Research On Key Techniques Of Cascaded STATCOM With LCL Filter

Staic Synchronous Compensatror (STATCOM) is an important power electronicequipment for both reactive compensation and harmonic suppression, which can solvethe problems of power quality, such as lack of reactive power, voltage fluctuation andflicker, harmonic pollution of power grid and so on. Compared with the multiplextransformer structure of STATCOM, Cascaded STATCOM is widely used andconcerned for its following advantages of a small footprint, high efficiency, modulardesign and so no. The design of main circuit parameters and control strategy hasimportant practical value as well as academic value. To solve the above keytechnologies, the research in this dissertation was put forward according to cascadedSTATCOM with LCL filter.Based on the analysis of main circuit and mathematicl model of cascadedSTATCOM with LCL filter, the dissertation deduces the mathematical model in thea-b-c and d-q coordinates under the principle of coordinate transformation. From themathematical model, it can be seen that cascaded STATCOM with LCL filter is atypical high-order, nonlinear systems with multiple variables, close coupling anduncertain parameters. The device choices and detailed designs of IGBT powermodules, dc bus capacitor of H-bridge and its reference voltage, number of H-bridgechain link and snuuber circuit of suppression of IGBT turn-off voltage are given inthis dissertation.LCL filter can achieve the same filter performance with smller inductance valuescompared with L filter, which not only brings cost savings and smaller size, but alsoraises dynamic performance. For third orders nonlinear characteristic, changes inparameters of LCL filter can affect filter performance, resonant frequency, ripplecurrent suppression performance and voltage loss, which is the reason of complexparameters design. Based on the further study of basic principle and characteristics ofLCL filter, a parameters design method suitable for engineering applications isproposed with the following four design steps. The parameters design methodproposed in this dissertation is simple and the physical meaning is clear, which is alsopractical for engineering application.The artificial fish swarm algorithm (AFSA) is adopted to optimize parametersdesign of LCL filter. The traditional parameters design method picks out a plan at thelowest cost on the condition that certain performance indicators and given constraints can be sastisfied, which can not guarantee the best filtering effect. An optimizationparameters design of LCL filter by AFSA is given with Matlab language. By theAFSA, the best filtering performance parameters of LCL filter can be picked outquickly and effectively, which has certain values for theory study and applicationresearch.A grid side current feedback control method using PI controller and typicalsecond order low pass filter as inner current loop and repetitive control as outercurrent loop is proposed. The open-loop magnitude margin and close-loop stability isimproved and ensured by inner current loop. Steady-state performance of current loopis improved by the above double-loop control, ensuring both fast dynamic responseand small steady-state error for current track and compensation in the banwidth ofcascaded STATCOM. For the demands of selective harmonic compensation, a methodfor detecting the harmonic command current is proposed with the combination of2-step band pass digital filters and2nd-order digital lowpass filter which can savememory resource of FPGA. Finally the detailed design of the two above filters ispresented.The mathematicl model of active power exchange between cascaded STATCOMand the grid is developed, from which capacitive voltage balance control influence ofnegative sequence current is gained. Under this circumusance, the balance control ofcapacitive voltage at the DC side under three-phase unbalanced condition is proposedby three levels: overall balance control, phase inside balance control and phase tophase balance control. The proposed method not only enhances the reliability ofcascaded STATCOM with LCL filter, but also solves the difficult capacitive voltagebalance control problem effectively.Based on the further research of related theory and key technique of cascadedSTATCOM with LCL filter in this dissertation, the prototype device is developed withthe capacity of±2.8Mvar and the voltage class of6KV. A detail design procession isgiven. Experiments are carried out to investigate effects of reactive power andharmonic compensation. The results show that the design in this dissertation is correctand effective. Finally, the propotype device is applied in a coal mine of SHANDONGENERGY GROUP CO., LTD. The measuing data before and after compenstationproves that the cascaded STATCOM with LCL filter designed in this dissertation canrealize an ideal dynamic reactive compensation and harmonic suppression effect,which reaches the design target.