AC vs. DC Powerlines and the Electrical Grid
- April 2, 2013
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Ever since the earliest examples of long distance electric power transmission, overhead transmission lines have been the preferred method for transporting large amounts of electric power. As the length of any conventional transmission line increases, both the energy transfer capacity of the line and the efficiency of energy transfer decrease. The main ways to fight this are to increase the transmission line voltage, and/or to increase wire diameter. At the time that power grids linking major cities were first built, there was no convenient way to change the voltage of DC power, whereas the transformer made that relatively easy for AC power. That is why AC power won the “war of the currents” in the 1880’s. Up until 1956, only AC power could be readily changed from one voltage to another (via transformers, which only work for AC power). In 1956, ABB built the first high voltage DC (HVDC) transmission line in the West (which is still in service, though it has since been upgraded) between Gotland Island and the Swedish mainland, via a subsea cable. The Soviet Union had built an HVDC line earlier than that, between Moscow to Kashira, which was based on technology taken from the Germans after WWII. These early projects were based on mercury arc valves. Since then, HVDC has evolved a lot, and is now the best way to transmit large amounts of power great distances.
There are different trade-offs for AC versus DC power transmission. Voltage can readily be taken up to about 765,000 volts (765 kV) for an AC powerline (this is the current maximum AC voltage in the US) but beyond that, power dissipation through dielectric loss becomes significant. (Dielectric losses are caused when dipoles in matter align with a changing local electric field. As the polar structures turn to follow the field, the movement causes local heating. This is the basis of microwave ovens. The dielectric loss during transmission is equal to the total heat that is generated in materials around the powerlines due to induced motions of electric dipoles.) At high voltage, non-resistive power dissipation via dielectric losses (for AC only) and/or through corona discharge (for both AC and DC) becomes severe. Voltage for DC overhead powerlines can be taken up to higher voltage than the maximum practical AC voltage; at present the worldwide maximum is ±800 kV for HVDC lines. Note that the way that voltage is reported for AC vs. DC powerlines is different; a ±800 kV DC powerline has 1600 kV conductor to conductor (800 kV conductor to ground), whereas AC voltage refers to the conductor to conductor root mean square, or “rms” voltage; roughly speaking AC rms voltage is comparable to the line-to-line voltage in DC in terms of transmission capacity. In effect, HVDC voltage can go about twice as high as HVAC voltage, which explains most of the advantage of overhead HVDC lines compared to overhead HVAC lines.
Wire diameter is limited for AC transmission lines due to the “skin effect” that prevents an AC current from penetrating to the center of a large wire, whereas a DC line can be arbitrarily thick. At 60 Hz, the skin effect becomes significant for wires greater in diameter than about an inch. Because of the skin effect in part, multiple wires arranged in a circular pattern and separated by polymer spacers are often used in high capacity high voltage AC transmission lines. Thus, overhead HVDC powerlines can transport significantly more power for greater distances than AC lines, for two main reasons: the effective voltage can be higher, and the wires can be bigger. But DC lines were not developed initially to be capable of higher voltage, nor to be able to move more power than AC lines, but rather to make it possible to put high capacity power lines underground (for security) or under the ocean (to bring power to islands initially).
To understand why undergrounding HVDC lines for great distances is feasible, while undergrounding HVAC lines for more than about 40 miles is not, it is necessary to consider the capacitance of air-insulated overhead lines versus cables, which are typically surrounded by polymer insulation and soil. Capacitance is a property of every electrical circuit, not just capacitors (which are designed deliberately for high capacitance). A wire suspended in air has much less capacitance (by about a factor of 50-100) compared to a cable, in which the wire is surrounded both by polymeric insulation and soil. The capacitance limits how fast the voltage responds at the far end of a power line when voltage is applied at the near end. Capacitance has only a small transient effect on a DC power transmission line, delaying the voltage rise at the far end of the line by milliseconds at most when voltage is applied at the near end. When capacitance of an AC line is too high though, it has a quite dramatic effect; this is the case because at 60 Hz, the voltage reverses 120 times per second (8.33 milliseconds for per reversal); each time this happens, the “line capacitor” needs to be charged up before any power can flow through the line. The much higher capacitance of a cable (especially one that is located underground or undersea) means that this limiting line capacitance is reached for a much shorter cable (50 to 100 times shorter) than an overhead line. Thus at most short bits of an AC power transmission line can be placed underground, whereas there is no problem in terms of power flow with putting a DC power line underground.
Another important property that differentiates AC from DC power lines is that for an AC line, the line power must be synchronized with the local AC grid at both ends of the line, whereas DC power can bridge between two different synchronized AC grids that are not synchronized with each other. For this reason, DC power lines are often referred to as “asynchronous links” by power engineers. Examples where this is important involve power links between the Quebec AC grid and the Eastern US grid; between the Eastern and Western US grids; between the Texas grid (ERCOT) and everywhere else; and between the incompatible 50 Hz and 60 Hz regions in Japan.
Nearly all of the above factors would seem to favor DC over AC transmission, so why are most transmission lines, and virtually all power distribution lines AC? Simply put: transformers (which change voltage of electrical power) and circuit breakers are dramatically less expensive for AC than for DC power. At the time that the first long transmission lines were built by Westinghouse between Niagara Falls and New York City, there was no such thing as a DC/DC transformer, and that hard technical limitation persisted for a hundred years (which is why we have a strictly AC grid). Today, electronic DC/DC voltage transformers are found on every computer motherboard, and can be built for high voltage, high power conversion as well…but these devices are a lot more expensive at present than conventional transformers. However, being electronic devices, these DC/DC transformers have been on a steeply declining cost curve for some time now, and it is probable that they will in the future reach cost parity with conventional AC/AC transformers. This could mean we will have a DC grid in 100 years or so, but don’t hold your breath. Meanwhile DC circuit breakers are also a huge problem, especially at high power levels above one megawatt (MW). ABB recently announced a breakthrough on HVDC circuit breakers that they say will allow HVDC circuit breakers up to one gigawatt (GW; equal to 1000 MW; still well below what will be needed to implement a supergrid). ABB has not published either a cost for their new breaker (which I estimate will be about 100 times as high as comparable AC circuit breakers), nor on-state power loss figures (which I estimate will be ~0.25% of transmitted power). We still have a way to go to having a DC circuit breaker that is capable of enabling a supergrid, in spite of ABB’s efforts to convince us otherwise. This is a problem I have been working on; I call my solution a Ballistic Breaker™.