Modeling And Mechanism Characteristic Analysis Of SSO In Large-Scale PV Generation Connected To A Series-Compensated Power System

With the promulgation of the Fourteenth Five-Year Plan,China has put forward higher requirements in terms of energy conservation,emission reduction and sustainable development.Photovoltaic as a major renewable energy has developed rapidly,and its installed capacity has been expanding.The main photovoltaic power stations and load centers in China show reverse distribution characteristics,and need to be transmitted over a long distance through series compensated lines.However,there is a risk of sub-synchronous oscillation in the dynamic interaction between series compensated lines and power electronic devices.In order to ensure the security,stability and economic operation of power system,the modeling and mechanism analysis of the sub-synchronous oscillation of large-scale photovoltaic grid-connected system has important practical significance.The paper main research contents and results include:(1)The equivalent model and small-signal dynamic model of large-scale photovoltaic series compensation grid-connected system with retaining and ignoring transformer excitation branch are established respectively.The eigenvalue analysis reveals the sub-synchronous oscillation mechanism of the system.The results show that the current-controlled PV grid-connected inverters have similar output characteristics to the current source,which will excite the parallel resonant points.However,the parallel resonant points generated by the series-compensated grid-connected system and the transformer excitation branch on the AC side of large-scale PV power generation may be the dominant factor affecting the oscillation stability of the system.(2)Based on the small-signal dynamic model of large-scale photovoltaic gridconnected system with series compensation considering transformer excitation branch,the inherent oscillation mode of the system is calculated and the oscillation characteristics are analyzed.The results show that the system has a subsynchronous oscillation mode dominated by the phase-locked loop and a hyper/subsynchronous oscillation mode dominated by the resonance point;the parameters of transformer excitation branch and the system series complement affect the oscillation frequency of the resonance point dominant mode;the parameters of the phase-locked loop in the photovoltaic control system have a greater influence on the oscillation damping of the resonance point dominant mode,the oscillation frequency and the damping of the phase-locked loop dominant mode.Unreasonable PLL parameters may lead to system self-excited instability.(3)The forced subsynchronous oscillation mechanism and dynamic characteristics of large-scale photovoltaic power generation system with series compensation are revealed.Based on the modulation theory,the causes of interharmonic generation among photovoltaic grid-connected inverters are analyzed.The interharmonic generated by the inverter based on disturbing MPPT is proposed as the excitation source,and the photovoltaic grid-connected control system as the forced oscillation system.The mechanism of photovoltaic forced subsynchronous oscillation is studied.It is found that when the frequency of photovoltaic interharmonic is close to the frequency of the inherently weak damped oscillation mode of the forced system,the interaction between the two will cause the forced subsynchronous oscillation of the large-scale photovoltaic series compensation system.