ADB EURODIM TWIN TECH, Installation Manual

Page: 55 / 166

EURODIM TWIN TECH
www.adblighting.com Installation manual - Page 53
Version 1.2
12.6 Main transformer, cables, switchgear, busbar systems
The main transformer, the switchgear and the supply cables to the dimmer cabinet carry
these non-sinusoidal currents.
This is valid for all phase-control dimmers (thyristor, triac, and some transistor dimmers),
independent of brand and manufacturer.
A 'classic' recommendation is
- Inform the (future) supplier of the power transformer about the presence of thyristor-
controlled dimmers. He will select a suitable, type taking into account the extra losses
due to the harmonics and the increased rms current in the N
- over-size the kVA rating of the main transformer; e.g. by 50% above the expected
max. load.
12.7 Voltage distortion and 'short-circuit voltage' of the main transformer
Another aspect is that the harmonic currents cause distortion of the sine wave voltage. And
with a distorted voltage wave-form, the dimmer electronics have a harder time to find the
'zero crossing'; unstable 'zero crossing' means that the light levels may flicker in some
instances.
In more technical terms: the combination of the source impedance of the supply system
(transformer, switchboard, busbars, and supply cables) with the harmonic currents will distort
the voltage at the input terminals of the dimmers.
Main transformer
– recommendation
The transformer plays an important role. Its output voltage will show less distortion of the
sine wave if the transformer's series impedance is low, i.e. if the transformer has a low 'short-
circuit voltage'. We recommend that the main supply transformer should have a 'short-circuit
voltage' of max. 4%. Again, this recommendation is valid for all phase-control dimmers
(thyristor, triac, and phase-control transistor dimmers), of all brands and manufacturers.