PowerStore - Flywheel energy converter

Background

powerstore SLD DIgSILENT Pacific was engaged by PowerCorp to develop a PowerFactory dynamic model for the PowerStore flywheel energy storage system. 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 an IGBT converter connected to a 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 dynamic model for time domain simulations of the PowerStore. The model captures the behaviour of the converter and also represents 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.

powerstore Block diagram

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.

powerstore simulation plot