Effects of harmonics - Increased losses
Losses in conductors
The active power transmitted to a load is a function of the fundamental component I1 of the current.
When the current drawn by the load contains harmonics, the rms value of the current, Irms, is greater than the fundamental I1.
The definition of THD being:
[math]\displaystyle{ THD= \sqrt{\left (\frac{Irms}{I1} \right)^2} -1 }[/math]
it may be deduced that :
[math]\displaystyle{ Irms = I1 \sqrt{1 +THD ^2} }[/math]
Figure M8 shows, as a function of the harmonic distortion:
- The increase in the rms current Irms for a load drawing a given fundamental current
- The increase in Joule losses, not taking into account the skin effect
Fig. M8: Increase in rms current and Joule losses as a function of the THD
(The reference point in the graph is 1 for Irms and Joules losses, the case when there are no harmonics)
The harmonic currents provoke an increase in the Joule losses in all conductors in which they flow and additional temperature rise in transformers, devices, cables, etc.
Losses in asynchronous machines
The harmonic voltages (order h) supplied to asynchronous machines provoke in the rotor the flow of currents with frequencies higher than 50 Hz that are the cause of additional losses.
Orders of magnitude
A virtually rectangular supply voltage provokes a 20% increase in losses
- A supply voltage with harmonics u5 = 8% (of U1, the fundamental voltage),
u7 = 5%, u11 = 3%, u13 = 1%, i.e. total harmonic distortion THDu equal to 10%, results in additional losses of 6%
Losses in transformers
Harmonic currents flowing in transformers provoke an increase in the “copper” losses due to the Joule effect and increased “iron” losses due to eddy currents. The harmonic voltages are responsible for “iron” losses due to hysteresis.
It is generally considered that losses in windings increase as the square of the THDi and that core losses increase linearly with the THDu.
In utility-distribution transformers, where distortion levels are limited, losses increase between 10 and 15%.
Losses in capacitors
The harmonic voltages applied to capacitors provoke the flow of currents proportional to the frequency of the harmonics. These currents cause additional losses.
Example
A supply voltage has the following harmonics:
Fundamental voltage U1, harmonic voltages u5 = 8% (of U1), u7 = 5%, u11 = 3%, u13 = 1%, i.e. total harmonic distortion THDu equal to 10%. The amperage of the current is multiplied by 1.19. Joule losses are multiplied by 1.192, i.e. 1.4.
