Harmonic Distortion (THD) in LED and VFD Installations

What are harmonics

A pure sinusoidal supply operates at the fundamental frequency — 50 Hz in Europe. When a non-linear load draws current that does not follow a sinusoidal waveform, that current contains frequency components that are integer multiples of the fundamental. These are harmonics. The 3rd harmonic is 150 Hz, the 5th is 250 Hz, the 7th is 350 Hz, and so on. In 3-phase systems, odd-order harmonics dominate, and the triplen harmonics (3rd, 9th, 15th) behave differently because they are in-phase across all three phases rather than cancelling as balanced harmonics do.

The primary sources of current harmonics in modern buildings are LED drivers (switched-mode power supplies operating at high frequency), variable frequency drives (VFDs) with 6-pulse diode rectifier front ends, uninterruptible power supplies (UPS), switch-mode power supplies for IT equipment and KNX device power supplies, and electronic ballasts in older fluorescent luminaires. Each draws current in pulses at the peaks of the voltage waveform rather than continuously.

THD-I vs THD-V

THD-I (total harmonic distortion of current) is the ratio of the RMS value of all harmonic current components to the RMS value of the fundamental current, expressed as a percentage. It is measured at the load terminals — at the input to the LED driver, VFD, or UPS — and describes how distorted the current drawn by that load is. A pure resistive load has THD-I = 0%. A typical switched-mode LED driver without power factor correction measures THD-I of 80–120%.

THD-V (total harmonic distortion of voltage) is the ratio of the RMS value of all harmonic voltage components to the fundamental voltage, measured at the supply bus. Current harmonics flowing through the source impedance of the supply system cause voltage harmonics — the harmonic voltage drop across the impedance appears as distortion of the bus voltage. THD-V affects all equipment connected to the bus, not just the harmonic-generating load. This is the distinction: THD-I is a property of the load; THD-V is a property of the network and affects every connected device.

Voltage harmonic at order h:
  Vh = Ih × Zh

Where:
  Vh = harmonic voltage component at order h (V)
  Ih = harmonic current component at order h (A)
  Zh = source impedance at harmonic frequency h (Ω)

Source impedance increases with frequency:
  Z at 250Hz (5th) = 5 × Z at 50Hz (fundamental)
  Z at 350Hz (7th) = 7 × Z at 50Hz

Higher source impedance (weak supply, long cables) →
more THD-V for the same THD-I load

THD-I measured: at load input terminals (per IEC 61000-3-2)
THD-V measured: at supply bus (per EN 50160)

IEC 61000-3-2 Class C limits for LED luminaires

IEC 61000-3-2 establishes harmonic current emission limits for equipment with input current up to 16 A per phase connected to public LV supply networks. Class C covers lighting equipment, including LED luminaires and their drivers. For luminaires rated above 25 W, the standard specifies harmonic limits as a percentage of the fundamental current, with the 3rd harmonic limit dependent on the power factor (cos phi) of the luminaire.

EN 50160 voltage quality at the LV supply point

EN 50160 defines the characteristics that utilities must provide at the low-voltage supply point. For voltage harmonics, the standard specifies that THD-V shall not exceed 8% as a weekly average of 10-minute measurement intervals. Individual harmonic voltage limits apply to harmonics up to order 25: for example, 3rd harmonic must not exceed 5% of fundamental, 5th must not exceed 6%, 7th must not exceed 5%.

Utilities measure compliance by recording 10-minute RMS averages over a continuous 7-day measurement period using class A power quality instruments. Enforcement varies by country — in the EU, voltage quality complaints are addressed via the national regulatory authority. For installers, EN 50160 sets the upper boundary of what the supply network delivers; if THD-V approaches 8% before you connect your load, adding further harmonic-generating equipment risks exceeding the limit and degrading supply quality for other consumers on the feeder.

Impact on KNX installations and panel components

Harmonic currents cause specific problems in building installations that KNX panel designers must account for. The most critical effect in 3-phase installations with single-phase LED loads is neutral conductor overloading. In a balanced 3-phase system, the fundamental currents cancel in the neutral conductor. However, triplen harmonics (3rd, 9th, 15th) are in phase across all three phases — they add arithmetically in the neutral rather than cancelling. In a fully loaded 3-phase installation with high-THD single-phase LED drivers, the 3rd harmonic neutral current can equal or exceed the phase current.

3 x 20A phase circuits, THD-I = 90%, 3rd harmonic = 70%:

  3rd harmonic current per phase: 20A × 70% = 14A
  Neutral 3rd harmonic current:   14A × 3 = 42A (adds in neutral)
  Fundamental neutral current:    cancels (balanced 3-phase)

  Total neutral current ≈ 42A (exceeds 20A phase current!)

IEC 60364-5-52 Table 52-D:
  For THD > 15%, size neutral conductor at 1.45× phase rating
  In high-THD LED installations: use same CSA neutral as phase,
  or upsize neutral by one cross-section (e.g. 4mm² phase → 6mm² neutral)

MCB false tripping:
  High-frequency harmonic currents cause MCB thermal element
  to see higher RMS current than 50Hz-only meters show.
  Derate MCB to 0.8× rated current in high-THD circuits.

KNX power supply units are SMPS-based devices. High THD-V on the mains input causes ripple on the 29 V DC KNX bus output, reduced SMPS efficiency, and accelerated aging of the input electrolytic capacitors. Capacitor lifetime halves for each 10°C rise in operating temperature — harmonics increase losses and therefore temperature. In high-THD environments, specify KNX power supplies with active power factor correction (PFC) input stages, such as MDT STC-0640.02 or ABB SU/S 6.5.1, which reject harmonic content more effectively than passive SMPS designs.

VFD harmonics and line reactor mitigation

A standard 6-pulse VFD drive has a diode rectifier front end that draws current only near the peaks of the supply voltage waveform. This produces a characteristic harmonic spectrum dominated by the 5th harmonic (250 Hz) and 7th harmonic (350 Hz), with the 5th typically at 25–35% of fundamental and the 7th at 8–15%. The THD-I of a standard 6-pulse drive without mitigation is typically 35–50%.

Measurement: when and how to assess THD

THD measurement requires a true-RMS power quality analyser capable of resolving individual harmonics to at least the 40th order. Suitable instruments include the Fluke 435 Series II (class A per IEC 61000-4-30), Chauvin Arnoux C.A. 8336, and Carlo Gavazzi EM24 energy analyser (continuous monitoring). Connect the analyser at the sub-distribution board serving the suspect loads and measure THD-I and the individual harmonic percentages under representative load conditions.

Critical measurement occasions: before installing capacitor banks (capacitors can resonate with supply inductance at harmonic frequencies, amplifying the harmonic voltage — always measure THD-V before adding capacitor banks to a VFD-rich installation); after major LED retrofit projects (baseline vs post-retrofit comparison); when MCBs trip without apparent overload (harmonic RMS current causing thermal tripping); when investigating KNX bus stability issues (THD-V reaching KNX PSU input).

Passive harmonic filters

A passive harmonic filter consists of a series LC circuit tuned to present low impedance at a specific harmonic frequency, connected in shunt (parallel) with the supply bus. Harmonic currents of the tuned frequency take the low-impedance path through the filter capacitor and inductor rather than flowing back into the supply. A 5th harmonic filter is tuned to 250 Hz: C and L values are chosen so that 1 / (2π × 250 × sqrt(LC)) = 1.

For distributed installations — multiple VFDs spread across a building — filtering at source (one filter per VFD, typically a line reactor or integrated input filter) is preferred over a single large central filter. Central filtering requires the filter to handle the combined harmonic current of all loads and adds complexity; source filtering keeps each drive's harmonics contained. For centralised solutions with large motor control centres, a tuned passive filter bank rated for the total drive kVA may be more economical.

Active harmonic filters

Active harmonic filters (AHF) — also called active power conditioners — measure the harmonic current drawn by the load in real time using a current transformer and DSP controller, then inject equal and opposite harmonic currents via a PWM inverter to cancel the measured harmonics at the point of common coupling. Unlike passive filters, an AHF adapts to changing load conditions and can simultaneously correct harmonics up to the 50th order and provide reactive power compensation.

Representative products: ABB PQFI (30–200 A units, Modbus TCP interface), Schaffner ECOsine Active (25–200 A). Both provide Modbus TCP registers for THD-I and THD-V before/after compensation, individual harmonic levels, and filter status. For KNX integration, connect the AHF Modbus TCP output to a WAGO 750-362 Modbus TCP gateway configured with the KNX coupler module. Map THD-V L1/L2/L3 to KNX group addresses with DPT 9.002 (2-byte float) and set threshold alert logic in the KNX comparator block to trigger an alarm when THD-V exceeds 5%.

KNX integration: Carlo Gavazzi EM24 Modbus registers

The Carlo Gavazzi EM24 energy analyser supports Modbus RTU (RS-485) and Modbus TCP (via optional Ethernet module). For continuous THD monitoring integrated into a KNX building automation system, the recommended architecture is: EM24 Modbus RTU → WAGO 750-485 RS-485 module in a WAGO 750-352 Modbus RTU master fieldbus controller → ETS6 KNX group address mapping.

Register  Address  Description                   Format      Scale
0x0028    40041    THD-V L1 (%)                 uint16      ÷10 (e.g. 35 = 3.5%)
0x0029    40042    THD-V L2 (%)                 uint16      ÷10
0x002A    40043    THD-V L3 (%)                 uint16      ÷10
0x002B    40044    THD-I L1 (%)                 uint16      ÷10
0x002C    40045    THD-I L2 (%)                 uint16      ÷10
0x002D    40046    THD-I L3 (%)                 uint16      ÷10

Suggested KNX group address mapping:
5/7/1  THD-V L1 (DPT 9.002, %)
5/7/2  THD-V L2 (DPT 9.002, %)
5/7/3  THD-V L3 (DPT 9.002, %)
5/7/4  THD-I L1 (DPT 9.002, %)
5/7/5  THD-I L2 (DPT 9.002, %)
5/7/6  THD-I L3 (DPT 9.002, %)

Alert logic (ETS6 comparator block):
IF THD-V L1 (5/7/1) > 5.0% → send TRUE to 5/7/90 (THD alarm)
→ KNX alarm indicator activation + HA push notification