Enabling fail-safe and initial charge limitation in uncontrolled rectifiers by applying virtual-air-gap variable reactors

In this paper, two beneficial features are enabled in uncontrolled rectifiers by controlling virtual-air-gap variable reactor (VAG-VR): 1) fail-safe function - the VAG-VR can automatically isolate the faulty rectifier from power source, to ensure the power quality of other devices connected to the power source and protect the faulty rectifier from large fault current and thus overheating; 2) initial charge limitation - the VAG-VR provides a large reactance initially to suppress high current to a specified level in the uncontrolled rectifier. Then the reactance of VAG-VR decreases to the minimum value gradually, which does not affect the steady-state operation of rectifiers. Compared with other existing approaches of initial charge limitation, the VAG-VR demonstrates controllable reactance rather than resistance. Moreover, the fail-safe function is a novel feature brought into the uncontrolled rectifiers. In the test, the transient voltage sag is reduced from 84 V to 23 V and the inrush current into the faulty rectifier is limited from 125 A to 34 A. These two beneficial features are verified by simulation tests as well as experiments.