Voltage unbalance in three-phase networks — NEN-EN 50160

A horticultural business with a mix of large three-phase drives (fans, pumps) and a lot of single-phase load (lighting, small equipment, charging points) on the same connection can be confronted with voltage unbalance: the three phase voltages are no longer equal in amplitude and/or exactly 120° apart in phase. NEN-EN 50160 ("Voltage characteristics of electricity supplied by public electricity networks") sets a quality limit for this.

What voltage unbalance is and how it is expressed

Unbalance is expressed as the ratio between the negative-sequence component and the positive-sequence component of the voltage (symmetrical-component decomposition). In a perfectly balanced three-phase system this ratio is 0%; the greater the asymmetry between the phases, the higher the percentage.

The standard's requirement

NEN-EN 50160 states that, under normal operating conditions, the negative-sequence component of the supply voltage must not exceed 2% of the positive-sequence component for 95% of a week (measured as 10-minute averages). In networks with a substantial share of single-phase or two-phase connected load, this limit may rise to 3%.

Causes

Uneven distribution of single-phase load across the three phases (common with lighting circuits, charging points, small single-phase drives).
Asymmetric network impedance between the phases, for example due to unequal cable lengths or cross-sections.
Faults or interruptions on one phase of the network.

Consequences for three-phase drives

Three-phase induction motors are sensitive to voltage unbalance: a relatively small voltage unbalance (a few percent, for instance) can cause a much larger current unbalance in the stator windings, which results in additional heat generation and accelerated ageing of the motor insulation. This makes unbalance particularly relevant in practice for businesses with large three-phase fan or pump drives.

Note: the exact relationship between voltage unbalance and current unbalance/motor heating depends on the motor design — verify this per situation using the motor specifications; do not rely on a fixed rule of thumb.

Who monitors this

The grid operator is responsible for monitoring voltage quality in accordance with NEN-EN 50160 at the point of supply. However, an installation owner who introduces unbalance themselves (for example through an uneven distribution of single-phase load within their own installation) can still contribute to a local problem, even if the grid operator remains within the standard's limits at the connection point.

Common mistakes

Distributing single-phase load randomly across the three groups without taking balance into account — as installations expand (new charging points, lighting), the risk of unbalance increases if this is not actively managed.
Immediately attributing unbalance-related complaints (overheating motors, vibrations) to the motor itself without measuring the voltage balance at the connection — the cause may lie outside the motor.
Confusing the 2% limit with an absolute guarantee against motor problems — even within the standard's limit, unbalance can, over time, contribute to accelerated motor wear in continuously heavily-loaded drives.