Five minute guide DC Power Five minute guide: DC Power Historical background Direct current (DC) is a continuous flow of electricity in one direction through a wire or conductor. DC is created by generators such as fuel cells or photovoltaic cells, and by static electricity, lightning, and batteries. By convention, electric current flows from a high to a low potential; for example, in a battery, from a positive to a negative pole. The “War of Currents” Early 20th Century, power systems were developed DC transmission could not be realized without a distribution networks. AC won out, and today all as isolated networks looking after local needs. But sizable loss of power. The solution was to generate utilities generate, transmit, and deliver electricity in as the demand for electric power and transmission AC power, transmit it at higher voltages, and the form of alternating current. distances grew, voltages had to be raised to maintain convert it to the desired DC voltage level near But while AC was perfectly adequate for the efficiency, and networks required interconnection the point of use when required. Initially, rotary conditions of the day – indeed for much of the converters were the only means available for for reliability. 20th century – the needs of the 21st century are conversion from AC to DC. Rotary converters DC systems were constrained due to the complexity exposing its limitations. The problem of DC were relatively expensive and required regular of motor and generator design and non-availability voltage transformation has been solved with power maintenance. of equipment for voltage transformation. Tesla’s electronics and the number of devices requiring invention of the induction motor simplified the Despite a vigorous campaign against the adoption DC power has increased dramatically with the design of machines for the majority of industrial of alternating current, Edison could not overcome expansion of digital technology. Today it is not a applications. This led to the dominance of the shortcomings of his DC system for large question of AC versus DC, but of AC and DC. alternating current (AC) over DC for generation, transmission, and to a large extent the utilization Increasing end use of DC of power. In spite of all this, DC continued in use for variable speed motors (for example, for city Cathode Wireless Vacuum Radio Radar Digital Transistors Micro railways and rolling mills that required better rays telegraphy tubes circuits processors Television Computer Integrated The controls) and for electrochemical processes such as circuits electron electroplating and electrolysis. 1880 1920 1940 1960 1970 Five minute guide: DC Power DC applications Several potential benefits are driving new interest in Data centres Embedded generation DC power delivery systems in the 21st century. Simplify power system and reduce conversion Distributed generation systems frequently produce The majority of progress in developing DC-based losses within data centres. DC power (e.g. PVs, wind turbines). technologies has occurred at either the high- or low-voltage level. At the moment it only makes Micro-grids Power electronics economic and technical sense at the two ends of Enhance micro-grid system integration, operation Increasing quantity of DC electronic equipment. the power chain where design standards are already and performance by making multiple sources of Low cost electronics has replaced traditional established: supply easier to connect and operate. lighting sources and enabled efficient variable speed • Continental transmission where Transport control of induction motors within equipment. very long transmission distances are Future transportation will be mainly electric: hybrid, involved (EHV, more than 200kV) Storage devices or battery EV across personal and public transport Devices such as batteries, flywheels and capacitors • Locally within buildings (LV: 380V and ELV: and commercial fleets. USB, Power over ethernet and lighting systems) store and deliver DC power. Microgrids and building scale nanogrids are an Lighting emerging application due to advances in equipment Lighting installations are adopting DC supply to and a need for more intelligent energy systems at achieve benefits of smaller luminaires with higher the edge of power networks. performance (e.g. flicker free dimming). Use of DC in electrical networks Scale of network Final devices Building Local utility distribution network Transmission grid Long distance transmission Actual/estimated 2005 2025 2100 2070 1980 year of introduction Five minute guide: DC Power Opportunities and challenges There are advantages and challenges associated with the introduction and usage of DC. For new markets, like electric vehicles and photovoltaics, it is very natural to introduce DC. For established markets like buildings, DC networks will become popular when the technical and/or cost advantages combine and become compelling enough to outweigh the challenges. Key benefits Addressable challenges Modern electronic loads are all inherently DC loads. Even appliances that require different Business case for DC power delivery in established markets voltage levels interface better with a DC supply, because of the elimination of AC-DC is not yet demonstrated in a completed installation. It is likely conversion (efficiency, materials and space benefits.) that the business case will be strengthened when coupled with other technologies such as network discovery, powerline With DC power, there is no reactive power loading the lines, there is no need for phase comms, metering/control, bidirectional flow, commissioning synchronization and no harmonic current mitigation required in the DC installation. This and error services which reduce operating costs and improve reduces the complexity of engineering networks with multiple sources of supply and helps to reconfigurability of installations. improve system efficiency. Safety and protection standards and equipment need to be Distributed generation systems, such as photovoltaic cells and fuel cells, and advanced developed. energy storage systems, produce energy in the form of DC power so it is more efficient and cost effective to connect with DC than converting to AC. Common practices for design, installation, and maintenance need to be established. DC power delivery can enhance micro-grid system integration, operation, and performance. High performance applications require fast-acting DC circuit High Voltage DC is economic and reliable and also economic for transmission distances in breakers. excess of 500km. Technologies to support grid controllers and network Installations are increasingly controlled through digital means rather than physical switching management need to be developed and standardised. of power. Where devices (such as luminaires or motors) are infrequently used the standby To mitigate transition cost, appliances will need to be suitable energy in power converters can be a significant proportion of overall energy use (in adverse for both AC and DC supply with agreement of technical conditions more than 50%). DC-DC power converters consume far less standby power than standards (voltage and connector). conventional AC-DC converters. Five minute guide: DC Power DC market The number of applications in which DC makes sense is increasing. The following pages of this guide provide insights into a selection of these applications. Edison’s original vision for a system that has DC generation, power delivery, and end-use loads may come to fruition – at least for some types of installations. Power over Ethernet Installations PV However the market for DC distribution networks is not a Nanogrids single, cohesive market. Rather, it encompasses several disparate Projects opportunities—telecommunications towers, data centers, grid-tied Office & commercial buildings, and off-grid military networks—that will Metro & railway consumer systems respond to specific market dynamics. gadgets Total DC Distribution Network Vendor Revenue by Scenario, World Markets: 2013-2025 Building refurbishment e.g. office Lighting $35 & factory environments Markets $30 Key for DC Aggressive $25 Off grid Conservative EHV $20 systems (rural transmission electrification Base systems ($ Billions) $15 projects) $10 Data centres Utility scale $5 distribution storage system systems $- 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 Batteries for Drives / building energy batteries management for EVs Source: Navigant Research Five minute guide: DC Power HVDC transmission Although in Edison’s time, direct current was HVDC in a nutshell DC power: lower losses for longer distances impractical for transmission beyond the distance of HVDC provides instant and precise control of the a mile, today high-voltage direct current (HVDC) power flow. Manual or automatic control determines HVAC: 2 x 400kV can transmit bulk power over very long distances the required power flows via the link. 10 Key and enables interconnection of incompatible power The HVDC link enables secure and stable HVDC networks. HVAC asynchronous interconnection of power networks HVDC: 1 x +- 400kV Since the 1950s, HVDC transmission has been that operate on different frequencies, or are 5 in practice worldwide as an effective means for otherwise incompatible. It also provides fast transmitting bulk power over long distances. Since response to change in voltage and frequency at Energy losses % the first use of mercury arc valves to modern solid either end. AC/DC conversion losses state IGBT devices