Research On Cascaded H Bridge Based STATCOM And Its Control Strategy

Rapidly increased in recent years, the nonlinear, impact and unbalance loads indistribution network have brought a lot of power quality problems. Among them, thereactive power, harmonic and three phase imbalance problems are comparativelymore evident. As the most advanced equipment for reactive power compensation, thestatic synchronous compensator (STATCOM) can simultaneously solve all theaforementioned problems. With a lot of advantages, the cascaded H bridge basedSTATCOM has wide application prospects in distribution network. This paper studiesseveral key technologies of cascaded H bridge based STATCOM.The instruction current detection algorithms which can be used for fullycompensation are researched firstly for cascaded STATCOM to provide syntheticcompensation ability. The commonly used method of p q method, synchronousreference frame transformation method and FBD method have been analysed andcompared. Improved methods are proposed to avoid the disadvantages of them.Simulation results verify the improved methods. The detection algorithms which canbe used for selective compensation are also researched to make the compensationmore flexible. Several normally used methods are introduced, and then a newdetection algorithm utilizing the combination of synchronous reference frametransformation and discrete Fourier transformation (DFT) is proposed. Simulationresults prove that high detection precision and good dynamic performance can beachieved by using this algorithm.It is difficult to eliminate steady state error by using the traditional proportionintegral (PI) controller when the STATCOM needs to provide synthetic compensationability. To solve this problem, the resonant controller is used in this paper. Resonantcontrollers are used in dq frame to reduce the number of controllers. The PI Rcontroller which based on traditional resonant controller and the PI VPI controllerwhich based on vector proportion integral (VPI) controller are researched in this paper.The excellent control performance of the PI VPI controller is well proved bycomparing with the PI controller and the PI R controller. The design method ofrelevant parameters is given in detail. The effects of different discretization methodson the performance of VPI controller are analized in detail so as to abtain theoptimum methods.To simplify the controller, an instruction current tracking control strategy basedon repetitive controller is also researched in this paper. Repetitive controller and PI controller are used together to form a composite controller so as to improve theresponse character. In order to solve the problem of poor frequency adaptability ofrepetitive controller, a software phase locked loop (PLL) with fixed number ofsampling points is proposed. Simulation results verify the strategy.PWM modulation strategy is also studied in this paper. Compared with thecarrier phase shifting PWM, carrier disposition PWM (PD PWM) has better outputcharacteristics and is relatively easier. Based on this, an improved PD PWM methodis proposed in this paper. By using this method, the design will be simplified becausethe carrier wave number is only half of the ordinary PD PWM method.In allusion to the establish process of the DC bus voltage, a soft start strategy isused in this paper. By using this strategy, the DC bus voltage can be increased from0to reference value smoothly without big shock current, which ensures the safety startof STATCOM. To solve the DC voltage balance problem of cascaded STATCOM,control method including overall control, clustered control and individual control isproposed in this paper. A zero sequence injection method is used to realize theclustered balancing control. In allusion to the STATCOM which using PD PWM, asoftware individual balancing control strategy is proposed. The software strategy isbased on the energy balance principle of the inverter bridge. By using this method, thecharging and discharging time of the capacitors can be controlled by changing thedistributive rule of modulation signal according to the capacitor voltages. Simulationresults verify the strategy.Take6kV STATCOM (±3MVar) for example, the paper also introduces thehardware and software design. Through constructing an experiment platform, thereactive and harmonic compensation performance is tested, and the selectivecompensation function is also tested. The STATCOM has been applied to practicalprojects. Experiment and engineering application show that the STATCOM devicedesigned in this paper can achieve a good compensation effect, which verified therationality and effectiveness of the proposed control strategy in this paper.