Small-scale wind turbines — grid connection and protection

The guide on PV inverters and anti-islanding and the guide on CHP grid connection cover two forms of distributed generation that must comply with the same grid-connection standard. A small-scale wind turbine (typically up to a few tens of kW) falls under that same standard, but has two risks that a PV installation does not have: flicker caused by gusts of wind, and rotor overspeed on loss of the grid.

The same grid-connection standard as PV and CHP

Like a PV inverter, a wind turbine inverter must comply with NEN-EN 50549-1 (requirements for generators connected to the low-voltage grid): voltage and frequency limits, ride-through of short-duration grid disturbances where applicable, and — essentially — anti-islanding: on loss of the grid, the turbine must disconnect within the prescribed time, so the grid does not unintentionally remain live during maintenance or a fault elsewhere.

Generator type determines the grid behaviour

Directly-coupled asynchronous (induction) generator: simple and robust, but draws a significant magnetising current from the grid on reaching cut-in wind speed — comparable to the starting current of a three-phase motor (see the guide on motor starting current) — which can cause a brief voltage dip.
PMSG (permanent-magnet synchronous generator) with a full power electronic converter: completely decouples the rotor speed from the grid frequency and gives the converter more control over the connection behaviour, comparable to a PV inverter.

Flicker: a greater risk than with PV

Wind power fluctuates with every gust, and a directly-coupled induction generator converts those power fluctuations almost directly into voltage fluctuations at the connection point. This makes flicker (see the guide on voltage fluctuations and flicker) a more prominent concern for small-scale wind turbines than for a PV installation, where power — apart from passing clouds — varies far more gradually.

Overspeed on loss of grid: why this is more than a grid issue

For a PV inverter, anti-islanding is primarily a grid safety requirement — the PV panels themselves are at no risk on disconnection. For a wind turbine, loss of grid is also a direct hazard to the turbine itself: in some turbine designs (notably directly-coupled generators), the grid carries part of the reactive counter-torque that keeps the rotor at speed. If the grid disappears without the turbine quickly detecting and compensating for this, the rotor can overspeed — with a risk of mechanical damage to blade, shaft or bearings, and, in an extreme case, of blade parts detaching.

For that reason, the electrical protection of a wind turbine on loss of grid or overspeed drives not only grid disconnection but also a mechanical or aerodynamic braking function — for example a mechanical disc brake, furling the rotor out of the wind, or adjusting the blade pitch to a braking position.

Lightning protection

A wind turbine mast and rotor blades are often the tallest point in the immediate surroundings, with an elevated risk of a direct lightning strike — comparable to the considerations from the guide on lightning protection separation distance, but applied to a moving, earthed structure whose blade tips sweep a considerable capture area during rotation.

Note: this article covers the electrical grid-connection and protection aspects. The structural, aerodynamic and permitting requirements for a wind turbine fall outside the scope of NEN 1010 and are covered elsewhere (building code, manufacturer standards, the IEC 61400 series).

Practical relevance

When assessing a small-scale wind turbine installation, it is not sufficient to check only the grid-connection standard (NEN-EN 50549-1) as one would for a PV installation — the assessment must also establish whether the electrical grid-loss detection is actually coupled to a mechanical braking function, and whether the expected flicker stays within the permitted limits for the local wind characteristics.

Common mistakes

Assessing a wind turbine connection as "the same as PV" without weighing the flicker and overspeed risks that are specific to wind.
Assuming anti-islanding only protects the grid — for a wind turbine, the coupled mechanical braking function also protects the turbine itself against overspeed.
Skipping the flicker assessment because the turbine is "small" — precisely a directly-coupled induction generator can cause disproportionate flicker relative to its power on a weak grid.