PowerStore - Flywheel energy converter

Background

DIgSILENT Pacific was engaged by PowerCorp to develop a PowerFactory stability model for the PowerStore flywheel energy converter. The PowerStore consists of a flywheel for energy storage of up to 18 MWs. The device has been successfully used for frequency stabilisation in islanded wind/diesel networks.

Electrically, the PowerStore consists of back-to-back AC/DC IGBT converters connected to a permanent magnet synchronous machine and coupled to a flywheel. The converter maintains the charge (speed) of the flywheel continuously by drawing a small charging current from the connected AC system. Through sophisticated control, the PowerStore is capable of independent frequency and voltage regulation.

PowerFactory model

DIgSILENT developed a detailed PowerFactory stability model for time domain simulations of the PowerStore. The model had to capture the behaviour of the back-to-back converters and also represent the flywheel state of charge so that the device current could be appropriately limited when the charge was either full or depleted. The model also represented the following additional critical controls:

• Power Factor control;
• Under/over frequency protection;
• Under/over voltage protection;
• Thermal limiting; and
• State of charge control.

Simulation results

The plots below shows the reponse of the PowerStore to a sudden decrease in system frequency caused by a relatively large increase in load in a small AC system.

The step load change 1 second into the simulation causes the frequency to begin to fall. In response, the PowerStore rapidly increases its output to stabilise the system frequency. When the system frequency starts to rise again after the load steps back to the original power level, the PowerStore output returns back to zero.