7.1 Emission of disturbances

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

This, however, gives rise to another type of disturbances. As the pulse width modulation on the input side »chops« the incoming current into many »thin slices«, this is equivalent with releasing a high frequency current into the network, which is largely attenuated, but not completely extirpated by a capacitive filter on the input side of each electronic ballast (Fig. 7.4). So the possibility of conducted as well as transmitted disturbances remains. It has happened, for instance, that the frequency was 77 kHz, equal to that of the Frankfurt long wave transmitter which broadcasts the time signal of the Braunschweig atomic clock. The interference caused radio controlled clocks to malfunction inside buildings equipped with these ballasts. Typically these disturbances occur at two different frequencies, for obviously the HF transformer for generating the lamp current and the electronic power factor correction work at different clock frequencies (Fig. 7.5): The former is responsible for the radiated and the latter for the conducted disturbances. Moreover, this high frequency, since it is not sinusoidal, for itself consists of a theoretically infinite spectrum of harmonics, so that the highest frequencies occurring nearly reach right up into the megahertz range. In the meantime standards have been released to restrict the maximum permissible levels of such disturbances. Unfortunately the tests according to these standards are carried out individually in a lab on one sample of the ballast in question, while in the field some hundreds or even a few thousands of those are operated on one site, so the disturbance levels to some extend add up. Adding to this, there is a frequency gap in the standards, leaving a certain range of frequencies without any limitations. Witty engineers now design their appliances in a way as to displace all disturbances into this blank, just as if only standards did matter and disturbances did not. Lots of interferences have so far been reported informally but on account of the special market structure they never ever appear in print.

Fig. 7.4: Input current of an electronic ballast at different time resolutions

Fig. 7.5: Frequency spectrum of the same electronic ballast

Inspectors and official surveyors repeatedly report about an oscillation of the voltage amplitude in installations where there is a large coverage of electronic ballasts. At the feeding point the same can be observed with the current but with opposite phase, so this current variance must be the cause for the voltage variance. The inspectors speak of frequencies up to 3 Hz but usually only 0.3 Hz or often even a lot less than that, one period per 30 seconds is typical. They see a coherence with the usually capacitive power factors they find in these installations, while this cannot really be the cause. Truly electronic ballasts usually have a slightly capacitive power factor (Fig. 7.1), and truly installations are usually not metered or monitored, so nobody realizes the power factor correction is no longer a correction but the opposite of that and should be switched off or stepped down, but an oscillation at such a low frequency would require tremendous lots of both capacitance and inductance. Rather, the automatic output power control of the ballasts may be the cause: When there is a voltage sag for some reason, the input current into the ballast must be increased to keep the output power stable, and if the share of the total power that goes into such lighting equipment is high enough, this will increase the sag palpably. The voltage will continue to drop, and the overall current will go on rising until the input current increase of the ballasts is offset by a decrease of the input currents to some loads where it decreases as input voltage decreases. This is even indicated on the rating plates of electronic ballasts (Fig. 7.6). Now the process is inversed, and a voltage swell starts. The surveyors say the problem is usually solved by replacing failing electronic ballasts with magnetic ones without adding any compensation capacitance. When the share of magnetics reaches about 1/3 not only the electronic ballast failures stop but also the voltage oscillation ceases. So they think the shift of the power factor slightly into the inductive range was the solution, while the true explanation is probably that the behaviour of lamps with magnetic ballasts is inverse to that of electronic ones: Input current, both the active and the reactive share, drop over-proportionally as input voltage decreases. A linear drop might not suffice as an offset to stop the oscillation.

A high frequency expert reported he had tested some electronic ballasts and found out that their HF emission frequency also varies. It periodically hops to and fro between at least 2 frequency bands, obviously deliberately, by design. The background is probably that the relevant standard allows a certain amount of radiated energy at a certain frequency band, integrated over a defined period of time. So this standard is dodged by dispersing the disturbance across a wider range of frequencies. Unfortunately the expert was not able to say which standard it is that defines these values and procedures.

In another case the surge diverters in a brand new supermarket kept on failing. The whole market was equipped with electronic ballasts and the feeding lines with a properly designed overvoltage protection, comprising coarse, medium and fine protection downstream. However, the protective devices at the last stage, the fine protection, continuously failed, looking charred after failure, without any tripping of the coarse and medium stages. So the protection would have to be built up the other way round, coarse indoors and the fine stage upstream, since the disturbance came from inside the installation in this case.