Research On Three-phase Three-level Grid-Connected Inverter In Distributed Generations

Distributed generations develop rapidly in recently years with substantial benefits, including flexible electricity production, high energy utilization and low environment pollution. Grid-connected inverters which are the interface of grid and wind power generation, photovoltaic power generation and fuel cell power generation play an important role in distributed generations. The grid connected by distributed generations is usually weak and may be distorted due to nonlinear loads and unbalanced due to short-circuit faults. With the increase of distributed generation power ratings, multi-level inverters are expected to be widely used in distributed generations since the voltage rating is increased without direct parallel switching devices. In the standard of grid-connected inverters in distributed generations, islanding detection, low voltage ride through, active and reactive power control involve two important technologies, which are the phase-locked loop (PLL) technology and the inverter control strategy. So in this paper, the research focuses on the PLL technology in distorted and unbalanced grid and the control strategy of neutral point clamped three-level inverter.Based on the fact that the low-power distributed generation is usually connected to single-phase grid, this paper compared several typical single-phase PLL methods, analyzed the advantages and disadvantages in theory and practice, and evaluated the performance in distorted grid. On the basics of above work, this paper proposed a modified PLL method especially for distorted grid. The proposed PLL method has a coupling feedback structure composed of a main module aiming to estimate fundamental component and multi notch filters aiming to get harmonic components. The main module is an amplitude-frequency-phase-locked loop (AFPLL) using gradient descent method and the harmonic modules are simple second order notch filters (NF). The proposed PLL method can be easily implemented in DSP and the experimental results show that the propose PLL not only can eliminate steady state error in harmonic distorted grid voltage, but also has a quick response when grid voltage suddenly changes.For unbalanced three-phase grid voltage, this paper firstly analyzed the basic principle of commonly used PLL methods, and then proposed a PLL method to estimate positive and negative components with gradient descent algorithm in stationary reference frame. The proposed method has a very simple structure and does not require transformation of grid voltage form stationary reference frame to synchronous reference frame. It can estimate the amplitude, phase angle and frequency of positive and negative components by solving a nonlinear equation set with gradient descent algorithm. The performance of the PLL method is verified by experiments, and the experimental results show that the proposed PLL has a quick response when grid voltage suddenly becomes unbalanced. The response time of the PLL is within two frequency cycles even if all the amplitude, phase angle and frequency have large changes.As for the control strategy of neutral point clamped three-level inverter, this paper proposed a finite set optimal predictive (FSOP) control method. In the proposed method, the optimal switching vector is selected from a finite set according to a cost function. The finite set is composed of27switching vectors, and the cost function is composed of the error of output current, the neutral point unbalanced voltage and the switching times of device. The state space model of neutral point clamped three-level grid-connected inverter is detailed derived and the flowchart of FSOP control is given. Finally, the performance of FSOP control is verified though simulations and experiments. Experimental results show that FSOP control has a quick response in tracking output reference current and balancing neutral point voltage, insensitive to harmonic distorted voltage and model parameters. FSOP control has a flexible control purposes and we can optimize the synthesize performance just by adjusting weighting coefficients of cost function.As for engineering practice, this paper proposed a high performance simulation methodology for multi-level inverters. In comparison with two-level inverters, the control algorithm of multilevel inverters is very sophisticated and many new control algorithms have been proposed recently. Without a full understanding of the control algorithm, the experiment will take both money and time even with safety issues. Therefore, an effective simulation methodology for multilevel inverters is needed. The simulation methodology can simulate the factors that have great impact on control performance such as dead band, digital delay, and quantization error. The simulation results are more convincing and can be easily ported to an actual system. The simulation methodology also provides an off-line debugging function, which will be helpful for exploring advanced control algorithms. This paper also described the implementation problem of PR controller in fixed point DSP. The software and hardware protection methods and some experiment photos are also demonstrated.