Research On Topology And Control Algorithms Of Z-source Three-level NPC Inverter

Traditional three-level NPC inverters have many problems,such as failing to provide voltage step-up and step-down functions,poor anti-interference ability,low voltage level,the existence of dead-time possibility,fragile breakdown for bridge arm,the possibility for leading to devices damage caused by electromagnetic interference,system collapse or interruption caused by sudden voltage increase or drop.Therefore,the traditional three-level NPC inverters have been greatly limited in the field of photovoltaic power generation and fuel cell,which need both boost capability and anti-interference capability because of the complexity of its environment.In order to solve these problems,a enhanced quasi-Z-source three level NPC inverter is proposed.Its advantages are as follows: firstly,compared with traditional Z-source three-level NPC inverters,the enhanced quasi-Z-source three level NPC inverters have higher boost capability and can be applied in situations with higher requirements for boost capability.At the same time,the higher boost capability which means that lower duty cycle and higher modulation can be used to obtain better output waveforms.Secondly,the input current of traditional Z-source three-level NPC inverters is intermittent,while the enhanced quasi-Zsource three level NPC inverters have continuous input current.Thirdly,initial surge current is restrained and the operation working for safety and stability are improved.Through the simulation analysis of MATLAB/Simulink,the correctness and feasibility of the corresponding the enhanced quasi-Z-source three level NPC inverter is verified.A new SPWM fuzzy control algorithm with neutral-point voltage balance capability is proposed for the neutral-point voltage deviation problem of Z-source three-level NPC inverter.The algorithm takes error and error change rate of the capacitance voltage as input of fuzzy logic controller,and the amplitude of DC reference signal is output of fuzzy logic controller.According to the direction and extent of the neutral-point voltage deviation,the algorithm regulates the upper-shoot-through time and lower-shoot-through time dynamically to achieve the neutral-point voltage balance,which can suppress the neutral-point voltage deviation effectively in the situation of achieving Z-source network boosting.This algorithm does not need to establish an accurate mathematical model.It is not only simple and practical in control algorithm,but also has strong reliability.It can suppress the deviation of the neutral-point voltage obviously.It can control the neutral-point voltage to fluctuate in a very small range with high control accuracy,fast control speed,and new equilibrium state can be achieved quickly with relative control accuracy remaining unchanged when voltage suddenly has dropped.It has excellent dynamic-state and steady-state performance,low total harmonic distortion of output voltage and good output waveform.At the same time,this algorithm does not increase the extra number of semiconductor commutations,and hence not increase the switching losses.Through the simulation analysis of MATLAB/Simulink,the correctness and feasibility of the SPWM fuzzy control algorithm to suppress deviation of the neutral-point voltage is verified.Aiming at the problem of neutral-point potential offset of Z-source three-level NPC inverters,an improved SVPWM control algorithm based on fuzzy logic controller is proposed.In vector state order,by inserting two upper-shoot-through vector states and two lower-shootthrough vector states into the traditional zero vector,the algorithm will increase the traditional seven-segment vector state order to eleven-segment vector state order.By inserting the shootthrough vector state of the minimum action times of switching devices,the loss of switching devices can be minimized,and the neutral-point potential balance control can be realized by controlling and adjusting the unique upper-shoot-through working state time and lower-shootthrough working state time of Z-source three-level NPC inverters in real time.The correctness and feasibility of the improved SVPWM control algorithm based on fuzzy logic controller is verified by the simulation analysis of MATLAB/Simulink.