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Is it time to ask UKPN to consider if HVDC works better in future networks ?
Helios 11001214249
48 Posts
Question
I have been wondering about the big line losses that are necessary in HV transmission systems , HVDC can nearly halve these to 4% but all the new renewable technology of generation and of storage is mostly DC . From the interconnector its all AC cant really change that , but if the electric car becomes reality then all the chargers will be converting DC to AC (in some quite high flows of electricity from low to full of some batteries) , so we are perhaps saying that the electric car will bring more line losses , it might not be that bigger deal, but if we electrolyse water to Hydrogen and Oxygen then the conversion losses from AC to DC will add up , so it has to be better to just transmit in DC , if we could generate in DC and I think we have brushless three phase generators now then we generate and transmit efficiently to the new big users of electricity . In the USA we see generation plant to city interconnector , I don't think they use a balancing grid , as cities are so far apart . I know any design has its problems but with the new uses of electricity any generation system will have to match (or think about the new denands) , its perfrectly possible to have designed and balanced generator to interconnector supply , but a shared transmission grid obviously allows you to arrange supply generator and demands in a different way .
One use I thought about is for a village to say have a battery and the line to it may only charge the battery once a week as DD line to DC battery and that electricity that is then spare can be used to say make Hydrogen ? It could give a completely different and more efficienct system ? Not really worked much on HV systems , but I can sort see a sketch of how it could work as a network . If only a short distance to a big demand then might as well use AC , but a 1000mw supply to an interconnector losing 7% over its life time is a lot . 
26 Replies
Simon Barker 22060613
653 Posts
HVDC may be more efficient, but we have transmission lines running at a wide range of voltages from 11kV up to 400kV.  So there would have to be DC/DC converters to replace all the current AC/AC transformers.  Plus there would have to be a DC/AC inverter to replace every local substation to generate the 400/230V 50Hz that houses and businesses currently run on.  That's an awful lot of new infrastructure to build.  And hence a huge cost.  Meanwhile, transformers are nice simple things that can run for decades with no maintenance.

Why would you want to charge a battery only once a week?  Where batteries are currently used, it's to handle peak loads when the grid is under stress.  The battery would be recharged again as soon as demand (and wholesale price) drops back again.  A battery that could power a village for a week would be enormous and expensive.  And why bother when you have already installed the supply to the village?
Helios 11001214249
48 Posts
Hi Simon , yes it would cost quite a lot of money , but it might pay for itself from the energy gains/ losses , 4% in MW terms is a bi deal , I think UK peaks and lows are about 20,000MW to 45,000MW , so that's something like 800 to 1600MW every hr , which is pretty much like a new power station , thrown in for free.
The energy use problem of efficiently generating power and low carbon thinking, wind and hydro are mostly in DC ,as is energy store in batteries (car or Grid scale), and water electrolysis are all DC systems and surely its best to optimise in DC before inversion into AC. We do have pretty efficient transformers and DC requires rectification and only relatively recently has the engineering made HVDC systems possible , siemens I think has managed 1000kv DC , but that was for a super hydro project .I think they use 400KV DC in the EU , so they must have efficient rectification devices, and I guess the cooled superconductor is pretty near (were at -60oC) so we really could have a system with much lower transmission losses, but as you say not much we can do past the interconnector as most domestic and business use is for AC . That's what worries about me about the car chargers being planned they are all  Dc to car battery but AC in feeds , so putting one in every house will add up ,, one calculation I saw put at least a 20% in crease  on MW required for the electric car to replace fossil fuels , so we really going to need an efficient transmission system sooner or later .
Simon Barker 22060613
653 Posts
Until you do the costings, everything is just speculation.  You would need to work out what the losses are now (in cost terms) and what the losses will be after the upgrade.  You would need to know how much it will cost to do the upgrade (lots of money), and the effect on maintenance costs after you've done it.

I suspect the upgrade would never be cost-effective, and it would be cheaper to put up a few more windmills and live with the losses.
Helios 11001214249
48 Posts
mmm not sure about that I mean from a costs point of view it beats windmills as its a continuous efficiency, or another way of viewing it is it makes every windmill feeding it seem to produce more energy . Transmission losses are well documented I think up to and interconnector and post transformation but it is quite a lot, but no denying efficiency it would attain over current system , depending on where you placed your new power stations. 
mapj1 80733779
2097 Posts
Yes, renewable sources generally create DC, or if they create AC, it is not at anything like 50Hz. But the DC voltages generated are not very convenient, or even constant.

The current strategy of then inverting to a constant voltage AC, and then stepping up by transformer to something high for transmission is hard to beat.  Losses in a line carrying the same DC voltage are indeed slightly lower than the same line carrying AC - but that is the wrong comparison, you need to factor in the inversion losses. (and the reliability ) Unless you are under the sea so losses are too high, or the lines are transcontinental, so phase shift is an issue, then a higher AC  transmission voltage is the easier way out.

For your isolated village, if you have no inertial generation at all, (or indeed less than about 30% inertial) the inverters need to be signifiantly oversized, as you will not have the luxury of a frequency falling with (over) load, and the automatic load shedding action this gives when driving induction motors - this allows you to ride out short duration transients associated with inrush.
Helios 11001214249
48 Posts
So the transmission network is designed in AC purely for motor powering ? so howcome most new netwroks are HVDC?
Simon Barker 22060613
653 Posts
HVDC tends to be used for underwater cables, and really long cable runs.  For your average run between a new power station or wind farm and the National Grid, it's not worth the effort.
Helios 11001214249
48 Posts
oh I agree at over 1000km other non electrical transmission systems start to be more efficient , but at under 1000km (remembering we now have much more design efficiency to think about with DC and moving and storing DC ) it should win out , for instance if you have a 100kw low head hydro , it has a higher availability than a 100kw wind turbine , both will generate in DC and if you want to store electricity as electricity , then its a DC battery , that is the most efficient electrical system , to feed say a small town , in turn the new demand for electric cars will be the biggest use of electricity from domestic supplies and the conversion losses will be important over time , so for a small village/town using a renewable it may be better if they charge there cars from the large battery . What I mean is with the electric car we will have very large use of DC electricity so it starts to change thinking on grid design and if we want to create Hydrogen from water electrolysis , this will also be using high amounts of DC electricity , for example a hydrogen car will do 400miles on 5kg of H2 , so at 60 kwh for 1 kg of hydrogen , 350 kw/hr (note conversion loss of electrolysis cell efficiency and efficiency of Hydrogen fuel cell, which are rarely quoted , which electric car companies see as the end game with Hydrogen as battery to electric motor is 80%), then a town of 1000 cars (doing 200 miles each) will need 175,000 kwh of new DC electricity each week , if a house uses 50 kwh of non car charging electricity each week , you can see that this new demand for the electric car is of a different order and well worth thinking about grids differently. Electrolysis DC electricity requirements are so high that efficiency becomes really important .  
mapj1 80733779
2097 Posts
I'm not sure that most new networks are DC, or even a modest fraction. Are you thinking of some specific cases ?

Generation at DC tends to be at voltages that are too low - hundreds of volts for solar panels, and a few kV for wind turbines, and both vary variable with the weather.
This is not a suitable source for transmission over any distance more than perhaps a few hundred metres.
To step up a variable DC to a higher DC for transmission, perhaps a few hundred kV and stable in voltage to perhaps 5%  involves generating an AC first, so it is generally easier and more efficiant to stop at that stage, rather than to rectify, transmit and invert back to AC again.
Helios 11001214249
48 Posts
Yes I agree the problem of lack of transformability is most cited obstacle , but my understanding came from an article on a big hydro project in china where siemens had built a 100kv HVDC system and it gave considerable savings , Most hydro I think generates in DC as does solar and wind so we are already losing some inversion losses , which I haven't seen any decent papers on for the UK although siemens has made a recent DC system for a wind array and that has worked really well in electrical efficiency improvement, The actual collection points for cables on DC wind arrays/cables has a device for "cleaning" all the mixed outputs into one. The efficiency is in the transmission itself , but looking to the future all battery and water electrolysis systems use DC and will be using a lot of DC , in some models if we make of Hydrogen , more than actual demands we measure at the moment , so if your taking about sending large amounts of power to different places in the country and you also have large regular battery use demands , you need a HVDC transmission system  , to the points off distribution/interconnector and then convert to AC . Large inverters have been developed to do this. Actually generating in DC is difficult , the gensets are usually larger and heavier than AC, and most electrical engineers give up on it , but I think it can be done if you use say a 6 or 12 pole genset , brushless 3 phase DC has already been done by a clever bit of shunt wiring , but not at say 500MW as everyone is stuck on AC for simplicity. The efficiency of HVDC is proven in transmission losses , the transforming or up stepping to HVDC from DC generators is not quite perfected , and I myself don't fully understand the equipment that is used , recently there was some talk of an idea by Nikoli Tesla on a transformer , which I think was called a Farday transformer, but no technical efficiencies were ever shown or proven. Once we have a large output DC generator it all fits together much better ,then I have asked GE about it , but they are a bit shy about discussing technology , but they are looking at HVDC transmission .       
Helios 11001214249
48 Posts
I think if you look at future demands , electric cars and water electrolysis a HVDC network does make sense , but at the moment the engineers that can show how it will work from HVDC transmission to AC distribution arnt getting the opportunity to show why it works better. The other option of course is to dispense with the UHV transmission system completely and generate AC locally to the interconnectors , which would require very different thinking but certainly could be efficient , I wouldn't like to do the maths on losing the UHV transmission system as there are some very good arguments for having a national grid for transmission from large generation centres. Renewables have come along pretty well , way beyond what I thought was possible 10yrs ago. I think solar PV will probably lose out in all but the high regular sun places or those who want to go off grid , because distributed electricity is so cheap with the right selection of generation. Hydro will probably move towards green hydrogen by splitting water as 500mw and over are not really common and some have caused environmental problems , but wind still causes grid management problems even though £30 per MW raw price is possible, and I think wind could go to green hydrogen also , at 1/3 rd of nuclear per MW it could provide quite cheap hydrogen .
When you see the figures for Hydrogen production , just even for Ammonia production let alone the Hydrogen car, the numbers are mind boggling.   
Simon Barker 22060613
653 Posts
Nobody wants hydrogen cars.  The overall efficiency is too low.  You take electricity, and use it to make hydrogen.  You them pump the hydrogen into the car, where it runs a fuel cell to generate electricity again.  Instead, we use the much simpler and more efficient approach of using the electricity to charge a lithium ion battery.
Plus, you can plug in a battery car wherever there's a mains electricity supply.  There's no infrastructure to support hydrogen vehicles, and nobody is investing to build one, because nobody is buying hydrogen cars.
mapj1 80733779
2097 Posts
It maybe would be worth you reading up on how a DC- AC inverter works at these voltages- it is not at all like designing an inverter for a few hundred kVA
Line Commutated conversion explanation it is not an elegant process in terms of waveform,  and as well as needing an AC grid to synchronize to, a lot of effort is needed to clean up the sine waves.
The cases where DC wins hands down over AC, are where you are linking regions so far apart that the AC phase offset is a problem (China's transcontiental links are like this), or the frequency is different at both ends (Japan's DC links between 50Hz and 60Hz regions are like this) or the AC losses are very high (under sea links are like this)
 
Helios 11001214249
48 Posts
Yeah totally agree I think fuel cell efficiency is around 60% , so if car battery gets sorted , then Hydrogen has a quite a way to go, problem is Hydrogen for chemicals , gas reforming produces CO2 around 12kg per kg of hydrogen , and if gov wants to put it in gas mains at 20% numbers get massive if from water electrolysis . 
Plenty of more cost and energy efficient H2 production technologies being developed today that don't use SMR that generates CO2, that needs CCUS, and they also produce value secondary cashflow by-products that reduce their whole life costs.
Simon, you might not like/want FCEVs and HICEs, but Hydrogen scooters, cars, buses, trains and trucks are being developed because e-vehicles don't meet all current fossil fuelled transport requirements. Many countries already have national targets for 100,000s H2 vehicles in the next decade.

You make green/blue hydrogen and put it in a H2 storage vessel that has the same life as a fossil fuel tank today - the life of the vehicle. BEVs need to replace their batteries at regular intervals at cost. Analysis of the wheel-to-well GHG emissions of a H2 vehicle is less than an equivalent BEV. Recharging FCEVs is faster, simpler and more 'BAU' as vehicles today. Plus, FCEVs have no charging cables lying all over the place. BEVs expend lots of their energy moving around the heavy batteries they have on board too, whether full or empty.  

The H2 transport hydrogen refuelling infrastructure is small but rolling out in the UK (17 stns as of today I believe) and more in the pipeline.

It is the classic 'chicken (infrastructure) or egg (OLEVs)' scenario that thwarted the take up of EVs for a decade.  This is due to a complete lack of national strategy and government leadership on low carbon transport. Build the infrastructure, reduce 'refuelling' anxiety, the demand grows and the vehicles will be invested in and produced. Vehicles can also be converted to H2 relatively easily. But all stakeholders need to be working collaboratively and coherently - two big words this government does not have in its lexicon.
Simon Barker 22060613
653 Posts
Maurice Dixon:

The H2 transport hydrogen refuelling infrastructure is small but rolling out in the UK (17 stns as of today I believe) and more in the pipeline.
 

By August 2019, there were 9300 electric vehicle charging stations in the UK.

EVs didn't really start being of interest or cost effective until 2006, and then the lack of EV charging infrastructure limited EV buying and choice - chicken (infrastructure) and egg (BEV) scenario - same for H2 vehicles. So it has taken 13 years to get to 9300 EV charging stations consisting of various electric charging technology, different connectors and cables, different BEV technology vehicles, many stations can only be accessed by membership or vehicle manufacturer, many are at home chargers not accessible to others, different payment methods, charging stations are denied to others if public chargers have cars left plugged in once fully charged, the charging slot occupied for hours by an BEV, or even occupied by a non-BEV vehicle. Range is still very limited on many BEV models, and range prediction is unreliable due to driving conditions, weather conditions, battery temperature, battery condition deterioration. BEVs are predominately focused on smaller commuter personal transport vehicles. How long would a fast charger take to recharge a large van or truck - BEVs currently not viable, or available, for most logistics and long distance uses? When charging on standard technology chargers, and for older vehicles, a full charge takes hours, etc. So, in realty, the BEV infrastructure has been a very slow burn expansion start to transition to low emission vehicles, but not yet the ubiquitous, widespread, universally accessibly, easily usable and convenient system many trumpet.
https://www.zap-map.com/charge-points/ 

Much for the H2 infrastructure rollout programme to learn from and do better, smarter, cheaper, quicker.

I don't think BEV v FCEV is the 21st Century equivalent of Betamax v VHS. We have a mixed fossil fuel transport system (petrol, diesel, LPG, hybrid) and we need a mixed LEV transport system (electric, hydrogen, hybrid) to replace it. Horses for courses as no clean energy technology is the silver bullet for all LEV transport needs - collaboration is better than competition.
Helios 11001214249
48 Posts
OK Mapj1 I know I have to show a DC generation and transmission system , just had a bit on the for the last few years perfecting a energy environmental solution for use of wastes as fuel☺ But you are right there is whole different area in large DC inverters which have only recently been shown to work ok. Phase interference is a big factor in losses , didn't know Japan ran 50 and 60hz systems that must be really difficult , I know USA settled on 60hz early on and it gave them a few advantages on motor power.
I don't think there is any problem in taking power from an HVDC to say a large DC battery (hoping they can get a in phase 3 phase output )and from there to invert to AC , its where you need to increase voltage in DC which is so problematic .
Apparently in early transmission experiments GE developed 12 phase transmission 🤩 !!!
Give me 6 months and Ill see if I can design the idea/improvedl thing !! If I cant then its just seeing where we can get measurable improvement of losses with reliability.
But thanks anyhow 
Helios 11001214249
48 Posts
I agree with you Maurice , my interest in EV design was when the first reports of water table lowering as the Lithium component was taking so much water and in the recycling and life LION batteries , When they melt down the Li it can catch fire. The occasional vid of a lion battery having a chemical fire had me utterly shocked about how they could work in a crash especially given the weight, in impacts. MY thinking was in design , was when I had a trip in a little smart car and was amazed with a number of design aspects . I started thinking we have railway , so lets make that the long distance means of travel , and consider small light 150 mile range EVs .True it puts charging may be more regular and probably 6hrs at night and that could pose problems for the electrical grid , but I never saw the fast charger coming , so you could say put it on charge while shopping . No one mentioned either about battery degradation , holding 80% charge after 5 yrs and failing individual cells , and the way car designers literally build the batteries into the floor pan meant a new battery was going to be a big expense and a trip to the manufacturers garage for guarantee after fitment as OEM .So yeah the gov made a lot of engineers sort of scratch there heads with what they were thinking , because if the battery is no good or doesn't recycle or costs a fortune for a new battery then poor old customer was never going to see any purchase/ownership value , nor for that matter fleet purchases or car dealers , because the rolling chassis starts to become worthless .
EV chargers I have so far seen ,most are in funny places and I haven't seen them in use at all , but  then whilst seeing plenty of hybrids I haven't seen many purely electric vehicles.  
The desertec project, solar farms in N Africa proving clean energy the Europe under the Mediterranean, was planning to use HVDC
http://www.desertec-uk.org.uk/elec_eng/grid.htm 

As is the proposed Iceland geothermal electricity IceLink to UK and N Sea energy grid.
https://www.landsvirkjun.com/researchdevelopment/submarinecabletoeurope 
Helios 11001214249
48 Posts
Well yes I suppose HVDC was put forward for these types of projects , but looking back I cant say that these were exactly well thought out and it was more of and EU type think tank thing , because the N Africa countries would need the clean energy themselves and not be in a position of export , got some quite glossy presentation material and academic backing too. But they seemed to miss the crucial aspect of local demands . 
I believe desertec was proposed with a number of national and international aims: replace dwindling oil and gas export revenues and GDP with enduring desert-generated solar heat and electricity; given the low energy use density of N African countries there would be plenty to export at a premium cost to Europe, whilst meeting national energy demand at. subsidised costs; solar would be a very profitable use of otherwise wasted resource (the heat in air and ground in the vast deserts); replace national use of oil and gas and deliver via a clean energy infrastructure for citizens; generate local jobs in renewable energy technologies, reduce GHG emissions from flare-offs and fossil fuel use, improving air quality and national health and wellbeing, etc.

Had they applied proper national clean energy strategy, transition and taken a systems design and leverage approach would have been a win win.
In Germany and Austria (probably Switzerland too) an HVDC network supplies the railways. Spot the two or four conductor pylons.
Helios 11001214249
48 Posts
Ok this is perhaps a sketch and thinking about what is possible , the final thing may well be quite different and of course we are missing some key yes or no technologies as to what is technically possible and generating in DC.
in terminology terms , transmission refers to means of transmitting electricity to the point of Distribution which is where all the different uses and loads are done , primelary transmission is concerned with carriage of electricity from generation to distribution. The main problem of electrical transmission systems over time , is that generation outputs have increased which in turn has led to problems in how to deal with these high outputs at distribution. In the 1990s materials became possible to insulate specially designed HVDC and HVAC systems , it is not disputed that higher voltages enable more power to be transmitted via a cable and that HVDC has considerable advantages over HVAC over distances.
The advent of the high output wind array , hydro or solar array led to a need for cabling to allow collection and collation of the various units of generation into a single or duel cable system  , to give a neat high output point of exit from generation to then transmit or merge with grid systems , which in itself has required some great work by electrical engineers.
Compatibility is always a problem and large amounts of electrical energy is wasted where this isn't thought through , Imbalances have further problems and once out of operational specifications imbalances can be catastrophic, so as any good engineer should tell you , specification and operational running all need a lot of thought , particularly where such electrical quantities are large and possibly centralised .
The transmission system as a transmission grid has the same advantages as separate circuits , if a generation unit fails , then generation to a common transmission system "should" mean that the demand is not affected in operation , equally failure of a demand "could" suddenly give a problem of overgeneration . Electrical flows in national systems are never static and the balances are always in lag or forward to within the parameters of the equipment and materials.
Is there a perfect system ? well yes and no .If you could make the demands vary less in theory you could match generation much more smoothly and specify equipment where the losses are less , the ideal system in my own thinking is similar to the idea of a constantly variable transmission system , where generation rises and falls in sympathy with demands , however perhaps only in very few situations can this sort of efficiency be achieved in real life.
The advent of the very large battery poses another technical possibility which may suite some thinking , where an HVDC transmission system is used in that a battery can balance demands in a very different way , batteries are DC technology requiring inversion to AC for distribution. If electric car vehicles are using DC then it does seem a bit of problem to be converting from AC to DC via rectification (as in the current system)  or in any future system from  HVDC to AC , the numbers are important the electric car could be consuming around 4-5000MW a day if all fossil fuel cars are replaced , losses of 7% from point of generation to connection to car battery don't seem unreasonable (they are more) but 450MW a day is a lot .
In cost terms when we start thinking about networks from the interconnector , we can see all sorts of different thinking and infrastructure and as nice it is to contemplate if we could relay and rewire all the losses out of the system I doubt , the disruption and cost would make most spending projections easy to put together . But none the less the efficiencies are there.
some of the more interesting future possibilities are around superconductors and perhaps graphene , in terms of voltage losses , however any large bit of electrical infrastructure that is overcomplex becomes a weak point , and engineers now have to think about the future engineers renewing or replacing what has been commissioned now as electricity is and in the future will be (I think) the main form of energy use.

NJK has recently announced a 650kv HVDC cable using its XPLE insulation , it uses copper or aluminium conductor and the breakthrough since the 1990s has been in quality of insulation and manufacturing process quality , 50yrs ago cable quality was limited by process .
The 525 KV cable has been around for a while and cant transmit 2600MW , the 650kv cable can transmit 3000MW and work is already being done on 700kv .The 650kv at 70oC operating temperature starts to suggest some sort of limits and they are designed as submarine cables (being cooled by the water) for large wind arrays . Given Siemens is speculating the 14MW wind turbine is in design  and GE Halide 12MW turbine under going tests , the cable specifications had to catch up !!!
None insulated cables are of course the large overhead transmission systems we see , using aluminium conductors for weight and cost saving . 
The future demands (depending on how generations systems are thought about)  if we do things like local large scale water electrolysis will be very different , and in some ways resolving energy sources for different aspects has to be worked through , as powering everything electrically may not be a good idea .
Converting DC to AC at MW scale has been achieved and it will be perhaps here where the next frontier of heavy electrical engineering will be , transformer efficiency for AC systems has been achieved at 99% , so as transformers are replaced we can recover some of the system operational efficiencies , however (and this real speculation of interest to mathematical modellers only!)  I am wondering if we could supply to individual properties as HVDC and have a circuit for car charging as DC and an individual , 99% efficient , super reliable transformer (and some way of metering) .
Hopefully that gives some aspects of the thinking 

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