Investigation On Equivalent Modeling Of Photovoltaic Power Plants

Because the photovoltaic(PV) power plants can use solar energy efficiently, the installed capacity has increased very fast in recent years, which has good prospects for development. A large-scale PV power plant is composed by many PV power generation units, so establish the PV power plant model by building every PV power generation units in detail to analyze the impacts of PV power plants on power systems will be too large-scale and very time consuming. In view of the above problems, an equivalent modeling method for PV power plants which the inverters are the same type is studied.The modeling of PV power generation unit is the basis of the investigation on equivalent modeling of PV power plants. The modeling methods of PV power generation unit including the PV array, maximum power point tracking(MPPT) and inverter control considering the low voltage ride through(LVRT) are analyzed, respectively. As for the MPPT modeling, the control strategy which combines the perturb and observe(P&O) method and the constant voltage tracking(CVT) method is adopted to improve the voltage fluctuation at the DC side of the inverter during the fault recovery. Based on the analysis and fitting of the measured data of PV inverter during the LVRT, an improved LVRT control strategy is proposed, which considering the active power ramp recovery process modeling during the fault recovery. By comparing the simulation results under different operating conditions, the accuracy of the simulation model is proved. Thus the preparatory work is done for the following equivalent modeling investigation.Aimed at reducing the modeling complexity of each generation units in the PV power plants, the investigation on model simplification of PV power generation unit is done. Because the detailed model for electromagnetic transient simulation considering the control of power electronic switching devices, the simulation step size is very small, which causes the simulation time is long. In this paper, two simplified models are proposed. The average model uses the mean value model to replace the power electronic switching devices, while there is no simplification in the control circuit. The simplified model for electromechanical transient simulation uses the controlled current source to replace the power electronic switching devices, in that way to realize the inverse process, and the current inner loop in the control circuit is ignored. The simulation results show that two kinds of simplified model proposed in this paper can reflect the output characteristics at the point of common coupling(PCC) of the detailed model well, and the simulation time is significantly shortened.In order to reduce the numbers of PV power generation units needed to be built in the modeling of PV power plants, the clustering equivalent method is investigated. The transient active power response characteristics of a single PV power generation unit under different working conditions are analyzed. Based on the law reflected by the transient characteristics at PCC, a clustering equivalent strategy for PV power plants is proposed. The research shows that when the active power ramp recovery control module has been put into operation during the fault and recovery periods, the clustering equivalent strategy need to be used, and when that module has not been put into operation or has not been considered, the PV power plant can be equivalent to a PV power generation unit. Also, the calculation methods of equivalent parameters and deviations are analyzed. The simulation results show that the clustering equivalent method proposed in this paper has good equivalent adaptability under the disturbance of irradiance, different fault time and different fault location, thus verify the feasibility of the proposed equivalent method.