ATV6xx / ATV9xx Published THDi versus Measured THDi

[ZGunt]

Hi, coming across a discrepancy I have not been able to understand/resolve.

In our application we have 5 VFDs with each one directly connected to a WEG motor. Total full load power for our application is 94kW. 

Specifically we are using 5 x ATV930 22kW (30HP), operating as a Normal Duty application. We have technical data for the THDi from SE that the 44% < THDi < 55% (see attached for 80%-100% load). Meanwhile during actual testing we see values of ~33%, for all 5 VFDs (see attached).

94kW/110kW = 85.5%

So I would have expected close to 50% THDi, but instead it shows ~33%.

Can someone explain why we are seeing a lower THDi than the technical data from SE?

This is a good result, but I need to be able to explain it.

Can you also explain at the much lower loads we see THDi of ~126%?

I know at partial load THDi is much worse but I have not seen such high values of 126% before. I did not know those are possible.

Thank-you in advance!

[ismailozinan]

Hi,

THDi is calculated as:

THDI = sqrt( I_2_rms^2 + I_3_rms^2 + I_4_rms^2 + ... ) / I_Main_rms

At low load, I_Main_rms is small, which can result in values such as 126%. This is expected behavior.

Additionally, grid impedance (Zgrid) can be approximated by:

Zgrid (supply impedance) = VLL / 1.7 x Isc

Isc is the short-circuit current. A low grid impedance (strong supply) can reduce THDi, which may explain your ~33% result.

We believe these measurements should be interpreted in the context of the actual supply conditions. Catalog values are based on standardized test conditions, so deviations in actual field measurements are normal.  Hope this helps.