Research On Control Strategy Of Current-source PWM Rectifier Under Unbalanced Grid Conditions

The development of super-conducting technology and advantages of Current Source Rectifier(CSR) itself such as controlling the output current more directly and implicit short-circuit protection make CSR have a widely application prospect. When CSR realizes constant voltage output, which means CSR has a voltage source load. This dissertation studies the Current Source Rectifier with Voltage Source Load and its control strategies. The main contents are as follows.Mathematical model of CSR is founded in a-b-c stationary coordinate, ?-? stationary coordinate and d-q rotating coordinate. The SVM modulation method of CSR is introduced. Moreover, the design of the parameters of the main circuit is discussed.The model in d-q coordinate can be simplified when the input capacitor voltage vector is served as the oriented vector,and on this basis a decoupling control strategy of dc-link and ac-link is proposed. State feedback control is used on the dc-side while active damping with virtual resistance is realized on the ac-side. The method makes the CSR system have fast dynamic response, good steady-state performance, low THD of grid-side current and run under nearly unit power factor.Under unbalanced supply conditions, direct use of aforementioned control method may worsen the performance of the CSR system. This paper analyzes the effect of unbalanced source voltage on the ac-side and dc-side of the CSR. Derived from complex power of the system, a compensation control scheme is presented to eliminate the harmonics in the dc current as well as suppress the distortion of grid-side current. In the proposed method, corresponding power components are set to calculate the ideal grid-side reference negative-sequence current, and then the corresponding control variables are superimposed on the state feedback control on the dc-side and active damping on the ac-side under balanced system, which makes independent control of three different frequency ranges. Comparing simulation results of before and after compensation proves that the method can eliminate the harmonics in the DC current and achieve sinusoidal input current.Finally, this paper describes a direct power control strategy of CSR based on hysteresis control. The key of the method is a direct selection of switching modes of the rectifier to control instantaneous active and reactive power. Therefore, a theoretical analysis on relationship between the instantaneous power and the switching modes of the rectifier is deeply investigated, which is essential to compose a switching mode table. Besides, a technique to maintain the switching frequency called dithering is introduced. Then, feasibility of the proposed strategy is verified by simulation. Particularly, the performance of DPC-CSR under unbalanced source voltage supply is discussed,furthermore, grid-side current distortion is suppressed by means of a notch filter, which is used to eliminate harmonics in the gird-side instantaneous power when makes calculation.