Research Of Volt-var Control Strategy For Integration Of Centralized Photovoltaic Power Station

In recent years,photovoltaic(PV)industry in China has developed vigorously,and the installed capacity of centralized PV power station has been increasing.The power generation mode of “centralized integration and long-distance consumption” has caused the voltage problems suffered by integration of centralized PV power station to become more severe.The fluctuation of PV output will cause large fluctuations in the nodes voltage at the point of common coupling(PCC)bus and the internal collecting power lines,which makes the centralized PV power station encounters the risk of voltage violation.Simultaneously,the external weak grid environment caused by long-distance power transmission severely limits the power transmission limit of centralized PV power station,and the large-capacity PV power transmission makes centralized PV power station have the risk of voltage instability.Therefore,the voltage control of centralized PV power stations has become an important part of its large-scale development and an important foundation for ensuring its safe and stable operation.This paper takes the centralized PV power station as the research object,and first analyzes the mechanism of the grid-connected voltage problem.Then,based on different external grid equivalent conditions,the researches on Volt-Var control(VVC)strategies of centralized PV power plants are carried out.The research purpose of this paper is to realize the reasonable distribution of nodes voltage inside centralized PV power station,so as to ensure the safe and stable operation of the centralized PV power station when connected to the external power grid.The research content of this article is the continuation of the related research content of the completed sub-project of the 863 project “Research on the Key Technologies of Large-scale Photovoltaic Power Station Grid Connection(2011AA05A301)”.The main research work and contributions of the paper are as follows(1)According to the grid-connected impedance model of centralized PV power station,the mechanism analysis of the voltage problems encountered by centralized PV power station is carried out,and the risks of high voltage violation and high grid-connected voltage instability for centralized PV power station are revealed,and the effect of VVC on regulating the voltage deviation inside the centralized PV power station and the margin of the static voltage stability(SVS)for integration of centralized PV power station is described.In addition,for the different scenarios of centralized PV power station connected to the grid,the analysis of the external grid equivalent conditions is carried out.The above content lays the foundation for the development of the following VVC strategy research on the grid-connected centralized PV power station.(2)Based on the external power grid environment where the equivalent voltage and equivalent impedance changes are small,in view of the differences in the operating characteristics and voltage regulation capabilities of the controllable VVC devices inside the centralized PV power station,a multi timescale VVC strategy combining centralized and local control approaches is proposed.This strategy builds a three-layer control framework including “hour level” and “minute level” optimization,and“real-time level” regulation.In the “hour level” and “minute level” control layers,the centralized control approach is taken to coordinate the operation of the controllable VVC devices and optimize the voltage distribution inside PV power station.In the“real-time level” control layer,the local control approach is taken to respond to the real-time fluctuation of the system nodes voltage.The proposed strategy can take advantage of the voltage regulation advantages of different controllable VVC devices,realize the coordinated operation of controllable VVC devices,and optimize the grid-connected voltage status of the system.(3)Based on the external power grid environment where the equivalent voltage and equivalent impedance changes are small,in view of the large-scale expansion of PV power station,and the centralized control approaches has high communication pressure and low communication reliability,a two-stage VVC strategy based on distributed optimization is proposed.By adding the coupled boundary constraints to the objective function for relaxation,the original optimization problem is splited into multiple optimization sub-problems for distributed iterative solution.This strategy constructs a two-stage control framework.In order to coordinate the boundary electrical quantity coupled between the PCC bus and the head end of each collecting power lines,the upper-stage control layer takes the analysis target cascading method for distributed optimization.In order to optimize the reactive power outputs of the PV power generation units,the lower-stage control layer takes the alternating direction multiplier method for distributed optimization.The proposed VVC strategy can alleviate the communication pressure of centralized PV power plants,improve communication reliability,and has a good convergence and the same optimality as the centralized control approaches in terms of control effects.(4)Based on the external power grid environment where the equivalent impedance change is small but the equivalent voltage change is large,in view of the estimation error of the equivalent voltage of the external power grid and the random fluctuation of PV outputs,a robust VVC strategy considering the uncertainties of equivalent voltage of the external grid and the PV outputs is proposed.The strategy continues the hierarchical control idea of multiple timescales,and establishes a three-layer control framework that combines optimal scheduling and real-time control.In the optimization scheduling of the first two layers,the external grid equivalent voltage and PV active power output are regarded as the uncertain variables,and a robust optimization model related to the actions of controllable VVC devices is established.Through the Lagrangian duality theory,it is realized that the conversion from robust optimization model with uncertain variables to traditional deterministic optimization model.On this basis,the third layer continues to use local control to adjust the real-time nodes voltage deviation.The proposed strategy is robust to the uncertainties of the external grid equivalent voltage and PV outputs,and ensures that it can provide a feasible voltage control planning when encountered with any operating conditions caused by the above uncertainties.(5)Based on the external power grid environment where the equivalent voltage and equivalent impedance changes are large,in response to the gradual increase in PV penetration and the increasingly close electrical connection between centralized PV power station and the external power grid,a collaborative VVC strategy that considers the interaction between centralized PV power station and the external power grid is proposed.This strategy emphasizes that the integration voltage problem of centralized PV power staiton can be solved through the coordinated operation of the centralized PV power station and the external power grid,and a large-scale integrated system including the centralized PV power station clustering structure and the specific topology of the external transmission and distribution network is constructed.On this basis,the strategy continues the hierarchical control idea of multiple timescales,and establishes a two-layer optimization model.Taking into account the data privacy and communication barriers between the sub-systems,parallel cooperative co-evolution and generalized Benders decomposition are used to realize the distributed optimization of large-scale integrated system,according to the characteristics of each layer optimization model constructed.The proposed strategy can flexibly dispatch the controllable VVC devices resources inside and outside the centralized PV power station.The proposed strategy not only reduces the internal nodes voltage deviation of centralized PV power station,but also improves the SVS margin of the external grid,thereby improving the external power grid environment for integration of centralized PV power station.