Advanced Frameworks to Aggregate and Unlock the Power Flexibility of Distributed Energy Resources Located in Active Distribution Networks

Environmental concerns are leading to a fast transition in electric power systems towards replacing fossil fuel and nuclear generation with renewable generation. This transition has two significant implications for electric power systems:1-The capacity of conventional dispatchable electric power plants, which are the main providers of power flexibility , is sharply falling. It is notable that the security of electric power systems can be preserved if and only if there is an adequate amount of power flexibility to guarantee the frequency stability, voltage regulation, power quality, and congestion management.2-The share of renewable energy sources (RESs) in the electric power generation portfolio is steeply soaring. It raises a significant stress on electric power systems due to the fact that the generated electricity from RESs is intrinsically intermittent and accompanied with uncertainties.The simultaneous realization of these two issues puts electric power systems in a turning point. This thesis devotes a deep attention to the power flexibility provision issue with the purpose of empowering transmission system operators (TSOs) and distribution system operators (DSOs) to steer electric power systems securely in this emerging architecture. To this end, it sets out to unlock the potential power flexibility of distributed energy resources (DERs) located in distribution networks. The first part of the thesis deals with the problem from TSO's perspective. It develops two decision making tools for TSOs to help them optimally quantify their required power flexibility from flexibility providers (including flexible distribution networks) in a decentralized energy and flexibility market structure. The first tool concentrates solely on active power flexibility and accordingly offers a framework to model aggregated active power flexibility of distribution networks from TSO's point of view. In contrast, the second tool introduces a holistic TSO-DSO coordination framework to enable TSO and DSO to exchange both active/reactive power flexibility. Both developed tools employ mathematical techniques to cast the TSO's decision making process as a two-stage linear stochastic optimization problem while accounting for risk associated to the uncertainties. The frameworks lay a ground for TSO and DSOs to exchange power flexibility without having to reveal their confidential grids data.The second part of the thesis deals with the problem from DSO's perspective. It first concentrates on an active distribution network (ADN) and constructs a set of linear scenario-based robust optimization problems to characterize the maximum active/reactive powers flexibility that the ADN can provide upon request at its point of common coupling (PCC) to the upper-layer grid. Second, it constructs a linear grid&uncertainties-cognizant ADN management method to determine the amount of active/reactive powers flexibility that should be provided by each DER during the real-time operation in such a way that the ADN can provide, with minimum deviation, the active/reactive powers flexibility requested by the upper-layer grid at the PCC.In addition to dealing with the problem from both TSO's and DSO's perspectives, a privileged feature of the thesis is that it offers linear tractable algorithms to deal with all above-mentioned tasks while considering grid's constraints and uncertainties stemming from the forecast errors of demand and renewable generation.

Assessment of Solar Thermal Power Technologies

James Spelling

In the present climate of growing awareness about the dangers of global climate change, the development of new renewable energy technologies is primordial in the efforts to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign fuel sources. Amid the plethora of option available, one of the more promising solutions is the increased use of solar power. Two main contrasting technologies exist for the conversion of the Sun’s energy into useful work, namely photovoltaic cells and solar thermal systems. The development of photovoltaic power has shown massive growth in recent years despite high costs, due mainly to the modular nature of the cells which allow their installation by private citizens, coupled with a number of government incentives. Development of solar thermal power generation has however been reasonably limited until recently, with the only continually operated commercial plants being the Luz SEGS I-IX plants at Kramer Junction in the United Sates. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, notably with the construction of the PS10 central receiver plant in Spain. The Spanish government plans to bring the total solar generation capacity to a total of 300 MW by 2013. In other countries around the world, interest is growing in solar thermal power, with increased government and private spending fuelling research and innovation. Most solar thermal conversion technologies are still at the experimental stage, and no single technology has yet to impose its dominance. Different countries and research groups hold different solutions dear, but if solar thermal power is to become a fully mature electricity generating technology, the best of these diverse solutions must be combined in order to achieve optimal performance of the power plants.

2008

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.

2008

Real-Time Control Framework for Active Distribution Networks Theoretical Definition and Experimental Validation

Lorenzo Enrique Reyes Chamorro

The great challenge of massively integrating the volatile distributed power-generation into the power system is strongly related to the evolution of their operation and control. The literature of the last decade has suggested two models for such an evolution: (i) the supergrid model, based on enhanced continental/intercontinental network interconnections (mainly DC) for bulk transmission, (ii) the microgrid mode, where small medium/low voltage networks interfacing heterogeneous resources, such as local generation, energy storage and active customers, are intelligently managed so that they are operated as independent cells capable of providing different services from each other and operate in islanded mode. Irrespective of the model that will eventually emerge, the control of heterogeneous distributed resources represents a fundamental challenge for both supergrid and microgrid models. This requires the definition of scalable and composable control methods that guarantee the optimal and feasible operation of distribution grids in order to satisfy local objectives (e.g., distribution grid power balance), as well as the provision of ancillary services to the external bulk transmission (e.g., primary and secondary frequency supports). Several control methodologies have been proposed to achieve these goals, and the majority of them have been inspired by the classic time-layered approach traditionally adopted in power systems that are associated with different time-scales and extension of the controlling area, i.e. primary, secondary and tertiary controls, ranging from sub-seconds to hours, respectively. In the context of microgrids, these three levels of control can be associated with a decision process that can be centralized (i.e., a dedicated central controller decides on the operation of the system resources) and/or decentralized (each element decides based on its own rules). In the current literature, the former is used for long-term, whereas the latter for short-term decisions. In particular, primary controls are typically deployed through fully decentralized schemes mainly relying on the use of droop control. With this in mind, in this thesis we propose, and experimentally validate, a novel control framework called COMMELEC â A Composable Framework for Real-Time Control of Active Distribution Networks, Using Explicit Power Set-Points. It controls a power grid in real-time based on a multi-agent structure, using a simple and low-bandwidth communication protocol. Such a framework enables a controller to easily steer an entire network as an equivalent energy resource, thus making an entire system able to provide grid support by exploiting the flexibility of its components in real-time. The main features of the framework are (i) that it is able to indirectly control the reserve of the storage systems, thus maximizing the autonomy of the islanding operation, (ii) that it keeps the system in feasible operation conditions and better explores, compared to traditional techniques, the various degrees of freedom that characterize the system, and (iii) that it maintains the system power-equilibrium without using the frequency as a global variable, even being able to do so in inertia-less systems. Our framework has been extensively validated, first by simulations but, more importantly, in a real-scale microgrid laboratory specially designed and setup for this goal. This is the first real-scale experiment that proves the applicability of a droop-less explicit power-flow control mechanism in microgrids.

EPFL2016

Assessment of Solar Thermal Power Technologies

James Spelling

2008

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

2008

Real-Time Control Framework for Active Distribution Networks Theoretical Definition and Experimental Validation

Lorenzo Enrique Reyes Chamorro

EPFL2016