Hierarchical Control of Droop-Controlled DC and AC Microgrids – A General Approach Towards Standardization Josep M. Guerrero 1 , Juan C. Vásquez 1 , and Remus Teodorescu 2 1. Technical University of Catalonia, Spain. 2. Institute of Energy Technology, Aalborg University, Denmark Email: josep.m.guerrero@upc.edu Abstract – DC and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical- controlled microgrid are provided to show the feasibility of the proposed approach. Keywords: Distributed Generation (DG), Distributed Power Systems (DPS), Droop method, Hierarchical control, ISA-95, Microgrid, Parallel operation. I. INTRODUCTION Microgrids (MG), also named minigrids, are becoming an important concept to integrate distributed generation (DG) and energy storage systems. The concept has been developed to cope with the penetration of renewable energy systems, which can be realistic if the final user is able to generate, storage, control, and manage part of the energy that will consume. This change of paradigm, allows the final user be not only a consumer but also a part of the grid. DC and AC microgrids have been proposed for different applications, and hybrid solutions have been developed [1]- [12]. Islanded microgrids have been used in applications like avionic, automotive, marine, or rural areas. The interfaces between the prime movers and the microgrids are often based on power electronics converters acting as voltage sources (voltage source inverters, VSI, in case of AC microgrids). These power electronics converters are parallel connected through the microgrid. In order to avoid circulating currents among the converters without use any critical communication between them, the droop control method is often applied. In case of paralleling DC power converters, the droop method consist of subtracting a proportional part of the output current to the output voltage reference of each module. Thus, a virtual output resistance can be implemented through this control loop. This loop, also called adaptive voltage position (AVP) has been applied to improve the transient response of voltage regulation modules (VRMs) in low- voltage high-current applications. However, the droop method has an inherent trade-off between the voltage regulation and the current sharing between the converters [8]- [13]. On the other hand, in case of paralleling inverters, the droop method consist of subtracting proportional parts of the output average active and reactive powers to the frequency and amplitude of each module to emulate virtual inertias. These control loops, also called P–ω and Q–E droops, have been applied to connect inverters in parallel in uninterruptible power systems (UPS). However, this method also has an inherent trade-off but between frequency and amplitude regulation in front of active and reactive power sharing accuracy [14]-[19]. To cope with this problem, an external control loop named secondary control has been proposed to restore the nominal values of voltage inside the microgrid. Further, additional tertiary control can be used to bidirectionaly control the power flowing when the microgrid is connected to a stiff power source or the mains (in case of AC microgrids) [14]. In this paper, we propose a general hierarchical multilevel control for DC and AC microgrids. The paper is organized as follows. In Section II, a general approach of the hierarchical control stem from the ISA-95 is adapted to microgrids. In Section III, the hierarchical control is applied to DC microgrids, solving the trade-off of the droop method by implementing a secondary control loop, and able to share the load together with a stiff dc source, which can be either a DC generator or a DC distribution grid. In Section IV the approach is applied over an AC microgrid consisted of droop-controlled inverters, able to operate in grid-connected and islanded modes. Finally, Section V gives the conclusions. II. A GENERAL APPROACH OF THE HIERARCHICAL CONTROL OF MICROGRIDS The need for standards in microgrid control is related to the new grid codes that are expected to appear in the next future. In this sense, the ISO-95 standard deals with the integration enterprise and control systems. In this standard a multilevel hierarchical control is proposed, with the following levels: ｩ IEEE 2009 4341 Preprint of IECON 2009 Proceedings