This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE SYSTEMS JOURNAL 1 Networked Control and Power Management of AC/DC Hybrid Microgrids Amr Ahmed A. Radwan, Student Member, IEEE, and Yasser Abdel-Rady I. Mohamed, Senior Member, IEEE Abstract—This paper addresses power management and control strategies of a hybrid microgrid system that comprises ac and dc subgrids. Each subgrid consists of multiple distributed genera- tion (DG) units and local loads. Both entities are interconnected by voltage source converters (VSCs) to facilitate a bidirectional power flow and increase the system reliability. The control of the interconnecting VSC can be achieved autonomously. However, it is shown that the autonomously controlled hybrid microgrid fails to operate following variations in the power generation charac- teristics of local DG units (such as droop coefficients, set points, or loss/connection of DG units, etc.) A centralized controller is therefore proposed and compared to the autonomous scheme. The centralized control strategy provides an accurate and optimized power exchange between both subgrids. Parallel operation of mul- tiple interconnecting VSCs is considered so that the transmitted power is shared according to their power ratings. Small-signal stability analysis is conducted to investigate the influence of the communication delays on the system stability. A hierarchical control strategy has been proposed by setting the autonomous controller in a primary layer whereas the centralized controller is set into a secondary layer to generate a compensation signal. Time-domain simulations results are presented to show the ef- fectiveness of the proposed techniques and the drawbacks of the conventional scheme. Index Terms—Autonomous control, bidirectional power flow, centralized controller, distributed power generation, microgrid, power control, smart grid, voltage source converters (VSCs). I. I NTRODUCTION D ISTRIBUTED generation (DG) systems are gaining a high momentum to facilitate the penetration of renewable energy resources into the utility-grid [1], [2]. The ongoing deployment of DG units into the distribution system creates active clusters or microgrids, which can operate either in grid- connected or intentionally islanded mode, and are classified into ac or dc types [3]. The distinct features of ac and dc mi- crogrids are as follows. 1) AC microgrids are more compatible with existing ac systems. However, synchronization procedures should be followed prior to connection [4]. On the contrary, the connection of a dc microgrid to an ac system is easier. 2) DC microgrids meet the inherent dc nature of most nowadays loads (e.g., home appliances, lighting systems, and motor drives, etc. [3]) Moreover, most of renewable energy resources are dc, such Manuscript received March 31, 2014; revised May 19, 2014 and June 19, 2014; accepted June 30, 2014. The authors are with the Department of Electrical and Computer Engi- neering, University of Alberta, Edmonton, AB T6G2V4, Canada (e-mail: aradwan@ieee.org; yasser_rady@ieee.org). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSYST.2014.2337353 Fig. 1. Hybrid ac/dc active distribution system. as photovoltaic (PV) panels, fuel cells, and energy storage units. 3) The protection of dc systems is more challenging due to the absence of zero-current-crossing that provides an inherent self-extinguish feature of the fault current in ac systems [5]. 4) AC systems are associated with transmission lines skin effects [6]. The concept of hybrid ac/dc systems is therefore emerging to combine the benefits of both types, reduce the losses by avoiding multiple power-electronic conversion stages, and increase the system reliability [7]–[19]. Different hybrid system structures are recently proposed to interconnect ac and dc subsystems using a three-phase voltage source converter (VSC) [7] or a back-to-back converter [8]. The dynamics and stability of hybrid systems are investigated and improved using a robust controller [9] or a supplementary second-order compensator [10]. The power fluctuations in the hybrid system are mitigated in [11] whereas the particle swarm optimization is utilized in [12] to design the controllers. In this paper, a generalized hybrid system, shown in Fig. 1, is considered. The system comprises an ac subgrid that is formed by aggregating ac DG units and distributed loads along a com- mon ac bus. Similarly, the dc bus aggregates the corresponding DG units and loads to form a dc subgrid. The dc subgrid is interfaced to the ac system by one (or more) interconnecting VSC. In the grid-connected mode, any dc demand that exceeds the generation capacity of the dc DG units can be imported from the utility-grid via the interconnecting VSCs [7]. In the islanded mode, the operation of the hybrid system is more challenging. The interconnecting VSCs have to be properly controlled to manage the bidirectional power exchange to accommodate the excessive ac/dc demands with minimal power disruptions or load shedding. This objective should be accurately achieved regardless of the control modes or parameters of local DG units in both subgrids. 1932-8184 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.