Research On Common-mode Circuit And Method Of Common-mode Current Suppression For Brushless Doubly-fed Generation System

Due to the cancellation of the brush and slip ring,the brushless doubly-fed machine is more reliable than the doubly-fed induction machine and it has broad prospects in the field of variable speed constant frequency power generation.However,similar with ordinary induction machine,the brushless doubly-fed machine will be damaged by common-mode voltage and common-mode current because its stator windings are excited by voltage-source PWM converters.For the special structure of brushless doubly-fed machine,it is necessary to study the common-mode circuit model,common-mode current's flow paths,characteristics and suppression method.Based on the structure and operating principle of brushless doubly-fed machine,the double-loop control strategy and the slow-start control strategy of the back-to-back converters,and the structure and parameters of a 50 KW experimental platform for brushless doubly-fed stand-alone generation system.Steady state and dynamic characteristics of brushless doubly-fed generation system are verified by start-up experiment and sudden load(unload)experiment.The common-mode voltage,parasitic parameters and common-mode current loops of the brushless doubly-fed generation system are analyzed.The parameters of equivalent circuit model are obtained by measuring the resistance between stator power winding and the stator control winding.The characteristics of the third common-mode current loop is analyzed with emphases because this loop is different from induction machine and doubly-fed induction machine.The relationship between common-mode current amplitude and phase-difference,frequency-difference of the machine-side and load-side converters' common-mode voltages is obtained.The equivalent common-mode circuit model,parameter extraction method and common-mode current characteristics are verified by simulation and experiment.The modulation strategy for reducing common-mode voltage is studied.In this paper,the carrier synchronization control strategy is proposed,which reduces amplitude and eliminates low-frequency fluctuation of the common-mode voltage in loop 3.The modulation strategies and characteristics of AZSPWM1 and NSPWM are analyzed and compared.The adverse effects and improvement of switching delay on low common-mode voltage modulation are studied.The effect of carrier synchronization control and AZSPWM1 modulation on the common-mode voltage is verified by experiments.The method of suppressing common-mode current by common-mode transformer is studied.The structure and working principle of the common-mode transformer are introduced.The influence of the damping resistance on the suppression performance of the common-mode transformer is analyzed through the root locus.The effect of common mode transformer on common-mode current is verified by experiments.