High-Energy Discharge Resistor Protects Wind Turbine Generators During Grid Faults

As renewable energy sources become more integrated into global power grids, the need for robust protection mechanisms in wind turbine generators has grown significantly. One such mechanism involves the use of a high-energy discharge resistor in the static excitation system to protect against overvoltage during grid faults or sudden disconnections.

This case study focuses on a wind farm located in Northern Germany, consisting of 45 wind turbines with doubly-fed induction generators (DFIGs). These generators rely on static excitation systems to control rotor currents and maintain stable output. However, during a series of severe thunderstorms, multiple turbines experienced grid disconnection events that led to uncontrolled voltage spikes in the excitation system.

Challenge: Overvoltage Risk During Grid Faults

During normal operation, the static excitation system maintains precise control over the generator's field current. However, during sudden grid faults or emergency shutdowns, the excitation system is abruptly turned off. Due to the inductive nature of the generator rotor, the stored magnetic energy must be quickly dissipated to prevent dangerous overvoltages.

Without proper protection, these overvoltage events could damage semiconductor components, including IGBTs and thyristors, leading to costly repairs and prolonged downtime.

Solution: Integration of Crowbar Circuit with High-Energy Discharge Resistor

To mitigate this risk, the wind farm retrofitted each turbine’s excitation system with a crowbar protection circuit , consisting of anti-parallel SCRs and a high-energy discharge resistor . This configuration allows for immediate conduction when overvoltage thresholds are reached, redirecting the energy from the rotor winding into the resistor bank.

The selected discharge resistor was designed to withstand repeated high-energy pulses typical of wind generator applications. It featured a modular design, allowing for easy replacement and scalability depending on the turbine size and generator rating.

How the System Works

When the generator is disconnected from the grid—either intentionally or due to a fault—the excitation current is interrupted. This causes a rapid rise in voltage across the DC bus. The crowbar circuit detects this voltage spike and triggers the SCRs, connecting the high-energy resistor directly across the rotor winding. This creates a controlled discharge path, ensuring that the energy is safely absorbed and dissipated as heat.

The resistor was engineered to handle peak energies of up to 25 kJ per discharge and had a thermal capacity sufficient to manage multiple discharges within a short period without degradation.

Outcome: Enhanced Protection and Reduced Downtime

Following the retrofit, the number of excitation system failures dropped dramatically. Post-event inspections showed no signs of damage to sensitive electronics, and the average repair time was reduced by nearly 60%.

Operators reported increased confidence in the system’s ability to respond to unpredictable grid events, particularly during storm seasons. The integration of the high-energy discharge resistor proved to be a cost-effective and reliable solution for protecting wind turbine generators during abnormal operating conditions.

High-Energy Discharge Resistor Ensures Safe Generator Shutdown - EAK RESISTORS