Isolation Transformer Cubicle

K – rated – Isolation Transformers , Cubicles and Distribution Consoles

Any computer Installation with number of Computers and peripherals generate  large value of  harmonic currents basically for the simple reason that they all have SMPS  power supplies at  the input. SMPS  power supplies have large nonlinear current demands with crest factors in the range of 5 -7.

These harmonic current loads can causes problems in the power system and transformers, such as overheating and premature failure. Conventional Transformers are designed for linear loads and merely de – rating them does not solve the problem.

What are harmonic currents? Harmonic currents are those which have frequencies that are multiples of  the fundamental (power supply) 50Hz. The harmonic currents are superimposed on the fundamental current resulting in non- sinusoidal current  waveforms associated with nonlinear loads.

What is K- Rating? Underwriters laboratory (UL) has  devised a rating system to indicate the capability of  a transformer  to handle harmonic loads. This is known as it’s K- rating. The ratings are described in UL1561 and are known  as transformer

K- factor. K- factor is a weighting of harmonic load currents according to their effects on transformer heating, as derived from ANSI/IEEE C57.110. K-factor of 1.0 indicates  a linear load(no harmonics).

The higher the K- factor the greater the harmonic heating effects. K-factor =(l ) xh,  Where l  is the  load  current at harmonic – h, expressed per- unit basis, such that the total RMS current equals one. This means that the higher the harmonic number its contribution to K factor is more as it is  squared in the formulae.

Design factors in K – rated Transformers

Double Sized Neutral: Triplen harmonics  which form the major part in non linear loads are of zero phase sequence variety and hence get cumulated in the neutral conductor, unlike fundamental currents getting cancelled. This Causes excessive heating of neutral conductors and terminals. For K- factor transformers UL  requires that the  neutral terminal  and connections are sized to accommodate twice  the rated  current than phase conductor.

Multi core conductors: With increasing frequency of harmonic currents there is a skin effect of the conductor cross section which results in the current concentrating on the surface of the conductors. This results in reducing the effective cross section conductors and causes additional heating of the transformer winding and cables connecting the system. Datsons make k- rated transformer use  electrolytic grade copper – multi – core conductors  for winding which virtually nullifies the skin effect and additional losses in the windings. It also adopts rectangular conductor cross section with large aspect ratio such as 5:1 resulting elimination of the skin effect further.

Low Flux Density magnetic design: In order to compensate and provide for core losses due to higher harmonics currents K – rated transformers are designed at lower flux density than conventional transformers. Essentially all K- rated transformers adopt High permeability low loss CRGO cores and for larger ratings use mitred joint core assemblies.

Built- in Derating: For transformer handling k- rated loads ANSI /IEEE C57.110 recommends method to derate them for different Harmonic load profiles. Transformers directly supplying single – phase power supplies used in all computers and  peripherals with SMPS at power input with K factor of 20, require derating of 20-35%

Harmonic Mitigation: K rated transformer design’s primary objective is to handle K rated loads and not to mitigate harmonics But transformers having Delta Star configuration do mitigate triplen harmonic currents in the load currents as they circulate in the Delta primary winding and do not reflect on source side.