A Permanent Magnet Variable Inductor for DC Fault Current Limiting Applications

A novel permanent magnet variable inductor (PMVI) is proposed for fault-current limiting applications. It is designed to fill the need in hybrid DC circuit breakers (DCCBs) to have low insertion inductance under normal operating conditions, but high inductance under fault conditions to slow down fault current di/dt. Below a designable ‘trip' current the magnetic core the device is driven into saturation by permanent magnets. Under a fault condition, the flux induced by the fault current flowing in the inductor winding drives the core out of the saturation region, creating a large increase of inductance. The operating conditions of the permanent magnets are analyzed to prevent partial demagnetization. Electromagnetic analysis is carried out in COMSOL FEA and compared to magnetic equivalent circuit analysis. An augmented MEC branch model is introduced to better capture eddy current effects that have a substantial effect on the PMVI inductance characteristic during high di/dt fault events. A PMVI was designed and built targeting 600V/20A with 50μH inductance at the nominal operating point. This PMVI device achieved 300% peak inductance change during the fault condition.