Fault Current Contribution of Medium Voltage Inverter and Doubly-Fed Induction-Machine-Based Flywheel Energy Storage System

One of the main concerns with inverter-based distributed generation (IBDG) is its negligible fault current contribution compared to synchronous generators. IBDG hardly affects fault current level, which shadows conventional protection schemes especially with a high penetration of IBDGs at high power levels and/or in island operation mode. This paper proposes a solution for IBDG in a medium voltage distribution network that allows active and adjustable IBDG fault current contribution without violating the nonoverload nature of the inverters. This is achieved by introducing a flywheel energy storage system based on a doubly-fed induction machine (DFIM) in parallel with the IBDG. Normally, the flywheel system is dedicated to power leveling; however, during faults, the flywheel DFIM storage system (FW-DFIM) has the ability to supply an exponentially decaying current to the grid. The parallel combination of the IBDG and FW-DFIM as a distributed generation (DG) unit is capable of providing a response similar to that of synchronous generators during a fault, but with additional control capability. The simulated system shows the effectiveness of the DG unit for downstream faults where it is capable of providing sufficient fault current to trigger the distribution network's protection devices when operating in island mode.