Studies On Coordinated Operation And Control Techniques Of Wind-Energy Storage Integrated System

In recent years,the growing penetration of wind power in the power grid has drawn significant attention from the academic and industrial communities due to its impact on the safe and stable operation of the system.On one hand,the rotor of a wind turbine generator(WTG)is decoupled from the system frequency,thereby unable to provide rotational inertia to the system.Furthermore,WTGs typically operate in the maximum power point tracking(MPPT)mode and lack the spare capacity for frequency control,resulting in a reduction in the system’s frequency support capability following largescale grid integration.On the other hand,its limited fault ride through(FRT)ability makes it easy to decrease the active power input to the grid due to its off-grid failure,leading to serious accidents.In this paper,domestic and foreign power regulatory authorities have issued requirements and criteria for wind power frequency control and FRT capability,in order to ensure the safety and stability of the power grid.The energy source of WTGs used for primary frequency control(PFC)is rotor kinetic energy or load reduction reserve.However,due to the limited rotor kinetic energy,long-term load reduction operation causes great economic losses to wind farms due to the loss of captured wind power.Therefore,further research is needed to improve the PFC capability of WTGs.In addition,in the process of FRT,there is unbalanced energy flow inside the WTG.How to buffer and release this unbalanced energy is the core challenge to ensure that the WTG has the ability of FRT.However,the existing methods such as overspeed load reduction and Chopper unloading circuit still face many technical problems.This paper proposes to explore solutions to the aforementioned challenges by configuring energy storage devices on the DC side of WTGs and building an integrated wind turbine generator and energy storage(WTG-ES)system.Based on the actual needs of the system for WTGs’ PFC and FRT capabilities,corresponding integrated PFC and FRT control strategies are designed to improve the PFC and FRT capabilities of WTGs.The focus of this study is the WTG-ES system,which combines a wind turbine generator(WTG)with an energy storage system.Building upon existing research findings,this paper firstly proposes a control framework for each converter of the WTG-ES system under normal operating conditions.Subsequently,specific control strategies are developed for Power Factor Correction(PFC)and the FRT process.The proposed strategies are then validated through semi-physical simulations to demonstrate their feasibility and effectiveness.The main contributions and innovations of this paper are as follows:(1)The control framework of the WTG-ES system in conventional operation mode is proposed.Firstly,through a comparative analysis of the performance of different types of energy storage,the lithium-ion supercapacitor is identified as the type of energy storage applicable to the WTG-ES system,and on this basis,the components in WTGES system are modeled.Secondly,with the control objective of smoothing the wind power fluctuation in the conventional operation mode,the WTG-ES system machine side converter(MSC),energy storage side converter(ESSC)and grid side converter(GSC)control framework is proposed,which is different from the conventional WTG’s converter control logic,The control framework of WTG-ES system with MSC,ESSC and GSC is proposed,and the control strategy of GSC under conventional operation mode is determined by investigating the influence of GSC’s power reference value giving method on energy storage.(2)The strategy of the WTG-ES system participating in the PFC system is proposed.In order to solve the problem of insufficient WTG’s frequency control reserve energy and the secondary frequency drop(SFD)caused by the withdrawal of WTG from frequency control due to low rotor speed,a control strategy based on the whole process of PFC is proposed.The strategy first estimates the energy demand of WTGs participating in the whole process of PFC and considers the releasable kinetic energy of WTG’s rotor and the energy loss during PFC,determines the actual releasable kinetic energy of WTG’s rotor under different wind speed operation scenarios,and proposes a configuration scheme for energy storage based on the actual releasable kinetic energy and energy demand.Finally,the sag control is attached to the MSC and GSC control of the WTG-ES system,and the MSC’s sag coefficient is reasonably adjusted according to the wind speed operation scenario to realize the comprehensive control of WTG’s rotor kinetic energy and energy storage during PFC,which realizes the effective support of WTG-ES system to the PFC process.(3)A low-voltage ride-through(LVRT)control strategy for WTG-ES system based on capacity assessment and power allocation control is proposed,and a power allocation control framework is designed to allocate unbalanced power in real time during faults in the order of GSC,ESSC and MSC,and to consider the unbalanced power dissipation capacity of the three sides under different operation states and different fault degrees to ensure that its capacity limit is not exceeded during power allocation.Finally,the control strategies of the GSC,ESSC and MSC are designed under this framework,so that they can correctly execute the commands issued by the power allocation system.The WTG-ES’s LVRT strategy based on power allocation realizes the coordination between energy storage and WTG during faults,and strengthens the LVRT and active support capability of the WTG-ES system.(4)The proposed control strategy is verified based on the RTDS hardware-in-theloop experimental platform,and the prototype WTG-ES system is developed and tested in field operation.The basic control architecture of the proposed WTG-ES system and the PFC and LVRT control strategies are programmed in the platform.The effectiveness of the basic control architecture,PFC and LVRT control of the WTG-ES system was verified.On the basis of the above-mentioned work on the overall structure of the WTGES system,control strategy design and semi-physical simulation verification,a prototype of the WTG-ES system was developed in cooperation with the WTG manufacturer,and the frequency regulation performance of the prototype of the WTGES system was tested in the new energy test base in Zhangbei,and the test results showed that the developed WTG-ES system could achieve the expected frequency regulation effect well.