Research On Key Technologies For Star Connected Cascaded H-bridge STATCOM

With the rapid development of the industry,more and more nonlinear,impacted and unbalanced loads are connected to grid,causing fluctuation,unbalance,flicker or even oscillation to the grid voltage.To improve the power quality and the stability of power system,the star-connected cascaded H-bridge converter is often chosen to implement STATCOM in medium/high-voltage and high-power applications due to its modularity structure,fewer devices,good scalability,improved harmonic performance,and good fault-tolerant performance.In this scenario,this thesis focuses on some key technologies on the cascaded H-bridge(CHB)STATCOM,as follows:Firstly,to explore the cluster voltage balancing control strategy,the cluster active power model of the CHB STATCOM is established.The conventional active power model is built in abc frame or dq0 frame,which includes lots of trigonometric functions,leading to a non-intuitive and nonlinear relationship between the cluster active power and the converter voltages.Hence,it is difficult to determine the required negative sequence voltage or zero sequence voltage to balance three cluster voltages.To overcome this demerit,this thesis establishes the cluster active power model in the dq frame.Not only the positive and negative sequence components but also the zero sequence components are converted to the dq frame.For the zero sequence components,we intentionally introduce an orthogonal zero sequence voltage,which lags 90° in respect to the original zero sequence voltage,and then applies PARK transformation for these zero sequence voltages to attain the dc-form expression in the dq frame.With this simple manipulation,a linear cluster active power model is achieved.In addition,it reveals that when three cluster voltage deviate from their reference,they are capable of reaching to stable voltage even without cluster voltage balancing control,which indicates that the CHB STATCOM has the cluster voltage self-stabilizing feature,which is verified by the simulation and experimental results,respectively.Secondly,to simplify the conventional cluster voltage balancing control,by using the dc-form expression of each variable in the dq frame and the linear cluster active power model,a simple cluster voltage balancing control is proposed,which directly adopts the proportional integral regulator to generate the control variables.In this scenario,the control variables do not need to be calculated indirectly by using the unbalanced active power,which thus eliminates the complicated inverse trigonometric functions and saves lots of sources for the DSP/FPGA-based controllers.Moreover,to improve the dynamic performance of the cluster voltage balancing control,a feedforward control is proposed.Based on the feedforward control,the safe range of the negative sequence grid voltage is derived by establishing the relationship between the negative sequence grid voltage and the maximum converter voltage.The proposed cluster voltage balancing control and the safe range of the negative sequence grid voltage are verified by the simulation and experimental results,respectively.Then,to reduce the negative sequence current introduced by the cluster voltage balancing control based on the negative sequence injection under the unbalanced grid,a new cluster voltage control scheme is proposed by regulating three phase cluster voltage to different values deliberately while maintaining three-phase modulation voltages balanced and their modulation indices highest under the unbalanced grid.Based on positive and negative sequence components extraction,it is interesting to find that the positive sequence voltages are highly related to the average value of three cluster voltages while the negative sequence voltages are highly related to the differences between the clustered voltages and their average value.To make three cluster voltages stable,the zero sequence voltages determined by cluster voltage differences are also extracted from the converter voltages to analyze the active power distribution.It is interesting to find that the total active power in each phase induced by both the negative and zero sequence voltages is equal to zero,which reveals the CHB STATCOM has the natural zero-active-power feature.Based on this feature,there is no need to inject any negative sequence current for the active power redistribution.Moreover,the algorithm for limiting the three different cluster voltage references within the safety range is investigated by establishing the relationship between the unbalanced grid voltages and the cluster voltages.The proposed cluster voltage control is verified by the simulation and experimental results,respectively.Finally,to improve the performance of compensating the negative sequence currents,a simple cluster voltage balancing control is proposed by using the dc-form expression of the zero sequence voltage in the dq frame,which avoids the complicated calculation,such as inverse trigonometric function and square root calculation.What is more,with the help of the proposed dc-form expression,an intuitive relationship between the negative sequence currents and the maximum converter voltage is established,which helps to achieve the accurate compensation region of the negative sequence currents in the dq frame.The proposed balancing control and the compensation region of the negative sequence currents are verified by the simulation and experimental results,respectively.