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This study firstly derives the relationship between the grid voltage and the rotor current of doubly-fed induction generator (DFIG) during the grid voltage swell. Then, an effective control strategy aiming to restrain rotor over-current is proposed and the control strategy of the grid-side converter is also improved. The proposed rotor current suppression and grid voltage suppression method, cooperating with the DC chopper, can not only solve the problem of short circuit of the rotor side converter resulting from the frequent switching of crowbar device, but also maximally improve the capability of wind turbine in dynamic reactive power support without adding any other power electronic devices. With the help of this control strategy, the wind turbine could maintain its normal operation even during the fault and the high voltage ride through of wind turbine can be achieved. As a result, the stability of the wind power integration is significantly improved. Finally, the simulations and analysis in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) verify the effectiveness and feasibility of the proposed control scheme.

This paper applies an electromechanical transient model to the analysis of ultra-low frequency oscillations (ULFO) in Chongqing-Hubei asynchronous power systems. The electromechanical model of Back to Back MMC-HVDC is established firstly. Emphasis is put on the contribution of hydroturbine governors to UFLO and additional damping controller design. The prolonged oscillations in the simulation is analyzed with power system analysis software package (PSASP) model, while additional controller in rector converter station is designed that proves to be effective in damping out the ultra-low frequency oscillations.

For the growing power rating and voltage level, modular multilevel converter (MMC) is a feasible solution in the application of voltage sourced converter based high voltage direct current(VSC-HVDC) transmission. However, due to its numerous independently controlled sub-modules(SMs), it is very difficult to represent the complete internal transient process of MMC or accomplish the hardware-in-loop (HIL) test of valve based controller (VBC) using digital simulation platform. Therefore, this paper deduces the similarity criterions and the parameter calculation method for the design of low-voltage physical simulation system. Taking the Xiamen ± 3 20kV/1000MW MMC-HVDC project in China as a simulation prototype, a physical simulation system is built, based on which the type test of VBC and system operation analysis are completed. Experiment results verify the correctness of the proposed calculation method and the designed system.

This paper introduces the electromechanical modelling method of modular multilevel converter based high voltage direct current (MMC-HVDC) system and studies the Chongqing-Hubei project based on the model. Firstly, the steady load flow model is established in this paper. Then, the transient stability model, which includes the AC side model, DC side model and control system model, is described in detail. For improving the transient stability, an additional control method, which combines a frequency controller and emergency power support strategy, is added on the control system model. Moreover, the modulation model is studied and discussed in case of fault conditions. The model is implemented on PSASP and its validity is verified through comparison with electromagnetic model. At last, the simulation is carried out based on the real grid date. The simulation results demonstrate the benefits of MMC-HVDC and the improvement in transient stability by applying the additional control method.