a b s t r a c tThis work presents a power flow strategy for multi-terminal HVDC grids. Energy is mainly generated via renewable energy sources and there are nodes in the network with the possibility to store energy. This energy is generated taking into account real weather conditions in order to make the best scheduling of the system in a realistic approach. An optimization scheme is proposed in which all these elements are included as well as real operation constraints. Distribution losses are minimized for the whole network. This gives as a result a control strategy being able to deal with the whole system and its inherent constraints giving the framework for a multi-objective optimization control.Ó 2014 Published by Elsevier Ltd. IntroductionThe difficulty of satisfying the constant growth in worldwide energy demand is exacerbated by a number of well-known factors regarding the supply and use of conventional energy resources (oil, gas, etc.): the inconvenient geographical location of many conventional energy production sites vis-à-vis their centers of use, the continuous price inflation accompanying these resources and, of course, the fact that their use releases undesirable emissions into the atmosphere. These disadvantages are turning the world's attention towards a number of renewable energy solutions, among which offshore wind farms present a particular interest. There are several advantages to developing offshore wind energy, notably: wind conditions are more favorable out to sea than on land; the average wind speed is higher therefore providing more energy, and the wind direction tends to remain more constant in the absence of obstacles [1]. Furthermore, the wind is less turbulent because variations in temperature between the different layers of air are smaller over the sea than over land. For these reasons, offshore wind turbines have a larger up-time than onshore turbines. These are some of the reasons why wind farm investment is chiefly being channeled to offshore facilities rather than to their landbased equivalent despite their higher initial installation costs.Energy transmission has historically been carried out in alternative current (AC). However high voltage direct current (HVDC) transmission is investigated in this study due to several advantages it possesses over AC lines. First of all the reactive power makes AC transmission losses greater than HVDC applications. Additionally, transmission capacity is also greater in HVDC lines due to the non-existence of the skin effect, and also the efficiency and controllability of DC converters are higher [2][3][4][5]. Another advantage of using HVDC is that the use of fewer cables in DC lines also implies lower costs and weight enabling therefore the possibility of operating in remote marine regions where wind conditions are even more favorable [6,7].Although multi-terminal HVDC systems are technically feasible, they have not been widely accepted as a cost-effective transmission solution. There are only two HVDC installations which have operated as mult...
This paper presents the modeling and control of a multilevel DC/DC bidirectional converter suitable for medium voltage and power applications, with a special interest in wind-power applications. The proposed multilevel topology has a modular structure constituted by base DC/DC converter cells. The multilevel converter is consequently based on Dual Active Bridge (DAB). The overall control of the DC/DC converter is achieved by using a nonlinear control based in Lyapunov theory.
This paper introduces a control induced timescale separation scheme for a multi-terminal high voltage direct current system, used for large scale integration of renewable energy sources. The main idea is to provide a detailed theoretical analysis, to the long stand practice that consists of empirical design of two control loops for the terminals. Experience has shown that such loops, i.e. current and voltage control loop, when heuristically tuned, often display very different dynamics. In the present paper, singular perturbation theory is applied to give explanation and fundamental analysis on why and how the two control loops work, and how to achieve the timescale separation between various state variables. Mathematical analysis is also carried out to illustrate a clear trade-off between system performance (actuator constraint) and the size of the region of attraction of the controller. Numerical simulations for a system with four terminals are presented to evaluate the system performance and illustrate the theoretical analysis.
This article provides a solution for the control of multi-terminal DC networks from the point of view of the network's transmission system operator (TSO), which includes local, primary, and secondary controllers. A new power flow technique is validated for this approach, which guarantees the stability and requires fewer calculations than conventional techniques. This study also describes an optimal control strategy for intermittent (renewable) energy producers, where the controller periodically transmits information about its state to the system operator. Its main goal is to optimize economic profit for the producer. This last controller is implemented via Model Predictive Control (MPC). The whole control strategy is validated in a scaled DC grid test-bench with 4 nodes. Real solar production (5 kW rated power), a storage system, as well as short-term weather and consumption forecasts are also included.
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