K-FACTOR TRANSFORMERS

This type of transformer is designed to support an additional load, represented by the high level of harmonic distortion existing in the installation. It has galvanic isolation between primary and secondary, with electrostatic shielding. It has special constructional characteristics that protect the transformer from current harmonics that cause losses and overheating in the windings of the transformers. The K-factor is a constant that indicates the transformer capacity to supply non-linear loads (for example, induction furnaces, motor speed variators, rectifiers, inverters, nobreaks, etc.) and support harmonic currents without exceeding its operating temperature.

Transformers with K-factor Classification

Nowadays, in industrial
workplaces, the proliferation of solid-state devices (lighting reactors, motor
drive and control systems, communications equipments, and other DC motor loads)
have created some relevant problems for specification engineering, contractors
and business owners. The non-linear nature of the power supplies of models with
switching by solid state systems, generate harmonic currents which, in turn,
generate additional losses, which make transformers (some of these are large
losses within the windings and some are closer to the surface) and the system
neutrals overheat and be destroyed.

Effect of harmonics on
dry-type transformers

There are several
situations that may create conditions for harmonic problems in transformers,
including the equipment addition to an existing electrical system, or the
addition of enhancement facilities or expansions to an existing power source.
Transformers with K-factor specifications are designed to reduce the heating
effects of harmonic currents created by non-linear loads. The K-factor rating,
assigned to a transformer, is an index of the transformer's ability to support
a harmonic index in its load current, remaining within its operating
temperature limits. A K-factor specific rating indicates that a transformer can
supply, in addition to the nominal KVA load output, a load of a specified
amount of harmonic index. In 1990, UL (Underwriters Laboratories) developed the
method of calculating K-factor rating to evaluate the ability of transformers
to support the harmonic effects. The K-factor does not mean that the
transformer can eliminate the harmonics. The UL test is oriented to the heating
of the windings, due to the general non-linear loads and overheating of the
neutral conductor.

There are two methods
of calculating the K-factor:

- The UL method

- The standardised
method

The UL method is based
on the “rms” effective rated current of the transformer. It is generally used
when the effective current is measured or measurable, and is defined as:

Where:

h = harmonic order;

Ih (pu) = rms current
of the harmonic expressed in pu (per unit) of the effective rated current of
the transformer.

The standardised method
is based on the fundamental load current. Harmonic measurements are often made
with a harmonic analyzer. Most harmonic analyzers have the output responses of
the harmonics in pu (values per unit) of the fundamental current. In
consequence, the method would be used. The standardised method is defined as
follows:

Where:

Fundamental current in
pu (the 1st harmonic = 100%);

Here is an example of
the two methods for the same harmonic spectrum of data. For example, we assume
that the measures were taken to obtain (pu):

Up to now, industrial
literature and comments refer to a limited number of K-factor ratings: K-1,
K-4, K-9, K-13, K-20, K-30, K-40.

In theory, a
transformer could be designed for other K-factor assessments among these
values, as well as for higher values. The commonly referenced classifications
are in accordance with ANSI/IEEE C57.110-1986 as follows:

K-1: This is the
assessment of any conventional transformer that is designed to support only the
heating effects of normal losses and additional eddy losses, resulting from
60Hz with the transformer loaded with sinusoidal current. Such a unit may or
may not be designed to support the additional harmonic heating in its load
current.

K-4: A transformer with
this rating was designed to provide nominal KVA, without overheating, to a load
provided by 100% of normal 60Hz frequency, sinusoidal current in the
fundamental, and:

- 16% of fundamental
current as the 3rd harmonic current;

- 10% of the
fundamental current as 5th;

- 7% of the fundamental
current as 7th;

- 5.5% of the
fundamental current as the 9th;

- Minor percentages
through the 25th harmonic.

The number
"4" indicates its ability to support four times the eddy current
losses of a K-1 transformer.

K-9: A K-9 transformer
can support 163% of the harmonic load of a K-4 transformer.

K-13: A K-13
transformer can accommodate 200% of the harmonic load of a K-4 transformer.

K-20, K-30, K-40: The
highest number of each of these K-factor ratings indicates the ability to work
with successively larger amounts of harmonic load indexes without overheating.

The table below gives
the example of K-factor loads

LOAD – K-FACTOR

Lighting with discharge
lamps - K-4

UPS with optional inlet
filter - K-4

Welding machines - K-4

Induction heating
equipment - K-4

PLCs and solid-state
controllers (other than variable frequency drives). - K-4

Telecommunication
Equipment (for example, PBX) - K-13

UPS without inlet
filters - K-13

Supply of general
multiwire receptacles, in areas with health-care instruments and school
classrooms, etc. - K-13

Circuit sources with
multiwire receptacles for inspection and testing equipment in productive
sectors or production lines - K-13

Loads of computer
servers (Mainframe) - K-20

Solid-state drives for
motors (variable frequency drives) - K-20

Supply of circuits with
receptacles in important safety areas and rooms for hospital surgery and
recovery - K-20

* Re-written with
permission from EDI Magazine

The K-factor must be
clearly marked on the identification plate of the transformer.

Technical information:

There are several situations that may create conditions for harmonic problems in transformers, including the equipment addition to an existing electrical system, or the addition of enhancement facilities or expansions to an existing power source. Transformers with K-factor specifications are designed to reduce the heating effects of harmonic currents created by non-linear loads.