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

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.

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

Or alternatively ensure that AC to DC rectification for car chargers is 99% efficient . rather like high quality audio equipment . you pay for the equipment inside the box.

The main problem for now I think is defining how much DC you could use in future infrastructure and perhaps more importantly where in terms of physical location you might use it . Single individual users e.g.a metals smelting and process may use induction melting and Dc motors in subsequent forgeing and rolling , it could generate its own supply.  For now the AC distribution system isn't really under any efficiency threat due to sheer problem and cost of changes , but generation (from central generation ) to a common transmission system would yield more electricity for whatever uses we want . you could generate locally to local interconnector but this would have to be very reliable (perhaps with backup units) , but then you would be specifying generation to demand and that would (unless some sort of standardisation can be done) create all sorts of different generation units , which without common/standardised critical parts makes some aspects of servicing/renewal difficult

The electrically heated home whilst not a problem for modern well insulated homes , will of course increase electricity use , as well as put more thought into service requirements when supply is lost , as the long standing thinking is at least if gas or electric failed you had some sort of energy supply, with gas being phased out where supply of electricity is lost , the ability to get it back up and running  is important , as is temporary supplies for vulnerable people , which with electricity can be done easily with portable generation direct to distribution board.

Of course not all homes/buildings can be heated electrically and with the car charger modelling peak winter demand isn't something we can afford to model wrong in terms of local grid effects a 15kw charger plus 6kw shower plus 6-15kw on heating a poorly insulated constructed building/home plus general electricity demand , over a towns local grid will put current distribution structures under strain. UK homes having standard 100A feed specification. Its true that night charging at home supplies evens this out but even so its a big change in what the distribution grid has been specified to do 

it is interesting to note that electrical efficiency losses have been in the UK energy public information for some time , some are difficult to get at , but some are not with the right strategy , with the MW scale inverter now perfected as well much more efficient switching equipment , there is a view possibility now that the Uk could redesign its generation and transmission networks to give vital future electrical efficiencies , as we will be using quite  a lot more electricity in the future in industrial processes. the difficulty is specifying what sort of kv system will work best , and underground or overhead cabling  .it all needs stability and balancing  as at these voltages any sudden changes can cause things to go wrong very quickly , we could use large water electrolysis banks as balances , or large batteries . If we do use MW scale inverters then we can recover the heat to supplement winter heating at interconnector , and when/if they fail a way of replacing them with minimal disruption 

Its just great the progress un UHVDC materials and conductors are showing what sort of efficient network is possible , add in the new electrolysis loads (and I am not sure hydrogen as transport fuel works , but h2 is an energy store for electricity for electricity production via fuel cell) 

I know they will say any switch to totally new system cant be done , new cabling to same points etc etc , and it is difficult , but imagine a future network that can handle much higher electrical quantities , and store it with minimal losses , if we could make use of inverter heat in heat in winter , we get bounus heating saving as well .