Research On Grid Side Converter Control Of Brushless Doubly Fed Generation System Under Unbalanced Grid Voltage

Brushless doubly-fed induction generator is getting more attention in the field of wind power due to the cancellation of the brushes and slip rings, improving the stability of the system operation and reducing maintenance costs. Its excitation converter consists of two PWM converters, respectively called machine side converter(MSC) and grid side converter(GSC). When the grid voltage is unbalanced, traditional control strategy will generate even order harmonics in the GSC DC side and odd harmonics in AC side. So, finding a control strategy for GSC under unbalanced grid is becoming a key issue. Based on GSC, this paper investigated the working principle and analyzed the power in AC side and DC side, designed the controller under unbalanced grid voltage in detail.Firstly, the mathematical model of GSC with L filter under unbalanced grid voltage under dq coordinate were derived. In order to control the DC bus voltage of GSC, double closed loops were used. According to overall goals of the generator sets and the freedom of grid side converter four grid side converter control strategies can be achieved, and accordingly inner loop current commands can be deduced respectively.The introduction of the R controller in inner loop increased the order of the controller, significantly increased the difficulty of controller design. Now widely adopted approach is to design PI controller firstly, then introduce a general R controller in paralell. Finally, adjust the parameters in order to obtain a better static and dynamic effects. This paper presented a comprehensive design method to design PIR controller parameter, avoiding the uncertainty of the former. In order to extract phase information of the unbalanced grid voltage, three kinds of phase-locked loop were introduced and compared.Finally, this paper designed a GSC experimental prototype, which consisted of the main circuit and the control circuit. To verify the feasibility of control strategies and the validity of the designed current inner loop controller, several experiments under balanced and unbalanced grid voltage were carried out.