Comparing the S&C PureWave UPS™ System to On-Line UPS Systems.

This article compares the S&C PureWave UPS System with commercially available on-line UPS systems, and highlights the many advantages the PureWave UPS System offers.

On-Line UPS Systems

An on-line UPS system continuously converts utility ac power to dc then inverts the dc power back to ac. The batteries are located on the dc bus between the rectifier and inverter. Since the UPS system continuously powers the load, this type of system is referred to as “on-line.” It’s also known as a “conventional” system or “double-conversion” system.

Refer to Figure 1 for a simplified single-line diagram of a typical on-line UPS system.

Figure 1. On-line UPS system.

In the event of an outage, the batteries provide power to the inverter, which produces ac power so that the load will not experience any disturbance. Batteries can be supplied in increments, providing as little as 5 minutes or as much as several hours of ride-through. Typical applications have ride-through times of 10 to 15 minutes.

Discrete systems typically provide protection for loads up to 1,000 kVA. Systems can be paralleled to achieve larger capacity or to provide parallel or isolated redundancy, as discussed below.

Parallel Redundancy

Parallel-redundant systems are the most common configuration of “multi-module” systems. Refer to Figure 2 for a simplified single-line diagram of a typical parallel-redundant system. Most such systems are designed with “N+1” modules, which allows the system to continue functioning if one module malfunctions; bypassing is not necessary.

In a parallel-redundant configuration, individual UPSs are paralleled together to serve a larger load than could be served individually. The practical limit for a parallel-redundant system is 3,000 kVA or 4,000 amperes at 480 volts. S&C PureWave UPS Systems can be operated in a parallel-redundant configuration by adding more power modules than actually required to carry the load, providing the ability to carry the load even if a power module goes off-line. It is more cost-effective to add a power module to a PureWave UPS System than to add an additional conventional UPS of the same capacity.

Figure 2. Parallel-redundant configuration.

For example, to serve a 2000-kVA load with N+1 redundancy, three 1000-kVA conventional UPSs totaling 3000 kVA are required. This load could also be served with N+1 redundancy by a 2500-kVA S&C PureWave UPS System. If one power module goes off-line, 2187 (2500 – 313) kVA of capacity remains available to serve the load.

Isolated Redundancy

An isolated-redundant system offers a higher level of availability and maintainability. Refer to Figure 3 for a simplified single-line diagram of a typical isolated-redundant system. In this configuration, each module is a complete UPS system serving its own respective load; one module is a reserve that can back up any of the other modules. Isolated-redundant systems have been installed with as many as six on-line modules and one reserve.

Figure 3. Isolated-redundant configuration.

The primary advantage of this configuration is that loads can be isolated to match the capacity of conventional UPSs, the largest of which is 1000 kVA. But with S&C PureWave UPS Systems, there is no need to isolate loads on the basis of UPS capacity. The PureWave UPS System is available in larger capacities, as well as in medium-voltage designs for very-large critical loads.

“Hot-Tie” Redundant Configuration

The most popular UPS configuration for very large Internet server-type facilities is the “hot-tie” or “double-redundant” arrangement. This arrangement includes features of both parallel-redundant and isolated-redundant systems. Refer to Figure 4 for a simplified single-line diagram of a typical hot-tie redundant system.

Figure 4. Hot-tie redundant configuration.

The term “hot-tie” comes from its use of two UPS systems that are electrically isolated from each other during normal operation but running in synchronization. Down-stream critical loads can be instantly switched between the two systems. Each UPS system offers “N+2” redundancy; the entire critical load can be run from one UPS system, if necessary, in a non-redundant mode.

For your information, two 10-MVA S&C PureWave UPS Systems are presently in service at a major semiconductor fabrication plant, in the hot-tie redundant configuration.

Advantages of PureWave UPS System

This table summarizes the advantages of the S&C PureWave UPS System compared with typical on-line UPS systems.

Feature	S&C PureWave UPS™ System	On-Line UPS System
Efficiency	The PureWave UPS System is 98 to 99% efficient . . . 6 to 9% more efficient than an on-line system, reducing operating costs	On-line UPS systems have continuously energized rectifiers and inverters. Efficiency is typically 90 to 92%, including air conditioning
Installation	The PureWave UPS System can be installed indoors and outdoors. Outdoor installation significantly reduces cost	On-line UPS systems are designed for indoor installation, requiring valuable floor space or a dedicated building
Battery Reliability	Batteries in the PureWave UPS System are not continuously connected to the dc bus, and thus don’t experience dc ripple. Equalize charging is possible, resulting in longer battery life and more reliable performance	Batteries in on-line UPS systems are continuously connected to the dc bus. The dc ripple degrades the batteries and the batteries cannot be equalize charged
Battery Maintenance	Batteries in the PureWave UPS System require no maintenance, reducing operating cost	Batteries in on-line UPS systems require routine maintenance, increasing operating costs
Battery Warranty	S&C offers a five-year non-prorated warranty	On-line UPS system manufacturers typically offer a one-year prorated warranty
Ride-Through Time	The PureWave UPS System provides 60 seconds of ride-through at full load . . . sufficient to support the load while back-up generator is started	On-line UPS systems typically provide 10-15 minutes of ride-through. Longer ride-through times offer data center operators the opportunity to shut down if the generator does not start
Generator Sizing	The PureWave UPS System does not inject harmonic currents into the system, so a back-up generator does not need to be oversized to compensate for harmonic currents and voltages	On-line UPS systems inject harmonic currents into the system, requiring that a back-up generator be oversized by 50 to 300%