Research On Cascade H-bridge Multi-level STATCOM's Control Strategy

There are lots of inductive loads and unbalanced loads in the power system,such as low power factor,harmonic pollution,voltage level is not normol and other power quality problems become increasingly serious.STATCOM can be a good solution to solve above-mentioned power quality problems,STATCOM also represents the latest reactive power compensation technology and the latest FACTS technology development direction,plays a vital role in the power system safe and stable operation.In this paper,the research object is cascade H-bridge multi-level STATCOM,discuss the control strategy in the equilibrium state and the control strategy in the load unbalanced state.The main research contents and innovations are as follows:This paper introduces the topology of cascaded H-bridge and the working principle of reactive power compensation,according to the equilibrium state of the topology and a series of mathematical derivation,get the control strategy below the balanced state.The analysis of STATCOM operation in unbalanced state is also analyzed,it is pointed out that the only solution of the phase current command of the STATCOM is the key to the reactive power compensation,and then gives the control strategy of the cornering type under the load unbalanced state.For the DC side of the capacitor voltage imbalance problem,this paper first analyzes the DC side of the capacitor voltage imbalance reasons,and then put forward control strategy which superimposed a signal that is deviated from the voltage after obtaining the modulated wave.At the same time,we analyze and discuss the overall control strategy of cascade STATCOM,that is the upper control strategy and the lower control strategy to achieve the STATCOM reactive power compensation function,and through the simulation to verify the correctness of the control strategy,both for conducts reactive power compensation,but also balances the DC side capacitor voltage.For the problem of the only solution of cornering type STATCOM phase current directive,this paper uesd the phase current and the line current of the compensations device to solve this problem.At the same time,using the instantaneous reactive power detection technology theory,first,the negative phase compensation phase current command calculation method is deduced,and then derive the reactive power compensation and harmonic compensation phase current command calculationmethod,finally,according to the calculation achieve synthetic compensation phase current calculation method.Load imbalance uses the above-mentioned comprehensive compensation phase current command calculation method,the correctness of the control strategy is verified by simulation,at the same time compensates reactive power,the negative sequence component,reactive component and harmonic component can be compensated,filters out the harmonics in the load current.