Why Voltage THD Decreases When Voltage Level Increases 1

[Mido5566]

I wonder if anyone has explanation for the following notices in Harmonics limit IEEE Std 519-2014 (as per attachment):

1- voltage THD decreases when voltage level increases.

2- current TDD (or THD) increases when short circuit level increases.

Thnx

 

[magoo2]

The voltage THD limits limits in Std 519 decrease as you go up in voltage because you're affecting a much larger number of customers at higher voltages. With more customers affected, you tend to want more conservative limits on voltage THD.

Also, as you go up in voltage, the system or source impedance becomes less and less. You see this in that the available fault current increases as you go up in voltsge and so do the circuit breaker interrupting ratings. So with a stiffer system at the higher voltages you can increase the current injection limits because the product of the higher injection current and the lower system impedance is what leads to the voltage THD and this product permits higher injection currents at higher voltages.