If the pulse rate is 1billion per second,then the signal frequency is 1GHz,not 50/60 Hz.
The signal may have a DC reference level,so that one 1/2 cycle returns to a set DC level,rather than being symmetrically distributed about zero volts,but it is effectively an UHF AC signal.
The DC component is present for such a short time compared to that which it takes for the transformer magnetic field to respond,that there could be no saturation effect.
In any case,for many other reasons,a powerline transformer will not operate at 1GHz,so you will not be able to do what you are attempting.
Even at these lower frequencies,a transformer designed for 50Hz will not
operate as a transformer----rather,it will look like a very large inductor.
Any coupling between primary & secondary will be due to capacitance,or some magnetic coupling outside the core.
A pulsed signal is effectively AC.
Even if the transformer could work,you would lose any DC component.
Power authorities do feed high audio & above control tones through their networks,but they need to use special circuitry to jump from primary to secondary of a power transformer.
I suggest you Google for "Electricity System control tones",or some similar term.