Research On The Analysis Method And Characteristics Of Wideband Oscillations In Power Systems With High Proportional Power Electronic Devices

With the vigorous development of renewable power generation represented by wind and photovoltaic,China’s power system is gradually transforming into a new system with renewable energy as the mainstay.The high proportion of new energy generation and the high proportion of power electronics are its main features.Under the background of “double high”,the multi-timescale control of power electronics devices and complex grid characteristics intertwine,resulting in significant changes in the system’s dynamic stability,the resulting wideband oscillation problems not only affect the stability of power electronics devices themselves but also may induce chain reactions,causing large-scale power outages,seriously threatening the system’s safe operation.Therefore,it is of great theoretical significance to carry out dynamic characteristics analysis for new power systems.Compared with traditional electromechanical oscillations,wideband oscillations involve multitimescale control dynamics of diverse power electronics devices and the significant increase of system calculation scale.Thus,on the one hand,more accurate and efficient system modeling and analysis methods need to be studied,and on the other hand,attention needs to be paid to the oscillation characteristics of different devices and the interaction between multiple devices.To this end,this paper investigates the characteristics and analysis methods of wideband oscillations in power systems containing high proportional power electronics,and the main work done and results achieved are as follows:(1)The method of formulating the discrete-time state-space matrix for power systems with highly proportional power electronics is proposed.According to the characteristics of the interface with the AC network,common components in power systems can be grouped into three categories: single-port,AC two-port,and DC two-port.By discretizing their continuous-time state-space models,the discrete-time state-space models of all components are established respectively,and each component is represented as a discrete-time equivalent circuit model similar to the equivalent circuit for electromagnetic transient programs(EMTP).By selecting the historical current terms in the discrete-time models as state variables,the corresponding state variables of an improper network in the discretetime domain are always independent of each other,and the relationship between the system state variables and the intermediate variables can be described by the discrete-time equivalent circuit equation.On this basis,the method is proposed to quickly generate the discrete-time state matrix according to the network topology using nodal analysis.This new method overcomes the problems of selecting independent state variables and the difficulty of eliminating intermediate variables caused by the improper characteristics of the transmission network in continuous-time state-space modeling method,and is easy to implement programmatically.(2)Based on the discrete-time model of the system,a further discrete-time domain modal analysis method is proposed for oscillation problems.The modal frequency and damping ratio are defined based on the discrete eigenvalues directly,allowing the discretetime eigenvalues to quantitatively evaluate system oscillation stability without conversion to the continuous-time domain.By analogy with the participation factor,the concept of component participation is proposed to indicate the contribution degree of different devices to system oscillations and thus to locate the oscillation source at the system level.Combined with the equations of the system’s discrete-time equivalent circuit,the response results of the oscillation modes in each node voltage and branch current of the system are derived,and the node voltage and branch current observability matrices are defined accordingly,which can be used for the delineation of the oscillation risk region and the identification of oscillation propagation path among the system,respectively.The effectiveness of the proposed method is verified by modal analysis and time-domain simulations of several real grid cases.(3)An example system with multiple wind farms,PV stations,DC transmissions,and SVG centrally connected to the main grid is established,and the wideband oscillation analysis is carried out for the system under a highly proportional power electronics operation scenario.The interaction pattern and mechanism between homogeneous multiple devices are revealed,and a grid simplification equivalence method that preserves the system’s dominant oscillation characteristics is proposed.It is found that there are various oscillatory instability scenarios in the system such as electromechanical oscillations,sub-synchronous oscillations between multiple wind farms,and super-synchronous oscillations between multiple flexible DC transmissions,and their oscillation characteristics are mainly affected by grid structure,system operation mode,converter control parameters,etc.When considering the structure characteristics of the grid to which power electronics are connected,the interaction among multiple devices may be dominated by anti-mode oscillations.The grid simplification equivalence that ignores the grid structure may cause misjudgment of the oscillation patterns among multiple devices,while the grid equivalence method based on nodal voltage distribution coefficients can retain the key nodes and grid ranges that influence the dominant instability patterns of the system,making the simplified model accurately reflect the dominant dynamic characteristics of the system.(4)The dynamic characteristics of two types of grid-forming converters and their effect on the oscillation stability of renewable power generation are investigated,and the mechanism of grid-forming converters to improve the oscillation stability is revealed.Besides,the low-frequency oscillation problems at the system level induced by grid-forming converters are clarified.The studies show that both the grid-forming direct-drive wind turbines with virtual synchronous control and the grid-forming SVG with DC voltage selfsynchronization control will introduce a low-frequency mode associated with the synchronous control link,and this mode may be instable under strong grid conditions.Virtual impedance control can improve the stability of this low-frequency mode.Both of the aforementioned grid-forming control technologies can significantly improve the subsynchronous oscillation stability of wind farms.To make the system stable in extremely weak grid conditions with a short-circuit ratio of 1,the required grid-forming converter is less than 20% of the total capacity of the wind farm.The grid-forming converters enhance the oscillation stability of wind farms by increasing the positive resistive component of the grid-side impedance,which is distinctly different from the synchronous condenser,whose mechanism is to raise the short-circuit current level on the grid.At the system level,as the control parameters of grid-forming converters change,there is a risk of interaction between grid-forming converters and other synchronous machines or grid-forming converters in the system,which may lead to different forms of low-frequency oscillations.