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An Introduction to Direct Current Distribution Grids

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An Introduction to Direct Current Distribution Grids AN INTRODUCTION TO DIRECT CURRENT DISTRIBUTION GRIDS Initiation of: DC – Road to its full potential JB Woudstra1 , P. van Willigenburg1 , BBJ Groenewald2 , H. Stokman3, S. De Jonge4,5, S. Willems4,5 1The Hague University of Applied Sciences (THU), Delft, Netherlands 2Cape Peninsula University of Technology (CPUT), Cape Town, South Africa 3DC Current b.v., Aalsmeer, The Netherlands 4GROUP T – University College Leuven, Leuven, Belgium 5CORE cvba-so, Leuven, Belgium ABSTRACT Electricity consumption worldwide is and regional operating Small and Medium continuously on the rise due to the load Entrepreneurs (SME’s) to improve their requirements for the electrification of innovative capacity and maintain their transportation, houses, offices, factories and competitive advantage in the Netherlands and many other facilities. However, the use of worldwide. sustainable electricity in urban surroundings is on the rise: for example, solar panels on roofs of The paper will conclude by arguing for the buildings. creation of low voltage DC distribution grids to avoid DC-to-AC and AC-to-DC conversions, These loads and sustainable electricity sources stressing the perceived advantages of low voltage have one thing in common - Direct Current. Solid grids in terms of sustainability. State appliances define the new electric world. The introduction of these DC loads and Keywords: knowledge transfer; industrial sustainable DC electricity sources to the current partners; efficiency; material saving; lifespan grid could contribute to changes in the behavior 1. INTRODUCTION of the current electricity grid – although not all these effects are beneficial to the grid. The The continual growing demand for electricity current grid provides a very stable and very necessitates the consideration of natural resources to reliable supply for all its users. This may not be a generate electricity. However, it is necessary not certainty in the near future, especially in densely only to generate electricity but also to consider the populated areas, such as The Netherlands and losses and cost involved with the different conversions between AC and DC power sources and Belgium, where AC transportation and loads.. The Netherlands has seen growing demand distribution grids are facing more and more for electricity due to, for example, the increase in: stability and reliability issues. Their common ground is the re-discovery of Direct Current and • Electrical transport: trains, cars, motor its applications. bikes and scooters • Cloud storage and data centers This paper aims primarily to illustrate the need • Use of Air-conditioning to consider Direct Current Grids in the In the future, a trend towards full electric is Netherlands. Additionally, it describes the expected. But also an increase in energy sources ensuing research and educational programs providing DC power is expected. being developed around Direct Current Grids to support this need. Finally, it promotes the creation of strong networks to transfer knowledge from knowledge institutes to local The growth of sustainable energy plants in urban and frequency and connection to the grid is only possible industrial areas creates new possibilities and new via a power converter (AC-DC-AC). If almost all problems (challenges). Among the possibilities is loads need DC as a supply voltage, why do we use the alleviation of electricity supply from current Alternating Current (AC) for the transportation and generators. One of the challenges is the introduction of DC into the current AC grids. This may cause an distribution of electrical energy? oversupply of DC leading to possible harmonic distortions and instability in the AC grid. Where It therefore appears to be logical to reconsider the does one start and what does one do? The common possibility of creating a low voltage DC distribution factor is Direct Current at the origin and at the load. grid to avoid conversions between AC/DC. The advantages of low voltage DC grids are higher Low voltage Direct Current grids are not something efficiency, material saving and longer lifespan of new. Thomas Alva Edison patented in 1880 the first apparatus. incandescent light bulb and initiated the construction of the first DC-power station in Pearl Street, New Due to this renewed interest in DC the authors and York, including a network that supplied 110 V DC fifteen members of various industries in the to 59 clients. In that time it was not possible to Netherlands met to decide on a strategy to revive DC transform DC to a high voltage level to make it grids. To do this, the consensus was to introduce a possible to transport DC-energy over a long new research and educational program to consider distance. the advantages and disadvantages of scaled DC grids. This new program aims to generate Nikola Tesla conceived the concept of alternating knowledge and strong networks to transfer current in 1886, together with the concept of using knowledge from knowledge institutes to local and transformers to step up the voltage. This causes a regional operating companies (SME’s), improving proportional reduction in current, therefore inducing their innovative capacity and maintaining their voltage drops and losses. This was the beginning of competitive advantage. the development of AC-grids. With the invention of high voltage power electronic 2. DC: BACK TO THE FUTURE devices, direct current has made a comeback. Long high voltage direct current (HVDC) lines and cables The electricity timeline is shown in figure 1. Before are on the rise. Direct current is also present in 1880 electricity was purely experimental. With the distribution grids from 230 V to 50 kV. Photovoltaic invention of the light bulb in 1879 it became more (PV) panels produce a DC output voltage and when relevant; everyone wanted to use electrical lighting. connected to current grids the Figure 1 Electricity time line DC output must be converted to AC. Wind turbines Batteries were no longer enough to satisfy the produce an AC output voltage with variable energy demands, raising the need for DC-generators. 2.1 War of the currents + Sustainable sources and users are working mostly with DC Thomas Alva Edison (1847 – 1931) initiated the + Lower losses with transport on high powers construction of the first DC-power station in Pearl (HVDC cables) Street, New York, including a network that supplied + Fewer conversions, on higher frequencies 110 V DC to 59 clients. In many other places DC + Possible to transport more power over a cable or networks were used as well. Soon after the line with the same cross section + No reactive power realization struck: DC power lines are limited in + No power factor problems their length due to voltage drop constraints. + Reliable power line communication + Longer lifespan loads and components Nikola Tesla (1856 – 1943) conceived the concept + of alternating current in 1886, together with the No short circuit power concept of using transformers to step up the voltage. + Reduced use of raw materials This causes a proportional reduction in current, − Safety (still) not standardized therefore inducing voltage drops and losses. An − Mechanical switching is difficult impolite campaign raged in the late 1880’s between − Standardization only for high voltage DC and Tesla and Edison, the so-called “War of the not for low voltage DC currents.” Tesla emerged as winner, marking the − Not commonly known beginning of the AC-grid. 2.2 DC Comeback After the invention of the transistor in 1947 by John Bardeen, the electronic revolution started. The invention of high voltage power electronic devices in particular has made a comeback for direct current possible. Long high voltage direct current (HVDC) lines and cables are now used more often. In addition to HVDC, direct current is present in residential and urban grids from 230 V to 50 kV. PV-panels produce a DC-voltage. Wind turbines produce an AC output voltage with variable frequency; connection to the grid is only possible via a power converter (AC-DC- AC). Almost all loads need DC as supply voltage. In figure 2 some DC sources and users are shown. 2.3 Advantages and disadvantages Below are some advantages and disadvantages of AC and DC. 2.3.1 Alternating current Figure 2 Some DC-Sources and Users 2.4 Saving, efficiency and lifespan + Efficient transformers This research program aims to search for savings of + Switching without sparks material and energy. Furthermore it is important to + Securing can be selective discover the lifespan of the DC tools. The lifespan + Widespread use of AC equipment is better defined.. Is it necessary − Reactive power that the lifespan of DC equipment is as long as some − Power factor AC-equipment (40 years)? Perhaps it is cheaper to − Losses with high power transport renew DC installations every 10 years. In this case, it is important that the source materials are easily − Short circuit power needed (certainly with obtainable and recyclable. decentralized energy production) − Difficult to connect different grids with each Figure 3 shows a normal three phase, 3 kVA, 50 HZ other AC transformer 400/20V 150A with a weight of40 kg and a DC transformer 350/16V 200A 3200W. 2.3.2 Direct Current The transformer used in the DC/DC converter weights 0,4 kg, the frequency is 30 kHz. The electronics and housing weights 2,6 kg, for a total of Table 2 DC world 3kg. PV ±25 DC 350 DC panels – transport – load 350 V 20 V DC DC DC transport is more efficient (±2,5%) than AC transport due to the higher voltage used: 350 V instead of 230 V. The overall efficiency is ±92%, resulting in an energy savings of around 10%. This figure gives the energy savings possible for a household, and considers only internal energy distribution. Additional savings are expected from Figure 3 AC and DC transformer of 3 kVA the used materials. Because DC systems require less current than AC systems, less copper, is needed.
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