Influence of Grid Connected Direct Drive Wind Farm on Subsynchronous Resonance of Thermal Power Units

The direct-drive wind farm (D-DWF) and thermal power bundled point-to-network system sent through series supply lines have become the main form of energy consumption in recent years because of its advantages of large transmission capacity and mature and reliable technology. However, there is a subsynchronous resonance problem in the thermal power series compensation system, and the direct drive wind farm system itself also has the risk of subsynchronous oscillation. Therefore, the oscillation problem of the baling system cannot be ignored. After D-DWF is connected to the thermal power series replenishment system, its influence mechanism on the subsynchronous resonance (SSR) of thermal power units is not clear. Firstly, based on the linearization mathematical model of point-to-mesh system with D-DWF and thermal power bundled and sent by series supplementation, the state variable elimination method suitable for modular modeling is used to establish a system small signal model based on modular modeling idea in MATLAB/Simulink, and its reliability is verified by step response. Secondly, the small signal model of the D-DWF system before and after grid connection is solved, and the SSR characteristics are analyzed. Then, the SSR mechanism of the D-DWF system after access is explained, and the SSR mechanism is verified by time domain simulation. Finally, taking the capacity, wind speed and string complement of D-DWF as the research objects, the risk of SSR in the system is analyzed from the perspective of damping characteristics and time-domain response. When the series complement is 23.5%, the corresponding resonant frequency is about 29 Hz, which is complementary to the frequency of SSR mode (corresponding to thermal shafting mode 4) in the system. At this time, when the system is disturbed, the subsynchronous component acts on the thermal power unit through the point of common coupling (PCC), inducing SSR. When there is a mode of about 32 Hz in D-DWF, after the disturbance occurs, the subsynchronous component will also act on the thermal power unit through PCC, which will excite the SSR in the system and intensify the oscillation. After the thermal power unit generates SSR, the thermal power unit feeds the subsynchronous component back to the PCC through the speed signal, and the subsynchronous component flows through the closed loop. The main conclusions of this paper are as follows: (1) Through step response, the reliability and accuracy of the small signal model of wind-fire bundle warp supply system are verified. (2) Before and after D-DWF access, the system has four negative damping torsional vibration modes, including an SSR mode. After D-DWF is connected, it will form a subsynchronous excitation loop with series lines and thermal power units. The oscillation mechanism shows the coupling phenomenon of "electromechanical resonance" and "modal frequency approach"; SSR mode damping is reduced, the risk of system SSR occurrence is increased, and the torsional vibration mode damping of other negative damping is improved. (3) The reduction of D-DWF capacity, wind speed decrease, and series complement decrease will increase the damping of SSR mode and reduce the risk of SSR. The string complement degree after D-DWF is incorporated is still the main influencing factor of SSR in the system, and the influence effect of D-DWF is limited. © 2024 China Machine Press. All rights reserved.