DRAFT    Rev 1 12/01/2011

DATA CENTER TRANSFORMERS (DCT)

The DCT are transformers primarily used to supply power to Information Technology Equipments (ITE). )  

  Requirements in Data Centers are characterized by high utilization (typically near 80%).  Efficiency at

these utilization levels is critical.  Data Centers also require the ability to perform without interruption.  

The ITE equipment is typically running 24/7 and supporting thousands to hundreds of thousands of

users.  Interruption of the equipment is very costly.  The cost for power in these facilities is very high

therefore they all have high voltage three phase inputs and they must have high power factor and

limited harmonics going back to the utilities.

DCT shall have following minimum features.

1) DCT shall be a Three Phase Dry Type Transformer with TWO or more shields.

2) DCT shall have K‐Factor rating as defined in UL standard 1561 and LISTED by NRTL. At 150 degrees C

Temperature Rise, K‐Factor rating shall not be less than

a) K‐13 for Transformers rated up to 300 kVA;

b) K‐9 for Transformers rated over 300 kVA and up to 750 kVA;

c) K‐4 for Transformers rated over 750 kVA.

3) DCT shall be provided with an Instruction Plate stating that DCT is specifically designed to supply

power to IT Equipments and it is unlawful to use DCT as a distribution transformer.

4) In addition to the above minimum requirements DCT shall have one or more following features

a) Magnetizing Inrush Current 4 times or less of rated full load current;

b) Rated Temperature Rise at full Load and at rated K‐Factor 130 degrees Centigrade or less;

c) Primary winding or Secondary winding or both windings have interwinding connection other

than DELTA or WYE;

d) TWO or more Transformers installed in the same enclosure;

e) Secondary phase to phase  RATED  NOMINAL Voltage other than 200 Volts to 218 Volts.

RATIONALE

1) All DCT are three phase. This requirement excludes single phase.
2) The DOE temperature rises are calculated at K‐Factor of 1. For higher K‐Factor transformer

the winding size increases rapidly. The winding losses in larger kVA transformers at higher K‐

Factor rating can be more than double than that at K‐Factor of 1. This increases winding

size, increases core size and increases cost much more than K‐Factor 1 transformers.

Customer will not buy high priced transformer if a lower cost transformer is available.

3) This to warn manufacturers and users against in appropriate use of the transformer.

4 a) To be cost effective, the Grain Oriented Materials (M6 and Better), have to be used at

higher peak flux densities than Non Oriented materials. The residual flux densities in modern

low loss steel can be as high as 80% of the Peak flux density in wound cores and up to 75% in

stacked cores. If we operate core at 17 kG we may get residual flux density of 14 kG to 13 kG. If

we use Non Oriented steel we to work with lower flux density to reduce losses and audible

noise. Further the residual flux density in Non Oriented cores is about 70% and core is always

stacked core design. The peak flux densities used in low inrush current transformers are from

12.5 to 11 kG in the leg and 9 to 11 kG in the yoke. The Yoke Cross section and Leg Cross section

is not the same. This arrangement cannot be done in Mitre cut or wound core construction. The

reason for increasing yoke section is to further help to reduce inrush current.  Clearly the

material used for the core and the winding is much more than that will be used by a transformer

which does not have low inrush current requirement. This increases the cost, size and weight of

the transformer.

4 b) Meeting the temperature rise of 150 degrees C with high K‐Factor load is difficult if not

impossible. The reduction of temperature rise below 150 C, requires bigger core and less

winding. This pushes best efficiency point towards higher load such as 50% or higher. The DCT

works at much higher load than 35% as the power density in the Data Center Installation

increases.  These transformers are not cheaper than garden variety LVDT.

4 c) IT Equipment generate undesirable high frequency harmonics. These harmonics have

detrimental effects on other Equipments and power systems in general. Utilities and industry

standards have limits as much harmonics can be transmitted back to utility. Some of the DCTs

are specified with winding connections on the secondary other than standard DELTA or WYE

connections. ZigZag and Double ZigZag are most common of these special connections. ZigZag

increases the winding losses and material cost by about 15% in addition to about 5% extra in

core losses.  

Double ZigZag is even more complicated and it works as a three winding transformer.

Two secondary windings are rated at about 60% each of Primary kVA rating. There is lot of

voltage between sections of the winding and lot more insulation, winding material, core

material and labor required to manufacture these transformers. They have more losses and

more cost, however, these disadvantages more than compensate in the overall performance of

the Data Center Installation by reducing harmonic induced losses and heating in other
Equipments, installation wiring and utility transformers.  These are very costly transformers and

nobody in right mind will use them in place of LVDT.

Final Remarks not part of the scope or definition.

    The DCT are normally installed in the standard enclosure. The size of the enclosure is

typically same from 50 kVA to 300 kVA. The Next size is from 500 kVA to 1000 kVA ratings. The DCT

may be supplied as core & coil assembly to eventual PDU (Power Distribution Unit) or DCT

manufacturer may build his PDU. PDU is very specialized equipment. The front is completely blocked by

input breaker/s (in case of dual input two input CBs), Output Panel Boards or output Breakers,

Metering, Remote Monitoring of each feeder on the panel board, TVSS ( Transient Voltage suppression)

etc. The PDUs can be installed in a line up next to each other with no cooling vents on the sides. The

transformer is therefore, cooled from back side cooling vents only without vents on the top cover. This

makes the design of the transformer much more conservative and very costly. Same DCT installed in a

LVDT cabinet with cooling on all sides can probably loaded continuously at 150 % load and if at is K‐

Rated it can take 200% load continuously. There some PDUs with two transformers in the same cabinet

side by side or second on the top of first transformer to save floor space in Data Center. There are new

developments in this field and the PDUs will be installed in shipping containers along with all Data

Processing Equipment to save the cost of a permanent structure. The life of a Data center is no more

than 10 years. The IT Equipment changes much faster than the transformer technology.   

Please note that there are some special exceptions in NEC for ITE Rooms.