Harmonics is a broadly used term to describe electrical distortion but as a subject is not universally understood within industry. Harmonic related issues became prominent with the proliferation of semiconductors and cause a variety of problems that are sometimes difficult to attribute and eliminate. Loads drawing their current through semiconductors have a non-linear characteristic and are primarily responsible for generating the harmonic distortion that is so damaging to supplies, equipment and sub-components.

What examples of loads are non-linear?
Single phase - switch-mode power supplies, PCs, monitors, photo-copiers, fax machines, high-frequency lighting, any electronic single phase load in fact.
Three phase - Variable-Speed Drives, Uninterruptable Power Supplies, 3 phase rectifiers.

Why are harmonics a problem?
Harmonic Current - causes overheating of conductors and their insulation, overheated transformers and increased losses, overloaded Neutral conductors, excess Neutral to Earth potential, overheating of capacitors and, ultimately, premature component failure. Additionally, exporting excessive harmonic distortion to the supply networks may well fall foul of the enforced requirements of G5/4-1.
Harmonic Voltage - causes linear loads to draw non-linear current (resulting in current distortion effects), torque pulsation in motors, capacitor dielectric failure, insulation breakdown, PC monitor and power supply failure, electronic lighting failure, malfunction of sensitive electronic equipment and, again, excessive distortion in distribution supply networks.

How are harmonics dealt with?
There is no single solution because, depending on the circumstances, some harmonic mitigation techniques will be much more advantageous than others. A successful resolution depends on accurate assessment of the many variables that first need to be taken into account.

G5/4-1 overview
Energy Networks Association Document G5/4-1 2005, details the maximum levels of harmonic distortion that electricity consumers are permitted to export onto the distribution network. The document forms part of the supply agreement between Electricity Company and consumer. Enforcement of the limits can include a refusal to supply until it can be demonstrated that harmonic levels have been adequately addressed, which remains the responsibility of the consumer.

Circumstances when G5/4 is exceeded.
Where the voltage distortion levels are exceeded, the DNO will give a conditional connection agreement which may need mitigation measures. However the final decision on whether connection is possible is left with the DNO (judge and jury).

Where circumstances are uncertain (possible resonance) then the DNO may give a conditional connection where remedial measures can be implemented within a reasonable time-scale.

In exceptional circumstances, where the site to be connected is remote from other customers, it may be considered at Stage 2 with computability levels appropriate to the system voltage.