Research On Key Technology For Grid-Connected Photovoltaic System Based On Switched-Inductor Quasi Z Source Network

Grid-connected inverter holds a core position in grid connected photovoltaic (PV) system, which acts as bridge between PV modules and power grid. Compared with the original voltage source inverter (VSI), Z source inverter (ZSI) has some evident advantages, such as shoot-through state is utilized between upper and lower legs to boost, single-state conversion is used to increase the efficiency, lower distortion rate of output waveform is achieved. Thus, the research on ZSI will be extremely important for new energy power generations, especially for solar power generation. However, ZSI still has some limitations, for example, input current is discontinuous, boost ability is limited, inrush current is huge, voltage stress on components is high, large shoot-through current is beard in high power applications, et al. In view of the issues above, improved ZSI topologies, mathematical model analysis, control strategy and applications will be discussed and investigated in this paper, and the contents are shown as follow:Considering the original switched-inductor Z source inverter (SL-qZSI) cannot achieve higher boost ratio in lower shoot-through time, a new SL-qZSI which improved switched-inductor cell is presented in this paper, where a bootstrap capacitor is used to replace a diode in original switched-inductor cell. In this case, the number of components and complexity of the inverter system is not changed, but the boost ratio in low shoot-through duty ratio is substantially increased. A contrastive analysis between the proposed new SL-q ZSI, original SL-qZSI and qZSI is made under maximum boost control method and maximum constant boost control method, respectively. Results of simulation and experiments demonstrate that the propose new SL-qZSI not only realize higher boost ability than other two topologies, but also has advantages in voltage stress of capacitors and switching devices, inductor current ripper, reverse voltage of diodes, conversion efficiency, et al.To investigate the steady-state and transient performance of proposed new SL-qZSI thoroughly, state space average method is used to build the small single model, where the load, components in Z source network, shoot-through duty, et al are all considered. Pole-zero plot map is used to analysis the steady-state and transient performance, which will be useful to design and choose\components of Z source network, as well as provide the theoretic foundation for controller design of inverter system. An improved direct dc link voltage control strategy is proposed by combining original direct and indirect dc link voltage control methods, where the sampling values of two capacitors are added to substitute for dc link voltage, and then implemented for close-loop control by holding dc link voltage constant. A three-loop control strategy is proposed for quasi Z source (qZSI) photovoltaic system by combining maximum power point tracking (MPPT) control method, dc link voltage control method and grid current control method. Results of simulation and experiments demonstrate that the proposed three-loop control method not only realizes MPPT and dc link voltage constant in inverter system, but obtains a good dynamic performance.Compared with original continuous-input-current (CIC) type qZSI, discontinuous-input-current (DIC) type topology possesses lower capacitor voltage stress. Nevertheless, the DIC type photovoltaic inverter system will not keep the best working statues due to discontinues input current. To deal with the problem mentioned above, this paper presents an extended switched-inductor quasi Z source inverter (ESL-qZSI), which combines DIC type qZSI with original boost converter to realize input current continuous. Furthermore, the boost ability is significantly improved owing to the two-stage boost topology. In maximum constant boost control method, lower capacitor voltage stress, dc link voltage stress and diode reverse voltage stress of the proposed ESL-qZSI is lower than those in traditional qZSI and SL-qZSI, the conversion efficiency is higher as well.The original ZSI requires higher duty ratio to boost when input voltage is low and output current is high. Thus, the shoot-through current of inverter will be larger, which will be unfavorable for the choice of components, lead to increased cost, volume and reduced efficiency as well. According to this, several isolated quasi Z source inverters will be connected as input-parallel and output-parallel (IPOP) type to reduce voltage and current stress of devices, while a three-loop control method is used to obtain power sharing between each modules. Compared with original isolated qZSI, the proposed two isolated SL-qZSIs hold stronger buck-boost ability, which will be more useable to provide electrical isolation between input and output, more applicable for inverter system with high voltage fluctuation. Based on original hysteresis control method, a quasi hysteresis control strategy with fixed frequency and constant shoot-through time is proposed for ZSIs and qZSIs, where increasing time and decreasing time of grid-connected current will be obtained by calculation, and the grid-connected current only has upper limit or lower limit. Results of simulation and experiments demonstrate that the proposed IPOP isolated qZSI system obtains steady dc link voltage and good current tracking performance when the quasi hysteresis control method is used.