Advanced control strategies for vehicle to grid systems with electric vehicles as distributed sources
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This thesis focuses on the control and implementation of the vehicle to grid (V2G) system in a smart grid. Important issues like structure, principle, performance, and control of energy storage systems for electrical vehicles and power systems are discussed. In recent decades, due to rapid consumption of the earth’s oil resources, air pollution and global warming (a result of the “greenhouse effect”), the development of electrical vehicles (EVs), hybrid electrical vehicles (HEVs) and plug-in electric vehicles (PEVs) are attracting more and more attentions. In order to provide regulation services and spinning reserves (to meet sudden demands for power), V2G services have a promising prospective future for grid support. It has been proposed that in the future development, such use of V2G could buffer and support effectively the penetration of renewable sources in power systems. This PhD thesis project aims to develop novel and competitive control strategies for V2G services implementation for EVs in smart electrical car parks or Smartparks. Through a comprehensive literature review of the current EV development and energy storage systems used for EVs, several energy storage technologies are compared and a hybrid energy storage system consisting of batteries and supercapacitors is proposed. This system combines effectively the advantages of high energy density of battery banks and high power density of supercapacitor banks. Supercapacitor and battery cells are tested in the laboratory using different charging and discharging procedures. Different supercapacitor and battery models are compared, discussed, and verified using the experimental data. For the energy storage system package, a cell voltage balance circuit is developed for the supercapacitor module. The principle of this circuit is also applicable to the battery module. The proposed balancing method is simple and reliable, and presents good performance for voltage balancing to prolong the lifetime of the energy storage system. The essential technology of V2G is based on the bidirectional power flow control of the charger. Besides charging the EV batteries, it can utilize the stored energy to feed electricity back to the power grid when there is a need. Three-phase AC/DC converters have been extensively used in industrial applications and also the V2G chargers. The power converters used for the V2G services are required to operate more efficiently and effectively to maintain high power quality and dynamic stability. Then the AC/DC converter used for the bidirectional V2G charger is developed and modelled. For the control aspect of AC/DC converter, a new control approach using a model predictive control (MPC) scheme is developed for V2G applications. With the advanced control strategy, the EVs in Smartparks can exchange both active and reactive power with the grid flexibly. The MPC algorithm presents excellent steady-state and dynamic performance. When a very large number of EVs are aggregated in Smartparks, the charging and discharging power should be a significant viable contributor to the power grid. New challenges will be introduced into the power system planning and operation. While discharging, the V2G power brings more potential benefits to enhance the power quality and system reliability. Using V2G services, EVs can provide many grid services, such as regulation and spinning reserve, load levelling, serving as external storage for renewable sources. An effective approach to deal with the negligibly small impact of a single EV is to group a large number of EVs. An aggregator is a new player whose role is to collect the EVs by attracting and retaining them so as to result in a MW capacity that can beneficially impact the grid. From the aggregator’ decision, the EVs are determined by the optimal deployment. The aggregator can act as a very effective resource by helping the operator to supply both capacity and energy services to the grid. By supplying active power and reactive power from EVs, the aggregation may be used for frequency and voltage regulation to control frequency and voltage fluctuations that are caused by supply–demand imbalances. Different case studies of EVs’ support to grid are carried out; the results show that V2G services can stabilize the frequency and voltage variations and have control flexibilities to fulfil system reliability and power quality requirements. The main attractiveness of V2G to consumers is that it can produce income to the vehicle owner to maximize car use. On the other hand, the utility companies can use EVs to stabilize the frequency in the power system and improve the utility operation. It also makes the utility companies more efficient with less loss because the energy is generated locally. From this point of view, V2G is a source of revenue in both electricity and transportation system, and it can help the environment reduce pollution and global warming. Various data of V2G systems have been collected for economic analysis, such as EV battery capacities, charging time, and grid electricity price and load demands. Then for the economic issues related to V2G services, optimal charging based on different objectives is presented. Dumbing charging, maximization of the average state of charge (SOC), maximum revenue and minimum cost are compared. Economic issues are a very special aspect of the V2G technology and how a large profit from V2G services can be produced is the main point of attraction to vehicle owners. Significant conclusions based on the research findings are drawn, and possible future works for further development including commercialisation of the V2G technology are proposed.
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