Coordinated Control Of The Wind Generation System Integrated By VSC-HVDC

The voltage source converter multi-terminal direct current (VSC-MTDC) transmission technology is suitable for power flow optimal control of the wind power interconnection system, however, DC network cannot provide effective mechanical inertia for dynamic support as AC network, thus the quick collaboration capability of active power regulation for both wind turbines and converters should be achieved to enhance the system stability. In this dissertation, an integrated control scheme of variable speed wind turbines for virtual inertia and primary frequency regulation is proposed to achieve the complete active power-frequency control ability. Following that, the linkage adjustment relationship between the system frequency and the dc voltage is established, and then the power coordinated control strategy of MTDC system with high wind penetration level is further presented, so as to fully utilize the mutual support ability of each network and the mechanical energy storage of wind turbines. In addition, the operation stability and the fault ride through (FRT) capabilty of the MTDC system connected with wind turbines can be enhanced as well. The main research results are as follows:1. The dynamic model of variable speed wind turbines and DC transmission system are established, the different topologies of MTDC system are analyzed, and then the suitable topology and operating mode of MTDC system for wind power interconnection is proposed, and the simulation platform of the wind power system is also built, which are used to provide the basis for designing the coordinated power control of wind turbines and each terminal converters.2. The virtual inertia control and the frequency regulation method of variable speed wind turbines are analyzed, and the integrated frequency control strategy for virtual inertia and primary frequency regulation is proposed. According to the definition of de-loading level and the pitch static difference coefficient, an optimized de-loading control scheme and a primary frequency regulation scheme with settable static difference coefficient are proposed respectively by variable pitch technology, which resolve the issue of the combination between virtual inertia and primary frequency regulation. With the proposed strategies, wind turbines achieve not only the inertia frequency response but also the system frequency regulation requirement, and thus the response ability to the grid active disturbances is enhanced.3. According to the analysis of the impact of grid faults on the safe operation of two terminal voltage source converter high voltage direct current (VSC-HVDC) system, an active power coordinated control strategy of wind turbines for fault ride through capability enhancement of DC system is proposed. During the current limit process of the grid side converter, the constant frequency control mode of the wind farm side converter is changed, and the relationship between the DC voltage fluctuation and the system frequency regulation is established, then the electromagnetic power of variable speed wind turbines can be regulated by the virtual inertia control, so as to provide the effectively support for DC system FRT using the mechanical energy storage. Moreover, the proposed scheme at low cost can be easily applied in MTDC system and without any communication.4. In the DC system connected with wind turbines, the inertia is low while power fluctuation is high. In order to improve the system stability, the decentralized cooperative control of each terminal network and the source network coordination control strategy are proposed respectively, and then the coordinate regulation ability of the interconnection systems is achieved. The decentralized cooperative control characteristics is explored firstly to regulate the dc network voltage in different operating modes effectively. And then, the dc voltage-active power control of the receiving end converters and the variable frequency control loop of the wind turbine side converter are designed respectively. Both the conventional power and the wind turbines under the integrated frequency control, which are dispersed in each terminal network, can develop their active power regulation ability to achieve the the source network coordination control. With the proposed coordinated control strategy, the mutual support ability of each network and the mechanical energy storage of wind turbines can be fully utilized, and thus the operation stability of the MTDC system with high wind penetration level is further enhanced. Moreover, the proposed scheme is easier to extended and without communication.