They are manufactured in a power range of 10-50,000 kVA, up to a high voltage level of 132 kV, typically three-phase, ONAN or ONAF cooled, and suitable for both external and internal use. The dimensions of the transformer tanks are calculated to store enough oil to cool the internal temperature. Protection elements such as Buchholz relays, contact thermometers, contact pressure valves, and contact oil level indicators are used to protect the transformer while it is operating in the network.