In this paper, an optimal energy storage sizing method is proposed for networked microgrids (NMGs) considering reliability and resilience enhancement. A bi-level optimization model for energy storage sizing is developed. The upper-level model is focused on the optimal energy storage sizing problem, aiming at maximizing annual profit. The lower-level problem is aimed at operation optimization for profit maximization under multiple operating scenarios, i.e., normal operation, typical fault, and extreme fault scenarios. The bi-level model is converted into a mixed-integer linear program (MILP), which can be solved by off-the-shelf optimization solvers such as CPLEX. For comparison, the optimal energy storage size schemes are obtained for the NMGs and non-networked microgrids (NNMGs), respectively. The results indicate that the required energy storage size can be reduced while the operating profit is improved by interconnecting the microgrids (MGs). The results also indicate that the energy interaction in NMGs enables the enhancement of both reliability and resilience during grid outages.INDEX TERMS Energy storage system (ESS), networked microgrids (NMGs), optimal sizing, reliability, resilience.

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This paper presents an integrated linear and nonlinear control for a unified power flow controller (UPFC). Three Pulse-Width Modulation (PWM) parameters of the UPFC are controlled by Proportional-Integral-Differential (PID) laws, and an analytic expression of the nonlinear control law for the UPFCphase angle ϕ2 is obtained by a feedback linearization method. This combined Downloaded by [Erciyes University] at 01:12 03 January 2015 336 H. Xie et al. linear and nonlinear control strategy enables the UPFC to improve power system transient stability and dynamic performance effectively. Simulation results illustrate the effectiveness of the developed control strategy.