Distortion
What causes harmonic distortion in a power system
Harmonics are not a fault. They are the predictable result of drawing current in pulses instead of sine waves — and they can be calculated before you buy anything.
Harmonic distortion is caused by non-linear loads that draw current in short pulses rather than continuously. The pulsed current contains frequencies at integer multiples of the supply frequency, and those extra frequencies flow back into the network, distorting the voltage waveform for every other load on the same supply.
On a 50 Hz system, the 5th harmonic sits at 250 Hz and the 7th at 350 Hz. In a three-phase six-pulse rectifier these two orders typically account for 70–80% of the total harmonic current.
Where harmonics actually come from
The distinction that matters is linear versus non-linear. A heater, an incandescent lamp, or an induction motor draws current roughly in proportion to the applied voltage, so the current waveform stays sinusoidal. A device with a rectifier front end does not: it charges a DC capacitor in narrow slices near the voltage peak, then draws nothing until the next peak.
That slice-shaped current is what generates harmonics. The load is not faulty — it is doing exactly what its design requires. The distortion is a side effect of conversion efficiency.
The loads that cause most of it
| Load | Typical current THD | Dominant orders |
|---|---|---|
| Six-pulse VFD, no reactor | 30–45% | 5, 7, 11, 13 |
| Six-pulse VFD, DC reactor | 25–35% | 5, 7, 11, 13 |
| Six-pulse VFD, AC line reactor | 15–25% | 5, 7, 11, 13 |
| Twelve-pulse rectifier | 8–12% | 11, 13, 23, 25 |
| Switch-mode power supply, no PFC | 80–120% | 3, 5, 7 |
| UPS with rectifier input | 25–35% | 5, 7, 11 |
| LED driver, passive | 15–30% | 3, 5, 7 |
| Arc furnace | variable | 2, 3, 4, 5 + interharmonics |
Note the pattern in the last column. A six-pulse rectifier produces orders of 6k±1, so 5, 7, 11, 13, 17, 19. A twelve-pulse rectifier produces 12k±1, which cancels the 5th and 7th almost entirely. This is why medium-voltage drives often use twelve or eighteen pulses instead of a filter.
How distortion spreads through the installation
Harmonic currents flow from the load back toward the source, because that is the path of least impedance. As they flow through the source impedance — transformer windings, cables, the utility network — they produce harmonic voltage drops. That is how a current problem becomes a voltage problem.
The consequence is that the damage is rarely confined to the offending load. Other equipment on the same board sees a flattened or notched voltage waveform, and responds in ways its designer never intended.
Vh = Ih × Zh
# THD, expressed against the fundamental
THDV = √(V2² + V3² + … + Vn²) / V1 × 100%
Because harmonic impedance rises with frequency, high-order harmonics produce disproportionately large voltage drops. A 25th harmonic current of modest amplitude can distort the voltage more than a much larger 5th harmonic current would.
What harmonics actually damage
| Equipment | Mechanism | What you see first |
|---|---|---|
| Power capacitors | Impedance falls as frequency rises, so the capacitor absorbs harmonic current and overheats. Resonance can multiply current 3–10 times. | Swollen cases, blown fuses, short service life |
| Transformers | Eddy current and stray losses rise with frequency squared; triplen harmonics also circulate in delta windings. | Hot spot, derating required, reduced capacity |
| Neutral conductors | Triplen orders add in phase in the neutral instead of cancelling. | Neutral hotter than phases, insulation failure |
| Motors | Negative-sequence harmonics create a counter-rotating field and additional rotor heating. | Derating, shortened winding life |
| Protection and metering | RMS and peak sensing disagree; zero crossings shift. | Nuisance tripping, billing disputes |
| Generators | Harmonic current raises winding temperature; AVR struggles with distorted feedback. | Derating below nameplate, voltage instability |
| Standby UPS | Distorted bypass supply prevents clean transfer. | Transfer failures during outages |
The capacitor case deserves its own warning
Capacitors are the component most often destroyed by harmonics, and the failure is usually misdiagnosed as a manufacturing defect. The mechanism is straightforward: capacitive reactance falls as frequency rises, so a capacitor is a low-impedance path for exactly the frequencies harmonics occupy. Adding power factor correction capacitors to a network that already has significant distortion can create a parallel resonance that amplifies harmonic current rather than reducing it.
The resonance order can be estimated from the short-circuit capacity at the point of connection and the capacitor rating. If that order lands near the 5th or 7th harmonic — which it frequently does — the installation will oscillate at that frequency.
# S_sc = short-circuit capacity at PCC (MVA), Q_c = capacitor rating (MVAr)
# h_res between 4.5 and 5.5 → the 5th harmonic will be amplified
This is what detuned reactors are for. A 7% reactor shifts the resonance below the 5th harmonic, so the capacitor never sees the amplified current.
Measure before you size anything
Every sizing decision downstream depends on one number that most projects never collect properly: the actual harmonic current in amperes, by order, at the point where the filter will be installed.
A power quality analyser recording for at least one full production cycle will show which orders dominate, whether the distortion is steady or load-dependent, and whether the existing capacitor bank is already in resonance. Ten-minute spot readings are not sufficient — they routinely miss the shift change or the weekend light-load condition that produces the worst distortion percentage.
| What to record | Why it changes the answer |
|---|---|
| Harmonic current by order, in amps | Filter rating is in amps. Percentages alone cannot size equipment. |
| Load profile across a full cycle | Distortion often peaks at light load, when the fundamental is smallest. |
| Transformer rating and impedance | Determines short-circuit capacity and therefore the applicable IEEE 519 row. |
| Existing capacitor rating | Sets resonance order. Adding a filter without checking this is how resonance gets worse. |
| Neutral current, if single-phase loads exist | Triplen harmonics may require a 4-pole or oversized neutral design. |
The four ways to fix it, and when each applies
There is no universally correct answer here, which is why the choice should follow the measurement rather than precede it.
- Reduce generation at the source. Line reactors, DC chokes, twelve-pulse rectifiers, or active front ends. Cheapest per unit when specified at the design stage, impractical to retrofit on existing drives.
- Passive filters. Tuned LC branches that present low impedance at one or two orders. Effective and inexpensive for a known, stable load, but they can interact badly with changing network conditions.
- Active filters. Inject a cancellation current across a wide frequency range. Adaptable to changing loads and effective from the 2nd to the 50th order, at a higher cost per amp.
- Hybrid arrangements. A passive branch for the bulk of the 5th and 7th, with an active filter handling the remainder and the higher orders. Often the lowest cost per amp on large installations.
Need the numbers for a specific installation?
Send the analyser export and single-line diagram, and we will come back with the harmonic spectrum, the applicable IEEE 519 row, and a filter rating in amps.