Generator total harmonic distortion
Total harmonic distortion (THD) expresses how much harmonic content a waveform contains relative to its fundamental component. For generator output, voltage THD describes distortion of the AC voltage waveform. Current THD describes distortion of the current drawn by a load; the two measurements answer different questions. [1]
Fundamental frequency and harmonics
A harmonic occurs at an integer multiple of the fundamental frequency. In a 60 Hz system, the third harmonic is 180 Hz and the fifth is 300 Hz. These components combine with the fundamental to change the waveform’s shape. THD compresses their combined magnitude into a single ratio. [2]
A common voltage definition is THDᵥ = 100 × √(V₂² + V₃² + … + Vₙ²) ÷ V₁, using RMS values of the included harmonics and the fundamental. The current version uses the corresponding current values. Measurement methods specify which orders and averaging intervals are included. [1]
Why THD changes with the connected load
Nonlinear equipment, such as rectifier inputs and variable-frequency drives, draws current that is not a simple sine wave. That distorted current interacts with the source impedance and can distort the voltage seen by other equipment. The observed voltage THD depends on the generator and the connected load mix. [3]
Alternator reactance and size are relevant in engineered generator systems. A no-load waveform result does not automatically predict performance with a large nonlinear load. Equally, high current THD in one device does not mean voltage THD has the same percentage. [3]
THD, regulation and transient response
Voltage regulation describes how voltage magnitude is maintained as conditions change. THD describes waveform distortion. Frequency stability and voltage dip during a sudden load change are additional characteristics. A specification such as a voltage regulation band cannot be substituted for a THD value. [3]
A useful THD claim identifies voltage or current, the measurement point, operating load and test conditions. Percentage limits also need an application context. The connected equipment’s manufacturer requirements and the system specification determine the necessary performance; there is no single threshold that proves compatibility with every device.
Inverter output and verification
Inverter generators electronically synthesize the final AC output and can provide low harmonic distortion. The model’s published specification and operating conditions establish the relevant result. An inverter label alone does not describe all aspects of power quality. [4]
A suitable power-quality instrument can measure waveform and harmonic content under the intended loads. An ordinary voltage reading reports magnitude without revealing the complete harmonic spectrum. Verification under representative operation is especially useful where nonlinear loads or demanding electronic equipment influence the design. [1]
Sources and further reading
- Fluke — 1760 power-quality recorder manual and THD measurement ↗assets.fluke.com
- Fluke — Harmonics and electrical equipment ↗www.fluke.com
- STAMFORD | AvK — Harmonic voltage distortion ↗www.stamford-avk.com
- Honda — What is an inverter generator? ↗powerequipment.honda.com
Use the manual and nameplate for the exact model. Local requirements and site conditions take precedence over general examples.