Large converter-fed synchronous machines used in pumped hydroelectric energy storage face several significant challenges related to their design, operation, and efficiency. These challenges largely stem from the interaction between the converter systems and the synchronous machines, particularly concerning current and voltage harmonics, losses, and mechanical aspects:
Key Challenges
1. Increased AC Copper Losses Due to Current Harmonics
Converters introduce current harmonics that increase AC copper losses in the stator windings of synchronous machines originally designed for traditional operation. These extra losses generate excessive heat, which can lead to winding damage or failure if not properly managed.
2. Stator Winding Design Adaptations
To withstand the effects of harmonic currents from two- or three-level converter topologies, the thickness of the strands in the stator windings often needs to be reduced. If this is not feasible, additional filtering equipment must be placed between the converter and the generator to mitigate harmonic effects.
3. Effects of Damper Bars
Including damper bars in converter-fed synchronous hydropower generators raises AC copper losses in the stator and causes significant losses within the damper bars themselves. Consequently, the use of damper bars is generally discouraged when employing two- or three-level converter topologies.
4. Mechanical and Electrical Design Trade-offs
The optimal synchronous reactance (around 1.2 to 2.0 per unit) and rated frequency (between 40 Hz and 50 Hz) need to be carefully selected to minimize total system costs and improve efficiency. These parameters directly influence converter and generator design and operation, impacting overall cost-competitiveness compared to fixed-speed machines.
5. Vibration and Operation Issues
Retrofitting synchronous generators into systems with different electrical configurations, such as doubly-fed induction generators, can cause mechanical vibration problems, which require careful engineering solutions.
Summary
In summary, the main challenges of large converter-fed synchronous machines for pumped hydro storage revolve around managing harmonic-induced losses, adapting machine design to converter characteristics, avoiding increased losses due to damper bars, optimizing electrical parameters for cost and efficiency, and addressing mechanical operation issues. These factors collectively complicate the design, operation, and cost-effectiveness of converter-fed synchronous machines in variable-speed pumped hydroelectric energy storage plants.
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