7 Possible disturbances with electronic ballasts

The former of these two styles of drawing electric power, the direct rectification of the incoming AC, generates extreme periodic current peaks somewhere in the proximity of the voltage maximum, while during the rest of each semi wave no current flows at all (Fig. 6.1). This current waveform includes a high harmonic content, especially of the third and its multiples, which add up on the neutral instead of cancelling out and cause a bunch of problems that have recently been analysed and described in detail in various sources: Neutral overload, transformer overheating, substantial distortion of voltage waveforms if network impedances are high, and in TN-C resp. TN-C-S systems these permanent operating currents on the neutral also intrude into all earthed metalwork, including the screens of data lines. There they can cause an additional bunch of problems such as magnetic stray fields, corrosion of pipework and earthing electrodes and especially malfunction and damage of IT equipment. While these harmonic currents in modern office buildings originate from the multitude of PCs, their screens and peripherals, electronic ballasts below 25 W including CFLs, because of their limited use, contribute only a smaller fraction to this problem. However, operating all fluorescent lighting following this simple principle would be virtually impossible, for which reason the upgraded electronic ballast technique with electronic power factor correction (PFC, Fig. 6.2) was developed. One source says about 30% to 50% of the price for an electronic ballast is spent on avoiding disturbances. Most of this obviously goes into PFC – quite successfully, as a comparison shows (Fig. 7.1): The input current of a CFL without PFC, rated only 11 W, has approximately the same crest value as that of a ballast rated 58 W with PFC. The total harmonic distortion of the currents is 80% in the former case, but barely 19% in the latter. Although less than 12% were measured with a magnetic ballast, this value is low enough not to encounter any harmonics related problems.

Fig. 7.1: Comparison of CFL without PFC (left) to electronic ballast with PFC (right)

Fig. 7.2: 3 electronic ballasts of old design operated on 3 phases

Fig. 7.3: Resulting neutral conductor current of phase loads as in Fig. 7.2

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