Harmonics & THD limits for large consumers

Large industrial installations — with variable-frequency drives (VFDs) on pump and ventilation motors, LED assimilation-lighting drivers, and CHP/PV inverters — are significant sources of harmonic distortion. The Netcode Elektriciteit sets binding voltage-quality requirements at the connection point, which these sources can run up against in practice.

What THD is and why VFDs/drivers are the source

THD (Total Harmonic Distortion) expresses how much the voltage waveform deviates from a pure sine wave, as a percentage of the fundamental (50 Hz) component. Non-linear loads — in particular the rectifier/switching front end of a VFD, an LED driver, or an inverter — draw current in short pulses rather than a smooth sine wave, which causes harmonic currents (and, via the grid impedance, also harmonic voltage distortion).

THD limits from the Netcode Elektriciteit

For low- and medium-voltage networks (up to and including 35 kV), the Netcode Elektriciteit, in line with NEN-EN 50160, applies the following quality criteria for the voltage at the connection point:

THD ≤ 8% (all harmonics up to and including the 40th) for 95% of the time.
THD ≤ 12% (all harmonics up to and including the 40th) for 99.9% of the time.

These limits apply to the voltage on the grid itself, not directly to the individual current draw of a single machine — but a large-consumer connection with many VFDs/drivers can itself substantially contribute to exceeding this limit at its own connection point.

Note: the exact article number within the Netcode Elektriciteit where these THD limits are laid down could not be reliably traced to a single citable article from freely accessible sources — consult the full, current Netcode text (Netbeheer Nederland) for the precise article reference for a concrete project.

Practical consequences of exceeding the limit

Structural exceedance of the THD limit at a large-consumer connection can lead to intervention by the grid operator or to a contractual non-conformity of the connection agreement. For the installation itself, there are additional practical risks: extra heating of cables and transformers, possible resonance with capacitor banks (see the guide on power factor correction for the VFD/THD nuance there), and disruption of sensitive control electronics (for example a climate computer).

Mitigation measures

Active or passive harmonic filters, sized to the dominant harmonic order of the load present.
Multi-pulse rectifiers (for example 12-pulse instead of 6-pulse) for larger VFD power ratings, which inherently suppress certain harmonic orders.
Spreading non-linear load across multiple phases/groups to reduce local peak distortion loading.

Common mistakes

Using a standard capacitor bank to solve a THD problem — a regular capacitor bank compensates reactive power, not harmonic distortion, and can actually worsen an existing THD problem through resonance.
Treating THD purely as a "grid operator problem" — a large-consumer installation with many VFDs/drivers can itself make a substantial contribution to exceeding the limit at its own connection point.
Sizing harmonic filters without first measuring the dominant harmonic order — a filter that is not tuned to the installation's actual order distribution is less effective and can itself lead to resonance problems.
Relying on a single fixed Netcode article number without consulting the current text — the exact article reference must be verified per project against the full, current Netcode text.