Research On Phase-lock Loop And Current Control Technologies Of Grid-Connected Inverters In Conditions Of Voltage Dips

Due to the energy and environmental issues caused by extensive use of fossil fuels, the utilization of renewable energy has aroused huge interest. The grid-connected inverter, which is the key energy processing unit in the renewable energy generation systems, needs to realize the functions of voltage-current converting and public grid matching. The installed capacity of renewable energy generation systems keeps on increasing in recent years, which makes its proportion of the public grid can't be ignored. Therefore, the low voltage ride-through (LVRT) ability is becoming necessary for the grid-connected inverter in the systems, which means that when the voltage dips appear, the inverter should be kept grid-connected according to certain requirements, or even provide frequency and voltage support to help the grid to recover from the fault condition.The phase-locked loop (PLL) and current control technologies are important for the LVRT of the grid-connected inverter. In this dissertation, the applications of these two technologies in the grid-connected inverters are summarized first. After that, theories and applications of these technologies in conditions of voltage dips are further analyzed, in Chapter 2 & 3. In order to solve the PLL error caused by the negative sequence component in the case of asymmetrical voltage dips and harmonic distortion, several sequence separation methods which can be used as PLL pre-filterserived, based on the performance analysis of the common used dq-PLL. The effects of PLL with sequence separation are simulated and analyzed. In order to eliminate the negative sequence current caused by asymmetrical grid voltage dips, current control methods are improved and simulated based on the analysis of PLL.The experiment results and a project are demonstrated in Chapter 4. Firstly, the experiments on a grid-connected inverter in conditions of steady state and symmetrical voltage dips in different levels are carried out to verify the effectiveness of the improved methods proposed in the Chapter 2 & Chapter 3. Secondly, an inverter-based energy-regenerative electronic load prototype is built to facilitate the burn-in test for a constant-current LED driver.