Secondary/Supersynchronous Oscillation Analysis And Its Damping Suppression Of Multi-wind Farm Access Power System

With the continuous increase of the installed capacity of grid-connected power generation of new energy,the power system will face an increasing risk of new type of oscillation,which is often related to the power electronic equipment connected to the power system.Among all kinds of new oscillations,secondary/super synchronous oscillation has attracted the most attention of scholars and operators.This oscillation phenomenon between 5 and 150Hz has caused hundreds of typhoon power units to be cut out by the power grid company,and thermal power units hundreds of kilometers away to shut down.Impedance analysis is the main method of secondary/hypersynchronous oscillation analysis.It does not have the problem of dimensional disaster of equation of state,and it has clear physical concept,which is very suitable for the analysis and suppression of large-scale power system secondary hypersynchronous oscillation.At present,the most suitable analysis method for power electronic devices is too complicated,harmonic linearization.In order to obtain the theoretical impedance characteristics of power electronic devices more easily,this paper studies the transformation method from state space equation to impedance characteristics,which greatly optimizes the difficulty of impedance modeling of power electronic devices.During the study,the author found that the traditional impedance model of chain SVG is not accurate,so the author considered the 100Hz harmonics of the DC side of chain SVG in the modeling process,established its impedance characteristics,and proved the correctness and necessity of considering the 100Hz harmonics of the DC side of Chain SVG from the perspective of impedance comparison and oscillation mode.The topology of a complex wind farm is very complex,and the study of its stability requires locating its oscillation source and oscillation path.Researchers can establish the circuit impedance of the system by graph theory knowledge and obtain the oscillation mode of the system according to the determinant criterion.Then,the branch participation factor of the main oscillation mode is analyzed.The branch with large participation factor is the oscillation path of the system,and the active component(power electronic device)on the path is often the oscillation source of the system.The secondary/hypersynchronous modes in the system appear in pairs.The reason why their amplitudes are different is that their observability is different,but there are master/slave modes.In the face of the difficult situation that the manufacturer refuses to provide the system control parameters because of the protection of intellectual property,the accurate analysis of the system oscillation mode is faced with unprecedented challenges.The characteristic trajectory has been noticed by researchers because of its superior frequency characteristics and mathematical characteristics,and it is highly suitable for the "black box problem".This method was first applied to the PSS design of thermal power units,and the damping control of wind power units can also use this kind of method.Different from thermal power units,the order of grid-connected system of multiple wind turbines is too high,which is not suitable for point-by-point design.Therefore,the author designs an SSDC gridconnected suppression device by presetting the phase Angle margin of special points(cut-off frequency points),and completes the suppression work of wind turbines through experiments.