Switching to a Better Switch

Innovative SuperSwitch provides smooth power transfer and addresses inrush current and phase problems caused by traditional static transfer switches (STS).

Governments and data centers have something in common; they both prefer  smooth transfers of power. In data centers the static transfer switch (STS) typically fills the vital role of transferring between the preferred and alternate power sources when necessary. STS-based thyristor technology is simple and robust, providing a reliable bridge between power sources and downstream power distribution systems.

That transfer, though, isn’t trivial.  A critical flaw in many STS designs can create high transient inrush – as much 11 times the rated current – in downstream transformers and devices. This inrush can degrade the power quality of the preferred source while creating intolerable forces in the transformer windings. Those forces can reach the short circuit rated value and last many cycles before they dissipate. The effects can also travel upstream. Further, the STS can ping pong back and forth between power sources overloading the UPS and eventually tripping circuit breakers resulting in server reboots and load drops.

An additional issue when switching between power sources is that their phase is going to drift over time,  complicating the task of the STS. Out-of-phase transfers are a concern that I hear about all the time from data center managers. They want better ways to smoothly switch between utility power, UPS, generators, and other sources regardless of how much the phase difference is.

Better STS technology

An innovative ABB solution is getting a lot of attention from data center managers. This new approach to STS technology solves both inrush and transfer-time problems. It’s based on what we call Real Time Flux Control™ for dynamic inrush restraint (DIR), the key technology in our SuperSwitch®4. This technology combines several engineering disciplines including magnetics, electrical, and embedded software.

We took advantage of the latest digital-signal processors and newly developed algorithms in order to stay ahead of power events. The technology monitors the condition of the transformer in real time processing, so the switch always has the flux data needed to instantly – within a single cycle – transfer the load when there’s a problem. The inrush value is lower than 1.2x the rated current, resulting in no damage to devices and exceeding the CBEMA and ITIC curve expectation.

Other STS approaches

The only other solution to switching problems is to move the transformer upstream from the STS. While effective, this solution is very costly because it requires additional transformers with the related maintenance and real estate costs.

Given the critical need for data centers to smoothly switch between power sources, equipment makers will continue to search for new solutions, possibly developing something very different than the current STS. For the foreseeable future, though, the STS is likely to be the technology of choice. The ability of the SuperSwitch 4 to smoothly transfer power to the loads, regardless of source-phase differences, makes this technology worth considering.

Data center managers interested in more detail about Real Time Flux Control, including formulae and waveforms, should take a look at ABB Review’s June 2015 article:  Preventing transformer saturation in static transfer switches (pages 17-26).

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About the author

Adil Oudrhiri

Adil is currently working in ABB Power Solution in Richmond VA as a Systems Engineer. He holds his Masters and Bachelor of science in Electrical Engineering from Montana State University in Bozeman. Adil has been involved for many years in the ground up development of multiple power quality and power conversion products. He is also a big soccer enthusiast.
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