Research On Damping Characteristics And Additional Damping Control Of Hybrid Multi-infeed HVDC System Containing Wind Farms

Line Commutated Converter based High Voltage Direct Current（LCC-HVDC）has the advantages of large transmission capacity,long distance.Voltage Source Converter based High Voltage Direct Current（VSC-HVDC）,which has better control characteristics than LCC-HVDC,and is especially suitable for wind power transmission,island power transmission,hybrid DC transmission,etc.With the large-scale access of clean energy,the Hybrid Multi-infeed High Voltage Direct Current（HMIDC）has a wide range of application prospects.However,the grid connection of Doubly-fed Induction Generator（DFIG）makes the coupling between AC and DC more complicated,and it is difficult to quantitatively analyze the stability of the system.Therefore,it is of great significance to analyze the influence of wind turbines in the HMIDC system from the perspective of power system stability and stable operation of wind turbines without losing synchronization to suppress Low-frequency Oscillations（LFO）in the system.This paper focuses on the research of the HMIDC system containing wind farms:（1）First,the HMIDC system model is established.Based on the basic principles of wind power generation and the principle of LFO,the doubly-fed wind turbines are connected to the sending end of the LCC-HVDC system,the sending end of the VSC-HVDC system,and the receiving end of the power grid.The impact of wind turbine’s access point location and different capacity on system damping characteristics and stability are studied.Second,based on the Total Least Squares-Estimation of Signal Parameters via Rotational Invariance Techniques（TLS-ESPRIT）,the system’s eigenvalues,frequency,damping ratio and other information can be known when wind turbines are connected at different access points.And the influence of the increase of wind turbine capacity on the system oscillation modes.Finally,through simulation analysis based on the model in PSCAD/EMTDC,the correctness of the eigenvalue analysis is verified.（2）In view of the fact that the wind farm is connected to the HMIDC system,the system is insufficiently damped and the LFO occurs.The DC additional damping controller based on H₂/H_∞hybrid control theory is designed.Firstly,the HMIDC system model with wind turbines is established.The wind turbines are connected to the LCC-HVDC system side and the VSC-HVDC system side,respectively.The speed difference of the synchronous generator is used as the input signal,the system’s eigenvalues,frequency,damping ratio and other information are obtained by TLS-ESPRIT.Secondly,use the Butterworth bandpass filter to filter the input signal,and design the LCC-HVDC and VSC-HVDC DC additional damping controllers based on the H₂/H_∞hybrid control theory,so that their characteristic roots are in a certain fan-shaped area.In addition,the balance cut method is used to reduce the order of the controller to realize the damping control of the system.Finally,the effectiveness of the proposed DC additional damping controller is verified in the PSCAD/EMTDC.（3）Aiming at reducing system’s LFO due to the access of wind turbines in the power system,this paper proposes a joint Power Oscillation Damping（POD）method for wind power and the HMIDC systems.At the DFIG converter control system,the rectifier side of the LCC-HVDC system,VSC-HVDC system,POD is used to suppress the LFO of the system.The validity of POD is verified by time domain simulation analysis and eigenvalue analysis method.