Research On Cascaded Multilevel Inverter Based STATCOM And Its Control

Together with the raised requirement of electric power quality, Flexible AC Transmission System (FACTS) has been rapidly developed as an efficient technology to improve electric power quality. As one of the key technology of FACTS, Static Synchronous Compensator (STATCOM) has currently become a hot topic of the research on Flexible AC Transmission equipment because of its effective control and good compensation performance. Most of the STATCOMs that have been in use home and abroad nowadays are made up of multipulse inverters and zigzag transformer, which can satisfy the demand of capacity to some extent. However, these equipments are expensive and bulky, and it is difficult to design the controller owing to the nonlinear characteristic of zigzag transformer. This dissertation presents a STATCOM which adopts cascaded multilevel inverters as main topology to replace the multi-pulse inverter and zigzag transformer, and carrier phase-shifted SPWM (CPS-SPWM) to replace the fundamental frequency modulation. The proposed STATCOM based on CPS-SPWM cascaded multilevel inverters can eliminate the zigzag transformer, and the high equivalent switching frequency can be obtained as well with low switching frequency devices, which simplifies the output filtering. In accordance with the strongly coupled and nonlinear characteristic of electric power system, this thesis presents a controlling strategy to apply the auto-disturbance rejection controller (ADRC) to STATCOM, which can not only shorten the time for dynamic response and produce good dynamic performance, but also ensure very good robustness. The studies carried out in the thesis and the corresponding results are elaborated as follows.(1) A unipolar dual-frequency CPS-SPWM method is presented to overcome the conflict between switching frequency and switching capacity in power devices. After a mathematic analysis of bipolar SPWM and unipolar SPWM, which are two methods of single inverter, it is found that unipolar SPWM has great advantage over bipolar SPWM. Then they are applied to cascaded multilevel inverter together with carrier phase-shifted technology, and the corresponding bipolar CPS-SPWM method and unipolar dual-frequency CPS-SPWM method are obtained. Based on a detailed mathematical analysis and comparing, it can be concluded that higher equivalent switching frequency can be obtained with low switching frequency devices. Bipolar CPS-SPWM heightens the equivalent switching frequency to N times of the switching frequency (N is the number of cascaded inverter per phase), and unipolar dual-frequency CPS-SPWM heightens the equivalent switching frequency to 2*N times. It can be clearly seen that the latter one can achieve better harmonics performance and more simple output filter.(2) A STATCOM topology is proposed based on unipolar dual-frequency CPS-SPWM cascaded multilevel inverters because the zigzag transformer in conventional STATCOM is expensive and bulky, creates much loss and it is hard to design the controller owing to the magnetic nonlinear characteristic. Because of such merits of cascaded multilevel inverter as the same cell structure, less power devices, and greater reliability to realize the modular, the proposed STATCOM can eliminate zigzag transformer and overcome the above disadvantages. At the same time, the DC side doesn't need active power supply, then the cascaded multilevel inverters can be taken greatest advantage of. The unipolar dual frequency CPS-SPWM method is adopted to replace fundamental frequency modulation method, then 2*N times of switching frequency devices can be achieved, which makes it more simple to filter low-order harmonics.(3) A mathematical model is created for STATCOM in abc three phase coordinates and dq two phase synchronous coordinates as well as corresponding equivalent circuits. After the reactive power compensation principle of the STATCOM is analyzed, the stability performance is carried out with the STATCOM transmission function in open loose. A conclusion can be made that the system can run steadily with any control angleδand magnification factor K. These analyses can provide theoretic basis for the designing of the controller.(4) Control strategy of STATCOM is thoroughly studied on the basis of the above theoretic analysis. Because of the charge and discharge of DC capacitor, the conventional PAM-based control scheme has the disadvantage of slow dynamic response. In the thesis, the PWM-based control scheme is adopted which can fasten the response. A kind of state feedback decoupled PI controller is presented on the basis of PARK transformation. It changes the three phase symmetry AC variables into two phase DC variables which could be controlled independently, with fast dynamic and non-steady error after stability. Simulation system is constructed for STATCOM with MATLAB SIMULINK. The simulation results are able to prove the presented STATCOM's ability to compensate for reactive power, and to verify the controlling performance of the state feedback decoupled PI controller.(5) ADRC based on extended states observer (ESO) is a nonlinear robust control skill for uncertain model. It regards the uncertainties and disturbances from the circumstance outside as the total disturbance that can be evaluated and compensated automatically. So ADRC possesses the advantages of adaptability and robustness. Owing to the strongly coupled and nonlinear electric power system, it is proposed in this thesis to apply ADRC to STATCOM. On the foundation of the study of the realizing method and principle of the parameters adjustment, a simulation model of ADRC for STATCOM is constructed with the analysis of operation principles. After comparing the ADRC with the PID, a conclusion can be drawn that ADRC possesses faster dynamic response and has no steady error with the same model parameters. What's more, ADRC's dynamic performance cannot be affected with the changing of model parameters and load. Therefore, ADRC has better robustness than PID.(6) An experiment prototype of the STATCOM based on unipolar dual-frequency CPS-SPWM cascaded 5 level inverters is designed on the basis of the theoretic analysis and simulation study. DSP and FPGA are adopted in the STATCOM control system, in which DSP is mainly responsible for signals sampling and control calculating, and FPGA mainly implements the 24 PWM pulse generator for 5 level PWM inverters. System experimental results prove that the STATCOM based on unipolar dual-frequency CPS-SPWM cascaded 5 level inverters can achieve excellent compensation effect at much low switch frequency, and can enlarge equipment's capacity in terms of system's voltage level, which is of perfect prospect in industry application.