c.Redundant system. The redundant static UPS system configuration is shown in figure 2-25.
This configuration is made up of two or more normally energized basic systems connected in parallel and synchronized with one another. The static interrupters used in this configuration are solid-state devices which serve to isolate a faulty inverter from the connected loads and to prevent the other inverters from supplying high fault currents to the faulted inverter. In this configuration, although the rectifier/inverter combinations are duplicated for higher reliability, only one common battery is used. This is due to the extremely high reliability of batteries. The rating of each of the parallel basic systems should be such that if one system fails, the remaining systems are capable of supplying the connected load. Therefore, for a two-parallel system, each system should be rated for 100 percent capacity and for a three-parallel system, each system should be rated for 50 percent capacity, etc. In this configuration, all the parallel systems are normally energized and share the load equally. It is also used where very high reliability is required. Should one system fail, the remaining systems supply the load without interruption. installations (400 kVA and larger) where the load exceeds the rating of available systems. This configuration is also commonly used in very large configuration is more costly than other configurations to purchase and install. In addition, it has a lower overall efficiency since all the parallel systems have to be operated continuously at part load where the efficiency is lower than at full load. However, this configuration is desirable where very high reliability is essential and the reliability of the ac power supply is questionable. It should be noted that C4ISR facilities require a system reliability level of 99.9999 percent.
d. Cold standby redundant system. The cold standby redundant static UPS system configuration is shown in figure 2-26. It is made up of two basic systems with one common battery. Each of the two basic systems is rated at 100 percent capacity. During normal operation one system is de-energized and the other system is energized to supply the load through a manually operated circuit breaker. Upon loss of the inverter output, the static switch automatically connects the load to the alternate source without interruption. The second system's static switch is then closed manually, also connecting the load to the alternate source. To transfer the load to the second system, the static switch of the failed system is manually opened. Next the second system's breaker is manually closed, its static switch is opened, and the load is supplied from the system's inverter. During the transfer operations, the load is continuously supplied fromthe alternate source without interruption until it is switched to the second system. In this configuration the two inverters are not intended for operation in parallel and their output circuit breakers are interlocked to prevent this condition. One disadvantage of this configuration, when compared to the redundant system, is that the load is supplied from the unregulated alternate source for a short duration before the transfer is completed. Also, the transfer from one system to the other is manually accomplished and requires an operator's action. This makes the use of this configuration undesirable in locations where the alternate power source has a low reliability.
However, this configuration has a higher efficiency than a comparable two-parallel redundant configuration. The higher efficiency is due to operating the energized system at 100 percent capacity as compared to operating each of the systems of a two-parallel redundant configuration at 50 percent capacity. Another advantage of this configuration over the redundant system is that the two basic systems are not susceptible to a single failure.
e. Dual redundant system with static transfer switches. The configuration of the dual redundant static UPS system with static transfer switches is shown in figure 2-27. Like the redundant system configuration, it is made up of two normally energized 100 percent capacity basic systems connected in parallel with one common battery. Each of the two basic systems is synchronized to the alternate source. The static switch provided at the output of each system serves the functions of a static interrupter as in the redundant system configuration, and a switch.
Operation of this configuration is similar to the redundant system. In addition, upon loss of both systems or deviation of the output voltage beyond acceptable limits, the static switches disconnect the load from the inverters and connect it to the alternate source in a make-before-break transfer.
The addition of the alternate source increases the reliability of this configuration over the comparable redundant system. In addition, the static switches make this configuration capable of supplying high in-rush currents by transferring to the alternate source. As with all systems with a static transfer switch to an alternate ac source, a regulating transformer connected to the alternate ac source is commonly used.