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

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 f lywheel 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- f lywheel configuration to complement standby power systems. Combining a hydraulic variable inertia f lywheel 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.