Inherent uninterruptible power supply via directly grid-connected machines and variable inertia flywheels

• A novel use of hydraulic variable inertia flywheels (HVI-FW) for inherent uninterruptible power supply (UPS) is proposed. • Direct connection of HVI-FWs to grid-connected motors enables instantaneous emergency power without power electronics. • Numerical simulations reveal that the combination of HVI-FW and electrically excited synchronous machines (EESM) offers the most stable voltage and frequency support. • The HVI-FW shows higher specific energy than conventional flywheels, reducing mass and material requirements. This paper analyses the potential of a novel hydraulic variable inertia flywheel to improve standby power systems when attached to directly grid-connected electrical machines. Conventional standby power systems often require several seconds to reach full operation after a grid outage. During this time, critical loads may be left without power, unless supported by an additional uninterruptible power supply system. By flange-mounting a hydraulic variable inertia flywheel to an existing grid-connected electrical machine, emergency power can be provided immediately without the need for power electronics. The ability of the hydraulic variable inertia flywheel to discharge at a quasi-constant speed offers improved frequency and voltage stability. The performance of the hydraulic variable inertia flywheel is compared with traditional flywheels of equivalent inertia and energy content across the two most common machines. The simulation results identify the most appropriate machine-flywheel configuration to complement standby power systems. Combining a hydraulic variable inertia flywheel with an electrically excited synchronous machine provides the most stable voltage and frequency support. The application of the widely used induction machine is found to be not preferable. Beyond acting as supplementary technology for standby power systems, combining directly grid-connected electrical machines with flywheels also offers secondary and tertiary benefits.