Research Of Grid-Connected Inverter Under Different Constraints Base On Unbalanced Grid

Since the 20th century, the world energy situation has undergone profound changes, the traditional fossil fuels has led to global warming, air pollution and resource shortages and other environmental problems to humans, people are looking for the appropriate energy to replace fossil fuels, wind, solar, geothermal energy, tidal energy, nuclear power and other new energy to get people’s attention. Because the new energy can not be used directly, new energy needs to be converted into electrical energy, the core of three-phase PWM Voltage converter as a power conversion is widely used as static var compensator, active power filter, unified power flow control, high pressure HVDC fields, solar energy, wind energy and power generation, etc.One of the main factors affecting the quality of the grid voltage is the performance of the control system, especially when the three-phase grid voltages are unbalanced, or when the three-phase grid voltages are disturbed by external factors, Control system can not output three-phase sinusoidal current, low frequency harmonics currents occur, resulting in an increase of the degree of distortion of the grid voltage, seriously equipment and other loads are damaged, so the research of grid-connected inverter control strategy under unbalanced grid voltage grid is necessary.In this paper, on the one hand study the phase-locked loop method, based on the existing main phase-locked loop method, the article focuses on the proposed detection method of grid voltage signal. In the method, a system of linear equations is constructed with a three-phase grid voltage and their micro-components, by solving the corresponding system of equations, not only the fundamental component amplitude and phase information of positive and negative sequence can be obtained, but also the amplitude and phase information of harmonic components can also be obtained. Because of differential operation would be resulting in greater error, then using the AC forgotten integration algorithm, so the signal can be separated into a smoother grid voltage by the filter.On the other hand study grid current control strategy,First, a mathematical model of a three-phase inverter is given under under unbalanced grid,Then introduced the mobile forms of active and reactive power between the converter and the power grid,Based on this grid current control policy is proposed under unbalanced grid voltage, The connotation of the strategy is to get the different control results by different control objectives,And accordingly design the control method based on positive and negative synchronous rotating coordinate system, namely to "suppress the negative sequence current," "suppression bus voltage fluctuations," "inhibit active and reactive power fluctuations." Consider the current limits of the power transistor, this paper focuses on the influence of unbalanced grid voltage to converter with a carrying capacity, This paper presents the optimal control strategy of the three-phase grid-connected inverter, the policy propose minimum active power fluctuations as the control objectives.In a 1kW laboratory prototype is built, the corresponding hardware design and software program is given. Theoretical issues of each chapter have been verified with the relevant experiments, including:converter experiments under ideal grid voltage, phase-locked loop experiments under unbalanced grid voltage, experiment and compare of the three control strategy under unbalanced grid voltage, converter optimal control experiments. Combined with the proposed phase-locked loop method and grid current control strategy, experiments show:In the three-phase unbalanced grid voltage, phase-locked loop in this paper can better lock the positive and negative sequence component of the grid voltage and the corresponding phase angle information, associated control strategies can effectively suppress power fluctuations or negative-sequence grid current.