Inverter grid synchronisation – Six AC system frequency facts

With inverter grid synchronisation the key problem is that the grid frequency can vary.  In fact the frequencies of AC systems around the world are different and they are also constantly changing.  This makes it a challenge to connect renewable generation to the network.

AC System Frequency

  1. Aircraft AC systems run at 400Hz three phase.  They do this to make the motors lighter with less iron due to the volt second integral being lower.
  2. Some railway locomotive AC systems are 16.7Hz single phase.  These frequencies are chosen to minimise the current needed to charge the capacitance of the overhead line and so minimises the number of substations needed.  The 16.7Hz  is chosen to so as to not be a multiple of 50Hz.  The was made by rotary frequency changers (this is truly what they were called as power engineers often lack imagination) but is now also made by static power electronic frequency changers.  With modern static frequency changers with inverter grid synchronisation 16.7Hz is achieved.
  3. In Switzerland the railway has their own separate distribution grid.
  4. There have been railway frequencies as low as 8Hz and some train locomotives even operate from very low frequency AC with a frequency of zero.  DC is the AC you have when you are not having AC.
  5. Early on GE decided that 40Hz would be good for AC distribution but it did not catch on.
  6. There are aluminium smelters in Australia where they have or had 60Hz, 50Hz, 25Hz and 16 2/3 Hz AC systems and reportedly all at the same time.

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Other useful information for Inverter grid synchronisation.

  • NATO ships are 60 Hz,
  • The Swiss, German, and Austrian rail network reportedly changed to 16.7 Hz from 16 2/3 Hz  in 1995.  This to prevent over heating in the rotary converters.
  • Japan has both 50 Hz and 60 Hz with the 50Hz built by the British and the 60Hz by the US after the second world war.  The two systems are linked by HVDC links.
  • 60 Hz is probably a better choice than 50Hz but it is too late now for the 50Hz countries.
  • Reportedly Tesla (not the car company) chose 50Hz first and then 60Hz.
  • There is a power station on the Argentina/Brazil border where half the generators make 60Hz for Brazil and half make 50 Hz for Argentina.
  • Sometimes, in some countries, on some days, 50Hz AC systems will run at 38Hz.  This presents an interesting inverter grid synchronisation problem.

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Grid frequency varies constantly as the mismatch between generator and load power changes.  In most countries the variation is not as large as the change to 38Hz.


The means that grid connected power converters must track or lock to the grid the grid.  The best way to do this is with a phase locked loop.

Inverter Grid Synchronisation

Phase locked loops to track grid frequency are an essential part of grid connected converters. Click here to ask us about using an ELMG phase locked loop for inverter grid synchronisation.

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