With the participating of electric vehicle (EV) aggregators in frequency regulation (FR) services, inevitable time delays are penetrating into power systems due to the open communication infrastructure and scheduling, which may lead to poor performance or even instability. The models of traditional FR system without EVs, FR system including EV aggregators without delay and FR system including EV aggregators consider-ing time delays are deduced and compared. And an effective damping ratio extraction method based on solution operator transformation is then presented for FR systems including EV aggregators with time delays. And by utilizing the proposed method, the dominant oscillation frequency and the minimum damping ratio are obtained. In consideration of time delays, we evaluate the effect of EV aggregators on the performance and stability of the frequency regulation system of the power system. Relationship between time delays, the gains of the PI controller and the dominant oscillation frequency and the minimum damping ratio of the FR scheme are explored. The model differences between time-delay and non-delay systems are revealed. Through comparisons, it is found that the mass utilization of electric vehicles in the frequency control might result in instability due to the aggregation delay, which induces oscillation for the whole system.
Keywords renewable energy resources, time delays, solution operator discretization, electric vehicle