The Association of Manufacturers and suppliers of Power Systems and ancillary equipment


Should even harmonics be included in Total Harmonic Distortion (THD%) calculations.

It is always accepted that even harmonics are self-cancelling and are therefore not necessarily included for harmonic distortion calculations (THD%), therefore only the 'odd' numbered harmonics are usually considered.

Here the objective is to identify the imposed heating effect associated with distorted waveforms where;

  • levels of excessive voltage distortion increase the operating temperatures of magnetic circuits (Lamination steel).
  • harmonic current content increase the operating temps of conductors (Skin effect).

The most important reason for calculating the resulting THD% for an application involving Non Linear Loads (NLL) is to ensure the resulting Voltage THD% will be within acceptable levels which will allow all the connected to function correctly and not suffer imposed life shortening stress. This topic is covered within AMPS FAQ 1008

Both V and I with THD% content combine to introduce a negative effect with regards to a distribution network’s provided Power Quality. This will then likely become an issue for an existing electrical load, particularly if a combined load incorporating power electronics based equipment. But it can also be an issue for building lighting equipment, and any building where the majority of the load consist of modern computer equipment.

There are waveform quality related calculations involving 'weighting factors'. These date back to the days of the need to consider the effect on analogue communication (Telephone) systems. The terms used here are THF (50 Hz) and TIF (60 Hz) networks. These calculations can include the even harmonic numbers.

There are military specifications, particularly with respect to shipping (STAN-AG, & US Coastguard) where the concern is on electrical waveform related issues which could interfere with communication equipment, where weighting factor related calculations are required.

Manufacturers of ac generators will be able to provide a harmonic analysis which details the Odd and Even numbered voltage harmonics for that generator under a no load condition. The current harmonic detail will always be closely related to the current ‘consuming’ characteristics of the specific electrical load.

FAQ 1015  

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