Total Harmonic Distortion (THD)
Total harmonic distortion measures the degree to which a voltage or current waveform deviates from a pure sinusoid, expressed as a percentage of the fundamental component. IEEE 519-2022 establishes harmonic limits at the point of common coupling to prevent power quality degradation. High THD causes overheating in transformers, nuisance tripping of protective devices, and interference with sensitive electronic equipment.
Detailed Explanation
Non-linear loads — variable frequency drives, switch-mode power supplies, LED drivers, and UPS systems — draw current in pulses rather than smooth sinusoidal waves, injecting harmonic frequencies (multiples of the fundamental 50 or 60 Hz) into the supply network. THD quantifies the total harmonic content as the root-sum-square of all harmonic amplitudes divided by the fundamental amplitude. IEEE 519-2022 limits voltage THD to 8% at the point of common coupling (PCC) for systems below 1 kV and 5% for higher voltage systems, with individual harmonic limits also specified. Current harmonic limits depend on the ratio of short-circuit current to load current at the PCC. The third harmonic (150 or 180 Hz) is particularly problematic in three-phase systems because triplen harmonics (3rd, 9th, 15th) are zero-sequence and add arithmetically in the neutral conductor rather than cancelling — potentially causing neutral conductor overloading even when phase currents are balanced. K-factor transformers and oversized neutral conductors are common mitigation measures. Active harmonic filters inject compensating currents to cancel harmonics at their source.
Standard References
| Standard | Clause | Relevance |
|---|---|---|
| IEEE 519-2022 | Table 2 | Voltage distortion limits at the point of common coupling |
| IEC 61000-3-2 | Clause 6 | Limits for harmonic current emissions from equipment |
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