The 'HDC' transformer: A liquid-filled transformer designed specifically for application within large data centers

Let me to introduce to you a new liquid substation transformer that I believe solves all of the problems we've been discussing for so many weeks.


If you've been following this series over the past five months, you'll know that there have been three major themes: 

Braided Flex Links Connect Transformer to Secondary Switchgear

1. Dry-type medium-voltage substation transformers have had a long history of catastrophic failures within data centers, due to switching-induced transient voltages when switched by upstream vacuum circuit breakers. I'm personally aware of dozens of such failures, and I'd have to believe there have been dozens more I'm not specifically aware of. Many of the failures have been well-publicized and thoroughly studied, and some solid research by IEEE committees has resulted in guidelines for mitigating techniques, published in IEEE Standard C57.142-2010.

2. Liquid-filled transformers, in contrast, seem far less susceptible to this problem. In fact, I'm unable to find a single documented case of a winding failure in a liquid transformer installed in a data center anywhere that could be attributed to this problem.


3. As data center loads have increased so dramatically over the years, and as 400 Y/230 V critical load distribution has gained popularity, the need to apply the "loadcenter substation" concept in data center power systems has become much more pronounced, in order to minimize the lengths of large cross-section secondary feeders and reduce the large tonnage of copper being presently installed in data centers (the title of this overall series of blogs is "Cut the Copper").

By and large, data center owners and their consulting engineers have been reluctant to use liquid-filled transformers inside their buildings. Many—perhaps even MOST—data centers used liquid-filled transformers, but generally installed outdoors. Relatively few liquid transformers have been used indoors, even though transformer manufacturers have offered very safe liquid transformers for that purpose for at least 10 years now. I'm surmising that this is due to a lingering distaste for liquids inside buildings after the production of PCBs was banned by the U.S. EPA more than 30 years ago, and companies were forced to spend billions of dollars in replacing and retrofitting Askerel-filled transformers.

3 MVA “HDC” Substation, 34.5 kV Primary, 200 kV BIL, Close-Coupled to Schneider Electric Secondary Switchgear. (Photo Courtesy Of Terremark Worldwide – A Verizon Company)

But now, let me to introduce to you a new liquid substation transformer that I believe solves all of the problems we've been discussing for so many weeks. I think that this new product is one of the most significant advancements in data center power infrastructure in the last 25 years.

A relatively new liquid transformer design, called HDC, for "Hardened Data Center Transformer," has been developed by Cooper Power Systems. I worked closely with Cooper for several years in refining this new design, in order to develop all of the features I wanted for my clients. It is the first liquid-filled substation transformer designed specifically for application inside data centers.

The secondary of the transformer is close-coupled directly to the secondary switchgear, and a set of very short braided flex links can replace literally 50 or more tons of copper in secondary feeder cables.

The HDC transformer is Envirotemp FR-3 fluid filled, with a compact footprint, and secondary flange/throat for coupling directly to secondary switchgear. Extra BIL (at least one level above ANSI/IEEE standard), is built into the primary windings as standard practice, and all primary leads within the tank are triple-insulated and carefully routed and securely spaced with correct isolation distances in between them.

In this design, it's easy to specify a higher BIL on the primary windings of, say 125 kV or 150 kV BIL, even on 15 kV-class units (more than double the IEEE standard of 60 kV for 15 kV dry-type transformers). I believe that his, in and of itself, would eliminate about 95% of the failures we've been discussing for so many weeks now.

2.5 MVA “HDC” Substation, 12.47 kV Primary, 125 kV BIL, Close-Coupled to IEM Secondary Switchgear (Photo Courtesy of Data Foundry Data Centers)

But beyond that, this design offers an optional feature that addresses switching-induced transients head-on—by preventing their very occurrence in the first place—by adding capacitance directly into the primary windings.

2.5 MVA “HDC” Substation, 24.9 kV Primary, 150 kV BIL, Close-Coupled to Schneider Electric Secondary Switchgear (Photo Courtesy of Sentinel Data Centers)

This feature operates very much like a "snubber," but without the need for any extraneous components like resistors or capacitors ... the capacitance actually becomes part of the primary winding itself. I believe that this technology has the potential to make the serious problem of switching-induced transformer failures a thing of the past, even if the transformer is fed by very short cable primary cables or by direct bus connection to the upstream vacuum breaker.

This new transformer is available with a full assortment of accessories, including internal liquid-submersed vacuum fault interrupter (VFI), 600 A load-break (under-liquid) incoming line switches or four-position loop switch, and viewing windows of the switches for visual confirmation that the transformer is isolated from primary voltage.

I've commissioned about 40 of these HDC transformers in indoor unit substation configurations at seven different data centers in the past three years, in ratings from 1000 kVA through 5000 kVA, with the majority of them in a 3000 kVA rating. No problems have been experienced in any of them so far, now at about 100 aggregate unit-years of operation.

The photos to the right show a typical 35 kV-, 25 kV-, and 15 kV-class unit, top to bottom.

The under-liquid VFI provides superior overcurrent protection, and the under-liqui

Under-Liquid Incoming Line Loadbreak Switches Replace Primary Load Interrupter Air Switches in Zero Footprint and Low Cost

d switches eliminate need for primary air switches generally used with dry-types, but without adding any size to the unit, because they are installed inside the tank.

These units have tested with very high efficiencies, typically in the range of 99.3% to 99.4% for most ratings at 100% of rated load, and around 99.7% at 50% loading.

With the new "voltage suppressor" feature in the design, this transformer seems virtually indestructible from any system problems it might encounter—from overloads, to secondary faults, to switching-induced transient voltages on its primary. It doesn't merely protect itself against switching induced transient voltages, but ELIMINATES them. 


And, these units are uniquely and extremely QUIET in their operation. In a typical installation inside an electrical room of a typical data center having typical ambient noise levels, they are virtually inaudible. More about this great new transformer next week.

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