13-06-2013, 02:41 PM
COORDINATED AND OPTIMIZED VOLTAGE MANAGEMENT OF DISTRIBUTION NETWORKS WITH MULTI-MICROGRIDS
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Abstract
This thesis presents a set of proposals for advanced control functionalities in order to achieve a coordinated and optimized voltage management of distribution networks comprising several Distributed Generation units, controllable loads, storage devices and microgrids.
The approach followed here is based on the exploitation and extension of the microgrid concept following a massive integration of these “active cells” in electrical distribution networks. Therefore, a hierarchical control architecture is proposed in order to manage all these Distributed Energy Resources located in the distribution system in a coordinated way, based on advanced communication solutions exploiting a smart metering infrastructure. This control structure is characterized by the inclusion of an additional controller at the Medium Voltage level – the Central Autonomous Management Controller – leading to the development of the Multi-Microgrid concept.
Large scale integration of Distributed Energy Resources, namely Distributed Generation at the Medium Voltage level and microgeneration at the Low Voltage level, poses several technical challenges for distribution network operation, especially concerning voltage control. Accordingly, the development of specific control solutions is required in order to maximize the integration of these units in the distribution system.
Consequently, the work presented in this thesis focused on the development of a conceptual framework model for regional ancillary services markets for voltage control. The approach developed is able to integrate the reactive power bids from the several providers (namely Distributed Generation units or microgrids) in order to satisfy the requested reactive power needs at the Medium Voltage level, which are set by the Distribution System Operator. The market settlement is achieved based on cost minimization for the Distribution System Operator from purchasing reactive power. An ancillary services market simulator for reactive power use was developed implementing this approach for a medium-term time horizon using data from generation scheduling and renewable generation and load forecasts.
Introduction
Motivation for the Thesis
Electrical power systems have been undergoing significant changes in the last few years. It is foreseen that these changes will mark an evolution of concepts and practices for the whole power system industry in a near future, especially concerning planning and operational procedures.
The traditional organization of power systems dated from the 1950s, based on large central generation units that supply electrical power through a transmission network to reach end-consumers in the distribution system, is beginning to be outdated. These large central generators are predominantly hydro power plants, fossil fuel-based power plants and nuclear power plants and in most of the countries there is a large dependency on imported fuels (mostly fossil fuels) since they do not have endogenous resources to fill their needs. According to [1], in Europe (27 countries), import dependency in 2007 was 82,6% for oil, 60,3% for gas and 41,2% for solid fuels. Moreover, according to the Organisation for Economic Co-operation and Development 1 and Eurostat2, the gross inland consumption in Europe (27 countries) has grown from 1662 Mtoe3 in 1990 to 1806 Mtoe in 2007 (more than 8%) while in the whole world the consumption has grown from 8462 Mtoe in 1990 to 12029 Mtoe (corresponding to an increase of over 42%).
Objectives of the Thesis
The work presented in this thesis involves the development of new functionalities for control systems for a coordinated and optimized management of distribution networks, by exploiting all major available resources such as DG units, controllable loads, microgrids, storage devices, reactive power compensation devices, On-Line Tap Changing (OLTC) transformers, etc. In particular, the issue of voltage and reactive power control is explicitly addressed. As previously explained, these functionalities are vital in order to maximize the integration of DG and microgeneration units, without them being a burden to the distribution system.
State-of-the-Art
In this Chapter a description is made of the most relevant research work and of the latest developments regarding the state-of-the-art on distributed generation, microgrids and overall active network management. The most important issues about these topics are analysed, namely the main advantages and drawbacks, the drivers and challenges, the technologies associated and the services that can be offered to the electrical power system. In addition, an insight to voltage control techniques and an overview of ancillary services (focusing on voltage support) in networks with Distributed Generation are given.
The Distributed Generation Concept
Although there is still no consistent and unified definition, DG (also referred to in the scientific literature as Dispersed Generation, Decentralized Generation or Embedded Generation) can be loosely defined as small-scale generation connected to the distribution network. Actually, several definitions for DG can be found in the available literature.
Between 1997 and 1999, DG was investigated by two working groups of CIGRE8 and CIRED9: the CIGRE working group on “Impact of increasing contribution of dispersed generation on the power system” (WG 37-23) and CIRED working group on “Dispersed Generation” (WG 04).