| Due to the "Carbon peaking and carbon neutrality" goal,the penetration rate of new energy power generation systems based on wind energy continues to increase in the power system,and onshore and offshore resources with dominant wind resources are basically being used.Construction of onshore wind power will gradually expand into more remote areas,and offshore wind power will gradually move to the sea or beyond the scope of the existing AC power grid.It is difficult for wind power to transfer energy externally using conventional DC without strong AC power grid support,and therefore only flexible DC is suitable.One of the challenges the power system face is the sub/super-synchronous oscillation(Sub/super-SO)happened in the wind farm connected with the high voltage DC transmission systems.Oscillation phenomena have been observed in wind power projects resulting in equipment damage and system instability domestic and abroad.It is urgent to study the mechanism and mitigation methods of sub/super-synchronous oscillation in wind farms since these phenomena are significantly different from sub/super-synchronous resonance(oscillation)in thermal power plants.This thesis investigates the sub/super-synchronous oscillation in three major issues in the permanent magnet synchronous generator(PMSG)wind farms connected to the voltage source converter-based DC transmission system(VSC-HVDC)system.(1)the occurrence mechanism and influencing factors of sub/super synchronous oscillation;(2)the mitigation method of sub/super synchronous oscillation of PMSG wind farms which causes the oscillation;(3)the control optimization of PMSG wind farms passively connected to the oscillating power grid.The main research contents and innovations of this thesis are as follows(1)The small-signal impedance models of PMSG wind farms and VSC-HVDC based on synchronized rotating coordinate are first established,and the bandwidth of each control loop is introduced into the impedance model as direct parameters.Then analyze the aspects involved phase and magnitude,and the Bode impedance stability criteria is used to reveal the mechanism of the sub/super-synchronous oscillation.The y-axis impedance of PMSG wind farm is the main cause of this system instability.Then the influence of each element on the system stability is analyzed,which include the bandwidths of PMSG wind power current control loop,DC voltage control loop,PLL and the bandwidths of VSC-HVDC current control loop,AC voltage control loop.Meanwhile,the stable operating point and the different type of grid-tied filters are all included.(2)This thesis analyzes the reason of the low phase characters of PMSG wind farm y-axis impedance,and the impedance reshaping methods would be applied to increase the phase margin of the y-axis impedance to mitigated the sub/super-synchronous oscillation in PMSG wind farm connected to VSC-HVDC system.Firstly,an impedance reshaping method to counteract the effect of a phase-locked loop with grid voltage feedforward is proposed.This method is available in some cases because it is affected by the current controller parameters.Secondly,in view of the shortcomings of this impedance reshaping method,another impedance reshaping method that equivalently reversed the effect of the current is proposed,which can provide a higher and stable phase margin without the affection of current controller parameters.(3)The influence of the oscillation voltage on the phase-locked loop and current control loop of wind power is analyzed.Based on the analysis results,a phase-locked loop based on weighted band-pass filter is proposed to eliminate the influence of the sub/super-synchronous oscillation voltage components on the output angle of the phaselocked loop.Active Disturbance Rejection Control is introduced to replace the traditional PI control to enhance the disturbance immunity of PMSG wind power converters as the current control loop.And the ADRC is optimized to further reduce the influence of the disturbance introduced by the coordinate transformation on the current loop.Finally,the simulation experiments prove the effectiveness of the proposed optimization method for phase-locked loop and PMSG wind power converter current loop. |
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