The Research Of Reactive Power And Voltage Control For Grid-connected Photovoltaic Generating Systems

Along with the advancement of sustainable energy development strategy,photovoltaic (PV) power technology has gained rapid development in recent years.From basic scientific research to engineering practice, PV power peremptorily hasbecome one of the most active research fields. Due to the rapid growth of PV powerpenetration in power system, the research subjects about PV power grid integration aredrawing more and more attention. This article will research on the voltage stabilityproblem of the point of common coupling (PCC). By means of reactive power andvoltage coordinated control, PCC voltage stability is enhanced and PV power gridintegration capacity is improved.Grid-connected PV generation system is divided into distributed PVgrid-connected systems and large-scale PV power plants based on installed capacity andgrid voltage levels. The paper analyzes the impact the varying output of the active andreactive power of grid-connected PV systems on network voltage. Grid-connected PVsystem should have independent control of reactive power to provide reactive powersupport to the grid. Basic theory of single grid-connected inverter for its mathematicmodel, vector diagram and reactive power constraint is also analyzed deeply, and set upthe structural diagram of the system at αβ two phase stationary coordinate frame, whichincludes active power control, reactive power independent control, current control andthe control method under voltage unbalanced and distortion. All of these lay thefoundation for the voltage and reactive power control of grid-connected PV generationsystem.The voltage and reactive power control strategy only uses the grid-connectedinverters as flexible and controllable source of reactive power in distributed PVgrid-connected systems. In the viewpoint of voltage drop the voltage variation atgrid-connected point before and after the grid-connection of PV system is researched,and the reactive power control strategy given by Verband Deutscher Elektrotechniker(VDE) is analyzed in depth. On this basis, a reactive power control strategy based onvoltage amplitude at grid-connected point and active output of PV system is proposed.Under the proposed control strategy all PV inverters participate power grid voltageregulation, and under the premise of maintaining the grid voltage within the specifiedrange the total absorbed amount of reactive power is the lowest and the active and reactive loss of power grid are minimized.The voltage and reactive power control strategy uses the grid-connected invertersand reactive power compensation device SVG as flexible and controllable source ofreactive power to regulate the voltage and reactive power of large-scale PV powerplants. Firstly, this paper has a detailed introduces the practical model of large-scale PVpower station. The effect of large-scale PV power plants to the grid voltage staticstability is studied. Secondly, set the real-time reactive power balance of the large-scalePV power plants according to Q(U) strategy of reactive power based on voltageamplitude at PCC and then distributed between PV Generation Unit and reactive powercompensation device, in order to achieve the the steady state value of grid voltage. Inorder to reduce reactive power transmission of the large-scale PV power plants, lowerpower loss, give priority to the reactive power compensation device of reactive control.Reactive power balance will be assigned to the PV Generation Unit when the reactivepower compensation device beyond the constraints. At the same time, voltage andreactive power sensitivity analysis is introduced into the reactive power distribution ofPV Generation Unit. And based on the amount of sensitivity, the PV Generation Unitwhich will be involved in the regulation of voltage or reactive power are determined.Thus, the accuracy of reactive power control of the large-scale PV power plants isimproved and the power loss is effectively reduced as well.The paper analyzes the impact on grid voltage static stability caused by differenttypes of grid-connected PV generation system, and proposes voltage and reactive powercontrol strategy according to their structural characteristics. The results will lay thetheoretical basis and provide decision support for improving grid voltage stability andraising the penetration rate of PV system on power grid.