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When I first heard this I must be honest - I did not know what these guys were talking about. "6 and 12 pulse feeds to UPS units" sounded like some controlling mechanism, but how was this affecting power quality? It has nothing to do with any controlling mechanism at all. It is simply the rectification methods used! When I first started with electronics the two methods were known as "3-phase full-wave" and "dual 3-phase full wave" rectification techniques, and have been used by the rail industries for years as the latter gave them a smoother output. I don't know when the names were adopted but they merely represent the number of rectification phases throughout one cycle. Still lost? Let's explain with the aid of a single phase circuit first. If we feed a voltage to a rectifier we will see a waveform as shown alongside. This would therefore, in some power engineering circles, be referred to as "1-pulse". If fed into a full-wave rectifier the output would now be available on both halves of the cycle and therefore, you guessed it, "2-pulse". As there are 3 phases to play with one can now add a few more diodes and have 3 x 2 pulses, or "6-pulse" rectification. Although it is possible to half-wave rectify a 3-phase system thus landing up with 3-pulse, it is unwise as half-wave rectification does magnetically unbalance a transformer which leads to early hysteresis. As rectifiers are being used the current curves are exceptionally non-linear and huge distortions, and therefore harmonics, are introduced on the system. The level of distortion will vary dramatically with factors such as source impedance, the level of rectification smoothing, and power output playing a huge part. But there is a rather strange trick that can be pulled with a 3-phase supply being the shifting of phase through a delta to star transformer. The resultant shift is 30°, and by supplementing the original set of rectifiers with another set fed from the output of this transformer we land up with a "12-pulse" output. Two advantages exist. First the ripple voltage (the lowest to highest points on the output waveform) is down to 3.4% at a ripple frequency of 600Hz (as opposed to 14.3% ripple at 300Hz for 6-pulse). This makes any smoothing required relatively inexpensive. The second advantage, and the more important one, is the distortion introduced onto the incoming supply is reduced by a factor of 4. Should the distortion have been 6% under 6-pulse, this is now reduced to 1.5% with a 12-pulse system.
© 23.06.01 |
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