Demonstrated Rapid Ballistic Synchronization From Rest of a Hydraulically-Driven Synchronous Generator

We present a small-scale pneumo-hydraulic energy storage system (ss-CAES) which can provide frequency support to a power grid, as fast instantaneous reserve. This system is designed to be maintained at rest until called, thus avoiding idling costs. It utilizes a hydraulic drive train which transmits very high torque directly to the shaft of a synchronous generator, thus enabling rapid acceleration of the rotor. When an under-frequency event occurs, the generator must be accelerated from rest and synchronized to the grid. Here, we show that this is achieved by a ballistic synchronization approach which computes and follows an acceleration trajectory which simultaneously synchronizes both phase and frequency. A two-stage strategy is employed which switches from the ballistic acceleration trajectory to a grid-following mode once the synchronization conditions have been met. Computer simulations indicate that this approach enables very rapid synchronization of a model system to the grid in <;1.5 s. We then demonstrate this approach in hardware by implementing the developed control system upon a 100-kW-rated ss-CAES prototype. The dynamic characteristics of the hardware prototype are experimentally determined to obtain and optimize the key control parameters. These values are then used to demonstrate the start-up and synchronization from the rest of the 100 kW generator to a 50 Hz signal in <;4 s. Our results prove the feasibility of utilizing distributed standalone synchronous generation units for frequency support in small islanded power grids.