The Transformer and Electrical Co Ltd,Constant Voltage Transformers, Harmonics, harmonic Cancellation, Neutral Current Trap, rectification, Transformers, Claude Lyons, Wilflo, TEC, Chokes, Reactors, CVT, Control, Power, Marine, Ships, Panel, Railway, Furn

Harmonic Cancellation Transformers

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 “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.

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. (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

QUALITY CATEGORY	TYPICAL EFFECTS AND PROBLEMS	MEASUREMENT METHOD	TYPICAL CAUSES	TYPICAL SOLUTIONS
Transients	Random equipment failures Computer data errors	Peak Hold Rise Time Pulse Duration	Lightning ESD Load switching	Surge Arrestors Ultra Isolation Transformers Line Voltage Conditioners Filters
Sags / Surges Slow > 100ms	Computer data errors Power supply failures	Peak / Trough Hold Pulse Duration	Load Switching Motor Starting	Voltage Stabilisers Ferroresonant Transformers
Dropouts/Surges 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 Voltages	Motor torque reduction Motor overheating Motor burnout Circuit breaker tripping Power supply failure	RMS vs Time Statistics Waveform 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) (L-N) (N-E)	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	kVA/kW kVAR	Motor Loads Lighting Systems	Power Factor Correction Capacitor Banks fixed/auto

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.

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.

PRODUCT INFORMATION

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 )