| China is rich in wind energy resources, while large-scale wind power bases are mainly located in the Three-North Area of China, such as Inner Mongolia, Xinjiang province, Gansu province, Jilin province and Heilongjiang province, etc. To solve the problem of inverse distribution of energy bases and load center, and reduce the impact of wind power fluctuations on the power grid, the concept of Wind-Thermal-Bundled (WTB) power system transmitted by high voltage direct current (HVDC) line has been put forward, which is an important development mode of large-scale wind power bases. With the increasing of wind power proportion, wind power fluctuations and wind farm failures are bound to have a great impact on system frequency stability. Therefore, it’s an urgent need to study on the frequency stability and control strategies for WTB power system.Firstly, based on real wind-thermal-bundled power system with Hami-Zhengzhou ±800kV ultra high voltage DC system, the actual situation and possible problems are introduced, including angle stability, voltage stability and frequency stability. Simulation models of WTB islanded mode and WTB interconnected mode are established in electric power system analysis software PSD-BPA, including wind turbine model, thermal power unit model, DC system model, wind speed model and etc. According to actual data, the probability density function of wind power variations is also analysed.Secondly, Based on simulation analysis, the influence of different factors on the frequency stability of sending-end system is studied, including different wind turbine types, the integration proportion of wind power, reserve capacity and inertia of interconnected network. With the analysis of mechanism of thermal power unit primary frequency regulation (PFR), the influence of key parameters of PFR on frequency regulation effect is studied. The requirements for PFR’s key parameters with large-scale wind power integration are proposed.Finally, the principle of DC modulation and various modulation control modes are introduced. The mechanism of DC modulation to improve the frequency stability of power system is analyzed. Considering WTB islanded transmission mode, whose frequency stability problem is more serious, suitable DC modulation control strategy is proposed, including modulation mode, modulation signal selection and the optimization of controller parameters. With comprehensive consideration of thermal units PFR ability, the necessity of coordinated control between DC system and thermal units is analyzed. Coordinated control strategies of DC system and thermal units are proposed, and the effectiveness of the proposed method is verified by simulation. The characteristic of two transimission modes and their frequency control strategies are summarized in the end. |
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