Calculation of Overpressure in the Enclosure of a Hydroelectric Generator during a Phase-to-Phase short circuit with electric arc

To estimate the overpressure generated by such internal arc faults and to design appropriate pressure relief systems, we adapted the methodology proposed by CIGRE Working Group

A3.24 (Report 602: Tools for the Simulation of the Effects of the Internal Arc in Transmission and Distribution Switchgear) to the specific context of synchronous generators. Key adaptations include:

1. Dual energy sources: During the fault, both the power grid and the generator contribute to the fault current until the protection systems operate.

2. Post-tripping contribution: After the circuit breaker opening, the generator continues to feed the arc for tens to hundreds of milliseconds.

3. Dynamic fault current: Unlike faults in switchgear cells, the short-circuit current in generators is highly variable over time. A campaign of more than 70 high-power electric arc tests were conducted at the EDF R&D laboratories. They enabled the development and validation of an arc model based on the shortcircuit current and arc length. This model predicts significantly higher voltages and energies than the default model proposed by the CIGRE working group.

The new arc model and overpressure calculation methodology were implemented in an electromagnetic transient simulation software and applied to several hydroelectric generators and to various operating conditions. Key findings include:

• Overpressure levels: Without venting, overpressure can rapidly reach several to tens of kilopascals, depending on operating conditions and fault duration. • Dominant contribution: The electrical grid constitutes the primary source of shortcircuit current, contributing to between 60% to 80% of the total overpressure.

Nevertheless, the contribution of the generator remains significant. • Mitigation potential: Properly and reasonably sized pressure relief vents, combined with adequate discharge volumes, can effectively reduce internal and external overpressure to acceptable levels. This study provides a validated modelling framework for assessing and mitigating the effects of internal arc faults in enclosed generators. The results align with ongoing efforts to improve the design of safer and more resilient hydroelectric generation systems.