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TRANSMISSION AND DISTRIBUTION Direct current is the future by J B Woudstra and H Stokman, The Hague University of Applied Sciences A world with direct current grids instead of alternating current grids. Is that possible? Sustainable generation of electrical energy is always linked with DC voltage. Solar cell systems produces a DC voltage, wind turbines always convert the AC voltage with a variable frequency in a DC voltage to convert it into an AC voltage with a constant frequency. Sustainable energy is becoming more and more important in the future. In northern Europe large offshore wind turbine parks are planned. Solar cell systems on roofs of houses, thermal heat installations in houses, small urban wind turbines will become more and more popular. When you look at the loads, then you see an explosive numbers of appliances in households, offices and industry that run on low voltage direct current. It is clear that a DC voltage grid is a logical next step. The choice to use alternating current for distribution of electricity was made in a discussion between two famous scientists Tesla and Edison around 1880. Tesla emphasised the practical advantages of alternating current. Transformers make it possible to easily increase AC voltage allowing electrical power to be transmitted via cables over great distances with very little losses. With DC voltage, this was not easy to achieve and there were great losses during transport. Tesla's practical arguments were at least for the time being the deciding factor. It is now however possible to design DC – DC transformers with a high efficiency to increase or decrease the DC voltage. So it becomes time to reconsider the decision for the use of DC voltage instead of AC voltage. Examples include halogen lighting, energy saving lamps, telecommunication and computer equipment and cordless/ rechargeable devices and tools. In the future the electrical car will replace combustion engined cars. To charge the batteries during the night will be a challenge. Sustainable generation of electrical energy is always linked with DC voltage. Solar cell systems produces a DC voltage, wind turbines always convert the AC voltage with a variable frequency in a DC voltage to convert it again in an AC voltage with a constant frequency. Sustainable energy is becoming more and more important in the future. Here in northern Europe large offshore wind turbine parks are planned. Another trend in generating electrical energy is a shift from central generation of electrical energy in large non sustainable power plants to more decentralisation in small local sustainable power plants. The energy flow was always vertical from large power plants to the users, but is becoming Fig. 1: Dutch electricity grid. more horizontal from users (with a small power plant) to other users. It is clear that a DC voltage grid is a logical next step. In the paper the transition from a vertical oriented grid to a more horizontal oriented grid will be explained, as well as from a fossil oriented production of electrical energy to a more or completely sustainable electrical energy production, and from AC grid to a DC grid. Electricity now and in the future In the Netherlands there are a small number of large power plants (28) fired energize - May 2012 - Page 32 by coal, gas or nuclear heat power plants. The total installed Electrical power is 25 264 MW and 119 818 GWh is produced. At this moment around 8,5% is produced sustainably and there is 15 583 GWh imported and 12 808 GWh exported. The import and export is with countries like Belgium, Norway and Germany. Most of the power plants are large industrial sites located at strategic locations, nearby the sea, rivers or lakes for cooling water, and close to energy resources or supply routes. These large power plants are connected to the transmission network by step-up transformers and are controlled TRANSMISSION AND DISTRIBUTION in order to take care of the voltage and frequency stability of the power system; this is what we call “centralised generation”. Until now, the power system is for a greater part supplied by this centralised generation and we therefore say that system is "vertically" operated, as illustrated in Fig. 2. We can see from the system layout that there is a "vertical" power flow in the system. At the top, power is generated by a (relatively small) number of large power plants. Via the transmission and distribution grid, power finds its way down to the consumers connected at the lowest level. Fig. 2: Vertically operated power system. Nowadays, the trend is to integrate more and more decentralised generation, also called distributed or dispersed generation, into the system, by means of connecting small-scale generators at the lower voltage levels. Examples of decentralised generation units are windmills, solar panels or combined heatpower units (producing steam for industrial processes and electricity as a by-product). When this trend continues, a large-scale implementation of these decentralised generation units will lead to a transition from the current "vertically operated power system", (Fig. 2), into a more "horizontally" operated power system' in the future, as shown in Fig. 3. Because of the increasing amount of decentralised generation units the most uneconomical and/or aged power plants are taken out of service, and this leaves a power system with the bulk of the consumption and the generation connected to the distribution network so that more or less "horizontal power flow" through the system results. Fig. 3: Horizontally operated power system. Possible developments and/or consequences of the transition from the current "vertically operated power system"into a future more "horizontally operated power system" for the power system are: Fig. 4: Horizontally operated power system without non-sustainable power plants. energize - May 2012 - Page 33 When most of the larger power stations have vanished, shown in Fig. 4, and the ‘horizontal' power system is a fact, the transmission/distribution network has lost one of its main purposes, namely to facilitate bulk transport of electrical energy from the centralized generators to the distribution networks. Such a situation requires different control and operating strategies in order to keep the system operation within safe margins. The voltage, for example, will no longer be imposed by the large (centralised) power stations, and voltage stability of the system becomes an issue. Traditional, the distribution network is a passive network, that depends totally on the transmission network for energy deliver y, frequency control TRANSMISSION AND DISTRIBUTION becomes two-way traffic. This has a fundamental impact on the protection of the system. Fig. 5: Sustainable energy sources in Europe and North Africa. Autonomous networks could be developed. When the total amount of power generated in a certain part of the distribution network is sufficient to supply the local loads, the network could be operated autonomously, by disconnecting it from the rest of the grid. From an operational view, this gives a system that can be controlled more easily. However interconnection of networks offers quite some advantages too. An important one is that the operation can be supported by others if there is a problem. For example with the local suppliers because of unexpected loss of generation. Therefore, such autonomous operating system will have to be equipped with a connection to the main transmission grid, or to neighbouring autonomous distribution systems, to safeguard the supply in case of a sudden emergency situation. This is not as easy as it looks: both systems operate in practice at a slightly different frequency and usually have a different voltage angle at the instant in time we would like to make the interconnection. Another issue is that both systems have their own frequency control, and the question arises: which system will act as the master and which system will be the slave? These problems can be avoided by having a DC-link connecting the systems; in this way, the systems have the possibility for power exchange but are frequency-wise and voltage-wise uncoupled. Sustainable energy links North Africa to Europe Fig. 6: Sun power plant in Spain. and voltage regulation. In a future "horizontally operated power system", the power is not only consumed by but also generated in the distribution network. Therefore, the distribution network needs to change into an active and intelligent network, which is able to control and regulate the system parameters, without strong support from the transmission network. It needs to become a "smart grid". In the "vertical power system", the direction of the power flow is more or less predictable: centralised generation → transmission network → distribution network → consumers. In the ‘horizontally' power system, with its active distribution network, the situation is different: (centralised generation →) transmission network distribution network consumers. The direction of the power flow in the network is not predictable any more: one-way traffic energize - May 2012 - Page 34 Sustainable energy will become ver y important in the nearby future. Fossil fuels are becoming very expensive and we can better use it for other things then burning it with a low efficiency. In the horizontally operated power system the sustainable sources are partly locally placed, solar panels, and partly in large offshore wind mill parks. But when you want to have a stable sustainable electrical power network in Europe it is necessary to connect all the sustainable sources in Europe and maybe North Africa, see Fig. 5. In North Africa and south Europe it is possible to build large sun-energy power plants. An example is shown in Fig. 6. Actually it is possible to use only sun-energy to provide the world from electrical energy. In Figure you can see what kind of surface you need to realise this. Contact J B Woudstra, Hagur University of Applied Science, [email protected]