The method of achieving reliable power supply and guaranteeing the power quality is currently a research hotspot for domestic and foreign scholars. In terms of power supply, as the main representative of renewable energy sources, photovoltaic or wind power is an effective way to deal with the fossil energy crisis and solve the environmental protection problems. In terms of power quality, due to a high controllability and rapid dynamic response, active power filter (APF), static var generator (SVG) and other active filter device, are the optimal solutions to improve the power quality. With the proportion of wind power and photovoltaic generation increasing year by year, and the popularization of other non-linear power electronic equipment, the paralleled multi-inverter system is the best way to expand the capacity. However, some key technical issues such as coordination control for multi-inverters, stability analysis and instability suppression, and ripple suppression need to be solved. The long-distance power transmission and multi-stage voltage transformation of photovoltaic and wind power will inevitably lead to the increase of power grid impedance. And these renewable energy sources with centralized or distributed grid-connection will bring flicker, harmonic pollution and other issues to power grid. In addition, a large number of impact loads (such as electric arc furnace, rolling equipment, etc.) are put into use, which results in the potential mutation of amplitude, phase and frequency of PCC voltage. The above problems will result in the following characteristics of the power grid: 1) grid impedance cannot be ignored, and changes with the operation of the grid; 2) there are serious background harmonics in the PCC voltage. On the one hand, the widespread use of non-linear power electronic devices will lead to the weak grid characteristics. On the other hand, the weak grid characteristics will affect the control performance of inverters under the application of new energy generation and power quality control. Under weak grid condition, due to the existence of unbalanced and harmonic components, the synchronization performance of PLL will be affected，which is one of the key technologies of the inverter. Due to the non-negligible grid impedance, the inverter in the parallel system is coupled with each other, which is prone to instability. This paper selects power quality control as the research background and studies the key technology of paralleled multi-inverters including grid synchronization, stability analysis and control, coordination control and ripple suppression strategy under weak grid condition.
Firstly, a novel unified digital PLL with multiple complex resonators for both single- and three-phase systems is proposed. Single-phase and three-phase voltage signal is transformed uniformly to a similar complex form that consists of positive and negative sequence components rotating in clockwise and counterclockwise directions, respectively. And synchronous rotating frame PLL with a generalized pre-filter is introduced. The proposed filtering structure consists of multiple complex resonators that can filter out harmonic frequency components and maintain fundamental positive sequence components accurately. Furthermore, a simplified structure of the complex resonator for single-phase system and an operation method for resonant factor are introduced.
Secondly, stability analysis and control strategy is put forward based on the global stability criterion. By establishing the two-port NORTON equivalent circuit, the current response expression of the parallel system is obtained. From the perspective of global admittance frequency domain, the paper reveals the intrinsic reason of instability in paralleled multi-modules system. In order to improve the damping property of the global admittance resonance point, a method of instability suppression based on admittance is proposed. Further, an assessment method for instability is proposed, which quantizes the influence of each inverter on instability.
Then, the coordination control strategy combining open-loop with closed-loop is proposed. The main feature of improved compensation system includes single CT sampling and double compensation points. The corresponding implementation method is introduced. Considering the harmonic content and spectrum distribution of the actual nonlinear load, the dynamic coordination control method with the optimal utilization is proposed. Based on DC side voltage control strategy, a new optimization method for comprehensive optimization of power loss and compensation precision is proposed. In the case of ripple suppression, the four-branch LCL-type (4B-LCL) filter is introduced, and the strategy combining active carrier phase shift with passive filter is proposed.
Finally, the experimental platform and analysis of the experimental results are introduced. The parameters design method of four-branch LCL-type grid interface is proposed. Dual-core control system based on DSP and FPGA is introduced, including the structure, the main components, and restart procedures under redundant fault conditions. The experimental results further illustrate the effectiveness and rationality of the proposed methods.