Why are the power transmission/distribution systems AC and not DC?

Question

Is there a good reason why we are not in the process of completely converting our electrical transmission system to DC? The main reason for using AC on the grid (no offense Tesla, I love you man) was to enable transformation to higher voltages in order to drop line losses (\$P=IE=I^2R\$) and if the conductor size remains the same, when \$E\$ is increased in the equation \$E=IR\$ then \$I\$ must necessarily decrease, in turn decreasing losses as the square of \$I\$). But now we have the ability to transform AC (at all thermal, hydro and wind generators) and DC (at solar generators) to any level of DC we desire and transmit, usually to residential or commercial loads which tend to use DC anyway. If need be it can be converted back to AC at industrial loads (motors usually).

In this way many transformers, capacitors, spacing issues, etc. can be eliminated from the electrical grid, increasing efficiencies dramatically, and in turn decreasing emissions and costs.

Am I missing something here?

Answer 1

There are several reasons. One: power loss in a wire is I^2 * R. Therefore it is better to transmit power at very high voltage and low current. AC is much more easily boosted to high voltage (no electronics are needed). To boost industrial loads using silicon electronics is not practical.

Another is ease of switching under load. If you turn off a load connected to DC, the arcing at the switch due to wire inductance and load inductance becomes problematic. This forces DC switches to be more robust.

The 60 Hz noise created by transformers is much less than the switching noise that would be created by all the electronics required to buck and boost DC and then convert it to AC at point of load as you propose.

Answer 2

HVDC is used: List of HVDC project. The two dominant technologies used for HVDC (thyristors and IGBT's) weren't invented until 1950 and 1968 respectively. In the mean time, countries were building AC transmission equipment. Why replace something that works when you've already spent a lot of money building a grid? Just wait until the existing system is no longer workable, and upgrade then.

The data appears to justify this: China is building a large number of HVDC transmission lines because they have money, and don't really have any existing network to interact/compete against. Similarly, there are projects in Europe and the Americas, but these appear to be more limited to areas where HVDC really shine (underwater systems) because there are existing networks so the cost of upgrading isn't justified yet.

Also, HVDC doesn't always make sense, notably when you need/want multi-point transmission. This makes routing a HVDC system more difficult than an AC system.

Answer 3

Mkeith has answered the question as asked, i.e. what are the main disadvantages of HVDC distribution. A "counter-answer" to that by helloworld922 (the next most-voted answer here currently) points in the direction of a bunch of cases where HVDC is/was used. All these engineers couldn't have been crazy, so I think it's important to actually explain here when HVDC makes sense. (That would have been a better question than what the OP asked, by the way.)

To start, there are some cases where AC would be almost infeasible. This includes connecting power AC grids that operate asynchronously with respect to each other, such as connecting 50 and 60 Hz systems; it happens in Japan for example: Eastern Japan uses 50Hz and Western Japan uses 60Hz. There are actually a few more niche applications where HVDC is the only reasonable choice, but they're not easy to explain to neophytes in a few words. If you want a more detailed list (with real-world examples), Delea and Casazza's Understanding Electrical Power System has a longer list.

Leaving aside such niche cases, I think it's important to emphasize that there is a total cost optimization that can (and in fact should) be performed when deciding whether AC or DC should be the transmission method for a power line. The two main factors are the cost of the line itself (cables, towers if applicable, e.g. not undersea) and the cost of the terminals. Generally, the DC transmission cables cost less than those of equivalent power for tri-phase AC. This happens for a reason that is easy to explain: you need fewer wires for DC than three-phase AC, but the insulation for the AC wires (and this may be just the air gap, but that translates into tower costs) needs to withstand the peak AC value, while you're only benefitting from transmitting "RMS power" (more correctly, average power corresponding to the RMS voltage) at AC. On the other hand, the terminating power electronics cost more for HVDC than the AC transformers, but why this happens isn't easy to summarize, none the least because the two terminating technologies are different.

This total cost optimization actually gives you the main application of HVDC today: transmitting large amounts of power over long distances (and by that meaning with no tapping/interruption). The typical values where HVDC is more economical than AC is transmitting more than 500MW over more than 500km (according to Delea and Casazza). Many (if not most) of the examples from the Wikipedia list (linked in helloworld922's answer) are of this kind. It shouldn't be a surprise than such examples are from China, Canada or Australia. In Europe, most of the medium/large HVDC transmission lines are undersea cables.

Below is what a synthetic (meaning textbook-level rather than real-world) optimization example looks like for a pre-determined power level, thus in which only the cost vs. transmission distance is plotted; it is excerpted from Kim et al. HVDC Transmission, the first chapter of which is freely available.

For a concrete cost perspective, here are some values (according to Larruskain et al..) for what is close to the lowest power for which HVDC terminal components are made:

• Thyristor converter, 50 MW, 100kV. Approximate per unit value is: 500 EUR/kW
• IGBT converter pair, 50 MW, +/-84kV. Approximate per unit value is: 150 EUR/kW
• Transformer, 50 MVA, 69kV/138kV. Approximate per unit value is: 7.5 EUR/kVA

Given the 20x-60x price ratio between a rectifier and a transformer at 50 MW, it is obvious why HVDC doesn't scale down to lower powers.

Answer 4

By using AC transformers (in this way), inverters, rectifiers, rotary transformers etc. can be eliminated from the electrical grid, increasing efficiencies dramatically, and in turn decreasing emissions and costs.

In Chicago and New York, the DC power grid was turned off in the 1990's. In Melbourne, Australia, the DC power grid was turned off around 2005. In the end, the main or only thing still connected to the DC grid was very old Elevators in old buildings. In Melbourne, after a transmission line failure, it was cheaper to give each remaining DC customer a rectifier, and connect the old equipment to the AC grid, rather than repairing and replacing the DC transmission grid.

Although AC power transmission has many advantages, DC power transmission continues to be used for inter-connecting HV grids: to maintain grid stability over long connections, and, particularly in underground/undersea cables, to reduce dielectric loss and skin effect.

Answer 5

Yes, you are missing something. With modern transistors and other electronic components, we can boost DC to a point, but not easily, economically, or with reaonable efficiency at MW power levels to the voltages required on major transmission lines.

Transformers are the only practical way to get 100s of kV at MW power levels, and transformers require AC.

Answer 6

Simply because Tesla vs. Edison 1880s. As a result, 99.9% of our generation and transmission infrastructure is AC. Changing over to DC is not something that can be done over the weekend. What about all people's appliances and factories with induction motors? DC won't work there. They'll need some kind of alternative developed. Substations will have to be completely redone. HVDC power electronics to handle all of this will need to be tested and certified. And perhaps most importantly, this all costs money. Lots and lots of money. Don't look for the switch from AC to DC to happen soon or quickly, if ever.

Answer 7

It's right there in your chart, item 6: "Multiple terminal/tapping : Difficult".

HVDC is already occasionally used for point-to-point links, but the more grid-like and multipath the electrical distribution system is, the less convenient it is. In compact European countries the average undisturbed length of a segment of grid is short, below the ~100km economic break-even point.

Personally I think we're more likely to see deployment of low-voltage DC microgrids fed by renewables and battery banks before we see a wholesale conversion of the AC grid to DC.

Answer 8

Here's what you are missing: You are thinking like an engineer, not a business person. Follow the money. When it makes economic sense to convert to DC, including all the costs of replacing existing infrastructure, etc., it will happen. In cases where DC does make sense it has happened and is happening.

Answer 9

I give you another good reason against DC networks besides:

• fault prone and expensive semiconductors and capacitors
• overwhelming EMC hassle at all those chopper and PFC circuitry
• increased corrosion when leakage occurs

Safety. It is very difficult to build circuit breakers for high voltage / high current DC network. Fuses have to be five times as big for secure quenching of the arc. Switches need much bigger and elaborate blast chambers due to the capacitance of the grid and the totally different arcing behaviour.

Answer 10

In the ac distribution system, all alternators have to be synchronized not only by frequency, but also by angle. Any time a load increases, it tries to slow the alternators down. That is not permitted, and power has to increase. If a load is too high, it has to be disconnected, and this puts extra strain on other alternators. In theory, HVDC is more stable and more forgiving. The reason we use ac is because it was the better method up to recently. As mentioned by others, changing to HVDC is costly.

Answer 11

All the previous answers cover the OP's questions but I thought I would just add to something said earlier regarding localised, short run DC networks. The next 'revolution' in power distribution will be Demand Response (https://en.wikipedia.org/wiki/Demand_response) systems that provide localised power through community grids fuelled by battery, solar and other renewables.

Tesla (the company not the man) are showing us where this is going with their domestic battery pack - imagine the domestic bill savings inherent in being able to switch to battery during peak energy cost times and charging batteries through PV et al during off-peak.

Get a few houses together to share that capacity in a community and then you also might have enough resource to sell your excess to other members/communities (you can already sell it back to the grid in the UK). Maybe this type of sub-grid could be HVDC if everyone in the community is a participant.

Answer 12

There are several reasons why high voltage DC is not practical yet, however it is slowly crawling back in some niche applicaions.

• AC transformers are very robust and proven technology with many years of research,improvement and optimization behind their back and are very cheaper than the DC/AC--High frequency Tranformer--AC/DC counterpart, and of course they are much more reliable
• Circuit breakers which are used for breaking circuits under load or short circuit are a serious problem in DC systems, since in an ac system currents inherently have to pass zero,it is much easier to break AC currents, AC circuit breakers are way ahead of the DC counterparts in price, breaking current capability, life and ...
• even if we get to the point that both technologies are on par with each other which is still many many years to that point, you gotta understand that AC distribution operators are very reluctant and cautious at applying new technologies

Answer 13

Off grid use in the home for lighting and computing is surely more efficient with dc. LED lighting uses a fraction of the power of incandescent and fluorescent lighting. LED must use DC, and for this reason each LED light has to have an AC to DC converter which is inefficient and prone to failure. Indeed most of the failures of LED lights are due to the conversion circuitry and very seldom to the LED light source itself.

All computers and electronics use DC. They work off a battery, or if connected to the AC mains must convert the mains AC to the DC required by the electronics through circuitry consisting of rectifier bridges, step-down transformers, capacitors, thyristors, etc.

Heating filaments for electrical heaters don't care if you use DC or AC as it is a purely resistive load. The fan for the heaters would have to be dc fans though.

AC would be needed for any appliances or equipment that use AC motors and/or compressors ie refrigerators, HVAC, fans, pumps, plug-in appliances, etc. Although more and more power tools are using rechargeable DC battery packs rather than plug-in, and the rechargers are DC.

Since the at site power generation is DC for solar power and can be DC for mechanical alternators for wind energy and biomass, it is not efficient to have to use inverters to convert the generated power to AC only to have it converted back to DC for uses cited above.

This is the system now, but as utility companies keep increasing the rates and the transmission infrastructure becomes more unreliable, more and more households will seek to use off-grid locally generated dc power. They will still use utility AC power or inverters from home battery stack for the equipment and appliances that must use AC.

While AC still the more economical choice for power transmission for over land transmission less than 500 km, the trend is toward local on-site electricity generation and storage, independent of the grid. Utility companies are already aware of this trend and partnering with municipalities and on site providers for grid buy-back, integration and other.

Answer 14

AC benefits from a critical mass of long experience, industry confidence, a wide variety of products at sane prices, and readily available service and support.

AC transformers are bulletproof. Let's say someone wants a 50A/240V RV receptacle on the far side of our property 2000 feet away. I can use common transformers to kick our 240V service up to 2400V, run a pole line, and another transformer. Cheap, reliable and off the shelf. Won't have to worry about the transformer failing, ever. And if it did need service, the number of electricians in my rural county who will know what they're looking at and can support it is definitely non-zero.

HVDC can't claim any of that.

There's an old adage from the 1960s mainframe world when outfits like Burroughs and Sperry were trying to break IBM's near monopoly: "Nobody ever got fired for buying IBM."

What facility manager is going to stick their neck out on HVDC? Not me today, I think. Maybe tomorrow. No boom tomorrow.