01 The History of the Electrical Grid
Early DC need for separate cables for each voltage resulted in cable chaos. AC power overcame this need by use of transformers from a shared line. Photo Credit: Michael Coghlan CC BY 2.0
As mentioned in my article “Electric Vehicles” electricity as a form of energy is, in historical terms, a relatively new form of energy having been developed as late as the early 1870s to mid 1890’s.
When electricity was first developed DC power was dominant. DC voltage however couldn't be easily increased or decreased over long distance power transmission or for use by various “power input” electric appliances on a shared/common line. As a result, supply companies had to use different cables for each voltage requirement ie. various street lights, streetcars or industrial motors, all needed separate cabling. It seemed for a while that the use of multiple small local power stations was going to be the way forward in the electrification of things. By the mid 1880 the technology to easily increase and decrease AC Voltage with the aid of transformers had reached the point where it could easily and inexpensively be commercially implemented. This was crucial for the transmission of electrical power over distances for city use. After a certain distance in both AC and DC power the amount of electrical energy lost due to the resistance found in transmission lines is significant. This is a serious problem with low voltages. In order to overcome this the voltage must be able to be significantly raised and lowered. The fact that one cable could be used to power various applications with varying voltage requirements along the same line because of the transformers, gave AC the dominance over DC Power. The combined need to expand electrification over great distances together with the use of the AC transformer enabled AC power to rapidly progress. The use of big AC centralized power stations became dominant by the mid to late 1890s.
It wasn't until the 1940s that the technology to reliably convert DC power to other levels was developed and improved. Technologies such as ‘rotary converters’, ‘mercury-arc valves and other such DC power rectifiers played key roles in improving DC capability but by then it was too late for DC power, AC had won and it dominated the world.
As electrification started to spread over Europe and the US due to its popularity one major problem that power supply companies faced was that of the ‘load factor’. Load Factor is derived by dividing the total electric power used in a specific time period by the value of the maximum peek power used multiplied by the specific period of time usually in hours. The closer the value of the load factor is to “1” the better it is for the grid as this means that the power demand is constant over that specific time period.
Back in the early days of DC power the load factor for the majority of the power stations was very low during the day when the power hungry carbon arc lamps used in street and factory lights were not being used. As a result power suppliers did not start producing power until the late afternoon when the street lights would light up. The introduction of Electric Street Railways helped to even out the load factor because as they worked during the day, they created a need for a constant electrical power supply across 24 hours – street cars during the day, arc lamps at night.
Today, more than 100 years later, our hunger for electric power is greater than ever but at the same time it still suffers restraints. Technology has made our appliances less power hungry and the environment has made us more climate conscious, as a result grid load factor is facing problems again. The old polluting coal & natural gas power stations are being phased out and are being replaced by alternative power sources such as wind, solar, hydro, thermal & nuclear. The problem is that most of the time these alternative power sources work 24/7 and are not easily turned off making the surplus power they produce a problem. Yet again this is where EVs and other new innovative developments can play a role in balancing the grid both at night and during the day. As an example, EVs are usually charged overnight when their owners are not using them, so they can draw off some of the night surplus. New ideas for solving old problems are emerging.
Smaller and re-imagined grid designs are one possible solution.
Denmark’s HUB & SPOKE design for its Renewable Energy Sources (RES) is one implementation. Denmark, as one of the world’s leading nations in renewable wind energy production and research is planning to build in two stages an artificial green energy island. In the first stage the island will have an area of 120,000m² controlling, distributing and storing the energy produced from the initial 200 MW offshore wind turbines. In the final second stage the area of the island will be nearly quadrupled to 460,000m² and the number of MW turbines it will be controlling will be 650.
The island will be the central hub for the renewable energy produced in the offshore wind turbine fields and the high energy cables conceptualized as the island’s ‘spokes’ running from the island to surrounding European nations. Some excess power from the fields will be used in the creation of raw materials, primarily hydrogen and some others such as synthesis gas (syngas).
Greece, is another example. Greece has about 6000 islands, islets and very small rock islets. Of these 166-227 of them are inhabited and only about 53 of them have a population greater than 1000 people. The other 5500+ islands or rock islets are uninhabited and could be potentially used in a RES (HUB & SPOKE) island configuration with solar or wind farms on them such as the one being created by Denmark.
Comparing HVDC v HVAC Transmission Systems.
The historical competition between AC and DC power and their respective advantages and disadvantages has changed over time.
As mentioned previously in the history of the electrical grid AC power became the dominant source of electricity after the development of the AC transformers in the mid to late 1890s. By the 1940’s, fifty(50) or so years later, when the technology to raise and lower DC voltage had been developed, the electrical grid had already developed with an ascendancy of AC power and appliance manufacture had adjusted production to that power source,
DC power however could be on a comeback now particularly in long distance transmission especially over large distances such as those between major cities, states or even countries. Over distances more than 600km it has been determined that HVDC or UHVDC (High / Ultra High Voltage Direct Current) is cheaper for transmission than HVAC or UHVAC (High / Ultra High Voltage Alternating Current). For distances smaller than 600km the cost of installing AC to DC Converter stations and DC to AC Inverter stations is too large to make it worthwhile.
However in AC transmission the electrical current primarily flows on the perimeter of the cable, in a condition known as the skin effect. In DC transmission the current flows uniformly in the entirety of the cable thus allowing for a significantly larger amount of current to be transmitted over the same size of cable. Transmission losses for HVDC power is about 3.5% per 1000km. 1500 to 2500km brings most of the North African shore to practically all nations in mainland Europe or from one end of Europe to the other. Such a grid connection could be beneficial for all involved.
In AC power the direction of the current is constantly being altered and thus when it flows along a conducting cable a magnetic field is created. This phenomenon is called induction and it contributes to the lose of current in power transmission. In very high AC transmission cables induction can ionize water particles in the air. The ionization causes an electrical discharge causing an audible hissing or crackling sound and sometimes creates a violet glow (the production of Ozone gas). This ionization phenomenon is called the Corona Effect or Discharge. AC power is also prone to short circuits due to its alternating nature and AC systems of different frequencies cannot be combined. These are two points that don't effect DC systems which are generally much easier to control and manage.
When it comes to the sharing of national grids to help with power demand or production UHVDC helps by not having to worry about different AC Frequencies (50 or 60Hz), the Corona Effect or the skin effect. A bigger diameter transmission cable can help with power transmission efficiency but it adds to the cable's weight and cost.
The historical competition between AC and DC power supply may well be entering a phase where both can be used for their strengths, and their weakness minimized through cooperation rather than forcing a choice between the one and the other.