The transformer is the heart of the substation. The transformer changes the relationship between the incoming voltage and current and the outgoing voltage and current. Power transformers used in substations are rated by their primary and secondary voltage relationship and their power carrying capability. For example, a typical substation transformer would be rated 69-13 kV and 20 MVA; meaning the primary or high voltage is 69 kV, the secondary or low voltage is 13 kV and the transformer has a power rating of 20 MVA or 20,000 KVA. Substation transformers, like most utility transformers, consist of a core and coils immersed in oil in a steel tank. The oil serves both as an insulator and as a coolant to keep the core at reliable operating temperatures.