Active Frequency Support Control Of Voltage Source Converter-based Multi-terminal Direct Current System Integrated Offshore Wind Farms

As offshore wind power has the advantages of stable resource conditions,high utilization rate,closeness to the load center,and small footprint,it has become an important direction for the development of wind power around the world in recent years.When the offshore wind farm is not far from the onshore system and its capacity is small,the alternating current integration is adopted.With the large-scale offshore wind farms integration and the long transmission distance,the voltage source converter-based multiterminal high voltage direct current(VSC-MTDC)system has attracted more and more attention,as an effective way to solve the cluster,long-distance,and multi-regional wind power friendly grid connection.However,due to the characteristics of the VSC-MTDC system,the offshore and onshore AC systems are decoupled,when the frequency fluctuation occurs in the onshore system,the offshore wind power can not directly provide frequency support to the onshore system,which may even affect the safety and stability of the whole power system.Therefore,in-depth research on the active frequency support control strategy of offshore wind farms connected with VSC-MTDC system is of great significance to realize the safe and friendly grid connection of large-scale offshore wind farms.Aiming at the VSC-MTDC integrated offshore wind farms,and considering the two different scenarios of close sea-land distance and small communication time delay,and far sea-land distance and big communication delay,this dissertation fully considers the coordination between the offshore and onshore systems,the coordination between the different onshore systems,the influence of communication time delay,the different working states of the wind turbines,and the smooth recovery of rotor speeds,and deeply study the active frequency support control strategy of offshore wind farm connected by the VSCMTDC system from the aspects of system modeling,theoretical analysis,control strategy,and simulation verification.The specific research contents are as follows.(1)The basic control strategies of the VSC-MTDC integrated offshore wind farms are introduced,which lays a foundation for subsequent research.The dynamic models of VSCMTDC integrated offshore wind farms are established in the MATLAB/Simulink platform,besides,the real-time simulation model of VSC-MTDC integrated offshore wind farms based on the RTLab platform is further established.The frequency support control strategies of VSC-MTDC-based offshore wind farms can be tested in the dynamic model and the realtime model.After that,the influencing factors of active frequency support from VSCMTDC integrated offshore wind farms are analyzed,including the control parameters for WTs and VSC stations,and the different communication time delays.Those influence factors are also verified in the proposed dynamic model.(2)Aiming at the coordination between the offshore wind farms and the VSC-MTDC systems for offshore wind farms with small communication time delay providing active frequency support,a communication-based coordinated control strategy for active frequency support from VSC-MTDC integrated offshore wind farms is proposed.At the offshore wind farm level,wind turbines provide frequency support successively through a step start-up control strategy to reduce the secondary drop of frequency caused by the recovery of rotor speed.At the converter station level,to reasonably distribute the frequency support power among the onshore converter stations,the droop coefficient is adjusted adaptively by measuring local frequency signals,which can also avoid the large-scale power flow change of the onshore AC system and help the onshore AC system recover the frequency quickly.Simulation results verify the effectiveness of the proposed communication-based two-level coordinated control strategy.(3)Aiming at reducing the influence of the communication time delay on the active frequency support effect,an improved communication-free coordinated control strategy is proposed.Adaptive droop control is adopted at the onshore converter station to convert the frequency change of the onshore AC system into onshore DC voltage change,and the onshore DC voltage signal is estimated by using the offshore local DC voltage and current signals.The offshore wind farm provides active frequency support by responding to the change of estimated DC voltage.After that,when the offshore wind turbine completes the frequency support and begins rotor speed recovery,the asymptotic recovery control strategy is proposed to reduce the second frequency drop.Simulation results verify the effectiveness of the proposed communication-free coordinated control and asymptotic smooth recovery control strategy compared with the communication-based control scheme.(4)Considering the different real-time operation states of offshore wind turbines caused by geographical distribution and wake effect,the frequency support capability of wind turbines are different,and a consistence algorithm-based distributed coordinated control scheme is proposed.The consistency state factor is used to characterize the operation state of each wind turbine,and the droop control coefficient is changed adaptively,the wind turbine with higher speed release more frequency modulation power,and fully taps the frequency support capability of wind farms while ensuring the safe operation.The simulations on MATLAB and RTLab real-time simulation platforms verify the effectiveness of the proposed distributed coordinated frequency support control strategy compared with the centralized control strategy.(5)Considering the situation that the VSC-MTDC integrated offshore wind farms are connected with different asynchronous AC areas,a communication-free coordinated frequency support control strategy between different onshore systems is proposed.By fast detecting,the disturbed and undisturbed AC areas,the proposed adaptive dual droop control can make the undisturbed AC system provide frequency support along with the offshore wind farms.However,the frequency support power provided by the undisturbed AC system is limited according to the estimation of the system inertia for safe operation.The greater the inertia,the more frequency support power can be provided.The simulations on MATLAB and RTLab real-time simulation platforms verify the effectiveness of the proposed adaptive dual droop frequency support control strategy compared with the conventional fixed droop control scheme.