The diagram below shows a Single Line Diagram (SLD) of the typical Electrical System in a thermal power plant. Thermal power plants could be a Coal-Fired Thermal Plant, a Geothermal Power Plant, Combined-Cycle Power Plant, or a Nuclear-Powered Power Plant; or at least it’s a power plant with steam cycles, circulating cooling water system, lube oil system, and should be high speed (around 3600 rpm).
Typically, the electrical system in these kinds of power plants would be composed of;
1. The High Voltage System. This is the system on the high voltage side of the step-up or main transformer. It is usually composed of the switch yard and the take-off to the bulk power transmission system. Typical voltage could be 230 kV, 500 kV, 138 kV, etc.
2. The Generator Voltage System. This is the system and voltage level where the main generator unit is connected. Typical voltage here is 13.8 kV, 16kV, etc.
3. The Medium Voltage System. The station service transformer or an auxiliary transformer steps down the generator voltage to a medium voltage, typically 4.16 kV. The Medium Voltage System and Switchgear powers-up the plants major auxiliary load, a mostly large motor, which includes the Circulating Water Pump, and Hot Well Pump and Cooling Tower Fan for Geothermal Power Plants.
4. The Low Voltage System. The voltage in the Medium Voltage System is further stepped-down to low voltage, typically 440 V and 220 V, to serve the smaller loads. These loads include small motors and lighting equipments.
5. The DC System. Using an AC-DC converter, the AC voltage of the auxiliary supply is converted to DC to charge a battery and to power DC loads which, typically, include Instrumentation and Control Devices. During total plant and grid shutdown or when there is no available feedback power from the grid to supply power to the Plant, the energy stored in the battery is used to continuously power the Plant’s DC loads like the control system. Some AC loads that need to be continuously running during this event, like oil pump, are also powered by the battery through an DC to AC inverter.