The THDi Total Harmonic Distortion of the current is the most used evaluation for equipment. The calculation is considering the sum of all harmonic currents up to the 40th.
This evaluation is used for many standards and is basically showing the harmonic currents in relation to the fundamental current. The voltage distortion is called THDv and the calculation is very similar using the individual voltage harmonics instead of the current.
Basically the THD is a good evaluation for Harmonic Distortion but it is not sufficient to give a full evaluation of the problems that may be caused by harmonics. One main reason for this is very simple and based on the following correlations for Inductance:
The impedance of an inductance depends on the frequency. This means: higher power losses for higher frequency components. Therefore equipment like transformer and other inductances are sensible to high order components. This is the reason why high harmonic distortion may cause overload on transformer, even though the actual nominal power is far below nominal power of the transformer.
This correlation also shows that the THD does not give sufficient description of the effects caused by harmonic currents. E.g.: If we compare a 100A distortion on the 5th Harmonic (I5=100A) with a 100A distortion on the 37th Harmonic (I37=100A), these currents will cause the same effect on the THDi value. But the power loss inside a transformer caused by I37 would be significantly higher due to the higher impedance on this frequency.
Typically the distortion from equipment is lower on the higher harmonics. Nevertheless this is the reason why standards and recommendation not only pay attention to the THD but also to individual numbers and higher harmonics. A common evaluation for higher harmonics is the Partial Weighted Harmonic Distortion PWHD which is used in different standards and recommendations to evaluate the higher harmonics, in this case all harmonics from the 14th to the 40th.
The following calculation shows the calculation of the PWHD of the current; the calculation for voltage distortion is equal but obviously refers to the individual harmonic voltage amplitude.
These harmonic evaluations THD and PWHD are used in most common standards, especially in different parts of the IEC 61000 and EN 50160.
The standard IEEE 519-2014 is the recommended practice and requirement for harmonic control in electric power systems. This standard gives recommended practice how to deal with harmonic distortion and uses a slightly different evaluation of the harmonic currents:
On the first sight this equation might appear equal to the THDi, the difference is the denominator, and use IL instead of I1. Both IL and I1 are defined as the fundamental current, or “1st Harmonic” so typically 50Hz or 60Hz. The I1 is the fundamental current for each individual operating point but IL is defined by the IEEEE as: “This current value can be established at the PCC and should be taken as the sum of the currents corresponding to the maximum demand during each of the twelve previous months divided by 12.“ Due to occasional overload of the systems this usually results in lower TDD values at nominal load compared to the THDi but especially in part load this leads to significant lower values (see Diagram 1).
The voltage distortion is caused by the amplitude of the individual harmonic currents not by the percentage value of the actual operation point. VFD equipment show increasing harmonic currents with increasing load therefore the TDD gives more accurate information about the effect on the mains supply.
The following diagram is showing the correlation between TDD and THDi and the values of a VFD with a Passive Harmonic Filter input.
Imagine the above diagram showing only the THDi. An inexperienced user might now assume that the harmonic distortion is worst at 18% motor load. The TDD curve is showing that the worst distortion is at 120% load. This is why the TDD is a more user-friendly way to evaluate or specify the harmonic distortion in a specific system.
The measurement of harmonic components is very complex but quality measurement equipment such as power analyzer are able to measure the harmonic component and show you THDi and THDv values, some equipment also show you the PWHD values. However the measurement equipment typically cannot know the IL as defined by the IEEE therefore common power analyzer are not able to show the TDD. If you are able to evaluate the IL you can calculate the TDD from any operating point and the corresponding THDi and I1 value.
However, these evaluations are only for interest or if user would like to check if the equipment provider is meeting the stated values. Basically the TDD is always lower than the THDi value.