THE NEW DC POWER GRID
by Paul Héroux, PhD
Occupational Health Program Director
Department of Epidemiology, Biostatistics and Occupational Health
McGill University Medicine,
1110 Pine Ave West, Room 307 Montreal, PQ, Canada H3A 1A3
Tel. (514) 398-6988 firstname.lastname@example.org
InVitroPlus Laboratory, Department of Surgery,
McGill University Health Center
General toxicology, in vitro toxicology of metals. Health effects of electricity
and electro-magnetic fields.
The time seems ripe to start the replacement of the electrical utility ac power system by a dc system for transmission, distribution and consumption of electrical power. The present ac system has long had many fundamental problems. Over long distances, cables had to overcome their capacitance, while overhead lines had to overcome their inductance, making power transmission inefficient or more expensive.
Another problem is that in an ac system, generators and electrical machines must be synchronized for power transfer to be effective. This is the network stability problem which makes it difficult to re-power electrical grids after a shutdown, and makes interconnection of large power networks difficult.
Another serious problem is that ac magnetic fields have been linked to cancer, particularly leukemia, which makes implantation of new power corridors extremely difficult, if not impossible. Epidemiological data gives opponents of power line projects powerful arguments to stop them.
Our own research at the Royal Victoria Hospital in Montreal has recently produced data supporting the role of extra-low-frequency magnetic fields in enhancing leukemia, as well as other cancers http://microwavenews.com/newscenter/unified-theory-magnetic-field-action). The effect is based on a disruption of metabolism by magnetic fields. We have also provided a physical mechanism to explain the effect, thereby removing the oldest and most powerful argument against the existence of magnetic field biological impacts.
In our view, extra-low-frequency magnetic fields are biologically active, and it is not prudent to expose the general population to such fields. For years, the electrical industry argued that ionizing action on molecules was necessary, while in fact, action on electrical charges, which can occur at very low levels, is relevant. We argue in the present article that the electrical industry can avoid a protracted implication in magnetic field health questions if it accelerates its implementation of a dc grid. If the past is a guide, the scientific community takes prodigious amounts of time to reach agreement on similar complex questions. For example, 21 years elapsed between the initial observation by Wertheimer linking leukemia in children to cancer in 1979, and confirmation that the effect was genuine by the International Agency for Research on Cancer in 2001
(http://monographs.iarc.fr/ENG/Monographs/vol80/mono80.pdf). Science is rich in diversity, but poor in unanimity. The electrical engineering community can act decisively such that it will remove the health risks of ac fields, while accelerating its own technical developments and modernization of the electrical grid. Elimination of extra-low-frequency magnetic fields to levels of 10 or 20 nT (such as recommended by the Austrian Medical Association) to avoid biological action is impractical for wide-scale, and in particular, industrial applications. Ac high voltage lines would need right-of-ways of 2 km to reduce fields to safe values.
The power distribution systems produce magnetic fields of hundreds of nT, comparable to the contribution of home wiring and appliances. Although coaxial conductors could be used to reduce magnetic field emissions, the alternative of distributing and consuming electricity as dc is far more practical. Already, parts of the electrical industry have been gearing up for a change to dc, based simply on a motive of energy conservation.
AC AND DC MAGNETIC FIELDS IN THE ENVIRONMENT
Pre-industrial ac magnetic fields were vanishingly small compared to the ac magnetic fields in the present environment, and to the 1000 nT (1 μT) typically
found at the limit of the right-of-way of a transmission line. If the line was converted to dc, a similar static field would only rate as 1/50th of the Earth’s magnetic field. All living systems have evolved to tolerate static magnetic field variations. Rotation of the human body on the Earth’s surface changes the direction of the magnetic field within tissues, and proximity to any ferromagnetic object perturbs static magnetic fields. The static magnetic field impact of electrical power would then be in the same league as the Earth’s, comparable to magnetic perturbations that have been with us at least since the iron age. Large steel masses, such as a car, cause a static magnetic distortion. For example, the static field in my laboratory is about 37 μT, but in close proximity to a sink, the field rises by a factor of 4.
ADVANTAGES TO THE POWER INDUSTRY
TRANSFORMERS TO POWER ELECTRONICS
The power industry adopted ac because transformers were an efficient way of altering voltage levels without large power losses, which could not originally be done for dc. This made transmission of electrical power possible over long distances.
But there are many practical problems with transformers. They are heavy, bulky, expensive to make, contain flammable materials, and are non-modular. Delivery
lead times for very large units are a year or more. Like the internal combustion engine, transformers have served us very well, but should be mostly retired. Since the development of semiconductors such as large-power thyristors, integrated gate-commutated thyristors, MOS-controlled thyristors and insulated gate bipolar transistors, it has become possible to alter dc voltage levels with an energy efficiency that rivals that achieved by transformers. The semi-conductors act as switches that charge groups of capacitors in series, and discharge them in parallel to reduce voltages. Output voltage can be fine-tuned by altering the charging time. Semi-conductor power electronics also present an opportunity to manage the magnitude and direction of power flow. Power dc converters can be assembled from components and modular circuitry, similar to a computer. This modularity means that, while transformers must be assembled as a single unit, power electronic systems can be assembled comparatively rapidly in many configurations from components or modules, and can be much more easily repaired, reducing downtime following failures.
DC IS ALREADY HERE FOR LONG TRANSMISSION LINES
The electrical industry, to serve economic expansion, has been looking at alternatives to ac power to strengthen its power grid. Most of these new techniques are based on dc power, which offers a low environmental impact both electromagnetically (dc magnetic field vs ac magnetic field), and visually (more compact structures).
High-voltage direct current has been used to transmit bulk power over long distances for the past 40 years, but electrical utilities are currently managing an unprecedented growth in dc installations worldwide. Back-to-back dc links have allowed the connection of large asynchronous ac networks, while avoiding stability problems. dc is now used exclusively to cover large distances to remote regions, islands, and across north-south (climate) and East-West (time-zone) boundaries, to attenuate crests in power demand. In 2008, 8% of electrical energy was exchanged across borders, but this is expected to increase sharply in the future. Europe will need wind and hydro-electric power from the north, and solar energy from North Africa, over long dc lines. A dc grid is, so to speak, ready for export.
According to a February 2011 article in IEEE Spectrum, up-front costs are about equal, but dc systems’ simpler components provide cheaper, more reliable power.
The transition to dc power is possible without compromising present structures. Ac transmission corridors can integrate dc lines, so-called hybrid transmission
corridors. An ac line, when re-configured to dc, can accommodate much more power flow with minimal investments. To understand why this is so, one considers that a dc power line (1) is always working at its maximum voltage (a gain of 30%), (2) has no skin effect (the tendency of ac current to form a doughnut on the section of conductors), and (3) does not need reactance compensation. In a conversion, a tripole dc configuration uses all three ac phases, without the need for earth return. Conversion from ac to bipolar dc is simple, and a well-known solution. Monopole and bipole convertors are used in parallel, such that the full thermal rating of all three ac phases is used for dc. This can be accomplished without requiring the use of any unproven equipment. During the conversion, it is even possible to keep an ac line in operation, while dc-rated insulators are installed. As transmission capacity requirements increase, dc transmission is becoming steadily more popular, as dc is the only effective means of increasing power flow on a corridor originally built for ac transmission. There is, of course, one aspect where ac power is superior: activation of a switch on an ac circuit benefits from periodic passage of current to zero. On a dc system, the switch has to diffuse the arc at full voltage to interrupt the current. This is not a problem for relatively small voltages, but utility high voltage dc circuit breakers are difficult to build, because a system must be included to force current to zero. In November 2012, the company Asea Brown Boveri announced development of the world’s first high voltage dc circuit breaker. Until such breakers become inexpensive, it will remain economical to rely on the present ac high voltage grid components for interruption. But industry is ripe to meet the challenge, with most companies at the ready to fill dc systems demands. dc will also increase the use of underground cables. The cost for an underground cable has traditionally been ten times the cost of an overhead line. However, it is claimed that the cost of a dc cable approaches the cost of an overhead line, if all factors are taken into account. A dc cable has an environmental impact of 64.5 kg of CO2-equivalents per meter, while the ac overhead line has an impact of 365.4 kg of CO2-equivalents per meter. In other words, the material used in the dc cable has only 17.6 percent the environmental impact of the ac line. The right-of-way for a buried cable can be 4 m, as opposed to 60 m for an overhead power line: a sidewalk, as compared to an autoroute (see figure). Although the traditional practice has been to use dc circuits for bulk transport of large amounts of power between distant locations, new developments have extended the economical use of dc to a few tens of megawatts (about 1000 houses), and lines as short as 40 km. There are already many commercial systems available: HVDC Light (ABB Group), HVDC PLUS (Siemens) and HVDC MaxSine (Alstom). dc facilitates energy exchanges not only between high level networks, but also between all types of sources, as only voltage level needs to be matched, rather than voltage and phase. The magnitude and direction of dc power flow can be directly controlled, inciting many power system operators to contemplate wider use of dc for its stability and simplicity benefits alone. dc improves compatibility of electrical networks with a number of smaller green sources such as wind and solar energies. Tomorrow’s power system will still depend on large central-station generation, but it should also integrate renewable energy generation, both at the commercial and private levels.Energy storage is essential for critical applications, such as airports, broadcasting, hospitals, financial services, data centers, telecommunications, and many finely tuned industrial processes. Many businesses maintain generators at the ready when power quality and reliability are critical. Such backup systems would be more economical under dc. More energy storage would be implemented by electric utilities, commercial operations and end users due to the lower cost of energy storage systems. Present commercial and industrial systems can supply power for up to 8 hours at 75% efficiency, and maintain performance through more than 5000 charge-discharge cycles. Residential versions typically involve two hour durations at 75% efficiency. Utilities may implement fast storage systems such as Superconducting Magnetic Energy Storage, Supercapacitor Energy Storage and Flywheel Energy Storage systems within their grid to enable fast active power control for the enhancement of stability through power electronics that include bidirectional dc/dc converters. Rather than utilities shutting off systems within an individual’s home, it may be preferable to offer consumers a means of avoiding crest-power costs, while contributing to grid integrity. If homes have a battery or car-based power autonomy, it may facilitate stability of the electrical power network as a whole, and considerably simplify routine maintenance operations in the distribution system. In this scheme, you may be able to re-sell electrical power stored in your home batteries or electric vehicle at a profit to the utility under certain circumstances, increasing consumer participation.
DC FOR DISTRIBUTION
Conventional transformers have poor energy efficiency at partial loads, use liquid dielectrics that can result in costly spill cleanups, and provide only one function: stepping voltage. They do not provide real-time voltage regulation. They require costly spare inventories for multiple unit ratings, do not allow supply of three phase power from a single-phase circuit, and are not parts-wise repairable. By contrast, power electronics equivalents eliminate the majority of the inductance and, along with it, all of the oil used as coolant, resulting in a substantial reduction in losses. It also offers a great deal more flexibility in voltage control.
The Electric Power Research Institute in the US has developed a first-generation power-electronic replacement of conventional distribution transformers. It includes a bi-directional power interface that provides direct integration of photovoltaic systems, storage systems, and electric vehicle charging. It provides an architecture that allows the operation of reliable local energy networks.
Deployment is expected to grow rapidly, from 10,000 25-kW units in 2015 to 1 million in 2030.
DC FOR CONSUMPTION
Most electrical devices are native dc devices. Lighting, heating and computing use dc. Large motors should (and already are) dc powered, and smaller ac motors can be economically upgraded to variable-speed increased effective power motors by power inverters. dc promises simpler equipment and significant energy savings. After more than a dozen beta installations worldwide, dc wiring is going commercial, as manufacturers have already started selling the first products challenging ac’s 120-year dominance. Standardization efforts are set to accelerate dc’s commercial adoption. The EMerge Alliance (http://www.emergealliance.org/), representing more than 100 manufacturers of power equipment, electronics, and building components, certified the first commercial products meeting its standard for 24-volt dc circuits, aimed initially at overhead lighting systems. LED lights running on 24-V dc lines will require up to 15 percent less energy than the same lights running on fixture level rectifiers. 24 V is for short distances (10 m), and below the shock hazard level, reducing human risks, so a second 380 V level is planned to cover entire buildings. Even the telecommunications industry (European Telecommunications Standards Institute, ETSI) is expected to join in, issuing draft standards for 380-V dc wiring for building-wide power distribution, thus making it potentially a world standard.
DC FOR COMPUTING
Telecommunications firms and data centers are likely to adopt 380-V dc immediately. Today, data centers take 480-V ac from the grid, and convert it to dc to charge the battery of an uninterruptible power supply. The secure dc is then converted back to ac and transformed to 208-V ac for distribution, only to be rectified back to 380-V dc by the first stage of each server’s power supply, to charge up power-smoothing capacitors. Dc distribution offers a comparatively simple scheme where 380-V dc that can both charge the uninterruptible power supply and supply the servers. A data center in Charlotte, North Carolina, measured a 15 percent reduction in power consumption in a test of a 380-V dc distribution system. Net energy savings could be twice that, once the cooler-running equipment’s reduced air-conditioning burden is factored in. Distributing dc enables replacement of AC-DC converters within individual devices with a smaller number of larger, more efficient converters. In effect, power losses are likely to be reduced by a factor of 5. The advantage of dc in feeding the explosion in the number of computers worldwide and of the Internet means that whatever improvements are made in ac power systems, dc will always remain superior, a cheaper, more reliable power source. A domestic electrical network based on co-axial cable would allow a computer to connect into a network without any problems of electromagnetic compatibility (no emissions) with a single wire acting both as a data and a power conduit.
IMPLEMENTATION OF THE NEW DC GRID
Given the natural tendency of industries to maintain habits, it is impressive that the dc movement is already spontaneously gaining credibility. It seems probable that communities that anticipate the change, and invest in future, as opposed to obsolete techniques, will ultimately gain a strategic advantage. In view of the long lifetime of electrical infrastructures, we propose that for reasons of public health, energy conservation (a global energy consumption reduction of 20%) and strategic industrial positioning, the government impose a moratorium on the construction of any new ac systems (at any voltage), and instead direct utilities to invest in a new dc grid.
This solution is extremely effective in reducing extra-low-frequency magnetic field exposures and would lead most electrical devices to slim down in size and weight by losing the weighty transformers that have served us until now. The electrical grid would be more effective, reliable and dependable, to the benefit of all.