Novel Over-current Tripping Device with Multi-level Adjustment Capability Based on Magnetic Coupling for Air Circuit Breakers in DC Traction System

In the DC traction power supply system in subway, over-current tripping device (OCTD) is essential for air circuit breakers (ACBs) as backup protection. At present, current threshold value (<italic>I</italic>_th) of OCTD is adjusted by the cooperation of magnetic circuit regulating block (MCRB) and spring. Depending on MCRB is installed or not, only two-level adjustment can be realized, resulting in low adjustment accuracy of <italic>I</italic>_th. In this letter, a novel OCTD based on controllable magnetic coupling is proposed. Instead of using MCRB, an auxiliary winding wrapped on the static iron core is added, with different resistors and thyristors attached. By controlling thyristors, a wide-range and multi-level adjustment of <italic>I</italic>_th can be achieved. Benefits of the proposed method are: reducing the stiffness factor of spring from 55N&#x002F;mm to 42N&#x002F;mm; improving the adjustment accuracy from 0.66kA&#x002F;mm to 0.33kA&#x002F;mm; adjusting <italic>I</italic>_th online without power supply system interruption.