Harmonics & IEEE 519 Explained

Harmonics are currents and voltages at whole-number multiples of the supply frequency. On a 60 Hz system, the 5th harmonic is 300 Hz and the 7th is 420 Hz. They come from non-linear loads, equipment that draws current in pulses rather than smoothly: rectifiers in UPS systems and drives, switch-mode power supplies in servers, LED drivers and battery inverters. The pulses can be broken down into the fundamental plus a series of harmonics, and those harmonic currents flow back through the system to the source.

Total harmonic distortion (THD) compares the combined harmonic content to the fundamental. It applies to current (THD_I), which is what the load draws, and to voltage (THD_V), which is what that current does to the bus as it flows through the source impedance.

Which harmonics show up

A rectifier's pulse number decides which harmonics it produces. A 6-pulse rectifier, the most common front end in drives and older UPS systems, produces harmonics at orders 6k ± 1: the 5th, 7th, 11th, 13th and so on. A 12-pulse rectifier cancels the 5th and 7th, leaving 12k ± 1: the 11th, 13th, 23rd and 25th. That's why raising the pulse number is one of the oldest mitigation methods.

Single-phase non-linear loads on a four-wire system add triplen harmonics (3rd, 9th, 15th). Unlike most harmonics, triplens from the three phases add in the neutral instead of cancelling. On a panel full of single-phase power supplies, neutral current can exceed phase current, approaching about 1.7 times phase current in the limiting case. That's why the NEC requires the neutral to be counted as a current-carrying conductor when most of the load is non-linear (NEC 310.15, see cable ampacity derating), and why shared neutrals on such loads are a known problem.

Worked example: THD, RMS current and TDD

A 6-pulse load draws 100 A of fundamental current, with harmonic currents of 30% (5th), 15% (7th), 8% (11th) and 5% (13th):

• Current THD: √(0.30² + 0.15² + 0.08² + 0.05²) = √0.1214 = 34.8%
• RMS current: 100 × √(1 + 0.348²) = 105.9 A. The harmonics add about 6% to the heating current, with no extra useful power.
• Total demand distortion (TDD): if the circuit's maximum demand current is 200 A, TDD = 34.8% × 100 ÷ 200 = 17.4%.

The difference between THD and TDD matters. THD is relative to whatever the load happens to be drawing, so a lightly loaded drive can show an alarming THD while injecting very little harmonic current in absolute terms. TDD is relative to maximum demand, which is why IEEE 519 states its current limits in TDD.

What harmonics do

Transformer heating

Eddy-current losses in transformer windings rise roughly with the square of the harmonic order, so a 7th-harmonic amp heats the windings far more than a fundamental amp. A standard transformer serving heavy non-linear load can overheat while its ammeter reads below nameplate. The response is either a K-rated transformer, built to absorb a stated harmonic heating (see K-factor), or derating a standard unit using the method in IEEE C57.110.

Capacitor resonance

A power-factor capacitor bank and the inductance of the source form a parallel resonant circuit. Its resonant harmonic is approximately:

Take the 1000 kVA, 5.75% transformer from the transformer impedance guide, which has a short-circuit capacity of 17,391 kVA. Add the 277 kVAR bank from the power factor correction worked example. Then h_r ≈ √(17,391 ÷ 277) = 7.9, just above the 7th harmonic a 6-pulse load produces. Near resonance, a modest harmonic current is amplified into a large voltage distortion and a large capacitor current. Blown capacitor fuses and failed capacitor cells are the classic symptom.

The usual fix is a detuned bank: a reactor in series with each capacitor step shifts the resonance below the 5th, commonly to somewhere around the 4.2nd to 4.7th harmonic. The bank then still corrects power factor but no longer resonates with the harmonics present.

Generators

A generator has a much higher source impedance than a utility transformer, so the same harmonic current produces much more voltage distortion when a site runs on standby power. Non-linear load also adds alternator heating. That's why generator compatibility with UPS and drive front ends needs its own check, covered in the UPS sizing guide.

IEEE 519 limits

IEEE 519 sets harmonic limits at the point of common coupling (PCC), the point where the utility's system meets the customer's, not at each piece of equipment. It divides responsibility: the utility is responsible for voltage distortion, and the customer for the harmonic current it injects.

Voltage distortion limits

Current distortion limits (120 V to 69 kV)

Current limits depend on how stiff the system is relative to the customer's load. That's the ratio of available short-circuit current (I_SC) to maximum demand load current (I_L) at the PCC. A stiffer system tolerates more harmonic current, because the same current causes less voltage distortion.

Limits for higher harmonic orders are progressively lower. Values are from IEEE 519-2014, Tables 1 and 2. Confirm them against the edition your utility applies, since the standard was revised again in 2022.

For example, suppose the PCC is at the secondary of the 1000 kVA transformer, with 20,919 A available and a maximum demand of 1,000 A. The ratio is about 21, which falls in the 20-to-50 row: the facility's TDD must stay at or below 8%. The load in the worked example above, at 17.4% TDD, would need mitigation if it were the whole facility.

Mitigation

• Line reactors and DC chokes on drives and rectifiers: cheap, and they noticeably reduce current distortion.
• Higher pulse numbers (12- or 18-pulse) using phase-shifting transformers, which cancel the lowest-order harmonics.
• Active front ends on drives and UPS systems, which draw a nearly sinusoidal current. Modern IGBT-rectifier UPS designs typically quote low input distortion.
• Passive filters tuned to specific harmonics, which must be designed with the system's resonances in mind.
• Active harmonic filters, which measure the distortion and inject cancelling current. Effective across many orders, and they follow changing load.
• K-rated or derated transformers and full-size or oversized neutrals. These don't reduce harmonics, but they carry them safely.

Common mistakes

• Judging compliance from THD at light load. IEEE 519's current limits use TDD, relative to maximum demand.
• Adding a capacitor bank without checking resonance. Check where h_r falls against the harmonics present before choosing plain or detuned steps.
• Applying the PCC limits to each device. The limits apply where the utility meets the customer. Equipment-level specifications are a design choice to get there.
• Sizing neutrals for balanced linear load on panels full of single-phase power supplies.
• Checking harmonics on utility power only. The same load distorts a generator bus far more.

Frequently asked questions