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AC/DC? - Time to switch the UK’s power distribution from AC over to DC

November 16, 2020 7:20 PM
By Geoff Harvey

Wind Power (GreenLibDems.org.uk)AC/DC? - A much needed shock to the system

It is the future. It is inevitable. So why not now? There will never be a more propitious time to start switching the UK's power distribution from AC over to DC. According the Committee on Climate Change, we will need a quadrupling of our electrical supply capacity to support electrification of heat and transport. The huge investment in new power distribution infrastructure is a once in a hundred-year opportunity to replace our anachronistic AC power distribution with something much better and truly modern: a DC power distribution system. Moreover, the timing is perfect to lobby for the Government's Ox-Cam Arc national infrastructure project [1] to be built as the UK's (and possibly the World's) first major DC power zone.

Working with the Green Lib Dems, I hope we can develop a policy motion for the Lib Dem Spring Conference, urging UK investment in the technology and for the Ox-Cam Arc national infrastructure project to be a trailblazing DC zone. It is hoped that a bill can then be introduced to Parliament and can gain cross party support, as has the Local Electricity Bill.

Back to the Future

It is an irony and an anachronism that our AC (alternating current) grid power distribution, first championed by Nicola Tesla over 130 years ago is completely unsuited to the job of charging Elon Musk's eponymous Tesla motor car! To charge the powerful DC (direct current) batteries of an EV, the AC power from the grid must first be rectified, a process that converts the 100 times a second huffing and puffing of AC power into the strong steady flow of DC, which is what batteries crave. The conversion process adds additional expense and turns some of the precious electrical energy into heat.

As time goes on, transmitting power via AC becomes more and more incongruous, since more and more power is both generated and consumed as DC. Photovoltaics generate DC power directly. Most domestic appliances are now DC internally and can easily be made AC/DC agnostic. The IEC (International Electrotechnical Commission) estimate unnecessary losses in converting to AC and back to DC as being up to 13% or total generation capacity. DC transmission is already used for long distance links (e.g. the +/- 515kV Norway-UK Interconnector) because DC power transmission, as an engineering fact, is inherently much lower loss. The IEC have been working on the new international standards that will support a new world that is DC [2]. In South Korea, a DC village has already been built as a technology pilot and now a small island is to made DC only. [3]

As a nation, we must leverage the huge investment that will be needed to upgrade the power networks to provide the fourfold increase in power predicted to be needed. It would be a missed opportunity to invest in more AC infrastructure when DC is quite clearly the future. By establishing the UK as a leader, not a follower, DC switchover can be part of the Green Recovery and an export opportunity.

Seizing an opportunity for change

Ideally, an entirely new housing and industrial development would be built as a pilot to eclipse the South Korean example in scale. The timing of the Government's Ox-Cam Arc national infrastructure initiative could not be better. Green LibDems should campaign to gather Party and local political support and to argue for the Government's Ox-Cam sustainable economic arc to be built as the UK's first DC zone - an initiative which might be called Ox-Cam Spark! Such an initiative could not fit better with the stated vision and aims proclaimed for the Arc. This is what a press release stated earlier this month, with the launch of the Ox-Cam Arc prospectus: -

Jeremy Long, Chair of the Arc Local Enterprise Partnerships Group, said: "Our vision is for the Arc to be a global hub for innovation, and home to exemplary models of green development that will inspire communities around the world. The Arc's place is at the forefront of the UK's green recovery and this will be made possible through bold leadership that focuses on the big opportunities. The Arc's world-leading innovation across multiple sectors can deliver prosperity for the UK. We now need the commitment from Government and international investors to make the ambition a reality."

The first step in this initiative could be to co-build a super-grid level HVDC clean energy spine by laying a high voltage DC cable alongside the forthcoming East-West Rail link. EWR is proposed to extend along the Arc and then to Norwich, but the cable at least, would continue to the North Sea coast to link up with new offshore wind capacity.

There are many technology synergies available combining a DC spine with East-West rail. One of them is that locos create huge power surges on acceleration/deceleration, so that this argues strongly for track-side battery storage. Batteries are also needed for e-bus and EV charging so batteries support a coherent and integrated clean transport infrastructure. Furthermore, the new railway is correctly orientated to have solar PV along its entire length. The special planning status of the Arc may allow onshore wind outside of and in advance of current NPPF restrictions.

So far, little work has been done on the energy infrastructure of the Arc, so an opportunity exists to make a truly visionary proposal. The HVDC spine will be significant enabler for the many grid connected clean energy assets that will be needed to meet 2050 net zero and give UK a technology foothold in an emerging market. As a second phase, LVDC (Low Voltage DC) expertise and hardware will be developed for last mile distribution into the home. I believe that HVDC and LVDC are technology areas that play very much to the strengths of the existing Ox-MK-Cam industries and universities and can be part of the Green Recovery.

There is also a developing countries and emerging economies aspect to this - the DC microgrids which are springing up in off-grid parts of the world will almost certainly be the seeds for future coalesced wide area DC power distribution in emerging economies. Since they have no AC legacy infrastructure, developing areas can entirely skip any involvement with AC and go straight for DC distribution. Therefore, the engineering expertise to connect and operate wide area DC distribution is a potential export opportunity for the UK.

Technical and engineering benefits and challenges

When we talk about 240 Volts AC, we actually mean a sine wave with a peak of 339V, which is the amplitude needed to have the same heating effect as 240V DC when hooked up to a simple load like an electric heater. A disadvantage of AC is that e.g. for 50Hz, power is transmitted in 100 pulses of it per second. Just like pushing water through a hose pipe, it is much more efficient to create a steady flow than to push a series of rapid pulses. Cables need to be designed to withstand a specified maximum voltage before the insulation breaks down. A cable that can withstand the 339V peaks of 240Volts AC would transmit exactly twice the power if fed with 339V DC. This simple fact alone would lead to significant reduction in the cost of power transmission cables and pylons, obviously. But it turns out there are other losses that occur with AC which are either absent or at a lower level with DC. Firstly, there is something called the skin effect. As a consequence of mutual inductive coupling of the filaments of current within a cable, the current flows preferentially near the surface of a conductor with AC, so the current carrying capacity of the inner cross-section is wasted. For 50Hz, roughly only the outer 9mm of a copper conductor carries significant current. For DC, the whole cable gets used equally. Corona loss, the slight glowing and crackling around high voltage lines is also higher for AC overhead lines. The combination of these factors means that if we were designing our national grid now, from scratch, we'd almost certainly use DC [4]

The main reason we use AC for power transmission is that, historically it made it easy to increase and decrease voltage using a piece of technology known to 19th century engineers, called a transformer. The losses in power transmission are lower, the higher the voltage that is used, because the current can then be lower for a given amount of power. Indeed, doubling the transmission voltage from 100kV to 200kV results in a fourfold reduction in resistive losses. These days we have advanced high voltage transistors that can be used with very much smaller magnetic components to step-up and step-down DC voltage, allowing us to have our cake and eat it. Moreover, where transformers can only step up or down in a fixed ratio, DC to DC converters can adapt that ratio on the fly. Many parts of the UK are now 'constrained' no-go areas for any further clean generation, because when power flows the 'wrong way' from consumers who now are generators, it upsets all the careful tweaks and adjustments, and makes it impossible for DNO's (Distribution Network Operators) to maintain voltage within their legally contracted limits. So, DC distribution is almost a prerequisite for flexible wide area clean energy networks.

Now that so much of our clean energy generation and consumption is inherently DC, there are benefits for cost and reliability when we switch to DC distribution and avoid ever having to go via AC-DC or DC-AC conversion. Because AC transmits power in pulses and DC, smoothly, internal energy storage is required whenever you convert from one to another. This means normally, a bank of capacitors to store excess energy from the AC peak and then use it to fill in the trough. Even so, in Cambridge recently, the City Council was refused permission by the DSO (Distribution Service Operator) to install EV chargers, because the AC-DC converters were causing too much harmonic distortion; a concern that would not arise within a DC system. And capacitors are not only expensive, they are also not on the steep downward price trajectory of semi-conductors and importantly are the least reliable of all electronic components you would find in an inverter. DC to DC conversion (to step up or down in voltage) can run at switching speeds 1000x higher or more, compared to 50Hz and therefore can use smaller, cheaper, more reliable capacitors. In terms of active devices, the simplest DC to AC inverter requires 6 to 8 transistors, whereas a simple DC-DC step-down converter requires one or two transistors and is increasingly, smaller, lighter, cheaper and uses much less copper and iron, compared to a transformer.

There are some safety concerns which differ between AC and DC. Firstly, it is an advantage that AC comes in pulses of power when, for example, a lighting strike causes a temporary power outage. The zero power crossing points of AC allow 100 opportunities per second for reconnection of tripped isolators to happen with minimal arcing. With DC, large resistors are needed to limit the surge current into the capacitors of the downstream DC-DC converters. Dealing with reconnection after trip is regarded as one of the most demanding technical challenges.

A second safety concern is more one of public perception since part of the mythology is that DC is inherently more dangerous since (so the argument goes) the muscles contract continuously under DC and if you gripped a live metal bar, you would not be able to let go, whereas AC would offer brief intervals when you could let go. But whether true or not, there are counter arguments. For example, AC is more likely to send one's heart into fibrillation (normally fatal). AC shock causes more sweating than DC, lowering the body's resistance, in turn increasing the current flowing through the body. Whatever the truth is, the IEC acknowledge that introduction of DC would be difficult unless it can be shown to be at least as safe as AC, and this is the aim. [5]

As with any technology that constitutes a system, we can peel away layers of complexity, only to reveal new complexities at a more granular level but I hope that I have shown enough to see that if we were to design a power distribution and sharing system today, from scratch, AC distribution would be regarded as an almost ridiculous complication and a liability. We should, therefore, take full advantage of unique circumstances, to replace it with something better.

References

[1] The Oxford Cambridge Arc was first proposed by the National Infrastructure Commission in 2016 with the aim to double the £115billion GVA economic output of the geographic arc from Oxford to Cambridge including other important settlements such as Milton Keynes and Bedford. Up to 1 million new homes are planned along the Arc, also the route of a new East West Rail infrastructure project, that is now at a detailed planning stage. The initiative has ongoing support at the highest level within Central Government. https://www.semlep.com/oxford-cambridge-arc/

[2] IEC (International Electrotechnical Commission) LVDC: electricity for the 21st century. https://www.iec.ch/lvdc/

[3] HongJoo Kim , Youngpyo Cho, Jaehan Kim, Jintae Cho, Juyong Kim Demonstration of the LVDC distribution system in an island https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8316218

[4] Gunnar Asplund. Sustainable energy systems with HVDC transmission http://desertec-uk.org.uk/reports/HVDC_Gunnar_Asplund_ABB.pdf

[5] Ohio State University [course notes] More about "The Fatal Current" https://www.asc.ohio-state.edu/physics/p616/safety/more_current.html


Geoff Harvey