Extinction of the Lambdasaurus vex

By J. M. Woodgate B.Sc.(Eng.) C.Eng.  MIET SMIEEE FAES Hon FInstSCE MIOA

Extinction of the Lambdasaurus vex

Some people may have been puzzled why the third harmonic current limit in Table 2 of IEC 61000-3-2 is 30 times the power factor ? (lambda). It is, in fact, a very long-term survival from a bygone age, not quite 60 million years, but 82 years. This led to the observation that the symbol even looks like a dinosaur:


and, having been described by manufacturers of lighting equipment as a nuisance in testing, appropriate genus and species names were coined, hence Lambdasaurus vex.

The origin was the conventional fluorescent tube luminaire, containing a current-limiting inductor and a power-factor correction capacitor, together with a gas-filled starter lamp. The tube itself has a non-linear resistance, thus producing harmonic currents. (The resistance is actually negative, as with most arc discharges, which is why the inductor is essential.) It is easy to show (and confirm by measurement) that the harmonic currents are not changed by the value of the capacitor (even at harmonic frequencies, the supply source has a very much lower impedance than the capacitor).

But the input current does vary with the capacitor value, typically halving when the capacitor is changed from zero value to the largest that makes a significant difference. The limits in Table 2 are based on the input current, so a luminaire without capacitor would effectively have limits roughly twice those for a better luminaire  with a higher power factor.  To overcome this for the important 3rd harmonic, the power factor ? was included in the limit value.

We now see a profound modernization of lighting technology, most forms of low-power lighting (even up to a few kilowatts) being replaced by LEDs. These have totally different current emission spectra, with switch-mode power supplies producing only low levels of low-order harmonics, and, in most countries, legislation requires a displacement power factor (cosine of the phase angle between input current and voltage) of at least 0.9.  While the Distribution System Operator (DSO) experts on the IEC committee took a lot of convincing (no change there, then), it has finally been agreed to replace the factor 30 x ? in Table 2 of IEC 61000-3-2 by a fixed factor of 27, and this change will in due course appear in the published standard.