Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) measures how much a voltage or current waveform departs from an ideal sine wave. Formally, it is the ratio of the combined energy of all harmonic components — multiples of the fundamental 50/60 Hz line frequency — to the energy of the fundamental itself, expressed as a percentage. A perfectly clean sine wave has 0% THD; the higher the number, the more distorted and "dirty" the waveform. It is one of the core power-quality figures for anyone feeding serious load from a panel, a transformer, or a generator — and a fleet of ASIC miners is serious load.

Why switch-mode loads create harmonics

Switch-mode power supplies — the kind inside every ASIC PSU — rapidly chop the incoming AC to convert it, which is an inherently non-linear way to draw power. Without correction, a rectifier-and-capacitor front end draws current only in sharp pulses near the voltage peaks rather than smoothly following the sine wave, and those pulses decompose mathematically into odd harmonics — 3rd (180 Hz), 5th, 7th and beyond — injected back into the building wiring. One small supply is harmless; banks of miners on a shared feeder add their harmonic currents together and can raise the feeder's current THD dramatically.

Consequences in a Hashcenter

Harmonics do damage in specific, predictable ways. Triplen harmonics (3rd and its odd multiples) from single-phase loads do not cancel in a three-phase system's neutral the way the fundamental does — they add — so a neutral conductor sized for balanced fundamental current can quietly overheat. Transformers suffer extra core and winding losses and must be derated (or specified with a K-factor rating) for harmonic-rich load. Distorted current degrades true power factor even when the displacement looks fine, inflates peak and RMS currents, causes nuisance breaker trips, and stresses power-factor-correction capacitors. Generators deserve special respect: a generator's source impedance is far higher than the utility's, so the same harmonic currents produce much worse voltage distortion on its output — feeding non-linear mining load from an undersized or poorly regulated generator can push voltage THD high enough to disturb the miners' own electronics and anything else on the circuit, which is why off-grid mining setups oversize generators generously relative to nameplate load.

The fix: PFC and good engineering

Measuring THD requires the right instrument: a basic multimeter reports RMS values and sees none of this, while a power-quality analyzer — or a clamp meter with harmonic readout — decomposes the waveform and reports THD directly, per phase and per harmonic. For a facility build-out, an afternoon of measurement at the panels under full mining load, comparing against the IEEE 519 guideline figures, is cheap insurance against transformer and neutral problems that otherwise surface as unexplained heat and nuisance trips months later.

The reason modern mining fleets behave far better than the horror stories is active power-factor correction. A PFC front end actively shapes the input current to follow the voltage sine wave, boosting it onto an internal high-voltage DC bus (in the APW12, the PFC stages regulate roughly 410–420 V DC across the bulk capacitors) before the main converter stage. The result is a power factor of about 0.99 and current THD pulled down toward single digits — a well-designed ASIC supply is, from the panel's perspective, a remarkably polite load. For large deployments, engineers still verify the aggregate: harmonic filters or line reactors on main feeders keep facility-level distortion within limits such as the widely referenced IEEE 519 guidelines, and a power-quality meter reading at the panel beats assumptions every time. For the home miner on a 120/240 V panel, the practical takeaways are simple: use miners with proper PFC supplies, respect the neutral in multi-wire circuits, and if you run from a generator, oversize it. THD sits alongside apparent power and inrush current as the three numbers that decide whether a mining feed is clean, reliable, and kind to the wiring that feeds it.