Research On Key Technologies Of Grid-connected Inverters Based On Power Quality And Efficiency

The energy crisis and environmental pollution around the world have become two severe issues that impede the sustainable development of human society. The distributed generation typified by solar and wind energy and the grid-connected inverting technologies possess both strategic significance and application prospect in developing renewable energy and realizing energy saving and pollutant reduction. The power energy is transmitted by grid-connected inverters in distributed generation systems. Hence, the realization of high efficiency and power quality in such system is a significant subject of studies. Focused on the two crucial tasks, and based on the overview of the existing studies and technologies, this dissertation has made thorough research on the following aspects:topology and modulation strategy of the transformerless grid-connected inverter, dead-time compensation and elimination method of the grid-connected inverter, modeling and control scheme of the two-stage grid-connected system, and current control technology of the grid-connected inverter.Firstly, based on the generation principle of the common-mode leakage current, a mathematical model of the common-mode resonant circuit in transformerless gird-connected inverters is derived in this dissertation. The condition to meet no common-mode leakage current is obtained by adopting the model. And a novel transformerless grid-connected inverter topology with symmetrical H6-type configuration is proposed, which can guarantee not to generate the common-mode leakage current. The unipolar SPWM and double frequency SPWM strategies can be both applied in the proposed inverter. The high efficiency and convenient thermal design are achieved by adopting the unipolar SPWM. Moreover, the higher equivalent frequency and lower current ripple are achieved depending on the double frequency SPWM. Therefore the harmonic contents and THD of the output current are reduced greatly, and the power quality is improved accordingly. Furthermore, the influences of the phase shift between the output voltage and grid-connected current, the configurations of the dead time among all power switches and the balance conditions of the switches'parasitical junction capacitance are discussed detailedly. Secondly, the generation principle of dead-time effects is analyzed, and an ideal model of dead-time effects is derived in this dissertation. Furthermore, the nonideal characteristics of the turn-on time, turn-off time and conduction voltage of power switches are discussed in detail. The mathematical functions for describing the loss and phase shift of the fundamental component and low-frequency harmonic contents of the output current, which are induced by the dead-time voltage, are formulated. Based on the above analyses, a novel online adaptive dead-time elimination method is proposed. Through tracking the load change on-line and determining dynamically the current direction, the zero-crossing current region and the non-zero-crossing current region are precisely distinguished and the optimal width of the zero-crossing current region is adaptively calculated. Therefore, the dead-time effects are eliminated respectively in the two different regions of the whole sinusoidal modulation cycle. The proposed method can significantly restrain the output distortion in the zero-crossing current region and reduce the low-frequency harmonic contents and THD. Moreover, the online adaptive arithmetic is implemented by DSP chip and has little dependence on the precision of the current detection devices, which improves the practicability and reliability of the proposed method.Then, the topological structure and control strategy of two-stage grid-connected system are explored in this dissertation. As the first power-processing stage, an accurate steady-state model of the high step-up interleaved Boost converter with active-clamp circuits is derived, which correlates the voltage gain of the converter with the duty cycle, the turns ratio and leakage inductance of the winding-coupled inductors, the switching frequency and the load. A full-bridge inverter with bidirectional power flow is used at the second power-processing stage, which employs the direct current control strategy based on PI regulators. And two feedforward compensation units are added in to perform in the control loops to restrain the disturbances of the DC-bus voltage and grid voltage, and then achieve the low THD of the output current. Meanwhile, the proposed control method has no influence on the system's stability. Furthermore, an improved predictive current control expression is proposed, which calculates the inverter voltages required to force the output current to follow the reference without the steady-state error. The algorithm not only has the advantages of precise current tracking, low harmonic contents and THD of the output current, but also reduces the probability of output current oscillation and enhances the system stability and robustness to the model parameter mismatch especially in the filter inductance estimate.Lastly, based on the above research results, the engineering applications of grid-connected inverters are achieved in the three projects of energy regenerative unit for the elevator system, grid-connected photovoltaic power system and regenerative electronic load. Furthermore, the production tests and the practical application have proved that the major technical parameters and performances of the grid-connected inverters have reached or exceeded those of the similar products in present industry.