Research Of The Control Strategy On The Large-Scale Photovoltaic Plants Restraining Low-Frequency Oscillations

With the growing of photovoltaic capacity and permeability, which is likely to be a harmful effect on the low-frequency oscillations of the power system, large-scale photovoltaic plants should have the ability to participate in restraining low-frequency oscillations. With the research object of single-machine infinite bus system including large-scale photovoltaic plants and based on establishing the system model which is used to analyze the low-frequency oscillations, this dissertation researches on large-scale photovoltaic plants impact on low-frequency oscillations of the power system, and proposing the control strategy to restrain low-frequency oscillations of the power system by large-scale photovoltaic plants. The main research contents include the the following parts:(1) Considering the dynamic behaviour of the photovoltaic battery output, maximum power tracking(MPPT) and power control, this dissertation establishes the large-scale photovoltaic plants model of PQ mode based on unit power factor, PV mode based on the P control and PV mode based on the PI control. Further more, combining with the model of synchronous generator, PSS, Π equivalent circuit of transmission line, the dissertation finally establishes the model of single-machine infinite bus system including large-scale photovoltaic plants which is used to analyze the low-frequency oscillations.(2) Based on the method of eignvalues analysis, and with the reference object of two synchronous generators that one has a PSS and the another has none, this dissertation researches on large-scale photovoltaic plants impact on low-frequency oscillations when the large-scale photovoltaic plants replaces the synchronous generator which include no PSS, when the large-scale photovoltaic plants replaces the synchronous generator which include a PSS, when the large-scale photovoltaic plants changes the power flow and when the large-scale photovoltaic plants adopts different control strategies. Theoretical analysis and simulation results show that: the large-scale photovoltaic plants replaces the synchronous generator which include no PSS will reduce the damping of low-frequency oscillations, the large-scale photovoltaic plants replaces the synchronous generator which include a PSS will significantly reduce the damping of low-frequency oscillations, it will reduce the damping of low-frequency oscillations with the increasing of the photovoltaic capacity, prolonging of the transmission line, closing to the synchronous generator of point of common coupling in the research system. the large-scale photovoltaic plants adopts the control strategy of PV mode based on the PI control provides more damping than the strategy of PQ mode based on unit power factor and PV mode based on the P control. The paper further points out that the large photovoltaic grid system carries the risk of leading to system instability under certain conditions, which is necessary for the large-scale photovoltaic plants to adopt appropriate control strategies to participate in restraining low-frequency oscillations.(3) Based on the analysis of characteristics of low-frequency oscillations, and refering to the PSS controller, this dissertation proposes the control strategy of restraining low-frequency oscillations by the large-scale photovoltaic plants. The adaptive damping control strategy using the differential signal of the active power on the wide-area transmission lines regulates the most active output in the large-scale photovoltaic plants. The strategy can offer active power as much as possible, and ensure smooth switching between low-frequency oscillation suppression mode and maximum power tracking mode. Considering the reactive power-voltage regulating ability in the related standards, the large-scale photovoltaic plants adopts the existing control strategy of power-voltage regulation. Theoretical analysis and simulation results show that: the large-scale photovoltaic plants which adopts the active damping control strategy can significantly improve the system’s ability in restraining low-frequency oscillations and guarantee the security and stability of the power system.