An electrical substation is a subsidiary station of the Waste-to Energy plant, where voltage is altered from low to high using transformers. Electrical energy produced in the generator is used to power plant equipment (accounting for 10 to 20% of total production, depending on the plant’s capacity), while the surplus is stepped up, usually to medium voltage, by the main transformer and sent out to the grid. The main transformer is usually immersed in oil and located near the substation.
Substations generally have switching, protection and control equipment and one or more transformers. In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may use re-closer circuit breakers or fuses for protection of distribution circuits. Other devices such as power factor correction capacitors and voltage regulators may also be located at the substation. The substations are placed on open surface in fenced enclosures, underground, or located in special-purpose buildings.
In case of turbine shutdown, it is possible to import electricity from the grid through the substation’s transformer. Power is stepped down to low voltage (690-415-230V) by each distribution transformer and fed to electrical loads in the plant. Each distribution transformer has the capacity of backup by another one.
Uninterruptable Power Supply (UPS) equipment is also available and capable of supplying all Direct Current (DC) consumers, but mainly it provides power supply security to the Distributed Control System (DCS) in case of failure of the main circuits, ensuring that the plant is properly controlled and supervised even in case of power failure.
An emergency diesel generator is provided for emergency loads in order to shut down the plant safely. The emergency diesel generator can be started automatically within 15 seconds of the network’s loss; return to the network will be manually performed. The main loads to be backed up by the diesel generator are:
• Furnace Fans;
• Feed water and condensate pumps;
• Turbine-generator control panels and oil systems;
• Continuous Emission Monitoring Systems; and
• Some of the building services.