Future active distribution grids (ADGs) will incorporate a plethora of Distributed Generators (DGs) and other Distributed Energy Resources (DERs), allowing them to provide ancillary services in grid-connected mode and, if necessary, operate in an islanded mode to increase reliability and resilience. In this paper, we investigate the ability of an ADG to provide frequency control (FC) in grid-connected mode and ensure reliable islanded operation for a pre-specified time period. First, we formulate the operation of the grid participating in European-type FC markets as a centralized multi-period optimal power flow problem with a rolling horizon of 24 hours. Then, we include constraints to the grid-connected operational problem to guarantee the ability to switch to islanded operation at every time instant. Finally, we explore the technical and economic feasibility of offering these services on a balanced low-voltage distribution network. The results show that the proposed scheme is able to offer and respond to different FC products, while ensuring that there is adequate energy capacity at every time step to satisfy critical load in the islanded mode.
It is well known that the presence of energy storage ameliorates the reliability challenges posed by intermittent sources. However, a quantitative assessment of the exact amount of storage required to meet a reliability target or guarantee in the presence of intermittent sources is not trivial. This paper describes a practical approach to achieving this. First, an analytical approach is developed for determining the amount of storage required to meet a reliability target at a specific load point. Then the method is extended to a more complex island-capable microgrid system where initial reliability assessment and final verification of reliability guarantee are performed using sequential Monte Carlo simulation. The necessary component and system models are developed and described, and a mock military base is used to demonstrate the method. The work reported was performed under a contract from Sandia National Laboratory to determine storage need in order to provide reliability guarantee on an island-capable military microgrid, but the approach applies as well to civilian systems, both islanded and grid-connected. A method for determining an optimal storage mix is also developed. The growing concerns regarding electric power quality and availability have led to the installation of more and more distributed generation. In parallel and in the context of an accelerating trend towards deregulation of the electric industry, the unbundling of services, many grouped under ancillary services, should create a market for some of these services. This paper discusses the potential of distributed generation (DG) to provide some of these services. In particular, DG can serve locally as the equivalent of a spinning reserve and voltage support of the AC bus. The main types of distributed generation with emphasis on the power electronic interface and the configurations appropriate to provide ancillary services are reviewed. The flexibility and features provided by the power electronic interface are illustrated. In addition to control of the real power, other functions can be incorporated into the design of the interface to provide services, such as reactive power, and resources associated with power quality. These include voltage sag compensation and harmonic filtering. The implications on the design of the power converter interface are discussed. A microgrid optimal scheduling model is developed in this paper to demonstrate microgrid's capability in offering ancillary services to the utility grid. The application of localized ancillary services is of significant importance to grid operators as the growing proliferation of distributed renewable energy resources, mainly solar generation, is causing major technical challenges in supply-load balance. The proposed microgrid optimal scheduling model coordinates the microgrid net load with the aggregated consumers prosumers net load in its connected distribution feeder to capture both inter-hour and intra-hour net load variations. In particular, net load variations for three various time resolutions are considered, including hourly ramping, 10-min based load following, and 1-min based frequency regulation. Numerical simulations on a test distribution feeder with one microgrid and several consumers prosumers indicate the effectiveness of the proposed model and the viability of the microgrid application in supporting grid operation. Usually, ancillary services are provided by large conventional generators; however, with the growing interest in distributed generation to satisfy energy and environmental requirements, it seems reasonable to assume that these services could also be provided by distributed generators in an economical and efficient way. In this paper, a proposal for enhancement of the capacity of active power reserve for frequency control using distributed generators is presented. The goal is to minimize the payments done by the transmission system operator to conventional and distributed generators for this ancillary service and for the energy needed to satisfy loads and system losses, subject to a set of constraints. In order to perform analysis, the proposal was implemented using data of the IEEE 30-bus transmission test system. Comparisons were performed considering conventional generators without and with distributed generators installed in the system. This paper analyzes energy saving effects of a combined heat and power (CHP)-based microgrid by an optimal sizing of energy supply system and optimal planning method. We also present economic feasibility and potential amount of reserve provision by microgrids. Reactive power control is a fundamental issue in microgrids, especially during islanded mode operation with no support from the main grid. Lack of infinite bus, tightly coupled generation and consumption, and existence of nondispatchable intermittent renewable power sources reinforce the need for a new VVC scheme. This paper presents a new model predictive control (MPC)-based dynamic voltage and var control (VVC) scheme, which includes the dynamics of the microgrid in the VVC formulation. The MPC-based controller uses a simplified voltage prediction model to predict the voltage behavior of the system for a time horizon ahead. The advantage of this method is that it can avoid unstable voltage conditions in microgrids by prediction of the instability ahead of time. This method can also avoid voltage drops or swells in any of the phases of the system since the model can predict the voltage of each phase separately. Also, the presented method can be implemented online so it can efficiently use the time-variant reactive capabilities of the distributed generators to compensate for reactive power needs of the system. This controller is tested for different operating conditions of the microgrid and the simulation results confirm that the MPC controller successfully keeps the system stable and achieves a smooth voltage profile.  Microgrids are Low Voltage distribution networks comprising various distributed generators (DG), storage devices and controllable loads that can operate either interconnected or isolated from the main distribution grid as a controlled entity. This paper describes the operation of a Central Controller for Microgrids. The controller aims to optimize the operation of the Microgrid during interconnected operation, i.e. maximize its value by optimizing production of the local DGs and power exchanges with the main distribution grid. Two market policies are assumed including Demand Side Bidding options for controllable loads. The developed optimization algorithms are applied on a typical LV study case network operating under various market policies and assuming realistic spot market prices and DG bids reflecting realistic operational costs. The effects on the Microgrid and the Distribution network operation are presented and discussed. a b s t r a c t Battery energy storage systems (BESSs), while at the moment still expensive, are from a technical point of view exceptionally well suited to support a distribution system operator (DSO) in the challenges created by increasing distributed, fluctuating and uncertain generation from renewable energy sources (RES), as well as by the unbundling of electricity retailing and grid operation. This paper presents three grid applications and measurement results from the grid-connected Zurich 1 MW BESS. Specifically, provision of frequency reserves, peak shaving and islanded operation of a microgrid are discussed in detail. The battery unit is able to perform these diverse tasks with outstanding quality. For the provision of frequency reserves the system has passed all prequalification requirements posed by the transmission system operator (TSO) in order to be granted access to the ancilliary services market. Along with these appli- cations, the paper aims to communicate the experience gained in designing proper control frameworks, communication infrastructure and the knowledge acquired concerning BESSs behavior. Among the ancillary services, the operating reserves are important researching aspects, which deal with supplying way and selling-buying prices. Currently, distributed generation (DG) rises as a new participant in providing ancillary services, therefore, it is of main importance in operating reserves subject. This work shows some advantages and technical drawbacks by using DG when operating reserves are supplied. Different sceneries were computed where the electric generation was performed in a centralized and non-centralized way respectively. A methodology for identifying the maximum DG, which can be allocated, is proposed; this maximum level can be assessed without carrying out a negative impact in the network. The operative reserves price provide by generation distribution is analyzed. Diverse DG technologies, which present better performance in supplying such reserves, are also presented. Because of the ever-increasing energy demand and petrol shortage one resorts to renewable energy supplies. Together with the advantages of having the generation sources physically close to the energy demand Microgrid has become a topical research area in recent years. A microgrid is capable of operating in parallel to the main power network or in intentional islanded mode. Such microgrids usually consist of several small coordinated generation units located closely together in the distribution network. The units can be solar, wind, CHP (Combined Heat and Power), fuel cells, etc. and are small in capacity. In a big physical region there can exist multiple micro- grids. Apart from participating in energy markets, an alternative of making profit is to take part in the Ancillary Services (AS) market. This study investigates the technical feasibility and difficulties of providing such services as well as the potential profitability of participating in the AS markets based on a setup of multiple microgrids. This paper presents a model for microgrid optimal scheduling considering multi-period islanding constraints. The objective of the problem is to minimize the microgrid total operation cost which comprises the generation cost of local resources and cost of energy purchase from the main grid. The microgrid optimal scheduling problem is decomposed into a grid-connected operation master problem and an islanded operation subproblem. The microgrid capability in operating in the islanded mode for multiple hours is scrutinized by a T-τ islanding criterion. The integer scheduling decisions determined in the master problem will be examined against the microgrid islanding feasibility in the subproblem. The scheduling decisions will be revised using proper islanding cuts if sufficient generation is not available to guarantee a feasible islanding. Islanding cuts will revise generating units, energy storage systems, and adjustable loads schedules. Any change in the schedule of adjustable loads outside the operating time interval specified by consumers is penalized by an inconvenience factor in the objective. Numerical simulations demonstrate the effectiveness of the proposed microgrid optimal scheduling model and explore its economic and reliability merits.