Executive Summary

• Under the UK Grid Code, turbines must remain stable and connected through balanced and unbalanced faults lasting up to 140 milliseconds, exposing sensitive DFIG rotor circuits to potentially damaging transient currents and voltages
• By rapidly diverting and dissipating excess rotor current during faults, they protect the converter and enable the turbine to ride through disturbances and return safely to normal operation
• Crowbar resistors must withstand high-energy pulses within milliseconds while managing thermal stress, low inductance and repeated faults. Robust performance helps minimise converter damage, maintenance costs and unplanned turbine downtime

Wind turbines are built to withstand harsh weather and operating stresses, but a less visible risk comes from the grid itself. When a fault occurs, rapid changes in stator flux can induce damaging transient voltages and currents in the rotor circuit. The power electronics housed in the nacelle, the unit at the top of the turbine containing the generator and converter, bear the consequences.

This is not a theoretical risk. The UK Grid Code defines what turbines must withstand, with both technical and commercial consequences for falling short. Clause CC.6.3.15 requires the generating plant to remain stable and connected through balanced and unbalanced faults lasting “up to 140 milliseconds in duration”, and to continue operating through longer voltage dips depending on retained voltage.

This Low Voltage Ride Through (LVRT) obligation exists for good reason. LVRT requirements define how turbines must remain connected and stable during grid faults — the very conditions that create damaging current surges in the rotor circuit. This ensures turbines continue supporting the grid rather than disconnecting at the moment they are needed most.

As inverter-based renewables increasingly replace conventional generation, the grid loses the rotational inertia that once buffered disturbances, increasing sensitivity to sudden changes in output.

An uncontrolled loss of generation during a fault can escalate quickly, risking frequency instability and wider outages. Turbine electrical systems must absorb fault energy without damage and return to operation promptly once the disturbance clears. Failure can result not only in regulatory issues, but costly converter damage.

When the grid pushes back

One of the most widely used architectures in onshore wind generation is the doubly fed induction generator (DFIG), where the rotor circuit connects to the grid through a power converter. This enables precise control of real and reactive power but also creates a direct electrical path to sensitive electronics. This connection is central to efficient operation but also increases exposure to electrical disturbances originating from the grid.

During a voltage dip or fault, rapid changes in stator flux induce transient voltages and currents in the rotor circuit that can exceed the converter’s rated capacity. Without protection, this current can reach the rotor-side converter at levels beyond its operating limits, risking damage to electronic components and control systems.

A crowbar resistor addresses this by switching across the rotor windings during a fault, creating a controlled path to dissipate excess current. This protects the converter by safely absorbing fault energy until normal conditions return. The approach creates a low-resistance path across the rotor circuit, with rapid response critical for effective protection.

While full-converter turbine designs use alternative protection approaches due to greater electrical isolation, crowbar resistors remain a proven and widely deployed solution across the global fleet of DFIG turbines.

Engineered for the split second

Crowbar duty is one of the more demanding applications a resistor encounters in power generation. Although fault events are brief, the resistor must absorb significant energy within a very short time. This places considerable stress on both the electrical and thermal characteristics of the resistor.

It must do so without mechanical or electrical degradation, particularly in the confined nacelle environment where surrounding equipment is both costly and sensitive. There is no tolerance for a protection component that fails under the conditions it is designed to manage.

Thermal performance is central to the specification. The resistor must have sufficient thermal mass to absorb pulse energy without compromising integrity, while cooling effectively to handle repeated fault events. Low inductance is also essential to ensure a fast and stable response. These requirements go beyond typical industrial resistor applications, making correct specification critical.

For operators managing wind farms over their operational lifetime, reliability is key. A crowbar resistor that performs consistently helps protect converters, maintain generation continuity and reduce the risk of unplanned maintenance while supporting long-term turbine performance and availability.

To find out more about crowbar resistors and protecting wind turbine power electronics, speak to Cressall’s experts.

About the author: Mike Torbitt is managing director of resister manufacturer Cressall.