Harmonics and THD from a fault record: what IEEE 519 does and does not tell you
How THD is calculated from a COMTRADE record, the IEEE 519 voltage-distortion limits by bus voltage, and why a record a few seconds long is a snapshot rather than an IEEE 519 assessment.
A fault record is a waveform, and any periodic waveform can be split into its fundamental and harmonics: components at whole multiples of the power frequency. Drives, rectifiers, LED lighting and arc furnaces draw harmonic currents, and those currents flowing through the supply impedance distort the voltage. A COMTRADE record lets you see that distortion directly, which makes it a good place to start an investigation, as long as you keep in mind what a few seconds of data can say.
How THD is calculated
The spectrum comes from a Fourier transform of an integer number of cycles of the waveform. Total harmonic distortion is the RMS of all the harmonics divided by the fundamental:
THD = √(V2² + V3² + V4² + … + Vh²) / V1 × 100 %
Two details decide whether the number is right. The analysis window has to hold whole cycles, or energy from the fundamental leaks into the harmonic bins. And the highest harmonic you can trust is limited by the recorder’s sampling rate: at 32 samples per cycle, nothing above the 15th harmonic can be measured, and anything above it folds back onto lower orders. SagSwell says how far up the spectrum a record can see and does not draw bars beyond it.
The IEEE 519 voltage limits
IEEE 519 sets harmonic limits at the point of common coupling, the point where a customer’s system meets the utility’s. The voltage limits depend on the bus voltage:
| Bus voltage at the PCC | Individual harmonic (%) | THD (%) |
|---|---|---|
| 1 kV and below | 5.0 | 8.0 |
| above 1 kV to 69 kV | 3.0 | 5.0 |
| above 69 kV to 161 kV | 1.5 | 2.5 |
| above 161 kV | 1.0 | 1.5 |
Current limits are set as total demand distortion (TDD), relative to the maximum demand load current, and depend on the ratio of short-circuit current to load current at the PCC, so the same measured current can be within the limit on a stiff supply and above it on a weak one.
Because the limit depends on the bus voltage, a single number is wrong somewhere: 8% is the limit on a 400 V bus and more than three times the limit on a 132 kV one. Check the bus voltage before reading a THD figure against a limit.
Why a fault record is a snapshot, not an assessment
IEEE 519 is assessed statistically over time: short-time values over days, compared as percentiles, for example over a week of monitoring. A COMTRADE record lasts from a fraction of a second to a few seconds, and it was usually triggered by something unusual. Its THD describes that moment. It is useful for seeing which harmonics are present, which phase carries them and how they change through an event, and it can show that a problem is worth monitoring properly. It cannot show that a site meets or fails IEEE 519.
Harmonics during a fault
During a fault, and for a few cycles after clearing, the waveform is not periodic: there is DC offset in the current, the voltage steps, and transformers may draw inrush current rich in the 2nd harmonic. A harmonic spectrum taken across those cycles mixes all of that together. Take the spectrum from the steady part of the record before the trigger, or look at how the harmonics change through the event, rather than reading one number across the whole record.
The Harmonics tab in SagSwell shows the spectrum and THD of every voltage and current channel, and the Phasors tab shows the fundamental and the symmetrical components at any instant. For the rest of the record, see the sags and swells guide.
References
- IEEE Std 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems