Voltimum Expert Topic – Power Harmonics

What are Harmonics?

They are waveforms with frequencies that are multiples of the fundamental frequency

(Typically 50Hz in UK & Europe,

60Hz in North America)

Causes of Harmonics

Harmonic currents are caused by the use of non-linear loads:

– Switched mode power supplies– HF fluorescent ballasts– Inverters

• Variable frequency drives• UPS systems

What do Harmonic waveforms look like?

The following 4 graphs show how complex waveforms develop when harmonics are present.

Fundamental WaveformsFundamental + 2nd HarmonicFundamental + 3rd HarmonicMultiple Harmonics

Remember the heating effect of the current flowing in the circuit is directly proportional to the area within the curve.

Waveforms - Fundamental

Fundamental Wave, 50Hz

Fundamental + 2nd harmonic

Waveform resulting from the fundamental plus the 2nd Harmonic

Note this shows little effect on the area within the curve compared to the fundamental

Fundamental + 3rd harmonic

Waveform resulting from the fundamental plus the 3rd Harmonic

Note the substantially increased area within this curve compared to the fundamental. This effect is seen with all odd number harmonic conditions (e.g. 3rd, 5th,

7th etc)

Multiple harmonics

Waveform

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

0 2 4 6 8 10 12 14 16 18 20

[ms]

Cu

rren

t [p

.u]

0

10

20

30

4050

60

70

80

90

100

1 3 5 7 9 11 13 15 17 19Harmonic order

%

These waveforms represent the effect of multiple harmonics.

Interestingly all these waveforms can represent the same content of harmonic distortion!

Note – above the 9th harmonic the distortion and therefore the heating effect is minimised

Typical Current waveform of Switched mode power supply

Some of the Effects of Harmonics

• Excessive heating in rotating machines– Causing premature failure and risk of fire

• Excessive currents in the phase and neutral conductors– Causing overheating and risk of fire

• Overloading of transformers, capacitors and other fixed loads– Causing premature ageing and degradation of function

• Overloading of switchgear and cables– Leading to unwanted tripping of protective devices

• Damage to sensitive electronic equipment– Leading to loss of functionality

Selecting Protective Devices

Points to consider in selecting Protective devices when Harmonics are present.

1. Heating effect

2. Shape of waveform

Selecting Protective Devices

1. Heating effect on Protective Devices

The thermal element of devices such as MCB’s, MCCB’s and HRC Fuse links will respond to the heating effect caused by the harmonic content. If incorrectly selected or set this will lead to unwanted tripping or premature ageing.

2. Shape of Waveform

Today’s electronic protective devices measure the true RMS current flow which takes into account the harmonic content rather than responding to the heating effect. The measurement of this true RMS can be affected by the shape of the waveform …..

Reference to manufacturers data is recommended when selecting these devices.

How to Accommodate the effects of Harmonics

Neutral sizingOdd series ( particularly the 3rd) harmonics within a three phase system do not

‘cancel - out’ but accumulate within the neutral conductor.

This can lead to rms current values equal to and in some cases, greater than that of the phase conductor currents even in balanced load conditions.

If a system is affected in this way, then neutral conductors throughout the installation should be sized accordingly

How to Reduce the Effects of Harmonics

• Good design practice is to look at methods to reduce harmonic content rather than to accept it. This could include the addition of one or more of the following:-

– Passive filters– Active harmonic conditioners– Transformer based solutions

Passive filters

• Capacitor and reactor combination

• Tuned to specific frequency (typically 189Hz)

• Requires higher voltage capacitors

• Designed for a fixed system requirement

Active harmonic conditioner

• Harmonic current compensation, 2nd to 25th

• Harmonic neutral current compensation

• Global or selective harmonic current compensation

• Site adjustable compensation parameters

Transformer based solutions

• 3rd Harmonic rejection transformers

• Phase shifting transformers

• Isolation or harmonic suppression transformers