Research On Cascaded H-bridge Five-level STATCOM With VFCB-PWM

As one of the important members of FACTS,Static Synchronous Compensator(STATCOM) has widely applied all over the world.Because STATCOM has advanced control performance,it has become the direction of development research in the world's SVC.At first,as equipment capacity becomes bigger and bigger,and then requirements of output waveform harmonic spectrum become higher,main circuit of the multi-level STATCOM becomes the direction of development in the future.The various five-level circuit topology are analyzed, such as,diode-clamped,flying-capacitor,cascaded H-bridge.The work type of cascaded H-bridge five-level topology is described in detail.The STATCOM main cicuit topology is presented.Then,a mathematical model is created for cascaded H-bridge five-level STATCOM in abc three phase coordinates and dqO two phase synchronous coordinates as well as corresponding equivalent circuits.After the reactive power compensation principle of the STATCOM is analyzed. These analyses can provide theoretic basis for the designing of the controller.Subsequently,the fast uniform rotation current detetion of vitual form is proposed.The problem of asymmetric three- phase power grid is resolved,as well as,the delay is reduced to 1/6 of source period.The rapid current detection is achieved.The response time is shortened.The speed of dynamic compensation is increased.The phase locked loop is eliminated.The computation is simple.Based on reactive current compensation as the main objective of STATCOM mathematical model,a direct current control,an indirect current control and other control strategies are given.Taking into account the size and capacity of applications,the constant voltage and real-time triangle wave comparison of direct current control strategy is proposed.In the next part,through the further investigation of five-level PWM control method and their output voltage harmonic characters,hybrid frequency carrier-based PWM control method (VFCB-PWM) suitable for cascaded H-bridge five-level inverters is presented.Furthermore,the harmonic characters of five level PWM methods are compared.Compared with these waves, VFCB-PWM can balance the upper,the bottom and the intermediate of the switching frequency and reduce output voltage harmonics.At last,the model of cascade five-level STATCOM is set up by simulink tools in MATLAB. With the fast uniform rotation current detetion of vitual form,the constant voltage and real-time triangle wave comparison of direct current control strategy and VFCB-PWM modulation method, the simulative results show that this method can achieve STATCOM of the fast current detetion, the harmonic suppression and the fast reactive power compensation under the imbalance system.