Research On Control Technology Of Applying Storage-Based Wind Farm As Black-Start Power Source For Power Grid

Due to the limitation of geographical resources, some areas such as Inner Mongolia and Northwest area in China are lack of water resource, and thus these areas are lack of water power plant as the black-start power source. Once a large-area blackout occurs in these areas, the restoration of local power grid would heavily rely on the main grid and the restoration time would be longer, and thus the large-area blackout would cause serious economic losses. However, these areas are rich in wind resource which has gradually become an important local power source. And the reliability and controllability of a wind farm would be effectively improved by configuring it with the high-capacity battery energy storage system (BESS) to build a storage-based wind farm. Therefore, tapping the potential of a storage-based wind farm to turn it into a black-start power source of the local power grid would increase the capacity and location points of the black-start power source and improve the restoration speed of local power grid.Based on the actual power grid, a black-start scheme applying storage-based wind farm as a black-start power source is proposed in this paper, and then the feasibility and some key techniques of the proposed scheme are studied. It could provide reference and theoretical basis for the power grid in water-shortage and windy areas to apply storage-based wind farm as its black-start power source. The main work of this paper is as follows:1. The simulation models of BESS and doubly-fed induction generator based wind turbine (DFIG) under different operating conditions are established. Through the analysis of BESS operating characteristics under grid-connected operation mode and island operation mode, the simulation models of BESS are built respectively. And the simulation models of DFIG under no-load and grid-connected operation mode are established respectively through the analysis of the operation principle of DFIG under these two operation modes. The established models could lay the foundation of the study of black-start technique with storage-based wind farm as a black-start power source.2. A control strategy of applying the storage-based wind farm to start the thermal power unit is proposed. Firstly, a local power grid is chosen as research object, an overall technical scheme using storage-based wind farm to supply the black-start of the nearby thermal power plant is designed, and the feasibility of the proposed technical scheme is verified. Secondly, a self-starting scheme of storage-based wind farm is designed based on the study of the starting strategy of BESS and the charging scheme of DFIG box-type transformers. After the self-start of storage-based wind farm, a control strategy of storage-based wind farm operating in island mode is proposed in order to guarantee system stability when the transmission lines and transformers are charged and the large-capacity auxiliary machines are started up.Finaly, in the case that wind farm has the maximum continuous output probability,the process of using storage-based wind farm to start up thermal power unit is simulated.3. A capacity configuration method of BESS when applying storage-based wind farm as black-start power source is proposed. Through analyzing the factors that affect the capacity configuration of BESS, the capacity of BESS is configured from three aspects: the rated capacity of the battery system, the maximum charge and discharge power of the battery system, and the rated capacity of the power conversion system. And on this basis, the feasibility and applicable conditions of using storage-based wind farm as the black-start power source of power grid are analyzed.4.To solve the problem that frequent alteration in DFIG's rotor speed and pitch angle might result in mechanical fatigue, an improved active power control strategy of DFIG when storage-based wind farm operates in island mode is proposed. Firstly,through analyzing the stability of wind turbine at different operation points,an optimization model is established with minimum comprehensive adjustment of rotor speed and pith angle as the objective function, and the power balance, operation point stability etc. as the constraint conditions. Secondly, a method dealing with no-feasible solution of the optimization model is proposed to obtain the feasible optimal transfer trajectory. Finally, A wind speed estimator and an operation trajectory controller are established and the control strategies for pitch system and rotor side converter are improved, which enables wind turbines to track the optimal transfer trajectory.5. A hierarchical coordination control strategy of wind-battery-thermal power system at the initial stage of black-start is proposed. This control strategy guarantees the wind-storage-thermal system to have not only no weaker active power output ability, but also stronger reactive power regulation ability than the ability of thermal power plant. Based on the analysis of the operating characteristics of the wind-battery-thermal system, the coordinated control model of wind-battery-thermal power system is established and differential evolution algorithm is adopted to attain the voltage regulation commands of thermal power plant and storage-based wind farm in real time. According to the voltage regulation commands, the excitation voltage of thermal power unit and the reactive power output of energy storage-based wind farm are adjusted, and the reactive power and active power output setting values of the energy storage-based wind farm are further distributed to BESS and each wind turbine. Through the control of BESS and DFIG, the storage-based wind farm could output the specified power and has the ability to participate in frequency and voltage modulation when operating under isolated operation mode.