Abstract-This paper describes and evaluates the feasibility of control strategies to be adopted for the operation of a microgrid when it becomes isolated. Normally, the microgrid operates in interconnected mode with the medium voltage network; however, scheduled or forced isolation can take place. In such conditions, the microgrid must have the ability to operate stably and autonomously. An evaluation of the need of storage devices and load shedding strategies is included in this paper.Index Terms-Dynamic response, energy storage, frequency control, microgrid, power system dynamic stability, voltage control.

This paper proposes a new methodology for coordinated voltage support in distribution networks with large integration of distributed generation and microgrids. Given the characteristics of the LV networks, it is shown that traditional control strategies using only reactive power control may not be sufficient in order to perform efficient voltage control. Therefore, microgeneration shedding must also be employed, especially in scenarios with extreme microgeneration penetration. An optimisation tool based on a meta-heuristic approach was developed to address the voltage control problem. In addition, neural networks were employed in order to decrease computational time, thus enabling the use of the tool for online operation. The results obtained revealed good performance of this control approach. Several authors have developed interesting work concerning the impact of DG on voltage control in distribution grids [2-5]. In [6], the authors formulate an algorithm for voltage control in distribution grids with DG by solving an optimisation problem, where active power losses are minimised, subject to a set of technical constraints. The control variables to consider are DG reactive power generation, On-line tap changing (OLTC) transformer settings and capacitor bank settings. One issue that frequently results from high DG penetration is voltage rise problems, especially in weak distribution networks [3, 7]. To overcome this problem, it is necessary to control both active and reactive power of the DG units and/or reduce the voltage at the HV/MV substation [3]. This voltage rise effect is partly the result of DSO policies based on a 'fit and forget' approach, which also requires DG to operate at a fixed power factor, thus limiting the

Abstract. The objective of this paper is to present novel control strategies for MicroGrid operation, especially in islanded mode. The control strategies involve mainly the coordination of secondary load-frequency control by a MicroGrid Central Controller that heads a hierarchical control system able to assure stable and secure operation when the islanding of the MicroGrid occurs and in load-following situations in islanded mode.

In this paper, a proposal for an ancillary services (AS) market framework addressing voltage control in multi-microgrid systems is presented. This var market proposal for MV distribution systems can be adopted to involve Distributed Generation (DG) units and microgrids in AS provision. In the approach that was developed each player is given the opportunity to submit its bid to the var market and the market settlement is performed using an Optimal Power Flow (OPF) formulation in order to minimize the price of reactive power purchased by the Distribution System Operator (DSO). This market is based on var capacity use and runs daily after the scheduling of the generation units for a period of operation of one day.

No abstract

SUMMARYThis paper addresses the extension of the microgrid concept, following a massive integration of these active cells in power distribution networks, by adopting a coordinated management strategy together with distributed generation units directly connected to the medium voltage distribution network. In order to achieve this, a technical and commercial management scheme must be developed for coordinated control of a distribution system with multi-microgrids, which should take into account the specific technical capabilities and characteristics of each type of generating source. In particular, tools for coordinated voltage support and frequency control, as well as for state estimation have been developed for this type of network. Concerning voltage support, a new methodology exploiting an optimization tool based on a metaheuristic approach was developed. For state estimation, two approaches were considered: multi-microgrid state estimation and fuzzy state estimation. Regarding frequency control, the hierarchical structure of the multi-microgrid is exploited to deal with the transition to islanded operation and load following in islanded operation. All these tools have proved to be efficient in managing the multi-microgrid system in normal interconnected mode and, in case of the frequency control, in islanded operation.

No abstract

Large-scale integration of distributed energy resources in low voltage distribution grids will have a serious impact on power system operation. The development of the microgrid concept is presented as a solution to overcome some of the negative impacts of massive microgeneration deployment. It has paved the way for an active network management approach within the smart grid paradigm. The microgrid concept is able to address the integration of geographically dispersed energy resources, thus avoiding significant technical problems that may affect the security of operation. C 2012 John Wiley & Sons, Ltd.