To the average hardworking person not in an electrical trade or profession, the difference between alternating and direct current can be insignificant if not an outright mystery.
Alternating current is what civilization has made the most use of since Nikola Tesla demonstrated the ease in which it could be transmitted great distances.
Direct current has the advantage of being stored in batteries and thus its domain has been mainly the engine industry and the starting of vehicles.
Thomas Edison was a proponent of direct current but his designs did not have the capacity to transmit power over long distances chiefly due to his inability to increase the voltage.
Doubling the voltage for a given wire size will deliver the same power at half the amperage, 15 amps as opposed to 30 amps over the same wire size.
Heat is generated with an increase in amperage due to the resistance of the wire.
Voltage does not increase heat generation; point of fact, doubling the voltage can reduce line loss (heat) by a factor of 4.
Another problem Edison faced was the inability to control the conversion of direct current for different uses; a light bulb requires far less voltage than power transmission over long distances.
Voltage control equipment, starting with mercury arc valves in the early 1920s, thyristor valves in the 1970s and presently solid-state equipment like insulated gate bipolar transistors (IGBT) have improved the performance and viability of high-voltage direct current transmission.
High-voltage direct current’s main advantages over alternating current transmission lines is lower capital costs as direct current requires one-third less wire, and the wire is smaller for the same amperage.
Direct current transmits electricity more efficiently than AC with estimated losses as low as three per cent per 1,000 km.
With the control now available, HVDC can provide stable power over long distance, is less prone to short circuits caused by weather and system overload and it can be converted to meet any requirement of alternating current.
The main disadvantages are the cost of and reliability of all the extra switching and power conversion equipment required.
These advances in control benefit the photovoltaic industry as well.
Previously, solar arrays for domestic use had to be arranged in the common DC configurations of 12, 24, or 48 volt.
These low voltages severely limited the distance from photovoltaic array to the charge controller and battery bank.
With the advances in solid state controls, we can now increase the array voltage, allowing longer distances to be traversed and more efficient power transmission and solar energy harvesting. These advances allow greater flexibility in array placement.
It has taken quite a while, but Edison would be happy to know the problems he could not solve in his time continue to be addressed and overcome as time and technology move on.
High voltage DC is a technology that has come into its own and is now taking its place in the world’s search for reliable energy solutions.
Lorne Oja is an energy consultant, power engineer and a partner in a company that installs solar panels, wind turbines and energy control products in Central Alberta. He built his first off-grid home in 2003 and is in the planning stage for his second. His column appears every second Friday in the Advocate.
Contact him at: lorne@solartechnical.ca.