Research On Topology And Control Strategy Of Three-Level Photovoltaic Grid-connected Inverter

With the development of world economy and increasing demand for energy, energy crisis and environmental pollution have become an inevitable problem for further development. Photovoltaic (PV) grid-connected power generation is one of the important strategies to solve energy shortage and environmental pollution, which developed rapidly with wide application at home and abroad.The main innovative research of the paper can be summarized as follows:First, as the interface device between PV array and the utility grid, PV grid-connected inverter plays a crucial role because it is the core component of the grid-connected system. In order to lower the cost and promote the efficiency of inverter, a new Interleaved-T type (Interleaved Boost and CCT, IBC) double three-level PV grid-connected inverter topology is proposed in the paper. The topology consists of DC/DC side Interleaved three-level Boost converter and the DC/AC side using CCT-type three-level converter components. The topology DC/DC-side switching devices and diodes withstand voltage stress little and has small loss and the DC output voltage ripple is small; the number of required power devices for DC/AC side is small, low loss, the inner and outer devices of each phase leg heat evenly, which is favorable for long-term stable operation; the system have the advantages of high efficiency and low cost.Second, the neutral-point potential balance is inherent problems of the three-level PV grid-connected inverter, a hybrid three-level neutral-point balance control strategy is proposed in the paper. The control strategy clearly clarifies sector selection principle based on the neutral-point voltage offset and three phases current under modulation index0<m<0.52, to overcome the voltage offset caused by non-redundant small vector; in high modulation index condition0.52<m<0.907, it fully considers the impact of the medium vector on the neutral-point voltage offset, the nearest-three-virtual-vector technique is introduced based on the definition of virtual vector, neutral-point voltage offset is controlled in the whole SVPWM.Third, islanding detection is one of the most important functions in the photovoltaic power generation system. In order to eliminate the non-detection zone (NDZ) and meanwhile minimize the effect on the power quality, a novel islanding detection method was proposed based on wavelet transform and Adaptive Network based Fuzzy Inference System (ANFIS).The simulation and experiment results show that the proposed method has high accuracy and effectively identify all kinds of load conditions such as the power of grid-connected inverter match and mismatch the one of local loads and can be used for single-phase and three-phase and multiple inverters.In addition, the paper also some work as follows:First, distributed generation systems often appear as a weak-grid, where the voltage is prone to be unbalanced, and the performance of the inverters within the system will be affected as the conventional control scheme can’t deal with the negative sequence (NS) component. A decoupled double synchronous reference frame (DSRF) PLL is adopted to obtain the positive sequence (PS) and NS phases of the unbalanced voltage, at the same time, instantaneous power model under the unbalanced grid is analyzed, and then an unbalanced control strategy based on the DSRF-PLL is designed by injecting an NS current into the PS reference current via a negative-sequence-to-positive-sequence transformation (NS-PST), which can well suppress the ripples of the DC voltage to guarantee the MPPT accuracy and smaller THD of the grid-connected current. Finally, the validity and reliability of the adopted scheme is verified via IBC photovoltaic (PV) grid-connected inverter.Second, in order to meet the grid-connected technical specification of PV inverter, a control strategy based on three-phase grid-connected voltage symmetrical and unsymmetrical sags for low voltage ride through (LVRT) is adopted. The voltage outer-loop was blocked and the current inner-loop of the specified value was given when three phases symmetrical voltage sag. The grid-connected voltage feed-forward control is adopted to suppress the current impact at the moment of voltage sags. Combining with fast extraction of positive-sequence and negative-sequence components of current and accurate synchronization signal of grid-connected voltage to ensure real-time control of the acquired signal, and take maintaining the balance among three-phase currents by suppressing negative-sequence current component, and adjust reactive current output to meet the reactive power support requirements. Finally, Matlab/simulink simulation and field experiments are completed based on IBC grid-connected PV inverter, the results show that the control strategy can effectively suppress the grid-connected current impact at the moment of voltage sags, and keep a high degree of sinusoidal current, which could be achieve security ride through during voltage sags.