The metal fence surrounding the substation ensures that almost no electric fields emerge from the substation itself. Any electric fields round the substation will come almost entirely from the overhead power lines entering it. But see also the electric field inside the substation.
There is equipment inside substations which produces magnetic fields. But the field falls with distance quite rapidly, and by the time you are at the perimeter fence or a few metres outside it, the magnetic field from inside the substation is usually approaching background levels. Again, the largest magnetic fields round the perimeter of a substation almost always come from the overhead lines and underground cables entering it. There are often several such lines and it can be difficult distinguishing the field from the substation itself from the fields produced by these lines.
Sometimes, if a transformer is located inside the substation close to the perimeter fence, the magnetic field produced by the cables or busbars supplying the transformer can cause an elevated magnetic field outside the fence. The same thing can happen if something called a reactor is very close to the perimeter - more on this. But generally, away from any overhead lines entering the substation, the magnetic field at the perimeter fence would be perhaps a microtesla.
Magnetic fields from a simple substation
Substations become very complicated, with many different sections of conductor carrying currents in different directions, and calculating the fields from them gets very complex. We illustrate the fields produced by an unusually simple example (actually Birkenhead Substation). One double-circuit enters the substation, and through a simple arrangement of busbars and switchgear is connected to three transformers (only two operating under the conditions of this modelling). The current leaves the substation through busbars (to an adjoining 132 kV substation, though this is not included in our model).
The following contour plot shows the magnetic fields produced by this simple arrangement, for a typical load flowing into the substation of 1300 A:
This confirms that the highest fields at the closest public approach (the perimeter fence) are where the overhead line enters the substation. The fields under the busbars crossing the perimeter fence are slightly higher, because these busbars are at a lower clearance than the overhead line, but this is not a publicly accessible area. All these fields are well below the public exposure limits.