Stability Mechanism And Power Oscillation Analysis Of Power System With High-Penetration Wind Power And Photovoltaic Generation

With the rapid development of renewable energy generation,the power system presents a characteristic with a high proportion of renewable energy and a high proportion of power electronic equipment access.This characteristic has led to the weakening of the grid,bringing great challenges to the safe and stable operation of the power system.Especially,the power oscillation accidents in grid-connected wind power and photovoltaic systems under weak grids occur from time to time,causing a baneful impact.Such accidents can be roughly divided into two stages,the first is the small-signal stable/unstable stage,and the second is the continuous power oscillation stage.In the first stage,whether the system is stable or not reflects its intrinsic stability mechanism,but the complex interaction between renewable energy sources and weak grids,with a wide range of influencing factors,makes the stability mechanism difficult to portray.In the second stage,the sustained oscillations reflect the characteristics of the dynamic behavior of the system after instability,mostly with nonlinear elements involved,and hence the traditional linear analysis methods are often inapplicable.To address these challenges,first,at the converter level,control methods for grid-forming and grid-following voltage source converters(VSCs)are improved to reveal the stability mechanism using a more concise approach.Then,at the system level,wind and photovoltaic characteristics are further considered.Based on the verification of the stability mechanism,considering the effect of the discontinuous nonlinear element,the relationships between power oscillation characteristics and system parameters are analyzed,and the role of the weak grid is further refined.The main purpose of this paper is to systematically elucidate the relationship between the weak grid and system stability under different control strategies,different system structures,and different source-side dynamics.In the second chapter,the stability of the grid-forming VSC under the weak grid is studied.The inner loop of the grid-forming VSC is often constructed by a voltage and current dual-loop control,and the strong coupling of the dq channels leads to a high order of the system model,which makes it difficult to establish a direct connection between system parameters and system damping.To address this challenge,an inner-loop direct decoupling control method is proposed to achieve tracking control while greatly simplifying the system model.The method has similar frequency response characteristics as the dual-loop control and can be regarded as a simplified characterization of the damping characteristic of the dual-loop control.On this basis,the negative damping effect of the outer-loop droop control and the positive damping effect of the inner-loop control are revealed,and the stability mechanism is revealed based on a series impedance model.For the grid-forming VSC,system stability is based on the fact that the positive-damping effect of the inner-loop control can completely reverse the negative-damping effect of the outer-loop control,and hence the inner-loop control is very important for system stability,and its influence should not be ignored.A weaker grid will enhance system damping,therefore grid-forming VSCs are more likely to keep stable under the weak grid.In the third chapter,the stability of the grid-following VSC under the weak grid is studied.The traditional synchronous reference frame phase-locked loop(SRF-PLL)leads to a high order of the system model and makes it difficult to derive an analytical representation of the system stability.To address this challenge,a grid-connected VSC control method based on algebraic operation synchronization(AOS)is proposed to ensure fast synchronization while reducing the order of the system model and making stability analysis easier.By comparing AOS with conventional phase-locked loop synchronization in terms of synchronization mechanism and system model,it is demonstrated that AOS is equivalent to an SRF-PLL with quite high bandwidth.On this basis,the analytical characterization of the system stability boundary is realized,and the feasible region of the system stability can be derived.Based on the feasible region analysis,the limiting effects of the current loop,PLL,and weak grid on the output current of the converter,and the limiting effects of the weak grid on the control parameters are revealed,which enriches the connotation of short-circuit ratio(SCR).For the grid-following VSC,the coefficient ratio of the current loop PI controller(K_i/K_p)mainly constrains active current output and the proportional coefficient(K_p)mainly constrains reactive current output.A weaker grid will enhance the limiting effect on output current and control parameters,therefore it is not easy for a grid-following VSC to keep stable under the weak grid.In the fourth chapter,power oscillations of the grid-connected permanent magnet synchronous generator-based wind turbine(PMSG-WT)system under the weak grid are studied.Based on the verification of the stability mechanism,the different effects of machine side converter(MSC)and grid side converter(GSC)on the power oscillations are revealed.When sustained power oscillations occur in a grid-connected renewable energy system,multiple nonlinear elements may be highly involved,and traditional linear analysis methods cannot handle discontinuous nonlinear elements,and hence the stability analysis faces challenges.To address this problem,a complete model of the grid-connected PMSG-WT system,including wind turbine,drive chain,MSC,GSC,and the weak grid is established,and the nonlinear element in the power loop is modeled and analyzed based on the describing function method.The effects of MSC control parameters,GSC control parameters,and grid strength on power oscillation characteristics are analyzed successively to reveal the causes of power oscillations,and also to obtain the relationships between parameters and the amplitude and frequency of the oscillations.With low PLL bandwidth,the weak grid has a suppression effect on the power oscillation,while with high PLL bandwidth,the weak grid may lead to instability.In the fifth chapter,power oscillations of the grid-connected photovoltaic(PV)system under the weak grid are studied,to reveal the reason why the weak grid has different effects on the system stability in different frequency ranges.In order to refine the influence of the weak grid on stability,a complete grid-connected PV system model,containing PV panel characteristics,VSC,and the weak grid is established.The stability within the sub-synchronous frequency range is analyzed based on the describing function method,and the relationships between the control parameters and the amplitude and frequency of the oscillations are obtained.Based on the impedance analysis method,the key elements affecting the high-frequency stability are extracted and the high-frequency stability of the system is analyzed.These two analysis methods both verify and complement each other.In the sub-synchronous frequency range,a weaker grid changes the linear part of the system and can reduce the amplitude of the power oscillations,while in the high-frequency range,the weaker grid will reduce the stability margin of the system,and it may finally lead to instability.