What is THDi?

THDi (Total Harmonic Distortion of Current) measures the ratio of unwanted harmonic currents to the fundamental current in an electrical system. Modern industrial equipment—such as Variable Frequency Drives (VFDs), rectifiers, and LED drivers—draws current in non-linear pulses, generating harmful harmonic frequencies (3rd, 5th, 7th, etc.).

When THDi exceeds standard limits (typically >5%), it acts as a silent killer in power grids. High THDi causes transformer overheating, premature failure of power factor capacitors, nuisance breaker tripping, and severe voltage distortion.

To control THDi and comply with IEEE 519 standards, modern facilities rely on Active Harmonic Filters (AHF). AHFs measure current distortion in real time and inject counter-phase currents to dynamically cancel harmonics (<5ms response), bringing THDi down below 3%.

Need reliable dynamic harmonic mitigation hardware to eliminate THDi issues in your next project? Explore factory-direct OEM/ODM solutions with Lafaelt.