Tendulkar DRAFT     11/23/2011

DATA CENTER TRANSFORMERS (DCT)

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

DCT shall have following minimum features:

1) DCT shall be a Three Phase Dry Type Transformer.

2) DCT shall have a K‐Factor rating as defined in UL standard 1561 and LISTED by NRTL.  The 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 that 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 125 degrees Centigrade or

less;

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

other than DELTA or WYE.

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 inappropriate 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
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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.

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%
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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.