TRANSFORMERS FOR NON-LINEAR LOADS

Non-linear loads generate high levels of harmonic currents. Typical non-linear loads include desktop computers, ac variable speed drives, inverters, and welders. Of these non-linear loads the main cause for concern is the switch mode power supply found in desktop computers and other office equipment.

When supplying power to these loads a special transformer design is necessary.

TEC Harmonic cancellation transformers utilise special winding techniques to minimise stray losses caused by the harmonic currents. A double sized neutral conductor is a standard feature designed to handle the excessive neutral currents found in non-linear loads.

TEC Harmonic cancellation transformers incorporate dual electrostatic screens to attenuate EMC noise and voltage spikes for a cleaner power supply.

K-FACTOR/LOAD TYPE

The amount of harmonics produced by a given load is represented by the term â€œKâ€ factor. The higher the â€œKâ€ factor the greater the harmonics present, hence the more harmonic current the transformer must be designed to withstand.

K-1 Resistance heating, motors, control transformers, distribution transformers.

K-4 welders, induction heaters. fluorescent lighting, solid state controls.

K-13 Telecommunications equipment, sub circuits in schools etc.

K-20 Main frame computers, variable speed drivesdesktop computers, data processing equipment.

HARMONIC EFFECTS UPON INSTALLATIONS

When linear and non-linear loads are connected to a common supply, harmonics generated by the non-linear loads are also applied to the linear loads.

There are several harmful effects harmonic currents can cause upon an installation.

The solution is to separate circuits supplying harmonic generating loads from those loads which are sensitive to harmonics.

BROWNOUTS RFI SAGS SPIKES SURGES TRANSIENTS

HARMONIC REDUCTION

It is impossible to predict the harmonic reduction because each installation will have a completely different harmonic profile.

There are three main types of harmonics that flow in three phase power systems these are :-

1. Positive Sequence

2. Negative sequence

3. Zero sequence

In the simplest terms, zero sequence currents are in phase with each other in all three phases and therefore add into the neutral wire return path. The worst of these are the 3rd harmonic (150Hz at 50Hz)

Positive and negative sequence currents have opposite rotational directions and do not flow in the neutral wire. The harmonics generated will also depend on the type of load. A 6 pulse rectifier will produce typically 30% to 60% (5th and 7th) current distortion whereas a 12 pulse will product only 15% (11th and 13th). Single phase power supply loads will produce a very high 3rd harmonic value typically 80% and can cause harmonic subtraction.

Note extra filters can be added to increase particular harmonic reduction.

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