The energy network revolution – Episode 1

A demonstration at Niagara Falls in the United States put an end to the  “War  of  the  Currents” that opposed Thomas Edison, a staunch defender of direct current (DC) for the transmission and distribution of electricity  and  Nikola  Tesla,  who  advocated  alternating current (AC).

The latter came out victorious and today alternating current is still transmitted through the electrical grid. However, the advent of renewable energies could change the game and give direct current another chance. In addition to electricity, every other energy network including those transporting gas and heat is concerned by this revolution that is a  consequence  of  the energy transition.

Today these networks  –  which are often invisible because they are underground - connect  consumers to (usually centralised) energy-producing plants. This vital infrastructure must adapt to accompany the energy sector towards a carbon neutral future.

But the questions raised by this  evolution  highlight the urgent need for technological  innovations :

• As  wind  and  solar  farms  increasingly  produce  direct  current,  is  it  interesting  to  continue transmitting electricity in the form of alternating current?
• Amongst the various low-carbon gases, hydrogen has the wind in its sails, but can it be injected into the existing gas grid?
• Is it possible  to  make  district  heating  (and  cooling)  networks even more sustainable than today?

Solutions do exist, however as these systems  are  complex  and  often  made  up  of  infrastructures  developed  over  decades,  these  questions  can  only  be  answered  by  adopting  a  nuanced  approach.  Let’s  take  a  look  at  a  few  examples  in  relation  to  the  aforementioned questions.

Nikola Tesla won the “War of the Currents” because it is relatively easy to convert alternating  current  to  higher  or  lower  voltages  by  using a transformer. In this way electricity in the  form  of  high  voltage  alternating  current  (HVAC)  can  be  transmitted  over  long  distances with minimal losses. However, the land-scape  of  energy  production  has  changed:  photovoltaic cells produce direct current and the AC production of wind turbine generators, whose speed of rotation varies, is unstable and therefore  does  not  comply  with  the  standard  grid  frequency  of  50  hertz.  To  correct  this  defect,  the  energy  passes  through  a  power  conversion system made up of a rectifier and an inverter and is converted to direct current at one step in the process.

DIRECT CURRENT GETS ITS SECOND CHANCE

Batteries  storing  electricity  for  mobility  solutions, portable electronic devices and grid services  all  operate  on  direct  current  as  well,  and all our electronic equipment also runs on DC.  In  parallel  to  this  expanding  offer  and  demand for direct current, power electronics, i.e.,  “energy  conversion  electronics”  has  become  a  mature  technology  and  it  is  easy  to  convert  DC  to  AC. 

At  the  end  of  the  day,  the  main reason for the original choice of alterna-ting current is now redundant. Greater distances between the installations where renewable energy is produced and stored and the population centres where it is consumed will require new electricity connections. Despite the potential increase in cost, under-ground cables should be preferred to overhead lines if technically feasible.

More  than  125  years  after  the  victory  of  alternating current for grid usage, power electronics technologies are now making it possible to transmit electricity effectively in the form of a high voltage direct current (HVDC) system, also  called  a  “power  superhighway”  and  thereby  provide  an  alternative  to  HVAC. 

HVDC  transmissions  systems  have  even  several  advantages when it comes to long underground or submarine cables, which could be the only way to connect offshore installations.

• Let’s start by considering the question of cost.  Although  it  is  true  that  HVDC  systems  require converter stations at the end of the line (which are more costly than the transformers used by HVAC), the cost of the line itself for the  same  capacity  is  lower.  In  other  words,  HVDC  links  are  actually  cheaper  than  HVAC  above  a  certain  critical  distance  (see  figure  opposite).  For  underground  connections,  the  breakeven point of HVDC is approximately 50 kilometres. HVDC transmission is cheaper as only two conductors are required - or even one in some offshore wind turbine network architectures  -  instead  of  three  or  even  four  in  HVAC  systems. Moreover,  the  energy  trans-port  capacity  for  a  given  cable  is  higher.  We  will return to this later.
• A  second  advantage  concerns  reactive  power, which can be defined as the dissipated power resulting from inductive and capacitive loads in an AC circuit. This can be contrasted to active or true power, which is the power that is  actually  consumed  in  an  AC  circuit,  power  that is transformed into movement or heat for example. Excessive reactive power causes over-loading and overheating of the electrical installation  (cables,  transformers  etc)  leading  to  additional losses, high voltage drops and trans-former overloads. As a result, the installation needs  to  be  oversized.  Long  underground  HVAC cables are another important source of reactive power that must be compensated for by ad hoc devices such as capacitors and coils. Reactive power only occurs in alternating cur-rent and therefore HVDC lines are not concerned  by  this  problem  (because  direct  current  only flows one way).
• One  final  advantage  is  that,  although  HVDC systems have higher losses at converter stations,  DC  line  losses  are  lower,  which  means that this solution is more effective over longer distances. In fact, if you compare AC and  DC  transmission  for  a  same  cable,  the  average voltage is higher and the amperage is lower  when  using  direct  current  and  energy  losses  are  therefore  less.  As  a  result,  for  a  given cable a higher power can be transmitted in DC than in AC, or seen differently, cables need  to  be  thicker  to  transport  the  same  amount of power when using AC. The advantages of HVDC power transmission systems must not eclipse certain disadvantages.  For  example,  a  converter  station  is  an  active  element  and  is  therefore  much  more  complex than a transformer, which is a passive component. Be that as it may, thanks to improvements in converter efficiency the advantages of  HVDC  systems  will  only  grow  over  time.  They  will  be  able  to  ensure  grid  stability  and  interconnect   networks   with   different frequencies  and  characteristics.  However,  HVDC  will  never  replace  HVAC  which  has  advantages of its own. HVDC is a complementary  technical  solution  for  new  underground  cables  transmitting  electricity  over  long  distances.

The capital costs for direct current (DC in blue) and alternative current (AC in green) vary depending on distance. Above a critical distance, DC is more competitive.  

Be that as it may, thanks to improvements in converter efficiency the advantages of  HVDC  systems  will  only  grow  over  time.  They  will  be  able  to  ensure  grid  stability  and  interconnect   networks   with   different frequencies  and  characteristics.  However,  HVDC  will  never  replace  HVAC  which  has  advantages of its own.  HVDC is ac complementary  technical  solution  for  new  underground  cables  transmitting  electricity  over  long  distances.

(To be continued...)