Harmonic Cancellation Transformers

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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.


The amount of harmonics produced by a given load is represented by the term “K” factor. The higher the “K” factor the greater the harmonics present, hence the more harmonic current the transformer must be designed to survive.

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 drives, sub circuits with loads
    of data processing equipment, desktop computers.


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. (see table below)

The solution is to separate circuits supplying harmonic generating loads from those loads which are sensitive to harmonics (see table below)


BROWNOUTS       RFI             SAGS              SPIKES            SURGES       TRANSIENTS


Problems / Solutions

Transients Random equipment failures

Computer data errors

Peak Hold

Rise Time

Pulse Duration



Load switching

Surge Arrestors

Ultra Isolation Transformers

Line Voltage Conditioners


Sags / Surges

Slow > 100ms

Computer data errors

Power supply failures

Peak / Trough Hold

Pulse Duration

Load Switching

Motor Starting

Voltage Stabilisers

Ferroresonant Transformers


Fast < 30ms

Lighting flicker

Computer data loss

Random equipment reset / errors

Peak / Trough Hold

Pulse Hold

Load Switching

Fuse Clearing

Arc Furnaces

Electronic Voltage Stabilisers

Ferroresonant Transformers

Mains Voltage Compensators

Line Voltage Conditioners

Under / Over


Motor torque reduction

Motor overheating

Motor burnout

Circuit breaker tripping

Power supply failure

RMS vs Time



Peak / RMS

Industrial Sites

System Overload

Load Shedding

Tap Changing

K-Factor Transformers

Ferroresonant Transformers

Mains Voltage Compensators

Voltage Stabilisers

Voltage Flicker Lighting problems / flicker

Transformer noise

Monitor screen wobble

Flicker Meter- Magnitude/Frequency Modulation Frequency Tap Changing

Load switching

Load Variations

Electronic Voltage Stabilisers

Ferroresonant Transformers

Harmonic Current Distortion 3 Phase Neutral cables overheating / fires

Transformer burn outs

High neutral to ground voltages/circulating currents

THD Spectrum Analysis (3 Wire / 4 Wire ) Non Linear Loads

Variable Speed Drives

System Resonance

K-Factor Transformers

Passive Filters

Active Harmonic Conditioners

Harmonic Current Distortion Single Phase Transformer overheating / fires

Monitor screen wobble

THD Spectrum Analysis Non Linear Loads

Variable Speed Drives

Passive Filters Series/Parallel

Ferroresonant Transformers

Active Harmonic Conditioners

Harmonic Voltage Distortion Random equipment resets / crashes

Clock problems

Synchronising problem due to to multiple zero crossings

THD Spectrum Analysis                  (L-L)



High levels of non linear current

Variable Drives

System Resonance

High Impedance

Electronic Voltage Stabilisers (Waveform correction option)

Active Harmonic Conditioners

Interruptions and Power Failure Black outs

Data loss

Production line down time

Duration against time Fuse fault clearing

Load Shedding

Auto reclosers

Uninterruptible Power Supply

Back up generator (local)

Bad Power Factor High maximum demand

Over sizes supply



Motor Loads

Lighting Systems

Power Factor Correction

Capacitor Banks fixed/auto


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.

In general the best case harmonic reductions that can be expected are :-

Harmonic No Phase sequence Harmonic reduction
3rd Harmonic Zero sequence 40-60% (Neutral current)
3rd Harmonic Zero sequence 20-30% (Line current)
5th Harmonic Negative sequence (10-15% Subtraction)
7th Harmonic Positive sequence (10-15% Subtraction)

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


DELTA / STAR -30 deg Phase Shift

DELTA / STAR +30 deg Phase Shift

DELTA / Interconnected Star 0 deg Phase Shift

DELTA / Interconnected Star + / - 15 deg Phase Shift

NEUTRAL CURRENT TRAP ( 3rd Harmonic Rejection Transformers )