Direct current (d.c.) and alternative current (a.c.)
A quick reminder for non-technical readers.
In d.c. the voltage does not change over time, in a.c. the voltage changes in a sinusoidal shape.
Moving from an a.c. to a d.c. world
Over the last decades as the use of electronics increased, most of our devices have evolved and now work with direct current. They are delivered with adapters which convert the a.c. available at the wall plug to d.c. The multimedia, mobile and information technology devices were the first to make the shift from one form of current to another. In recent years, LED lights have replaced a large number of traditional bulbs, and their use continues to expand. The latest evolution concerns the electric motors included in our home appliances and many other devices: they have also moved to d.c. with a power supply which adds speed control and improves energy efficiency.
Today’s private homes and larger commercial and office buildings are mostly equipped with d.c. loads. In developed countries, the main driver for moving to d.c. is energy efficiency.
On the power generating side, the move from a.c. to d.c. has followed the generalization of renewable energy power systems with the extremely fast development of solar and wind energy. Currently there is an ever increasing proportion of d.c. power generation systems.
A more recent technology shift is the development of energy storage systems. The improvement of smaller capacity batteries was a key factor for the development of mobile devices. This technology is now applied to larger storage devices which are used for electric vehicles, and more recently as in-house power sources.
Moving from a top-down power distribution system to a networked structure
The traditional a.c. power distribution system is to generate power through large capacity power plants. The long distance power distribution is made by high voltage overhead lines, which feed power substations for conversion to medium voltage. Medium voltage overhead lines dispatch the power to smaller areas and the final low voltage conversion is made to connect individual households and buildings to the public power distribution network. This traditional top-down structure is now challenged by the emergence of so-called distributed power sources.
Now the proportion of d.c. power generation systems. is increasing. These power generation systems have smaller capacities, compared to traditional power plants, and can be distributed in a large number of locations.
How could d.c. change the future power distribution structure?
Power generation is increasingly happening at the place where it is consumed. The houses and buildings that were connected to the public power distribution network are now able to generate power via solar panels, small wind turbines or micro hydro. With d.c. power generation, storage, and consumption they are able to run autonomously. The link to the public distribution network can be used to trade power in and out depending on periods where additional power is needed and those where more power is generated than consumed. Energy trading could be made at the neighbourhood, town, or even at a regional level.
The future structure of the power distribution system will comprise a large number of small power sources that will also trade power. Most of these entities will be able to generate power for their own needs, and will trade power to address periods of over or under capacity. This could result in some form of Internet network for energy.
Installations for d.c. power distribution
Another key technology shift which paves the way for d.c. power use is the ability to change d.c. voltage by using efficient d.c. to d.c. voltage converters. Before using these power converters, there was no easy way to modify d.c. voltage.
In future d.c. installations, these converters can be used in fixed systems to control the voltage delivered to home devices and thereby adapt the voltage to the internal requirements of each device. These electronic power converters can be embedded with solid state switching devices for functional safety purposes and broadband communications for power management and home automation purposes.
The development of d.c. power distribution installations therefore provides a unique opportunity to switch to intelligent power distribution systems.
Direct current, an opportunity for electricity access in developing countries
More than 1,2 billion people do not have access to electricity in the world. The top-down power distribution strategy did not succeed in delivering electricity to the numerous remote locations that can be found in the developing world. The cost reduction of photovoltaic panels, the development of LED lighting, and the recent availability of low cost high performance batteries, all play a part in the rapid development of local power supply installations that are generally not connected to the main grid. These forms of self-sufficient installations are also quickly growing in suburban areas. This trend is encouraged by significant governmental investment and support programmes.
The success of these systems is mainly due to their low initial installation cost, long term sustainability – components are often sourced locally – and the speed in which these systems can be operative.
The standard voltages to be used for the distribution of d.c. power are not the same as the ones used in a.c. The standardization of d.c. voltages is therefore a key and urgent priority. The ranges of voltage variation also need to be reconsidered, mainly for installations which include batteries. The output voltage of batteries changes depending on their load level, state of charge and other parameters, so the d.c. installation needs to allow for extended voltage ranges.
Plugs and sockets
The traditional a.c. plugs and sockets cannot be used in d.c. installations. New plugs and sockets need to be standardized. These new products also need to address a safety issue relating to the disconnection of an active load. The current in the line can create an arc between the plug and the socket and this could be dangerous. With a.c., the direction of the current is inverted at twice the frequency of the power supply and therefore periodically reaches zero reducing the arcing risk.
Effects on the human body
The effects of d.c. current on the human body are different from a.c. This needs further investigation, most notably for hazards such as burns and chemical effects.
A slightly different approach to d.c. also needs to be implemented in other safety areas:
– Overvoltage protection
– Overcurrent protection
– Earthing principles
– Fault detection
Most of these requirements can be addressed by adding provisions about d.c. into existing Standards for a.c.
Advantages of d.c. over a.c.
Nowadays, with renewables, energy is often produced in d.c. and then converted to a.c. . A large majority of electrical and electronic devices are able to use d.c. without the need for conversion. The conversion process from d.c. to a.c. to d.c. generates energy losses that could be avoided. A d.c. grid would do away with the need for the usual conversion steps and result in lower material costs and higher energy efficiency, reducing conversion losses. Some d.c. to d.c. conversions would still be required.
Storage and uninterruptible power supply (UPS) can be provided by batteries using d.c.. A clear advantage of the d.c. grid is that it works without interruption. The 5 to 8 milliseconds switching time needed for detecting deviations in phase length, phase angle and the amplitude of bypass and transfer switches is no longer necessary.
Thanks to d.c., it’s possible to connect different d.c. sources to a d.c. grid without any synchronization procedure. The sources and loads are therefore completely plug and play.
Grid quality can also be improved by d.c. networks. The problem of a.c. harmonic oscillations is eliminated. Apart from the lack of reactive power losses, d.c. has the further advantage of being more energy efficient regardless of the field of application. This is because it provides more efficient use of the existing wire cross-sections. The current density is evenly distributed across the entire cross-section. Current displacement (skin effect) occurs only when an alternating voltage is applied, leading to higher near surface current density. There may also be some advantages in cases where higher cross section busbars are used. Further explorations are needed.
Advantages of a.c. over d.c.
For the transmission of electricity over long distances, high voltage levels are preferable. This helps reduce transport losses compared to lower voltage levels. Up until recently, alternating current was converted using transformers. Historically, this was the main advantage of a.c. systems. Developments in the field of semiconductor technology have made it possible to generate higher frequency a.c. voltages easily and highly efficiently. Another benefit of a.c. is that user and component safety is ensured by proven protection concepts and protective devices which still have to be implemented with d.c.. Alternating current systems also benefit from being tried and tested systems which have been in use for a long time. We have an advanced knowledge of the system design, construction and operation of a.c., as it has been used for more than a Century.
Written by Vimal Mahendru, Chair, Organizing Committee Convener; Systems Evaluation Group-4 – LVDC