UNINTERRUPTIBLE POWER SUPPLY (UPS) SYSTEMS
Introduction
The UPS systems, also known as guaranteed instrument supplies (GIS) or no-break supplies, provide battery-backed ac and dc power supplies having higher reliability, quality, and continuity than the normal supplies provided from the electrical auxiliary system. They supply essential instruments, controls, and computers required for safe and reliable operating conditions. The loads supplied from the UPS systems include the following:
● Those required for postincident monitoring and recordings following a unit trip and loss of normal ac supplies.
● Those requiring very high reliability of power supply.
● Those required for the shutdown of the unit. These systems are normally supplied with dc power because it offers higher reliability than the ac power supply does.
● Those required for “black start,” i.e., when the normal ac power supplies are not available.
Battery-backed motor-generator (MG) sets and static inverters are used to provide UPS in power stations. Figure 24.6 illustrates a typical motor-generator scheme. The following are its main disadvantages.
● Excessive maintenance required, e.g., shutdown every few months for inspection and maintenance.
● Frequent replacement of the brushes due to excessive wear.
● Difficult parallel operation when the load is low.
● Unreliable frequency control.
● High failure rates due to large number of components. The mean time before failure
(MTBF) of an MG set is around 2 years.
Figure 24.7 illustrates a UPS system that uses static inverters. This system has proved to be more reliable than the one that uses MG sets. It consists of one inverter for a 500-MW generating unit. The unit electrical auxiliary system normally supplies the load via a single- phase step-down 415 V/110 V transformer. The 240-V dc supply to the inverter was derived from the 240-V dc station system. This system supplies also the emergency dc motors (e.g., emergency turbine lube oil pump motor and emergency generator seal oil pump motor), emergency lighting, switchgear and solenoids, etc.