Research On Control Strategies Of Star-Connected Cascade STATCOM In Unbalanced Condition

The cascade STATCOM has a very broad prospect in industrial application because of its advantages of small volume,low cost,easy redundancy and so on.In this paper,the main circuit of cascade STATCOM and its control strategy,especially the control strategy under unbalanced grid conditions are studied.Firstly,this paper introduces the background and significance of the research,summarizes and compares the reactive power compensation devices commonly used in the power system,and summarizes the outstanding advantages of STATCOM.Then introduces the cascade STATCOM structure,summarizes the research status of cascade STATCOM in two aspects.Besides,the main circuit structure and work principle of the cascade STATCOM are analyzed.The mathematical model of the decoupling device in the d-q synchronous rotating coordinate system is established by using the mathematical tools,and the steady state characteristics of the system are obtained.Secondly,in order to play the advantages of cascade STATCOM easy redundancy,this paper establishes a general simulation model of cascade STATCOM whose chain link is adjustable and control strategy uses the unitary design so as to adapt to different voltage levels,which making itself has different performance.And analyze the impact of the chain link change on parameters section of cascade STATCOM,providing a basis for selection for parameter selection.Then,according to the topology and working model of the chain STATCOM,the design of control strategy in the balance system is discussed in detail.For the realization of the dc capacitor voltage balance control,adopt the layered coordination control method,the upper control adopt voltage and current double loop control based on direct current decoupling control,the lower control adopts two levels of voltage stability and improved method of the recent level of approximation.Through the coordinated control of the upper and lower layers system,realized the reactive power transmission between device and the system and the balance of the voltage of dc power capacity of each link.The simulation model was established in PSCAD to verify the results.Finally,This paper analyzes the running characteristics of the chain STATCOM in the unbalanced system,and establishes the mathematical model.improves the control strategy of positive and negative double current inner loop.By reducing the output of STATCOM at the high degree of unbalance,the problem of over current of cascade STATCOM device and voltage unbalance of converter chain under unbalanced system condition is solved.PSCAD simulation was carried out on the model,it change the unbalanced degree by controlling the negative sequence voltage,negative sequence current reference within 200 ms to achieve stability,balance of three-phase converter chain effect significantly,it can satisfy fan net to a certain extent imbalance.Simulation verifies that the device is still not out of operation and has a certain amount of reactive compensation ability when the unbalanced degree reaches 50%.