As well-isolated buildings and homes require little heating but more cooling power, heat pumps will be increasingly used in the built environment. As a consequence, the electrical power consumption in the distribution grid will increase substantially. Local, short-term storage for grid support can be provided by batteries, for example, by intelligently charging and discharging batteries of electric vehicles (EVs). Studies show that with intelligent charging algorithms EV battery life can be improved while providing grid support >8, 9@. Hence, the interoperability with the transport sector, in particular the electric vehicle sector, becomes on the one hand a challenge for the distribution grid, but also offers opportunities to support the electrical grid infrastructure. Clearly, to minimize power consumption and maximize utilization of renewable power, next to storage and automation systems, ƅexible electrical distribution grids are essential to balance supply and demand in an economical way. The following sections will focus on how to make electrical grids more ƅexible, considering a CO2-neutral electrical energy production based on a mix of volatile renewable power sources (wind and PV) and controllable power sources, such as gas turbines in CHP units with gas storages, as depicted in Fig. 3.
Requirements of Electrical Distribution Grids with Ultra High Level of Decentralized Power Production One can ask the question why electrical grid structures have to change when electrical power production changes from highpower central power stations towards more decentralized small-scale power plants that may include a large fraction of volatile, renewable power generators. To provide an answer, the next section brieƅy explains the inventions and the design concepts that led to the classical alternating current (AC) grid structure that was developed over the past century. Next, the shortcomings of the classical AC distribution grid are pointed out. Solutions that are based on modern power electronic devices can provide not only a higher level of ƅexibility, but also improve efƄciency. It is shown that the proposed multi-terminal direct current (DC) grids offer the potential of substantial cost savings.
Major Inventions that Determined the Structure of the Classical AC Grid The IEEE Online History >10@ and the book “Empires of Light”, authored by Jill Jonnes >11@, are must-reads for anyone interested in the early history of electrical power systems. Both sources support the fact that the most important invention that led to the electrical transmission and distribution grid, in effect was the “secondary generator”, nowadays called the transformer3. In those days, no one except for Westinghouse, a mechanical engineer, understood the importance of the transformer as a medium to step up voltage of (low-voltage) AC generators to enable transmitting electrical power over greater distances. A tenfold increase in voltage reduces the current by a factor of ten but reduces the losses in the same wire by a factor of 1004. In practice, the conductor cross-section of cables or overhead lines can be made smaller to reduce their cost signiƄcantly. At the time the “war of currents” was being fought between Edison and Westinghouse no technology existed that could transform the DC voltage of (low-voltage) DC generators to high-voltage DC. Hence, DC generation was bound to the “one mile” power stations, which was ultimately proven to be expensive and impractical. Indeed, DC generators were driven by steam engines, which had only 6 efƄciency and were belching too much smoke and soot while causing too much noise in any urban environment. As such, Edison was Ƅghting a loosing battle even when it was clear to many that DC transmission would have been more efƄcient than AC at the same voltage level. The transformer Ƅrst made this voltage conversion for AC possible and its operating principles, which go back to fundamental laws of physics, worked only for AC. Its practicality for electrical transmission was Ƅrst demonstrated at the 1893 World Expo in Chicago. In the same year, the Niagara Falls Power Company awarded Westinghouse the contract to construct AC generators and an AC transmission system from Niagara to Buffalo >12@. In the following decades, many famous engineers contributed to the further development of the AC grid. For example, the development of three-phase transformers, three-phase transmission systems, ultra-high voltage transformers and isolation systems (up to 720 kVac), circuit breakers, protection gear and grid topologies. Over time the transformer fulƄlled not only the voltage conversion function but also played an important role in the galvanic isolation between voltage systems and the coordination of protection and grounding schemes in electrical grids. One such function, i.e. the short circuit limiting function, played an important role in the topological design and protection of the medium-voltage AC distribution grid used to date. For electrical engineers, it is a well-known fact that, when a short circuit occurs at the secondary side of a transformer, the transformer limits the short circuit current due to its so-called short-circuit reactance. 3. First transformer was demonstrated by Gibbs and Goulard in 1881 and patented in 1986 (US Patent 351,589) 4. Electrical power, being the product of voltage times current p = v.i, leads to the fact that, for a given amount of power, higher voltages lead to lower currents. When less current needs to be transmitted, the Ohmic power losses p con, which cause heat, are reduced substantially. Indeed, these losses are caused by the wire resistance R and are proportional to current squared p con = R. i2.
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