K-Factor Transformer.

A K-Factor transformer can be auto-wound, double wound, single phase or three phase. It is simply one that is designed to withstand the effects of harmonic rich loads currents without overheating. In basic terms, the higher the K-Factor, the greater the level of harmonic load current that the transformer is able to handle.

To achieve a cost effective solution, the design of a K-Factor transformer not only allows capacity to accommodate the self heating effects due to the harmonic currents, principally within the conductors of the transformer, but also seeks to minimise that heating by the choice of winding conductors and their arrangement.

Harmonic Mitigating Transformer.

A harmonic mitigating Transformer (HMT) not only uses the techniques employed in the K-Factor transformer but goes further by the use of winding arrangement and or connection to bring about the cancellation or mitigation of some of the effects of the harmonic currents. To do this requires the use of more than one phase current and so HMT’s are usually three phase.

One of the major weaknesses of the three phase K-Factor transformer, which is typically a delta primary to star secondary, is that where triplen harmonics are present they appear in the primary delta winding as a circulating current proportional to the balanced portion of the triplen harmonics.

Circulating Currents

Such circulating currents can cause significant problems in poorly designed transformers. The weakest point in most transformers with tappings is the tappings themselves. In the case of a delta / star transformer as described above with tappings in the delta winding, if the tappings are only rated for the nominal fundamental current rating it is likely that in operation they would overheat due to the effects of the circulating current. It is for this reason that some client specifications state that tappings must be sized at 1.5 times their nominal current rating.

An HMT avoids this problem by the use of an interconnected star secondary rather than the basic star connection of the typical K-Factor transformer. The interconnected star or zig-zag winding as commonly known has two major benefits:

The connection brings about the cancellation of the balanced portion of triplen three phase line bourn harmonic currents.

The connection has no phase shift between primary and secondary which makes this arrangement ideal for an isolation transformer, where a by-pass facility is to be provided for security of supply, such as in Power Distribution Units (PDU’s) used in Data Centres.

The HMT utilises the phase shift between the sections of the interconnected zig-zag windings to create fluxes which cancel each other out.

In an HMT transformer the balanced portion of triplen harmonics flow in the secondary but are cancelled out before reaching the primary therefore the HMT transformer does not suffer from the problem of additional heating within the primary and any taps, as the conventional delta / star K-Factor transformer does.

In operation, on a like for like basis, an HMT transformer will normally operate with lower losses, in the primary winding than its K-Factor counterpart due to its ability to cancel triplen currents, whilst the K-Factor transformer circulates them. This has the potential to provide energy saving for the client, a benefit which is often not appreciated.

PDU Power Source

Generally the power source for PDU systems is of high quality having dedicated supply transformers. The supply is often supported by generators that have control systems to stabilise the voltage. However if the PDU transformers are specified with tappings and by-passing facilities are provided, circulating currents can be set up through the input circuit breaker, the transformer, the output circuit breaker and the by-pass switch, when closed, if the tapping selected alters the primary voltage relative to the secondary. In an extreme case this could result in spurious operation of circuit breakers during a by-pass operation, which is undesirable.

View our PDU (HMT) Transformer product.