters are estimated from transient open-circuit test waveforms. Winding parameters are estimated using particle swarm optimization technique based on the fundamental component of CTMI output voltage measured at different frequencies. Compared to conventional methods, the proposed amplitude-frequency-based method achieves higher accuracy in estimating cascaded transformer parameters. The transformer output characteristics derived from these estimated parameters exhibit excellent agreement with measured values. The proposed method eliminates both the need for dedicated excitation signal equipment and transformer disconnection from the CTMI. Simulation and experiments in a 17-level CTMI operating from 5 to 30 kHz confirm its accuracy and practicality.