Attributes of a generator-friendly UPS

Load transfer is an important factor that has been given little attention until recently

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Load transfer is an important factor that has been given little attention until recently.

In the August issue of Cabling Installation & Maintenance, an article titled "A generator-friendly uninterruptible power supply" (page 37) was written about the generator-friendly attributes of a UPS. There is a bias nature to the article, which did not fully examine or clearly explain all of the attributes generators seek in the loads they protect. These areas of bias are:

  • Minimizing harmonic effects on generators;
  • Neglecting to mention the impact load transfers have on a generator;
  • Proposing that SCR (silicon-controlled rectifier) technology with an input filter is an equivalent and sometimes better alternative to IGBT (insulated-gate bipolar transistor) technology.

THD and input filter factors

The article makes some very good points regarding the need of a generator to have a low current total harmonic distortion (ITHD) load. But typical double conversion systems range anywhere from 30% to 35% ITHD on a fully loaded system having a 6-pulse rectifying circuit on the front end. If you choose to lower the ITHD of a system, an input filter needs to be added. By adding this filter, you introduce another component into the system, ultimately having an adverse effect on the system's overall availability. With the delta conversion on-line topology, you do not have to add an input filter to minimize the reflected harmonics because the topology inherently does this in its design.

At the right are three input waveforms. The top figure is a double conversion system with a 6-pulse rectifier. The second is a double conversion system with a 12-pulse rectifier and an input filter. The third is a delta conversion on-line system with a patented IGBT front end. All UPSs have been loaded up to 50% of their capacity with a non-linear load.

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In these three input waveforms, the top figure shows a double conversion system with a 6-pulse rectifier, the middle one showing a 12-pulse rectifier and an input delta, and the bottom figure showing a delta conversion online system with an IGBT front end.
Click here to enlarge image

As you can see, there is no need to "marginally" oversize the generator that feeds the delta conversion on-line UPS. That's because the reflected harmonics are below the 5% ITHD the article references as an acceptable tolerance for a generator.

The article also suggests that the load the UPS is protecting is highly harmonic; therefore, if the UPS goes to bypass, the generator will still have to contend with the highly harmonic sensitive electronic load. Right? Wrong. Modern IT loads, especially the type found in data centers, consist of equipment that is power factor-corrected for distortion (ITHD) and displacement. So, if the UPS does not have a power factor-corrected front end, the UPS becomes the weakest link and not the load. In this scenario, the generator has an easier time with the load than with the UPS.

Loading step-by-step

Load transfer is an important attribute, which has been given little attention until recently. To your right, you will find a comparison on how a typical SCR front end draws cur rent as it comes out of an on-battery state, and in the following figure, how an IGBT front end-found in all delta conversion on-line systems-draws current as it comes out of an on-battery state.

By comparing the figures, you can see the sharp curves of the SCR front end relative to the more gentle, linear curve of an IGBT front end. As an analogy, if you are loading a truck with potatoes, would you rather throw a 200-pound bag, or 5-pound bags thrown 40 times (assuming both ways will load the truck in the same amount of time)? If you had to load that truck eight hours a day, five days a week, you would opt for the second scenario. This is precisely how a generator would choose to be loaded-not with a huge inrush current followed by two step loads as seen in the figure Typical Load Transfer Of an SCR Front End, but rather in a very controlled linear way as seen in Typical Load Transfer Of an IGBT Front End.


There is some truth to IGBTs con suming more energy than an SCR rectifier. But when implemented in the right UPS topology, such as delta conversion on-line, the losses are negligent relative to the overall ef ficiency gains. In general, the IGBT losses, due to switching, are relatively inferior when compared to the gains made by the delta conversion on-line topology.

The voltage ripple created by SCRs is greater then those created by IGBTs; therefore, batteries on IGBT circuits last longer because they generate less voltage ripple on their UPS' DC bus. Also, SCRs create commutation spikes that affect generators, especially when the SCRs are creating double or multiple zero crossings.

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In this load transfer comparison, the figure at left shows how a typical SCR front end draws current as it comes out of an on-batter state, while the other figure shows the process for an IGBT front end.
Click here to enlarge image

The only way SCRs can attempt to approach the characteristics of IGBTs is through the use of an input filter. But as mentioned before, input filters create an additional component that can fail and drive down availability. Furthermore, an input filter typically is tuned to a typical load characteristic. When the load characteristic changes, however, the input filter starts to lose some or all of its benefit.

Finally, you need to look at the input filter's impact on the rest of the building's electrical distribution. For instance, some input filters will actually have a negative impact on some power factor-correction capacitors that exist closer to the service entrance, otherwise known as "upstream" from the UPS.

So, questions to ask when choosing a UPS for your generator should include:

  1. How does the UPS transfer from an on-battery state to an on-line state?
  2. Is the input ITHD of your UPS lower then your data center's ITHD?
  3. Which solution gives you the most value with the least amount of components, thus increasing your overall availability?

Michael Proffittis product marketing manager for American Power Conversion, W. Kingston, RI (

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