In order to improve the stability of doubly‐fed induction generator (DFIG)‐based wind turbines under weak grid, previous works have proposed the power synchronization control strategy in the rotor side converter (RSC). However, the potential instability risk of dc‐link voltage control in the grid side converter (GSC) has been neglected. Here, DFIG with power synchronization and phase‐locked synchronization for RSC and GSC respectively, is called as hybrid synchronization mode (HSM)‐controlled DFIG and, the full‐order state space model of HSM‐controlled DFIG are established. Eigenvalue and participation factor analysis shows that the HSM‐controlled DFIG is unstable under extremely weak grid due to the interaction of phase‐locked loop (PLL) and current‐loop in GSC. Furthermore, dominant pole analysis indicates that the HSM‐controlled DFIG exhibits weak damping at the AC side in strong grid. To enhance the stability of HSM‐controlled DFIG under extremely weak grid, an improved hybrid synchronization control strategy is proposed. The improved control strategy utilizes active power and dc‐link voltage to achieve PLL‐free operation of DFIG. The effects of the parameters in dc‐link voltage synchronous loop on control bandwidth, system stability and dynamic coupling are analysed. Finally, the validity of the theoretical analysis is investigated with experiments based on 2MW control‐hardware‐in‐loop platform.
It is of great significance to reduce the consumption of fuel oil for pulverized coal fired boilers in China. Minimal-oil ignition technology is an effective solution to save fuel oil. The principal, system components and industrial applications of the minimal-oil ignition technology are introduced in this paper. The concretely adopted schem involves that the small gasified oil gun is adopted to replace the starting-up oil gun, and the concentration device is arranged to realize the concentrationstaged combustion of pulverized coal, without any influence to the boiler's performance. The numerical simulation and experiment results indicate that the minimal-oil ignition technology can be applied in startup, stopping and aided combustion in low load condition of pulverized coal fired boilers. The average fuel oil consumption after retrofit is reduced by over 90%. The minimal-oil ignition system is simple, operation convenient and little maintenance is needed. Its extensive application can bring prominent economic and environmental advantages.
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