Analysis of the Medium-Frequency Oscillation Issue in a Medium-Voltage High-Power Wind Energy Conversion System

One promising solution for future large wind turbine power conversion is to use medium-voltage power conversion systems for high efficiency and high power-density, e.g., by using a cascaded structure presented in this paper. However, a medium-frequency (MF) oscillation issue was observed while testing the experimental prototype of the above system. Given the complex multivariable coupling models of the multiwinding transformer and multiple inverters, conventional single-variable theories are not suitable. Therefore, this paper presents a new decoupling analysis method. Using a proposed linear transformation matrix, the output currents of multiple inverters can be transformed to a summation (grid) current and difference currents, where the former is the summation of inverter currents and the latter are the differences between currents of any two inverters. This transformation has enabled the use of single-variable theories, e.g., Bode diagram to analyze the summation current and difference current separately. It was found that the MF oscillation issue is caused by the equivalent impedance of the summation current being much larger than that of the difference current. Specifically, using small controller parameters cannot ensure the grid current quality and using large parameters may cause oscillation in the difference current. The proposed method can provide guidelines to design the parameters of current regulators and filters in multiple-inverter systems. Experimental results of a downscaled wind energy conversion system are presented to verify the proposed method.