Instability Mechanism And Stabilization Methods Of MMC-Based HVDC For Wind Farm Integration

With the fast development of large-scale and long-distance offshore wind farms,an important issue is how to deliver the electricity from offshore to onshore.Modular multilevel converter-based high-voltage dc transmission(MMC-HVDC)technology is regarded as a promising solution due to its advantages,such as modular design,low distortion of output voltage,high efficiency,decoupling control of active and reactive power,and so on.However,compared with two-level voltage-source converters(VSCs),MMC has much more complex internal dynamics,e.g.,internal harmonic circulating currents,submodule capacitor voltage ripples,etc.,which might have harmful effects on the operation stability of the interconnected system and even lead to oscillations or instabilities.Currently,very few studies reporting on the stability of MMC-HVDC systems connected with wind farms are available.Hence,this paper focuses on the small-signal stability of MMC-HVDC system with wind farm.The main research contents are as follows:(1)The dynamic modeling,operation and control of wind farm integrated with an MMC-HVDC system are studied.According to the topology and working principle of MMC,the continuous mathematical models of the MMC with compensated modulation and direct modulation are built,respectively,including internal dynamic model,ac-side mathematical model,and dc-side mathematical model.Based on this,the internal resonance phenomena of the MMC with direct modulation are analyzed and the mechanism of the harmonic circulating current resonance is also revealed.The analytical expressions of the harmonic circulating currents are derived,and the design principles of the main circuit parameters of the MMC are also proposed to avoid the internal resonance.To improve the efficiency of research and simulation,the dynamic aggregation model of the wind farm is proposed for studying the electromagnetic interaction stability of wind farm integrated with an MMC-HVDC system.The instability phenomena are reproduced by the time domain simulation model of the interconnected system.(2)The small-signal impedance modeling methods of the interconnected system are studied.To represent the multi-frequency coupling characteristics,the harmonic state space(HSS)modeling method is introduced to build the harmonic state space model of the MMC.On this basis,the ac-side small-signal impedance models of the MMC are derived by using harmonic linearization method,which have taken into account the internal dynamics of the MMC.To study the impact of the inherent internal dynamics and the closed-loop control on the stability,the ac-side small-signal impedance models of the MMC with open-loop control,circulating current closed-loop control,and ac voltage closed-loop control are derived,respectively.In addition,the ac-side small-signal impedance models of the wind farm are established by considering the PLL dynamics based on the dynamic aggregation model of the wind farm.The analytical impedance models are validated by the measured impedances in the simulation.(3)The small-signal stability of the interconnected system is studied by using impedance-based stability analysis approach.The generation mechanism of the subsynchronous oscillation(SSO)phenomenon in an MMC-HVDC system with wind farms is revealed from the point of frequency domain,that is,the magnitude-frequency characteristics of the ac-side small-signal impedances of the wind farm and wind farm side MMC(WFMMC)intersects within below the fundamental frequency range.Furthermore,the impact of the controller parameters,main circuit parameters of MMC,circulating current control,modulation,voltage-balancing control,and swiching frequency on the stability of the interconnected system are analyzed.The study results indicate that,the controller parameters,including the ac voltage controller of WFMMC,current controller and PLL controller of wind power inverter,have great impact on the stability of the interconnected system;the MMC system is lack of stability margins if no any additional damping measures have been taken in the MMC;the stability of the interconnected system can be improved by properly increasing the arm inductance or submodule capacitance;the circulating current suppression controller of the MMC can not only suppress the circulating currents,but also increase the system damping;those factors,such as modulation,voltage-balancing control,switching frequency,and so on,have little impact on the stability of the interconnected system.(4)The propagation mechanisms of the SSO current injected from the wind farm in the MMC-HVDC system are analyzed by using instantaneous energy method.The impacts of the SSO current on the WFMMC,GSMMC,and ac grid are derived and analyzed theoretically.From the point of the HVDC converter control,an additional control strategy in WFMMC for the SSO current suppression is proposed,by which the SSO current can be effectively suppressed.(5)The optimized design method for the controller parameters and system stabilization methods are proposed to enhance the stability of the interconnected system.From a point of view of single converter,the design methods for the controller parameters of the WFMMC and wind power inverter are proposed,respectively.Based on this,from a point of view of system,an optimized design method for the controller parameters is proposed to enhance the stability of the interconnected system.Besides,several stabilization methods of the interconnected system,viewed from both WFMMC and wind power inverter,are proposed to make up for the deficiencies of controller parameters optimization.These stabilization methods are able to shape the converter impedances by adding additional control loops in order to make the interconnected system stable.Finally,both the simulation results and on-site recorded waveforms validate the theoretical analysis in this paper.