Research On Control Strategies Of Grid-connected Photovoltaic Inverter Systems

Grid-connected photovoltaic generation will be the dominant form of solar photovoltaic utilization in the future. With the appearance of terrific digital signal processor, it is possible that more advanced control strategies can be applied to photovoltaic generation systems. Besides, an inverter is an important part of grid-connected photovoltaic systems, new functions should be extended with the development of renewable energy technology. This thesis concerns with control strategies of grid-connected photovoltaic inverter systems, and targets enhanced reliability, control performance optimization, higher grid power quality, function expansion, and power quality regulation. Aiming at the existing problems in single-phase grid-connected inverters, three-phase grid-connected inverters and double-mode inverters, and according to the new demands of project application, the thesis gives the thorough research and investigation. The main contents include:(1) An improved all-digital closed-loop phase-locked method for synchronization signals in three-phase grid-connected inverters is presented even considering seriously distorted utility and abrupt frequency variation conditions. The implementation of the presented method is based on synchronous reference frame transformation and advantages of open-looped and closed-looped methods. The algorithm with the frequency adaptation function can effectively suppress the line negative-sequence fundamental and harmonic components and quickly track the line positive-sequence fundamental phase with little distortion. By comparing with existing phase-locked methods, simulation and experiment results show the correctness and feasibility of the proposed method.(2) Firstly, aiming at the risk of bus short circuit in single-phase grid-connected H6inverters when grid voltage is near zero, this thesis designs a PWM distribution circuit with dead zone control. Secondly, considering large dc-side voltage ripples for single-phase grid-connected photovoltaic inverter systems, a novel array filter is presented to suppress the disturbance of ripples to output current reference at steady state. The proposed filter can easily eliminate ac components of the voltage-loop controller output, and thus decrease the distortion of output current. Thirdly, an inductance compensation scheme in series is added to the inner current loop for better performance of the inverter at low output power. The algorithm can regulate the gain of current-loop controller in real time with the change of inductance current. Thus, the tracking ability at low power is improved by optimizing the current-loop controller. Finally, simulation and experiment results verify the feasibility of the proposed optimization methods.(3) Double-model photovoltaic inverters can operate not only in grid-connected mode, but also in stand-alone mode by connecting to local load. The functional integration is studied for one individual three-phase three-level inverter circuit. This thesis first deduces the mathematical model in stand-alone and grid-connected operation modes. Based on vector decoupling control under multiple rotating reference frames, the controllers are designed as to realize the good output performance of double-mode inverter. For wireless parallel inverter group in stand-alone mode, a vector droop algorithm is proposed to achieve power-sharing control. Simulation and experiment results verify the feasibility of the proposed vector control strategies.(4) Grid-connected photovoltaic inverters with active filter function can output maximum active power from photovoltaic arrays, as well as compensate reactive power and suppress harmonic due to the non-linear loads connected to the electric networks. For multiple grid-connected inverters, a centralized compensation control strategy is proposed for the purpose of high facility utilization, good compensation ability and unwanted overcompensation condition. The solution is to allocate the compensation task for every inverter according to the instantaneous power margin of each. Simulation results verify the effectiveness of the proposed compensation strategy.