Study Of Islanding Detection Metrhod For Grid-Connected Photovoltaic System

Islanding detection is an essential function for grid-connected photovoltaic system.Islanding detection method(IDM)generally has non-detection zone(NDZ)when the local load power is matched with the output of the inverter.Injecting perturbation into the output of the inverter could decrease the NDZ.However,the power quality is affected due to the injected perturbation when the inverter is connected to the grid.Conventional IDMs cannot ensure the effectiveness and power quality at the same time.Therefore,the IDM with smaller NDZ and less adverse effect on the power quality is studied for three-phase grid-connected photovoltaic system.The main work and achievements are as follows.The mathematic model of the grid-connected photovoltaic system is summarized.The parameter tuning method of the proportional-integral regulator of the grid-connected control system is analyzed.As the theoretical basis for the study of IDM,the relationship between the voltage amplitude,the frequency and the active power,reactive power and load characteristics before and after islanding is systematically studied.The methods which could quantitatively describe the NDZs of IDMs by different parameters are compared and analyzed.The principle of the slip-mode frequency shift islanding detection method(SMS-IDM)which is applicable to the grid-connected photovoltaic system is systematically summarized.In order to decrease the NDZ of the SMS-IDM under constant-power-control strategy,a hybrid islanding detection method(HIDM)combining the SMS and reactive power disturbance together is given.Simulation results indicate that the added reactive power disturbance could break the equilibrium effect of constant-power-control strategy upon phase deviation and thus the NDZ is decreased.Increasing the positive phase angle feedback 0m of the SMS-IDM could decrease the NDZ,but it degrades the power quality at the same time.For the sake of reducing the adverse effect on the power quality while decreasing the NDZ,a HIDM based on combination of the SMS as an active method and rate of change of frequency as a passive method is given.It is analyzed that the HIDM can not only detect the islanding for a broad range of load quality factors but also reduce the adverse effect on the power quality.A novel passive IDM based on inductance estimation is proposed.The proposed method does not have any NDZs.Meanwhile,this method has no adverse effect on the power quality.The continuous-time model of the SVPWM based grid-connected converter in the rotating dq-reference frame is transformed to discrete model.The inductance estimator utilizes three fixed consecutive current and voltage samples measured within every SVPWM sampling period.For the sake of accuracy,the inverter adopts sensorless grid-connected control strategy.The reconfiguration of grid voltage and the process of synchronous phase angle estimation are analyzed.The effectiveness of the proposed IDM is verified by simulation results.