Research On Z-source Three-level Inverter And Its Control Methods

In order to exploit and utilize the intermittent renewable energy resources, such as solar and wind energy, micro-grid technology is widely concerned. As the interface parts of micro-grid, inverter is the bridge of energy interaction between distributed powers, grid and loads. For its advantages in the quality of output waveforms, switching losses and voltage stress, three-level inverter has the widespread application in micro-grid. However, as the traditional voltage source inverter, three-level inverter can only step down the output voltage. So a two-stage inverter topology, which adds a boost link to the original inverter topology, is necessary for the inverter to operate in grid-connected mode. But this topology increases the complexity of the control and decreases the efficiency of the system.Z-source three-level inverter is proposed to solve the problem mentioned above, which is combined by Z-source network with the characteristic of voltage-boost and three-level inverter. So, this topic is a study on Z-source three-level inverter, and the main research content and goal are as follow:Firstly, this paper elaborates the topology and work principle of Z-source three-level inverter. The equivalent circuits in shoot-through and non-shoot-through state are analyzed with Kirchhoff’s circuit rules, and the mathematical model of Z-source three-level inverter is established by using the volt-second balance principle of Z-source inductance. Then the relationship between the output voltage of inverter and shoot-through duty ratio is confirmed and the control objectives of Z-source three-level inverter is explicit.Secondly, by analyzing the basic theory of SVPWM and neutral-point unbalance mechanism of Z-source NPC three-level inverter, two conlusions can be gotten: keeping upper and lower shoot-through time equal is the condition to make shoot-through state have no impact on neutral-point potential; changing the working time of redundant small vectors can balance the neutral-point potential. In order to realize decouping between the neutral-point potential balance control and Z-source boost control, this paper proposes control strategy for Z-source NPC three-level inverter to section the working time of small vectors dynamically by researching unified voltage modulation method and SVPWM method. The strategy calculates the relationship among shoot-through duty ratio, modulation index and balance factor, and offers the optimal solution of balance factor.However, the low-frequency fluctuations of neutral-point potential caused by SVPWM not only add the volume of DC capacitors, but also impact the quality of output waveforms. By analyzing the VSVPWM method and calculating the working time of each phase in P state and N state, a control strategy based on VSVPWM for Z-source NPC three-level inverter is proposed. Without changing the working time of virtual medium vectors, this strategy balances the neutral-point potienal and eliminates the low-frequency fluctuations by sectioning the working time of small vectors.Thirdly, a Z-source hybrid clamped three-level inverter is proposed by researching the topology and control strategy of hybrid clamped three-level inverter, which realize decouping between the neutral-point potential balance control and Z-source boost control from a hardware perspective. By analyzing the voltage deviation of clamping capacitors caused by electric charge accumulation, the relationship between charging and discharging circuits of clamping capacitors and the directions of phase currents is confirmed, and a control strategy based on phase disposition PWM for Z-source hybrid clamped inverter is proposed. The strategy chooses the switching states reasonably according to the directions of phase currents, not only balancing the voltages of DC capacitors, but also keeping the voltages of clamping capacitors constant, and increasing the range of Z-source output.At last, this paper elaborates the design of hardware and software of Z-source three-level inverter in detail, and the experimental platform for Z-source three-level inverter based on DSP is set up. The previous research contents have been confirmed by experimental results.