[broadgage]

Some loads draw DC current from AC mains, and the resultant voltage drop appears as a DC voltage superimposed on the AC supply.

For example, if a load draws more current on the positive half cycles, then the voltage drop between substation and consumer might be 8 volts on the positive half cycles and 7.8 volts on the negative half cycles, or a difference of 200mv.

Some cheap imported appliances draw substantial DC currents from AC mains by use of a silicon power diode in series with a heating element to give reduced heat.
Hair dryers in particular seem to use straight mains on the element for full heat, and half wave rectified for low.
Many later valve TV sets used a half wave rectifier in series with the valve heaters and therefore draw about 0.3 amp DC.
In some countries stairway lighting on delay switches is required to give warning that it is about to go out, normally this is done by half wave rectification.
Failed gas discharge lamps can work as crude rectifiers and the larger sizes can draw many amps of DC.

Some specialist theatre lighting dimmers are actually designed to give a half wave rectified output so as to drive one lamp on the positive pulses and another lamp on the negative pulses, thereby giving two control channels from one dimmer and cable run. If only one channel is used then several amps of DC can be drawn.