Railway earth (2)

       

…when DC and AC powered railway lines are paralleled, such as here in Berlin Spandau station (and all across Berlin). It came as a great surprise, even to one of the so-called harmonics gurus, when he was measuring the no-load power intake of a 200 VA toroidal core transformer and turned on a hot air blower to half power on the nearest socket.

Suddenly, the no-load power leapt up from 1.8 W to 38 W! In the long run, this would have charred the transformer even without any load. The no-load current even rose from 9.2 mA to 1.26 A! What had happened? The power of such blowers and hair dryers used to be halved by simply inserting a diode into the current path. This caused an asymmetric voltage drop, i. e. a DC component, in the mains of, say, 0.5 V. Now, if the transformer's no-load current, which already brings the core close to magnetic saturation, is only 10 mA and the ohmic resistance of the primary winding is some 20 Ω, then the 0.5 V DC component of the line voltage will already drive a direct current of 25 mA in the primary winding and thereby fully saturate the core.

Therefore, this approach is now in effect prohibited by the latest release of the EN 61000-3-2, limiting the second harmonic to 1.05 A, but which applies to domestic appliances and not to locomotives. One may wonder whether the same could not happen to the transformers in the AC locomotives when the return currents from the DC and AC railways share one rail and cause a DC voltage drop along the rail of the AC driven train.