Drive controllers and sine-wave filters control energy flow in new flywheel energy storage units

Jon Lawson

The efficient and optimised use of electricity generated from renewable energies calls for an appropriate form of storage. In the recently developed EnWheel flywheel energy storage units from German company Stornetic, Combivert F6 drive controllers from KEB control the energy flow together with specially developed sine-wave filters.

Leitmarkt Agentur NRW has achieved this with the help of the EU-funded Quirinus project, which focuses on developing sustainable security of supply. This is ensured by high and efficient system stability in increasingly decentralised generation plants that feed their power into the grid on a fluctuating basis. The surplus energy is stored in electrochemical storage systems (LiIon/SuperCap) or as kinetic energy in flywheels. These forms of storage are particularly attractive in order to absorb peak loads and therefore keep the supply network constant. Flywheel energy storage units (see diagram below) are particularly suitable for ensuring consistent capacity over many charge and discharge cycles. Stornetic develops and produces storage systems of this kind with DuraStor.

In the event of an energy surplus, the EnWheel accelerates an active magnetically mounted carbon-fibre rotor up to 45,000 revolutions per minute. The drive is disabled for interim storage of rotational energy (idle mode). As the rotor rotates freely in a vacuum, there are virtually zero friction losses. If energy is required, the drive synchronises to the rotating rotor and feeds the “braking energy” back into the network.

In addition to shutdown and synchronisation on the rotating rotor, the cooling of the motor with low heat transfer in the vacuum is one of the particular challenges.

KEB Combivert F6 drive controllers: an ideally tailored package

In the new generation of flywheel energy storage units, KEB Combivert F6 drive controllers function in combination with specially developed sine-wave filters to control the energy flows in the flywheel energy storage units. The efficiency optimisation in the overall system is based on an ideally tailored package consisting of a drive controller, sine-wave filter and synchronous motor in the EnWheel with minimised harmonic content (THD) and therefore the lowest electrical losses on the rotor side of the drive.

The Speed Search function, which is further optimised by its use in the F6 drive controller, is responsible for the load application and release of the flywheel rotor in ‘idle’ mode, which ensures an immediate connection to the rotating rotor up to maximum speed. Due to the possible load release in this mode, the complete switching losses of the IGBTs in the drive controller are eliminated for yet another contribution to better energy utilisation.

Fields of application are increasingly created by the concept of “microgrids” to compensate for peak loads and thus the stabilisation of local grids.

The benefits of this solution

In summary, the system is a smart solution with drive controller and sine-wave filter for THD values <1.5%. The idle mode via Speed Search function up to 45,000 revolutions per minute. Operation of the motor with optimised switching frequency is done at output frequencies up to 750Hz.

The novel system boasts fast response times and high power gradients without diminishing performance over the entire service life of the system.

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