What Is Harmonic Distortion (THD)? Its Effects

What Is Harmonic Distortion?

In an ideal AC grid, voltage and current have a sinusoidal waveform — at a single frequency (50 Hz). Harmonic distortion refers to the components at multiple frequencies that are superimposed on this sinusoidal wave. The 3rd harmonic is 150 Hz, the 5th harmonic is 250 Hz, and the 7th harmonic is 350 Hz.

Harmonics are produced by nonlinear loads:

• Variable-speed motor drives (VFD/inverter)
• Uninterruptible power supplies (UPS)
• LED drivers and electronic ballasts
• Computer power supplies (SMPS)
• Welding machines and arc furnaces
• Solar inverters

What Is Total Harmonic Distortion (THD)?

THD (Total Harmonic Distortion) expresses the ratio of the harmonic components to the fundamental-frequency component. The IEEE 519-2022 standard recommends keeping grid voltage THD below 5% in industrial systems.

The Harmful Effects of Harmonic Distortion

• Transformer heating: Harmonic currents increase eddy current losses in the transformer. As THD rises, the transformer heats up faster and its lifespan shortens.
• Neutral conductor overloading: 3rd harmonics (150 Hz) accumulate in the neutral conductor as a sum of the three phase components, leading to cable heating and a fire risk.
• Damage to the power-factor correction panel: In a harmonic environment, capacitors carry a risk of resonance. An unsuitable correction panel fails within a short time.
• Sensitive equipment faults: CNC machines, medical devices and measurement equipment can malfunction in a high-THD environment.
• Energy metering errors: Older-generation meters may take inaccurate readings in a harmonic environment.

How Is THD Measured?

Energy analyzers connected to Argus EMS (Schneider PM5350, Siemens PAC3220, ABB B-series, etc.) measure THD continuously on a per-phase basis. Models that also offer harmonic spectrum analysis report each harmonic component separately.

Harmonic Mitigation Methods

• Passive harmonic filters: L-C circuits tuned to specific harmonic frequencies. Low cost, fixed frequency.
• Active harmonic filters (AHF): Measure harmonics in real time and inject the exact opposite current. High effectiveness, wide frequency range.
• 12-pulse or 18-pulse drives: In VFDs, multi-pulse rectifier arrangements naturally reduce harmonic content.
• K-factor transformers: Specially designed transformers that withstand harmonic loads.
• System design: Harmonic loads are fed from a separate supply point to isolate them from sensitive equipment.

EPDK and Harmonics

In Turkey, the EPDK Quality Regulation sets the voltage THD upper limit at the connection point at 8%. If this limit is exceeded, EPDK may impose a penalty; more importantly, you may become the subject of complaints from neighboring subscribers. Industrial facilities are obliged to take the necessary measures within their own premises.