Transformer Saturation

Transformer saturates because of increase primary voltage. Increase primary voltage has a consequence increase of intensity of magnetic field and magnetic induction until "knee point voltage" at the magnetising curve. From "knee point voltage", if voltage increases, then intensity of magnetic field also increases, but magnetic induction stays almost equal like as before increasing of voltage or it increases a little (a lot less than in linear area)... This area of magnetising curve is called area of saturation. Area of saturation is very dangerous area for work of transformer, because in this area ferromagnetic core is warming more than in linear area, so it has a consequence damaging of ferromagnetic core.

The transformer core can saturate due to, High Voltage, Low Frequency or a combination of both. Protection element V/Hz (ANSI 24) can be used to protect the transformer against over excitation as this can cause damage due to overheating / excessive core loss. This protection element is commonly used on generator step-up transformers.

Another reason that I can now think of is if you have a DC component in you excitation voltage. This can happen if your neutral point is at an elevated voltage. This can happen due to various reasons. One is geomagnetic induced currents caused by the interaction between the sun's flares and our earths magnetic field. Other reasons may be purely a bad quality neutral point which is subjected to sources of induced currents. Railway transformers sometimes have problems with this since they are installed near railways -which if not properly grounded -may cause an elevated neutral voltage.

In case of Generator transformers, an overfluxing protection is provided to safeguard the transformer against over excitation by generators and during load throw off condition. Generator transformers are also specified to have the capability to withstand the over fluxing condition viz 110%- continuous; 125%- for 60 seconds; 140%-for 5 seconds.