| Since the 21 st century,the development of renewable energy,such as solar energy and wind energy,has been identified as a strategic emerging industry.Wind energy,as clean and renewable energy,has a broad application prospect.In recent years,the grid-connected capacity of wind turbine generators has risen steadily.At the same time,power electronic converters,as a kind of flexible and efficient grid interface to connect wind power systems,are being widely used in power systems.This leads to the power electronics dominated power system.However,the interaction between the power electronic converters and the AC power grid has greatly changed the system dynamics and caused a new type of sub-synchronous oscillation problem.This new type of sub-synchronous oscillation occurs frequently,and its mechanism is complex and diverse,which has an adverse impact on the stable operation of power systems.Therefore,studying the mechanism and characteristics of this new sub-synchronous oscillation and putting forward the suppression strategy has a great significance in theory and engineeringThe direct-drive PMSG incorporated into the AC power grid is used to research by harmonic impedance frequency characteristic analysis method.Based on the harmonic impedance characteristic,a new sub-synchronous oscillation suppression strategy is proposed.Firstly,the mathematical models of direct-drive PMSG,power electronic converters and AC power grids are established by studying and summarizing the research results of the predecessors.Based on the models,the research system is built on the PSCAD/EMTDC simulation platform,which lays the foundation for the follow-up research.Secondly,a sub-synchronous oscillation analysis method for harmonic impedance frequency characteristics is proposed in this paper.Based on Thevenin's equivalence theorem,the target system is equivalent,and then the sub-systems to be studied are divided from the target system to measure harmonic impedance frequency characteristics.Thirdly,the equivalent impedance of sub-system is solved according to network structure.Furthermore,the sub-synchronous oscillation mechanism is analyzed based on the circuit principle.The correctness and effectiveness of the proposed method are validated through the IEEE first benchmark model.Finally,based on the proposed analysis method,the risk of sub-synchronous oscillation of AC power systems connecting with direct-drive PMSG-based wind farm is studied.The results show that the power electronic converter of the direct-drive PMSG exhibits a small negative value at oscillation mode frequency.This means that the converter behaves like a capacitor with negative impedance.However,the capacitive impedance of power electronic converter forms a resonant loop with the inductance impedance of the AC power grid.In order to solve this problem,the control system parameters of power electronic converter are optimized to change the harmonic impedance characteristics,so that the capacitive impedance is further changed to reduce the risk of sub-synchronous oscillation of wind power system.The proposed sub-synchronous oscillation control strategy in direct-drive wind power system is verified on the PSCAD/EMTDC platform. |
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