In general, most protective relay control scheme circuits operate infrequently, and contaminants can accrue on the contacts, thus reducing or preventing the full voltage from being delivered to the operating coil in a control scheme. Where the control circuit contacts are exposed to the environment, higher control voltages are required. Typically, 125 Vdc or 250 Vdc is selected. In the case where the control scheme contacts are protected from the environment, lower voltages may be utilized, provided excessive voltage drops are not encountered.
Because of pollution condition of power plants environment, the 220 Vdc is utilized as auxiliary control and protection voltage.