DC transmission on a comeback

Existing solutions within the AC/DC conversion system topology are struggling to provide even a few percentage points of large scale improvement, despite localised performance boosts. The answer may be to look instead to a very different AC/DC structure, based on new approaches, rather than merely incremental changes. Using high voltage DC for power transmission, in conjunction with new conversion approaches, offers benefits for both sourcing options and system end to end performance. Ironically, this approach circles back to the earliest days of electric power generation and distribution. Edison favoured DC generation and distribution, while Tesla advocated AC, with its availability of transformers for voltage step up and step down to reduce transmission losses. The battle was big and the stakes were high, with technical, economic and political consequences. AC won that battle, but developments in components and devices, along with additional power system objectives, are positioning DC based systems as a better and available alternative. These developments include innovative conversion, control and distribution approaches, many of which are enabled by advanced semiconductors and conversion topologies which function effectively in ways not previously possible. As a result, high voltage DC (HVDC) systems are now practical for distribution. It should be noted that, between the IEC, NECA/NEMA, ANSI and IEEE, there is no agreed definition of 'high voltage'. Here, we use power electronics industry vernacular to describe a voltage up to 1kV Why use DC at all? Why should HVDC (380V nominal, 400V peak) be considered instead of the established and field proven AC approach? There are several aspects to the answer. DC does not require source synchronisation and can draw upon wind, solar and the grid as each source is available. There are no phase balancing or harmonic issues, and no 'stranded' equipment issues; all costly investments in infrastructure which may become obsolete or redundant. DC offers a lower total cost of ownership in building wiring, copper, and connectors, along with an increase in efficiency of between 8 and 10% – truly significant. A properly configured DC system offers higher efficiency and more potential for power extraction from multiple available sources. Other benefits are not as immediately apparent. Most back up energy sources, such as batteries and flywheels, are inherently DC. Further, telecom and server loads run on DC, so there are fewer intermediate, stages, along with fewer potential points of failure. HVDC has industry wide support and is supported by industry consortia – such as ETSI, ITU, IEEE and IEC – which have developed standards and interoperability specifications. Topology makes a difference Before looking at the topology and implementation of 380V DC distribution, let's look at the existing approaches. In the data centre (see fig 1), an incoming high voltage AC line is stepped down, then converted to DC so it can be paralleled with a battery back up system. The DC is then converted back to high voltage AC for distribution within the building, then converted yet again from AC down to lower voltage DC and then to voltages for the circuitry rails via DC/DC converters. Thus, there are four major conversion stages.




For existing telecom systems, there are two stages, but with major inefficiencies. The line AC is converted to 48V DC and combined with the backup batteries; this 48V DC line then supplies an array of DC/DC converters which provide the local, low voltage rails (fig 2). A typical data centre, meanwhile, has four major conversion stages from incoming AC line to final DC rails.




The HVDC system also has two major conversion stages, but there is more to the end to end performance metric than the number of stages alone, as the efficiency of each stage is also critical. In the HVDC approach, the stages are both more efficient and more reliable. The HVDC topology begins with the line AC rectified to 380V DC (nominal), with the battery back up also operating at that voltage (fig 3). The DC voltage is then distributed throughout the facility and stepped down by local DC/DC converters to supply the various loads. The system can draw on the outside AC line, batteries and onsite renewable sources simultaneously or individually. Similar implementations are seen in micro-grids and in commercial buildings, as seen in the work of the DC Components and Grid (DCC+G) Consortium (see footnote). Getting down to single volts Most circuitry operates from less than 12V DC; even as low as 1V in some instances. The challenge for any distribution/conversion system is to develop and deliver those low voltages (and their high associated currents) efficiently and reliably. HVDC can meet this requirement using several available building blocks. One is a Sine Amplitude Converter (SAC), used in the form of a BCM Bus Converter; an isolated, non regulated DC/DC converter which uses a zero voltage/zero current switching architecture. The SAC resembles a traditional AC transformer, except that it has a DC I/O and an I/O voltage ratio fixed by design. For example, with a transformer ratio (K) of 1/8, it produces a 48V DC output from a 380V DC input. The SAC reaches efficiencies up to 98%, partially due to its fixed, high frequency (greater than 1MHz) soft switching topology. The result is a power density of 110W/cm3; a Vicor 6123 ChiP bus converter measuring 61 x 23 x 7.7mm – around one third the area of a credit card – can deliver up to 1200W. The second block is the non isolated buck-boost regulator, also using zero-voltage switching at 1MHz, resulting in small size and an efficiency of 97%. Together, the SAC/BCM and buck-boost regulator provide an equaliser (adaptor) function over the full span of input voltages for the normal service range, as defined by ETSI. At the normal 380V point, the bus converter can drop the line down to 48V, with the equaliser operating in a power through mode (with a bypassed buck-boost). Thus, system efficiency is enhanced because the unit converts only when needed. If the DC voltage from the line or battery drops towards 260V, the buck-boost converter maintains the 48V rail. In either case, the architecture maintains high efficiency and allows for seamless use of multiple sources – a rectified DC line, battery and renewables – as they become available. The use of multiple modules ensures that legacy equipment is supported. In this way, HVDC can be phased in without ripping 'everything' out, which would be costly and impractical. After the transitional period, the intermediate voltage conversion after a power distribution unit could be unnecessary: the 380V DC would go directly to the loads to be down converted in a single step (see fig 4).

Challenges and opportunities A combination of factors is making high voltage DC an attractive solution to the energy consumption dilemma. The merging of voice centric telecom with data centric networking (voice, video, data) is driving increased power usage from information sources through end users. The electronics industry will be a big part of responding to these initiatives and meeting these goals. It must be inventive, with radical solutions, rather than incremental upgrades, and must collaborate on an industry wide basis with alliances among various vendors and organizations to set comprehensive standards, define commonalities and minimise barriers to adoption. Stephen Oliver is vice president of Vicor's VI Chip product line. 1) Seminar series on HVDC in applications such as data centres and commercial buildings. Go to: www.vicorpower.com/about-vicor/news-and-press/new-hvdc-webinars