Predictive Current Control For Three-level Voltage Source Pwm Converter

Three-level voltage source PWM converter has been widely applied inhigh-voltage and high-power fields, because of their attractive features, such as smallcurrent harmonics, adjustable power factor, low voltage stress on each switch device,and low switch frequency. The voltage source PWM converters usually containcurrent inner loop control whose control objective is to ensure quick and accuratecontrol of the current and to reduce the tracking time as far as possible. Modelpredictive control has the fast tracking performance, and compared to other methods itdoesn’t need complex parameter design, doesn’t need to increase modulation moduleand is easy to realize in digital signal chip. So the model predictive control is wellsuited to apply to current inner loop for three-level voltage source PWM converter. Inthis paper the predictive current control for three-level source PWM converter isfocused on. For the delay in time, huge calculated amount and the voltage amplitudehigh jump in ac side when switching states switch in the conventional method, theimproved method is presented in this paper.Firstly, the mathematical model of three-level voltage source PWM converter isestablished, and the reason of DC side neutral-point potential imbalance are analyzed.Then, this paper analyses the principle of model predictive control, and based on thisanalysis establish the system of three-level voltage source PWM converter. In order toeliminate the predictive current control error by the delay, this paper adopts two-steppredictive control, which needs to make the mathematical model of the system shiftone step forward in time. In order to reduce the calculated amount, the improvedmethod adopts the strategy of reducing the selecting range, which means that theoptimal switching state applied at the next cycle only need to be chosen from a subsetof all possible switching states. This subset consists of the current state’s adjacentstates. By adding or subtracting two-level circuit switching states on the current state,the current state’ adjacent switching states can be achieved. Because the number ofthe states in the subset is less than half of the total number of states in the three-levelvoltage source converter, the calculated amount is greatly reduced. Because the stateonly switches between the adjacent states at every adjacent cycle, the amplitude jumpwill not be high. At last, the experiment system for three level voltage source PWM converter isbuilt. A contrast experiment was made between the conventional predictive currentcontrol and the improved predictive current control. The results show that theimproved method can effectively reduce the calculation time and avoid the high jumpin the phase voltage and line voltage’s amplitude, meanwhile the improved methodcan obtain good steady state and dynamic performance.