Modal resonance mechanism investigation and mitigation in power systems integrated with full converter-based wind power generation

Abstract

In recent years, oscillation instability contingencies happen more and more frequently with the increasing penetration of wind power generations. One important reason leading to oscillation instability is the modal resonance between wind power generation and the external power system. The integration of full converter-based wind power generation (FCWG, e.g., permanent magnet synchronous generator (PMSG)) not only introduces FCWG oscillation modes (FOMs) but also might excite severe resonances with electromechanical oscillation modes (EOMs) of the power system. To dig the essential resonance mechanism, a two-open-loop-subsystem dynamic model is firstly established to investigate the modal interactions between the FCWG and the external power system. The two-open-loop-subsystem model divides the entire power system into two subsystems: 1 the subsystem of FCWG, and 2 the subsystem of the external power system. On this basis, a modal shift evaluation (MSE) method by using bilateral damping torque analysis (BDTA) is proposed to accurately quantify the interaction effect of FOMs and EOMs on each other and effectively explain their complex interaction process. Then two important concepts, i.e., modal shift sensitivity (MSS) with respect to various FCWG controller parameters and resonance excitation index (REI) according to a per-unit open-loop modal distance indicating the intensity of modal interactions, are derived to dig the essential modal resonance mechanisms. Meanwhile, to further clarify the modal resonance and facilitate the understanding of the modal interactions between FCWG and the external power system, a novel modal superposition theory is proposed to classify the modal interactions between FOMs and EOMs in the complex plane for the first time. The potential of suppressing modal resonance with proper modal coordination strategy is exploited. With flexibly modified FOMs, FCWG has the potential to actuate conducive dynamic interactions with EOMs of the external power system. The modal coupling mechanism is graphically visualized to investigate the dynamic interactions, and the eigenvalue shift index is proposed to quantify the dynamic interaction impact on critical EOM. Based on different manifestos in the modal coupling mechanism and eigenvalue shift index, a novel modal coordination optimization strategy to improve the dynamic interactions between the FCWG and the external power system is proposed within the existing control frame. The optimized dynamic interactions (i.e., modal counteraction) can significantly enhance the oscillation stability of the power system, the effectiveness of which is verified by both modal analysis and time domain simulations. Nonetheless, the off-line modal coordination optimization strategy is highly model-dependent, which relies on the system model with the full details to facilitate the resonance analysis and identify the related oscillation modes. On the one hand, the very accurate modeling of a full power system might be either technically or commercially unavailable, especially for large-scale power systems. On the other hand, even if the system model is available, the operating conditions may vary a lot in practice, and hence the optimal solution based on the modal coordination optimization cannot always guarantee optimal performance. Consequently, to overcome the above obstacles, with the aid of PMU measurement, a novel on-line modal coordination strategy is proposed as an attractive alternative to alleviate modal resonance. Furthermore, FCWG, though normally considered to be decoupled from the external power system can be actuated as an inertia source to suppress modal resonance in wind power generation penetrated power systems by installing auxiliary resonance controllers (ARCs). Three possible options for ARC installation are first identified based on some derivations of the conventional control model of FCWG. The damping support mechanism of ARC is revealed, a suitable and generic configuration structure of ARC is then established, and optimal parameter tuning is conducted on the basis of this ARC configuration. The three ARC alternatives are equipped to contribute to damping by utilizing the potential energy and dynamics hidden in different inertia source components (i.e., the wind turbine rotor and DC capacitor, respectively) of FCWG. Both modal analysis and simulation results validate the effectiveness of the three proposed ARCs in suppressing modal resonance and improving system oscillation stability. Most importantly, extensive comparison investigations are carried out to fully evaluate the pros and cons of the three ARCs and thus provide constructive application guidance for system operators and wind farm owners.