Six-Pulse UPS vs. Twelve-Pulse UPS

In a 6 Pulse UPS, there will be harmonics from 5th onwards. And in a 12 Pulse Drive, it will be from 11th onwards - PROVIDED the Input supply to the UPS is delivered through a three winding transformer, where the primary can either be Star or Delta and the one secondary will be Star and the other secondary will be Delta. Each of the seconadry is connected to each of the two 6 Pulse Rectifiers which comprise the 12 Pulse UPS.

If you don't do this there is no point in going for a 12 Pulse UPS.

To put what electricaexpert65 said in layman's terms (if I may be so bold):

The difference has to do with the "Total Harmonic Distortion" that the UPS will create on the LINE side, which may affect other equipment or wiring in your facility.

Power electronic equipment, such as UPS and VFDs, consume electricity in "gulps" rather than continuous smooth sine waves.These gulps of power represent "harmonics" on top of the nice 50 or 60 Hz sine waves the utility is supplying to you. This means that power is being consumed in multiples of the fundamental frequency. So for instance if you have 60Hz power, that is the "first order" harmonic, or the "fundamental". Power consumed at a 2nd order is being pulled at 120Hz (60 x 2), 3rd order is 180Hz etc. etc., you get the picture. Even order harmonics on a poly phase (3 phase) system cancel each other out from phase to phase, but odd orders do not and the sum total of all of the different odd orders put together is what we call THD. In particular though, 3rd order odd harmonics and it's multiples (called "triplens), such as 3rd, 9th, 15th etc. are additive from phase to phase, and that is where the biggest danger lies because they end up circulating in transformers and overloading neutral conductors, potentially causing damage or even fires inside the walls.

So end users must deal with this issue or subject themselves to serious risk. Some equipment manufacturers will take extra steps in their equipment design to mitigate harmonics up-front, in other words, help the end user deal with it by designing the equipment to mitigate its own harmonic contributions. One of those ways is to create a phase shift in the input power, which shifts more of the lower order odd harmonics into even orders or non-additive odd orders. So a standard power conversion scheme, one that does nothing to address this, is a "6 pulse" rectifier; it has 6 diodes in a bridge configuration to rectify the AC into DC for the UPS to use. Putting a transformer (as electricaexpert65 described) ahead of the UPS and using 2 rectifiers (2 x 6 pulses = 12 pulse) will reduce the THD, usually by 1/2. Further reduction can be done by using 18 pulses, but on UPS systems the cost tends to outweigh the added benefits. 18 pulse VFDs however are becoming more and more common because VFDs tend to have worse THD than UPS systems to begin with. This is sometimes called "point source THD reduction" because it reduces the contribution to the entire facility's THD issues at the point(s) where it is being created. This is widely considered to be the best approach.

So for you then, the 6 pulse UPS will be cheaper from an initial purchase cost standpoint, but you should add to that the cost of harmonic mitigation, i.e. more filters, reactors etc., when comparing it to a 12 pulse UPS. That cost must also include the services of a power quality engineer who can make a site survey and design a proper mitigation solution. Then in addition, every time you change something, that solution will need adjusted to suit the new conditions, with a new survey etc.

Or you can get cheap and take your chances.

http://www.pes.ee.ethz.ch/uploads/tx_ethpublications/nishida_PCIM07.pdf......this might help u.......

It is not the Output of Inverter for the user but it is the input to Rectifier to UPS which has some benenfits for the design.

Generally for non-tech it is:

6 pulse is generated by all power units Large-supply systems & Generators, ie 3Φ [3 phase] & design from this supply is quite simple as 99% designers large & small using it.

12 pulse is only for high-power systems as these are costly & has somewhat coplicated design

12 pulse : Additional 3Φs are generated by using a special type of transformer to produce 3 additional Φs at a phase-shift of 60 degree.

6 pulse rectifier has 3 full-wave rectififiers while 12 pulse has 6.

Filteration is better in 12 pulse.

Good information can be found in the "Chapter 7 - How to reduce harmonics by structural modifications in the AC drive system" (page 17 to page 23) from the ABB documents available in the link below:

http://library.abb.com/global/scot/scot201.nsf/veritydisplay/518a84b65bb2ff40c1256d280083acbd/$File/Technical_Guide6_EN.pdf

- MS

Thanks