DC SYSTEMS
Introduction
The dc supply systems are considered the most reliable power source in a power plant. They are normally supplied from a reliable ac power source through a rectifier. They are also battery-backed to supply essential loads when the normal ac power supply fails.
DC System Functions
The dc systems have the following functions:
● To supply equipment requiring dc power during normal operation and which is also required to operate following loss of normal ac supplies, e.g., essential instruments, control equipment, switchgear closing and tripping, telecommunications, protections, interlocks, and alarms
● To supply standby equipment required to operate following loss of normal ac power sup- plies, e.g., emergency lighting, emergency turbine lubricating oil pumps, and emergency generator seal oil pump
● To supply the equipment required to start the standby power generators, which consist of gas turbines or diesel generators
Mission Time of DC Systems
The dc systems are designed to operate from batteries for a least 30 min following a com- plete loss of ac power supplies. This period is known as the mission time for these systems. The standby batteries must have sufficient capacity to meet the mission time. A period of 30 to 45 min is considered acceptable for the mission time because the ac supplies are expected to be restored during this period. The nominal voltage for these systems is 48, 110, 220, or 250 V dc.
The dc systems provide reliable power to the following equipment:
● Emergency lighting
● Emergency auxiliary drives, e.g., turbine emergency lubrication oil pump and generator emergency seal oil pump
● Emergency valve operation
● Fire sirens
● Switchgear and control gear tripping mechanism
● Interlocks and protection
● Control equipment
● Sequence equipment
● Alarm and indication
● Remote control equipment
Figure 24.8 illustrates the dc scheme for the Heysham 2 power station which has two 660-MW generating units. A battery and a charger are provided for each unit. A standby battery and charger can also be made available to either unit when necessary. The charger is rated to supply the entire dc load for one unit plus 25 percent capacity to cover unknown and future loads.