Sensing weak DC components of strong AC currents in railway vehicles

Innovative equipment useful for detecting DC currents as low as 1 A immersed in strong AC currents typically in the range 400-500 RMS, needed to allow the correct operation of the rail protection and signaling equipment of the Portuguese Railways, is presented. The equipment must trigger a protective device if a DC fault component exceeds 2 A for 2 s or more and also must count this failure for statistical purposes. The authors describe the gapped current transformer with compensation winding and Hall effect detector, together with the signal processing circuit, intrinsic protection, and self-test. The experimental dynamic and static behavior of the current detector is also presented. Some dozen units have been correctly operating in the field for about 4 years, with some mechanical problems at the beginning due to the current transformer 500 A cable, but with no electrical failures