INFLUENCIA DE LAS CARACTERÍSTICAS DEL VINO EN LA

dissertation

In the search for low carbon, reliable and affordable ways to pro- vide electricity, an increased attention is going to the microgrid, a small-scale power system that uses a combination of energy gener- ation and storage devices to serve local customers. Microgrids are expected to revolutionise the existing electrical grid by allowing two- way communications to improve efficiency, reliability, economics, and sustainability of the generation and distribution of electrical power. However, several issues associated with planning and management must be addressed before the full benefits of the microgrid can be achieved.

14 _Introduction

There are a large number of economic and technical factors raised by both DER units and distribution network properties that have an impact on the operation of the microgrid. An active management of DER is needed to achieve a least-cost solution while satisfying all technical requirements. In general, this can be seen as a down- sized version of the unit commitment and economic dispatch problem that is traditionally applied to large central generators. However, ex- ceptional features of distribution networks have introduced further restrictions to this classic optimisation task. In comparison with unit commitment problem on transmission level, the microgrid scheduling task features the following major differences:

1. As a result of the climate issue, the environmental awareness becomes more important. CO2-emissions caused by combus- tion processes of distributed generation power plants need to be considered when developing the power system of the future. The challenge is to find an optimum which provides emission reductions while keeping costs at a reasonable level. An enviro- nomic (environmental/economic) optimisation model needs to be implemented in the energy management system of a micro- grid.

2. A complete formulation of the microgrid scheduling task in- cludes modeling of storage dispatch, demand side management (DSM), RES control measures and spatio-temporal uncertain- ties (e.g., electric vehicles), while the traditional unit commit- ment problem faces none of them.

3. Microgrids are constructed as small-scale distribution networks, which are generally radial or weakly-meshed, and are likely subject to technical problems such as over-/under-voltage, volt- age imbalances, or overloading when load/generation conditions vary.

The main contribution of this PhD thesis is the design of a multi- layer control strategy for microgrids which tackles the issues listed above. A decision making model is developed for a microgrid day- ahead scheduling, and fits in the tertiary level of the microgrid hierar- chical control structure. The tertiary control is conceived to achieve global controllability of the microgrid according to different criteria and will be discussed in § 2.2.2. The model works on a day-ahead time frame and will attempt to optimise the microgrid operation, based on merits of interests, over the complete time horizon. It provides

Introduction 15

optimal solutions by assuming that the future user and grid infor- mation are known in advance. The optimal solutions, also known as reference set points, can be provided to the underlying secondary and primary controllers.

The first issue relates to determination of the optimal mix of power generation in order to provide the performance needed at the least cost, or with the lowest possible emissions. Unfortunately, both ob- jectives are often contradictory. In general, low costs mean high emissions, and vice versa. A microgrid system operator may care more about achieving lower costs rather than the lower emissions. Given the preferences, the operator needs to decide how to operate the microgrid. In Chapter 3, an analytical method based on a multi- objective hybrid genetic algorithm was developed in order to help the microgrid system operators in the decision making process. The method optimises both objectives simultaneously, determining the costs and emissions associated with all possible options. Based on the results, the method generates a set of optimal operating strate- gies that will minimise costs and emissions.

When designing a new optimal planning tool for a microgrid, a major challenge (and opportunity) is to decide the units to operate while providing the performance needed. The second issue above intro- duces a major modelling complexity to the microgrid unit commit- ment task. The unit commitment views all components of a microgrid as a whole and attempts to converge to a global optimum which deliv- ers the best compromised result to all relevant entities. In Chapter 4, an analytical method based on a multi-objective hybrid genetic algorithm is proposed and demonstrated for a microgrid day-ahead unit commitment model. The model aims to schedule the power among the different microgrid units while minimising the operating costs together with the CO2 emissions produced. The approach is

demonstrated on a test case including a variety of DER units which are likely to be found in a microgrid. A storage device is added where the charge and discharge schedule is calculated according to both ob- jectives. The presence of a storage element, as well as the possibility of exchanging energy with the utility grid adds flexibility to the mi- crogrid operation and besides, it increases the solution space (set of feasible solutions) of the microgrid unit commitment problem. The charge and discharge commands of the storage device are allocated according to the lowest cost and the lowest CO2 emissions. Besides,

the commands of charging and discharging are associated to the en- ergy prices as well, and where the storage element takes advantage of

16 _Introduction

purchasing power from the upstream grid and selling it back accord- ing to the most favourable microgrid revenue. In addition, as a part of the demand side participation strategy, a charging schedule was determined for the electric vehicles. A 24-hour microgrid simulation has been performed with an interval or time span of 15 minutes where the objectives where considered simultaneously. These reference set points, delivered by the day-ahead unit commitment, can form an input for controllers which work on a shorter time frame (such as the primary controller) around which they may vary to maintain stabil- ity.

The third issue listed above relates to the fact that microgrids are inherently more fragile as compared to transmission networks. A complex and uncoordinated integration of DG units can easily cause technical problems in a weak microgrids. Additionally, the different types and sizes of DG units makes it difficult to apply some traditional control methods adopted for transmission networks such as frequency control and voltage control. These added uncertainties must be taken into consideration when solving the scheduling problem in order to obtain reliable solutions. In Chapter 5, a voltage control approach was developed and added to the microgrid unit commitment strat- egy. Due to the introduction of this voltage control approach, active power reference set points can be provided (to the underlying con- trollers around which they may vary to maintain real-time stability) not only according to the economical and the environmental objec- tives, but also taking into account the voltage level at every bus in the microgrid.

In order to demonstrate this multilayer control strategy for micro- grids, case studies were performed involving the planning of a micro- grid including a variety of DER units and loads which are likely to be found in a microgrid. The proposed microgrid includes renewable energy resources, a battery storage unit, micro turbine generator sets, a fuel cell (FC) and electric vehicles (EVs). The system should run as much as possible on its renewable technologies, and when more power is needed, the distributed generators or batteries will be used. Finally, the system should provide perfect reliability that is, it should never fail to meet total customer demand.

2

Microgrid multilayer control structure

This chapter is written to expose the proposed structure of this PhD thesis. The main shape and the necessary elements of the microgrid multilayer control approach will be outlined to reveal the structure. Each layer will be described and discussed briefly, and later through- out this thesis, a more detailed presentation will be provided, chapter by chapter.

2.1

Introduction

As explained before, microgrids are subsystems of the electrical grid consisting of an aggregation of (controllable) loads and DER units including dispatchable sources, RES and DS devices. A coordinated control and management of the DER units is crucial within the con- cept of microgrids [31]. The optimal operation of individual elements in the network can provide distinct benefits to the overall microgrid performance. Therefore, efficient control and management techniques have to be developed with a proper coordination strategy among the various microgrid elements. Besides, since the architecture of micro- grids can change on a regular basis, and in order to increase their po- tential of scalability, these control strategies should be implemented in a plug- and-play way.

18 _{Chapter 2}

controllers installed on each microgrid component. By use of these power electronic controllers, cooperative actions can be taken in or- der to control the microgrid [32].

The goal of this research is to develop a power system modernisation tool with the ability to optimise the microgrid. To attain this goal, an efficient multilayer control approach is developed which proposes a day-ahead unit commitment (UC) method in order to provide an optimal power generation schedule for microgrids. The objective or intention of this multilayer control concept is to provide an economi- cally and environmentally viable unit commitment that is physically feasible in terms of voltage violations.

This chapter starts with an introduction of energy management sys- tems (EMS) in microgrids. Subsequently, the multilayer control struc- ture will be presented and the different levels of the hierarchy will be discussed briefly. Each level will be extensively discussed later in this dissertation.