Harmonics and Total Harmonic Distortion (THD)
A clear explanation of how non-linear electrical loads create harmonics, what Total Harmonic Distortion (THD) measures, the problems harmonics cause, and the practical ways to control them in modern buildings.
A clean electrical supply is a smooth sine wave at the system frequency — 50 Hz in the UAE. Harmonics are additional sine waves at whole-number multiples of that frequency (100 Hz, 150 Hz, 250 Hz and so on) that ride on top of the fundamental and distort its shape. They are created by loads that draw current in pulses rather than smoothly.
Modern buildings are full of such loads: variable frequency drives, LED drivers, computer and server power supplies, UPS systems and EV chargers. As the share of these electronic loads grows, harmonic distortion becomes a real engineering concern — it wastes energy, overheats equipment and can disturb sensitive systems if it is not measured and managed.
How it works
The starting point is the fundamental. A linear load such as a simple resistive heater draws a current that is a faithful copy of the voltage sine wave. A non-linear load draws current only during part of each cycle, so its current waveform is distorted. Mathematically, any repeating distorted wave can be broken down (by Fourier analysis) into the fundamental plus a series of harmonic components at multiples of the fundamental frequency.
Harmonic order and naming. The 3rd harmonic is three times the fundamental (150 Hz), the 5th is 250 Hz, the 7th is 350 Hz, and so on. Odd harmonics dominate in most building loads. The 3rd and its multiples (the triplen harmonics) are important because, in a three-phase system, they add up in the neutral conductor instead of cancelling.
Total Harmonic Distortion. THD expresses how distorted a waveform is as a single percentage — the combined size of all the harmonics relative to the fundamental. Engineers distinguish current distortion (THDi), which describes the load, from voltage distortion (THDv), which describes the supply at a point in the network. A high THDi load drawing current through the system impedance is what creates THDv for everyone connected nearby.
The damage harmonics cause. Triplen harmonics overload the neutral conductor, which can run hotter than the phases. Harmonic currents cause extra heating in transformers and motors, so they must be de-rated. They can overheat power-factor-correction capacitors and even excite resonance. They also distort the voltage that sensitive electronics, metering and protection see, leading to nuisance tripping and measurement errors.
Mitigation. The first step is always measurement with a power-quality analyser to find where distortion is generated and how severe it is. Solutions then range from passive harmonic filters tuned to specific orders, to active harmonic filters that inject a cancelling current in real time, to drives with built-in low-harmonic front ends, to oversized neutrals and de-rated transformers. Good design also separates sensitive and polluting loads onto different feeders.
Main types
In the UAE
- UAE distribution authorities such as ADDC and DEWA require connected installations to keep harmonic distortion within recognised power-quality limits (commonly aligned with IEEE 519 or IEC 61000 series) to protect the shared network.
- The Gulf's heavy use of VFD-driven chillers, pumps and large UPS plant in cooled, electronics-dense buildings makes harmonic management a routine part of compliant MV/LV design.
- Estidama and energy-efficiency goals favour high-efficiency, low-harmonic equipment, since distortion wastes energy as heat and forces transformers and cables to be oversized.
How GPR applies this
GPR carries out power-quality surveys across Abu Dhabi using calibrated analysers to quantify THDi and THDv at boards and feeders, then designs the right mitigation — passive or active harmonic filters, low-harmonic drives, correctly sized neutrals and de-rated transformers. We coordinate harmonic limits with ADDC/DEWA connection requirements and integrate monitoring through the building management system so distortion is tracked over the life of the installation.
Frequently asked questions
What causes harmonics in a building?
Non-linear loads that draw current in pulses — VFDs, LED drivers, computer and server power supplies, UPS systems and EV chargers — distort the current waveform and inject harmonics back into the supply.
What is the difference between THDi and THDv?
THDi is the harmonic distortion of the current and describes how non-linear a load is; THDv is the distortion of the voltage at a network point and affects every load connected there.
Why do harmonics overload the neutral conductor?
Triplen harmonics (3rd, 9th, 15th) do not cancel between phases in a three-phase system; they add together in the neutral, which can then carry more current than the phases.
How are harmonics reduced?
By measuring first, then applying passive or active harmonic filters, low-harmonic drives, oversized neutrals and de-rated transformers, and by separating sensitive from polluting loads.
Are there limits for harmonics in the UAE?
Yes. Distribution authorities require harmonic levels to stay within recognised power-quality limits, commonly based on IEEE 519 or the IEC 61000 series, to protect the shared network.