Research On Control Strategy Of Cascaded H-Bridge STATCOM

As our country's modernization process is accelerating,the industrial load and the living load of urban and rural residents are increasing,and the problem of reactive pollution introduced is becoming more and more serious.As a kind of reactive power compensation equipment,STATCOM has good dynamic performance and compensation effect,making it the mainstream of the market.Among them,the cascaded H-bridge STATCOM is widely used in high-voltage and high-power applications due to its advantages such as modular multi-level structure and high compensation accuracy.In this paper,the 10kV/1Mvar cascade H-bridge STATCOM is taken as an example to study the control strategy under balanced conditions and unbalanced conditions.The star-connected cascaded H-bridge STATCOM has the advantages of a small number of modules and easy control at the same voltage level.In this paper,a mathematical model is established for the star cascade H-bridge STATCOM,and the main circuit parameters are designed.In order to further analyze the internal power unit principle,a switching function model of each H-bridge unit is established.In this paper,the characteristics and applicable occasions of different modulation strategies are compared and analyzed.Finally,the CPS-SPWM modulation strategy with low output harmonic content and high equivalent switching frequency is selected.For balanced operating conditions,the capacitor voltage imbalance caused by factors such as differences in capacitance losses.This paper adopts a three-level equalization control strategy,namely overall voltage equalization control,phase-tophase voltage equalization control and phase-in-phase voltage equalization control.The current inner loop adopts direct current control for tracking compensation of reactive power,and the voltage outer loop is used to maintain overall active balance.For the problem that the original phase-to-phase equalization control strategy has static error,this paper improves its proportional control link and gives the basis for parameter design.In this paper,an improved phase-integrated capacitor voltage equalization control strategy is proposed to solve the problem that the active vector superposition control strategy is prone to oscillation due to the introduction of output current.A parameter design scheme is proposed based on power compensation and maximum adjustment range.Aiming at the error problem of positive and negative sequence component separation strategy under unbalanced conditions,this paper proposes a separation strategy with good dynamic performance and high accuracy based on the principle of double dq transformation.This strategy realizes the accurate separation of each sequence component.At the same time,it is also possible to achieve accurate phase locking of the asymmetric grid voltage.In this paper,the power model is established and analyzed for the phenomenon that the phase-to-phase capacitance voltage is unbalanced under unbalanced conditions.The zero-sequence voltage injection strategy is used to solve the problem,and the control link is designed.Finally,the effectiveness of the proposed strategy and the accuracy of the parameter design are verified by simulation and experiment.