A Grid-connected Operation Technique In Voltage-controlled Mode For Photovoltaic System

With the development of worldwide industry and lack of energy supply, the distributed generation using of renewable energy has been attracted more and more attentions. The inverter is an important part for the distributed generation and is commonly in current-controlled mode, which can achieve good performance. However, output characteristic of the inverter in such control mode is like a current source that is not suitable for the public custom and it is only able to work in grid-connected operation mode,which lacks flexibility for the various demands. Focused on such facts, this paper puts forward a novel control scheme in voltage-controlled mode based on repetitive control, which can lower the output current THD by injecting harmonic positively.The mathematic model and characteristic of the controlled target is the basis of the study, so a linear model is established for a single half-bridge PWM inverter with the method of state-space averaging and the characteristic of power flow between tow voltage sources connected by an inductor is analyzed. After that, a digital calculation method of active power and reactive power is introduced.Based on a simplified model of two connected voltage source containing harmonic, a voltage-controlled algorithm is proposed for the first time which can decrease the harmonic voltage on the grid-connected inductor to lower the distortion of output current of the inverter by injecting harmonic. A comprehensive theoretic analyze is carried out on repetitive control system. System stability, error convergence and steady-state error are discussed and a design example aimed at a specific single inverter is given. A real time detection method of harmonic current is studied and an approach to detect harmonic voltage in single phase-circuit making use of orthogonality of triangle functions is proposed. The control algorithm is verified by the software simulation and actual experiment.Islanding effect must be prevented for any grid-connected distributed generation. The potential harm and detection principle is discussed and the main problems about the common detection method are concluded. After that, a simple active anti-islanding method is proposed and the influence of this method to the original system is discussed. The results of simulation and experiment indicate this method is feasible and effective.Such a harmonic compensation control scheme for the grid-connected inverter is introduced to the field of output voltage regulation of the PWM inverters and a supposition of separation control is put forward. According to it, a novel parallel synthesis control algorithm is proposed, which makes use of repetitive control to decrease harmonic and PI controller to regulate fundamental voltage. The simulation results prove its feasibility.