This study proposes an enhanced model of the doubly fed induction generator (DFIG)-based wind farm (WF) for small-signal stability (SSS) studies of weak power system. This model consists of a wind turbine, a drive train, a simplified generator/converter module and associated controls. As analysed and verified in this study, the dynamics of the phase-lockedloop and the rotor current loop are important for accurate SSS studies of the weak power system involving the DFIG-based WF and thus should be modelled appropriately. The enhanced model is validated by eigenvalue analysis and time-domain simulations. Compared with the detailed model, the WT3 model provided by General electric (GE) energy and the conventional model, the enhanced model presents high precision and satisfactory simplification for SSS studies. As the application of the enhanced model, a case study is carried out to explore the impact of WF ancillary voltage/frequency controls on SSS of the weak grid. As indicated in this study, the WF ancillary controls exert distinct impact on SSS. New lightly damped power oscillations can be introduced. Therefore extra measures should be considered if the ancillary controls are required by the WF in order to satisfy the future grid codes.
To overcome the real-time problem of maximum power point tracking (MPPT) for partially shaded photovoltaic (PV) systems, a novel nature-inspired MPPT controller with fast convergence and high accuracy is proposed in this paper. The proposed MPPT controller is achieved by combining salp swarm algorithm (SSA) with grey wolf optimizer (GWO) (namely, SSA-GWO). The leader structure of the GWO algorithm is introduced into the basic SSA algorithm to enhance the global search capability. Numerical simulation on 13 benchmark functions was done to evaluate the proposed SSA-GWO algorithm. Finally, the MPPT performance on PV system with the proposed SSA-GWO algorithm under static and dynamic partial shading conditions was investigated and compared with conventional MPPT algorithms. The quantitative and simulation results validated the effectiveness and superiority of the proposed method.
In this study, small-signal stability of the power system integrated with ancillary-controlled large-scale doubly fed induction generator (DFIG) based wind farm (WF) is studied. A model which considers grid code requirements and ancillary controllers is presented to indicate important DFIG dynamics for the study. The ancillary control of the WF which aims to improve power system voltage/frequency stability is evaluated from the small-signal stability perspective. It is shown that the ancillary controller deteriorates power system low-frequency oscillations and/or induces new lightly damped oscillation modes especially in a weak grid. The potential risks may restrict the WF to fulfil gird code requirements and threaten small-signal stability of the power system. Meanwhile, this study reveals that wind power penetration level and WF connection impedance are two main factors which affect dynamic interactions between the WF and the power system and thus affect system smallsignal stability. The principle of the influence on the small-signal stability with different grid weakness and ancillary control schemes is evaluated in this study by eigenvalue analysis and verified with time-domain simulations.
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