article · Energies
Variable speed hydroelectric pumped storage plants frequently employ doubly fed asynchronous machines to provide operational flexibility. Standard start-up procedures for these units function similarly to singly fed machines and waste considerable electrical energy. A novel start-up method addresses this inefficiency by using the rotor-side back-to-back voltage source converter for starting, braking, and speed control. Rather than short-circuiting the stator windings during starting, the technique injects a low-voltage direct current into the stator circuit. This injection lowers slip losses and cuts the required magnetising current, with direct current levels calculated from reactive power demands. Rotor speed dictates when a changeover switch transitions the stator between short-circuiting, direct current injection, and the electrical grid. Simulation of a 250 megawatt system and experiments on a 2.2 kilowatt machine prototype demonstrate that this approach conserves over 26.1 percent of electrical energy relative to conventional methods.
Pumped storage hydroelectric plants are critical for grid flexibility, but starting large asynchronous generation machinery typically involves substantial energy waste. By cutting start-up energy losses by more than a quarter, this technique can help hydroelectric operators boost net plant efficiency and reduce power demands during frequent starting cycles, supporting cleaner and more cost-effective grid management.
This technology applies to variable speed hydroelectric pumped storage facilities utilising doubly fed asynchronous machines, targeting plant operators, turbine manufacturers, and power utilities. The approach is at an applied and tested research stage, having been evaluated through computer simulations on a 250 megawatt machine and validated on a small 2.2 kilowatt laboratory prototype, meaning full-scale industrial trials would be needed before commercial deployment.
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The use of a Doubly Fed Asynchronous Machine (DFAM) provides attractive characteristics and offers operational flexibility in many variable speed generation applications, such as in a hydroelectric pumped storage plant. In a variable speed hydroelectric pumped storage plant, the start-up process of DFAM is identical to the conventional singly fed asynchronous machine, wherein a significant amount of energy is wasted. This paper introduces an energy-efficient start-up strategy in DFAM based hydroelectric pump-turbine. The back-to-back voltage source converter connected to the rotor side is amenable for speed control (real power), braking (regenerative/dynamic), and starting the unit. Further, in this starting technique, the stator circuit of the machine is injected with a low voltage DC supply at starting instead of short-circuiting the windings. This DC injection reduces the slip losses and cuts down the magnetizing current requirement. The magnitude of the required DC supply is estimated based on the machine’s reactive power requirement. Also, the switching of stator winding between the short circuit connection, DC injection, and grid supply is carried out using a changeover switch and determined by the speed of the rotor. The proposed starting strategy is investigated with 250 MW DFAM in Matlab/Simulink environment and experimented with a 2.2 kW DFAM prototype. Test results show that the proposed starting method can conserve more than 26.1 percent of electrical energy in the example application compared to the conventional V/f start-up strategy.
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DOI: 10.3390/en15093138
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