Study On Control Of Qusi-Single-Stage High Frequency Link Inverter And Grid Connection Technology

The Quasi-Single-Stage (QSS) High Frequency Link (HFL) inversion can mitigate the voltage spike and complex control logic in the single-stage HFL cyclo-converter inversion as well as the current stress in the current source single-stage HFL inversion. Therefore, it is promising for medium or small scale power applications. In this dissertation, the advantages and disadvantages of the typical HFL inversion techniques are analyzed based on its development and application. Detailed analysis of the research meaning of the QSS HFL inversion and overview on the research status are presented. This dissertation presents a generalized research on the key techniques involving inverter topology, closed loop control, grid connection control, and so on.A novel QSS push-pull forward HFL inverter topology is proposed by adding a forward capacitor to the QSS push-pull HFL inverter. It has some similar features to the push-pull inverter such as simple configuration and few power switches. This unique forward capacitor suppresses high frequency transformer magnetic saturation, absorbs leakage inductance storage energy, clamps power switch voltage and mitigates voltage stress. The Combined Aligned Unipolarity PWM strategy is adopted to achieve soft switching. A 1kVA prototype is implemented and the experimental results demonstrate that the proposed inverter enables four quadrants operation with the efficiency over 85%. Considering the higher voltage input and power application, a 3kVA QSS full-bridge HFL inverter is designed further and its efficiency reaches over 88%.The double modulation waveforms with unipolarity PWM strategy enables four quadrants operation without altering the control logic according to the voltage polarity, which simplifies the logic control circuit and avoids the output voltage zero crossing distortion. The voltage-current double-closed-loop control is employed to regulate the QSS full-bridge HFL inverter. The load current feed forward control is introduced to restrain load disturbance and improve the output voltage quality under nonlinear loads. The THD of output voltage is 2.02% under resistive loads and 4.17% under nonlinear loads. Two-Degree-of-Freedom (2-DOF) PI controller has two independent parameters which can be adjusted according to the command tracking and disturbance rejection performance of the system. Using the control strategy mentioned above, the command tracking and disturbance rejection performance of the controlled system is optimized in contrast to that under the conventional PI control, and the waveform quality of the QSS HFL inverter is improved.In order to extend the application field of the QSS HFL inverter, grid connection control is discussed further. A novel quasi-Proportional-Resonant (PR) with grid voltage feed forward control method is proposed. PR control enables better sinusoidal current control with zero steady-state error. The voltage feed forward control decouples the grid voltage with the grid current, so the gird voltage distortion and disturbance have no effect on grid current at all. Compared with PI control, experiments are carried out to verify the effectiveness of the proposed control.The Anti-islanding control is indispensable in the grid-connected inverter operation. A novel hybrid islanding detection method is proposed to avoid the fault trip when grid disturbances occur. It has less impact on the utility than the conventional active islanding detection method because the current disturbance is triggered only when the probable islanding is detected. In order to eliminate the Non Detection Zone (NDZ) of the proposed method mentioned above, the voltage feed forward positive feedback disturbance method is proposed to force the grid-connected inverter into the positive feedback operation after islanding. So the voltage of the point of common coupling is driven to an excursion from the normal condition. By evaluating the excursion, the islanding can be easily detected. The proposed method eliminates the NDZ and it is also effective when multiple grid-connected inverters operate simultaneously.The dissertation is supported by the National Natural Science Foundation of China (No. 50237020).