Research On Network Energy Analysis Method Of Low Frequency Oscillation For Power System With Voltage Source Double-FED Wind Power

In recent years,with the continuous improvement of grid-connected capacity of new energy generating units represented by wind power and the increasing scale of interconnection system,the dynamic characteristics of power system have undergone profound changes.The power grid will gradually develop into a low-inertia and under-damping network dominated by power electronic converters,and the stability problem is becoming more and more serious.Under this background,virtual synchronous generator control technology comes into being.It is applied to rotor side converter of doubly-fed wind turbine to construct voltage source doubly-fed wind power,which can simulate traditional synchronous generator to provide inertia and damping support for power grid.However,due to the similarity between mechanical and electromagnetic equations,the traditional low frequency oscillation problem is also introduced.At the same time,due to the deepening degree of electric power electronization in power systems containing a high proportion of new energy,the difficulty of system modeling becomes greater,and "source-side" analysis faces challenges.Aiming at the low frequency oscillation problem of doubly-fed wind power system with voltage source,this paper transfers the "source side" analysis to the "network side" analysis from the perspective of network and branch,and proposes an analytical method of network energy based on the energy function.Firstly,the construction of voltage source doubly-fed wind power grid-connected system is carried out,mainly focusing on the dynamic modeling of voltage source doubly-fed fan under electromechanical time scale.The mathematical model of doubly-fed fan,grid-side converter control link,machine-side converter control link,phase-locked loop control model,virtual synchronist-power outer loop control and voltage and current double inner loop control are introduced respectively.The virtual power Angle theory is proposed and the synchronization and operation characteristics of voltage source doubly-fed fan are studied by time domain analysis.Secondly,the theoretical basis of the energy analysis method of low frequency oscillation network is introduced.First,the conservation of oscillation energy of the system under small disturbance is proved.Then,the branch oscillation energy is defined,which is analyzed and simplified.After verifying the correctness of simplification in the time domain and frequency domain,the branch mode oscillation energy is defined.The relationship between the branch mode oscillation energy and the actual low-frequency oscillation phenomenon,the system mode state variable and the generator speed state variable mode observability is analyzed,and the applicability of the network energy analytical method is illustrated.Then,the network mode oscillation energy is defined,the relationship between each index in the energy analysis method and the network energy structure are clarified,and the theoretical basis of the low frequency oscillation network energy analysis method is improved.Finally,based on the network energy analysis method,the interaction characteristics of the machine and network in the voltage source doubly-fed wind power system are studied.By comparing with the traditional doubly-fed wind power system,the network mode oscillation energy,network oscillation energy and branch mode oscillation energy are taken as the quantitative indexes,and the interaction characteristics of the machine and network in the low-frequency oscillation are focused.On the other hand,according to the network energy analysis results,a low frequency oscillation region location method based on the branch mode oscillation energy is proposed,which can locate the regions participating in the oscillation in different modes online and observe the low frequency oscillation phenomenon more accurately.Compared with the previous energy function method based on dissipated energy,this method does not need to calculate the variation of each electrical volume.It is more suitable for application in combination with Wide Area Measurement System(WAMS).Meanwhile,the extraction method of branch mode oscillation energy under multi-mode is illustrated,and the effectiveness of the method is verified under load fluctuation disturbance and three-phase short-circuit disturbance.