Current variations in the power supply causes severe problems in the electrical devices i.e if it increases or decreases in both cases it causes overheating which melts the conductors. High current is said when the current is above the ratings of any electrical devices, this rises the temperature of the transformer similarly when the current is below the min. rated value of the transformer but the load remains constant because of which it draws heavy amount of current to meet the load current requirement, which damages the core. Therefore to protect the transformer fuses are used at H.V and L.V sides.
Fuses do an excellent job of protecting transformers from overloads cause by slow line surges and high current overloads on the primary or secondary as long as they last long enough to melt the medium of the fuse without undue stress on the windings.
The other issue which can destroy a transformer much quicker and too fast for a fuse to respond is a lightning strike to either the primary or secondary side of the device. Protection from this cannot be offered by any electro-mechanical device as they all would operate too slowly to prevent arc over between windings in either the primary or secondary.
The judicious use of stationary sparkgaps across the primary and secondary stages of the transformer set a third below the maximum surge voltage for which the transformer was designed will do the trick. These can be open to the air or in the case of very dirty environments they can be hermetically sealed in a clean environment. These, even if they completely short are much more reasonable to replace than a very high capacity power transformer.