Research On Three Phase Inverter Digital Control Technology And Short Circuit Current Limiting Strategy

As of providing high quality of power supply,stand-alone three-phase inverter is widely used in many areas,such as ship,aerospace,urban railway system,and renewable energy generation,etc.A high quality inverter needs a reasonable circuit design,structure design and other hardware design,moreover,the reasonable design of software structure,control strategy design and protection strategy design are also essential.Especially,the design of control strategy has great influence on the performance of inverter.Nowadays,with the development of digital control platform,digital control is widely used in inverters.And due to the mature analysis methods and convenient implementation,the traditional linear control strategies,such as PID control,are directly applied to the design of digital controllers in industrial application.However,as the control delay and the designing process approximation,most digital controllers do not achieve the desired performance indexes and its parameters need to be tested repeatedly when simply adopting traditional control strategies.Therefore,it is necessary to put forward a control strategy that can be applied directly to the experiment by the precise parameters tuning method of the controllers.Meanwhile,like the traditional power system,the supply system based on inverter may also occur symmetrical and asymmetric short-circuit fault in the load side.The consequences of short circuit faults may be fatal.Three-phase three-leg inverter is widely used because of its simple structure and low cost.However,due to its own structure constraints,three-phase three-leg inverter can not achieve the independent control of three-phase current.When a simple current limiting strategy based on internal loop limits is used in an asymmetric short circuit fault,the voltage limiting will occur in the healthy phase,which will affect the control of the short circuit current and directly affect the reliable action of the relay protection device.Therefore,the research of current limiting strategy of stand-alone three-phase three-leg inverter is significant.Based on the above introduction,the digital control and short circuit current limiting strategy of the three phase inverter are studied,and the main works of this paper are as follows:1)First,the continuous domain mathematical model of inverter is derived,including the model of no transformer and the model with ?-Yn output transformer.On this basis,the discrete mathematical model of the inverter is derived,which converts the digital control delay into an augmented state variable,and finally obtains the three order discrete domain state space model of the inverter.Besides,as the two experimental platforms in this paper are applied three level topology,a simplified three level SVPWM method is also introduced in this paper.2)As the control delay and the designing process approximation,most digital controllers do not achieve the desired performance indexes and its parameters need to be tested repeatedly when simply adopting traditional control strategies.In order to design the controller accurately,this paper proposes a double-loop control strategy in which proportional controller is used for current loop and PI plus state feedback controller is used for voltage loop.Particularly,the designing process of current loop and voltage loop is relatively independent,and the parameters of the voltage loop controller are precisely calculated according to the indexes of overshoot and bandwidth.The proposed control strategy and its parameters tuning method is verified through experiments.3)Based on the three-phase three-leg topology of inverter,this paper theoretically analyzes the mechanism of output voltage limiting when asymmetric short-circuit fault occurs.Then,a current limiting strategy proposed in an existing literature is introduced,and the shortcomings of this method are analyzed.Furthermore,this paper proposes a current limiting strategy based on virtual impedance,which suppresses voltage limiting by equivalently reducing load impedance by virtual impedance.And this method does not need to judge the type of fault and fault phase,thus simplifying the design process.In addition,this paper derives the implementation method of virtual impedance in the control system,and discusses the selection principle of virtual impedance from two aspects of restraining voltage limiting and ensuring system stability.Finally,the effectiveness of the proposed current limiting strategy is verified through comparative experiments.