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 .

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 .