Total Harmonics - THDs 145% Neutral to Ground

Can someone please help me to figure out why i get 145%+ THD on Neutral to Ground?
Tested from FLUKE 435-II 

Is this normal? 
I reckon this can be due to poor neutral earthing at generator side (its a set of diesel generators which power these DBs i tested).  

Higly appreciate your support in this regard and many thanks in advance. 

  • 145% of what ? I'd not expect a great deal to be present at the fundamental between neutral and ground as presumably they are strapped at the genset or very near, and phases will be balanced to first order.

    It is quite possible that if you are looking at 145% of something that is only a fraction of a volt, then you are making a precision measurement of something that is mostly noise and interference, and in power terms  not really there .
    Can you get an absolute figure - i.e. one with real units ? Then you will know if there is a problem. Also what sort of loads do you have - if it is mostly electronics like LED lights or computers it may well be correct.


  • Hi 

    Thank you for your reply. this is the output i got from FLUKE machine and I reckon 145% is the current THD and the graph on bottom is the Voltage THD of Neutral to ground.
    I cant figure out why it shows this high THD value.  


  • but these are all just quoted as percentages relative to the fundamental being 100, - which as you have not told me I have no idea if that may be kilovolts or microvolts ;-) It is probably neither.....

    -so  how much fundamental have you got in volts, or if the instrument does  not do that (shame on it if it can't) then what is the total voltage between N and E and then we can divvy it back up ? My money is on 'not a fat lot' of fundamental, so you probably do not really have a lot of harmonic either.
    Generally at the neutral of a 3 phase system in reasonable balance,  50Hz is not the dominant component.


  • I agree a high THD between the neutral and the earth could indicate a high impedance in the neutral-ground connection. However, other factors could also contribute to the harmonic distortion in the power system. One of them is the nature of the loads connected to the distribution boards. Non-linear loads, such as electronic devices, motors, and lighting systems, draw current in a non-sinusoidal manner, creating harmonic currents that distort the voltage waveform. Another factor is the balance of the three-phase system. If the phases have different impedances or loads, the resulting current imbalance can also generate harmonic currents. These harmonic currents can cause a voltage drop in the neutral conductor, which is reflected in the THD measurement.

  • I agree that measuring the voltage drop across the neutral and the ground would be beneficial, as the harmonic voltage is 1.45 times the magnitude of the fundamental voltage, indicating a severe distortion of the voltage waveform.

  • Agree NE is not a clean 50Hz sinewave - but that does not matter if it is really saying there is only  a very small fundamental 50Hz component, and correspondingly small harmonics. We have no idea of the scale.

    I'd be more worried about what the phase currents are doing, or  if measuring current is not easy and we can only measure voltages, then Live to something- these should all be much bigger, and tell you something about the loads, rather than just the degree of imbalance, -after all there should be no fundamental voltage  present at all in the neutral of a perfectly balanced 3 phase load..

    If the line currents or voltages  have THD more than a few tens of % then there really is something to comment about.