Mechanism Analysis And Risk Assessment Of Wideband Oscillation Of Direct-driven Wind Turbines Interfaced With MMC

Our country has accelerated the construction of a clean,low-carbon,safe and efficient energy system.The energy production and utilization patterns are being revolutionized,which promotes the rapid development of renewable energy represented by wind power.The traditional energy system supported by fossil energy will be gradually replaced by the new energy system focused on the renewable energy.Given that the wind energy base and load intensive area are generally far apart from each other,MMC-HVDC technology has become an effective solution with its unique advantages for large-scale wind power transmission over long distances due to its unique advantages in new energy integration.However,the interaction between the MMC converter station and grid-side converter connected with wind turbines is easy to cause wideband oscillation from sub-synchronous frequency range to mid and high frequency ranges,which seriously affects the reliable transmission of wind power and the safety of the interconnection grid.In this context,this paper focuses on the direct-driven wind turbines interfaced with MMC,establishes the impedance model of the interconnected system,deeply discusses the wideband oscillation mechanism of the system,comprehensively analyzes the main influence factors of each frequency band,and proposes a quantitative risk assessment method for wideband oscillation suitable for the black box system.The details are as follows:(1)Impedance modeling of the interconnected system.Firstly,the impedance models of MMC converter station and grid-side converter connected with wind turbines are established by modular modeling method.Then,based on the models,the mechanism of frequency coupling phenomenon is demonstrated through the mathematical derivation.Furthermore,an equivalent SISO impedance modeling method involved the coupling between positive and negative sequence and that between source and load,is proposed.This method makes the frequency coupling characteristics embodied in the sequence impedance model in the form of analytic expression.(2)Wide-band oscillation mechanism analyses.Based on the established impedance model of direct-driven wind turbines and MMC,the wide-band oscillation mechanism of the direct-driven wind turbines interfaced with MMC is revealed from the variation of impedance amplitude-frequency and phase-frequency curves.At the same time,the mathematical relationship between the controller parameters and the system impedance model is derived,so as to divide the frequency band of the system impedance characteristics and summarize the key factors affecting the oscillation of each frequency band.The correctness of the analysis results is verified by electromagnetic transient simulation in MATLAB/Simulink.(3)Improved quantitative risk assessment method for wideband oscillation.Firstly,the applicable range of the quantitative risk assessment methods based on Nyquist criterion and equivalent RLC circuit is deeply analyzed and compared.Secondly,the conservativeness of the latter is demonstrated from the perspective of time domain simulation.Finally,due to the fact that the actual systems are usually high-dimensional,the fitting objective function is selected as the fractional polynomial,leading to an improved quantitative risk assessment method based on equivalent RLC circuit with wider application range and higher accuracy.