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PM8240 Harmonic module setting value question
Metering & Power Quality
Schneider Electric support forum about Power Meters (ION, PowerTag, PowerLogic) and Power Quality from design, implementation to troubleshooting and more.
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THD is computed as the relation of the quadratic sum of all harmonic components (in this case voltage components) in relation with the fundamental voltage, this is,
where V1 is the fundamental component, and V2, V3 and Vh are the second, third and h-th voltage harmonics.
As you can see, for a value greater than 100%, the sum of the harmonics components, that is, the total harmonic voltage magnitude must be greater than the fundamental. This would result in a heavily distorted wave.
THD could be tricky, as it is a ratio between magnitudes, and it doesn't take into account if the Vrms magnitude is nominal or very low. Depending on the type of load, when computing THD for current, for example, is common to see very high values of THDi at no nominal load, because of parasitc currents flowing in power transformers or switched mode power supplies. The same could be true for voltage. For example, a 167% voltage THD could be true for a fundamental of 0.025 V and a total harmonic voltage of 2.04 V, for a total Vrms of just 2 V aproximately.