What is Harmonic Distortion?

When we think of ac voltages and currents, we normally visualise perfect sine waves with a frequency of 50 Hz.  Unfortunately, given the rapid increase in IT and process control equipment, this is now rarely the case.

Figure 1 shows the three-phase voltage and current waveforms captured on a major shopping centre incoming LV supply.  It is quite apparent that the current waveform is far removed from the ideal sinewave, being noticeably distorted.  A waveform such as that shown in Figure 1 is quite typical for large commercial / industrial sites and it is the presence of harmonics that results in such distortion.

Figure 1 – Voltage and current waveforms captured at major shopping centre

Harmonics are frequencies that are superimposed upon the 50 Hz signal that are an integer multiple of 50 Hz; i.e. the 3rd harmonic is 150 Hz, the 5th harmonic is 250 Hz and so on.

These harmonic currents are produced by the load and flow back to the supply, passing through the impedance of the cables, transformer etc. as they do so.  As these harmonic currents flow through the system, they produce harmonic voltage drops which in turn distort the supply voltage.

What Causes Harmonic Distortion?

The need to drive down energy costs has seen a marked increase in the use of non-linear loads such as LED lighting and variable speed drives.

Within industrial processes and HVAC applications, variable speed drives provide many valuable benefits; namely complete control of motor speed and voltage, which can result in noticeably reduced kWh consumption of the motors – particularly at times of lower load.

Whilst the benefits can be plentiful, there are drawbacks too. A result of the rectification / inversion process is that these non-linear loads produce significant levels of harmonics. The level of harmonic current produced by variable speed drives can be more than 40%!  Listed below are a few common causes of high harmonic distortion.

  • UPSs
  • Inverter drives
  • IT equipment (the power supplies therein)
  • DC drives

What Issues Can Harmonic Distortion Cause?

Harmonics can cause a multitude of different issues, the most common of which are detailed below:

  • Damage to power factor correction due to harmonic absorption of capacitors or even harmonic magnification due to harmonic resonance
  • Overheating of ac motors and even mechanical damage due to pulsating torques on motor shafts
  • Interference induced into comms / IT cables if not correctly segregated from power cables carrying distorted current, resulting in noise on telephony systems and possible data issues
  • Premature failure of light fittings
  • Premature failure of dc power supplies

Are There Harmonic Limits?

Yes – in terms of actual limits, these are set out by ENA G5/4-1 recommendations.  See the table below for a summary of the current G5/4-1 guidance regarding harmonic distortion limits:

Harmonics (non-multiple of 3) 400V System Harmonic Voltage % Harmonics (multiple of 3) 400V System Harmonic Voltage %
5 4.0 3 4.0
7 4.0 9 1.2
11 3.0 15 0.3
13 2.5 21 0.2
17 1.6
19 1.2
23 1.2
25 0.7
Total Harmonic Distortion (V) 5%