With the rapid development of wind power, the effects of doubly fed induction generators (DFIGs) on the transient stability of power system have attracted more attention. However, the effects are still not clear due to the lack of deep and theoretical analysis. A control method for enhancing system rotor angle stability enhancement is proposed with considering the power constraints of DFIG. The rotor angle oscillation of the synchronous generator (SG) can be reduced and overspeed and the overcurrent of DFIG can be avoided. In this paper, the effects of the DFIG on the rotor angle characteristics of SG are analyzed. The change law of system rotor angle varying with transient power of the DFIG is obtained. Then, the power equation in the emergency pitch control process is explored. The reactive power constraints of the stator and grid-side converter are deduced. Finally, the system rotor angle stability enhancement control method is proposed based on the power constraints of DFIG. The effectiveness of proposed method is proven by simulations.

With the increased use of high voltage direct current (HVDC) systems, line commutated converters (LCC-HVDC) and voltage source converters (VSC-HVDC) tend to feed into AC systems over short electrical distances. The flexible power control of VSC-HVDC systems provides an effective approach to suppress the commutation failure of LCC-HVDC systems. A suppression method, based on a controllable operation region, is proposed in this study to reduce the probability of commutation failure in a hybrid dual-infeed HVDC system. First, the quantitative transient reactive power control requirement of VSC-HVDC inverter that could fulfill the suppression control boundary condition of commutation failure was analyzed. Given the maximum current constraint of the VSC-HVDC inverter and the primary frequency modulation constraint of the sending-end grid, a controllable operation region of the hybrid dual-infeed HVDC system was proposed. Furthermore, a commutation failure suppression control method, based on the controllable operation region, was proposed to mitigate continuous commutation failure. Finally, the validity and accuracy of the proposed method was verified by the simulation of PSCAD/EMTDC. The proposed control method can realize reasonable use of the reactive power control capability of the VSC-HVDC system, which effectively improves immunity to commutation failure of the LCC-HVDC system under grid fault.

Wind generation is increasing rapidly in the grid, which has greater impacts on the power system than ever. The impacts are more serious under grid fault. The low‐voltage ride through (LVRT) controls of doubly fed induction generators (DFIGs) may change characteristics of synchronous generators (SGs) and threaten the transient stability of power system. However, the influence on the transient stability and even the controllable powers of a DFIG has not been considered in the existing LVRT method. The contributions of the reactive power supplied by a DFIG in the oscillation process of power angle of SG were analysed. The controllable range of powers of a DFIG was researched with consideration of the rotor speed limit. An LVRT control method of a DFIG based on the controllable range was proposed to improve the transient stability of power system. Simulations show that the proposed method could enhance the power angle stability of power system under the condition of avoiding overspeed and out of service of DFIG.