Introduction

In major applications like major computer installations, process control in chemical plants, safety monitors, IC units of hospitals etc., even a temporary power failure may lead to large economic losses. For such critical loads, it is of paramount importance to use UPS systems.

But all UPS equipments should be completely de-energized for preventive maintenance at least once per year. This limits the availability of the power system. Now there are new UPS systems in the market to permit concurrent maintenance.

High-Availability Power Systems

The computing industry talks in terms of "Nines" of availability. This refers to the percentage of time in a year that a system is functional and available to do productive work. A system with four "Nines" is 99.99 percent available, meaning that downtime is less than 53 minutes in a standard 365-day year. Five "Nines" (99.999 percent available) equates to less than 5.3 minutes of downtime per year. Six "Nines" (99.9999 percent available) equates to just 32 seconds of downtime per year. These same numbers apply when we speak of availability of conditioned power. The goal is to maximize the availability of conditioned power and minimize exposure to unconditioned utility power. The concept of continuous availability of conditioned power, takes this concept one step further. After all, 100 percent is greater than 99.99999 percent.

The Road To Continuous Availability

We determine availability by studying four key elements:

o Reliability
The individual UPS modules, static transfer switches and other power distribution equipment must be incredibly reliable, as measured by field-documented MTBF (Mean Time Between Failures). In addition, the system elements must be designed and assembled in a way that minimizes the complexity and single points of failure.

o Functionality
The UPS must be able to protect the critical load from the full range of power disturbances, and only a true double-conversion UPS can do this. Some vendors offer single- conversion (line-interactive) three-phase UPS products as a lower cost alternative. However, these alternative UPS's do not protect against all disturbances, including power system short circuits, frequency variations, harmonics and common mode noise. If your critical facility is truly critical, only a true double conversion UPS is suitable.

o Maintainability
The system design must permit concurrent maintenance of all power system components, supporting the load with part of the UPS system while other parts are being serviced. As we shall see, single bus solutions do not completely support concurrent maintenance.

o Fault Tolerance
The system must have fault resiliency to cope with a failure of any power system component without affecting the operation of the critical load equipment. Furthermore, the power distribution system must have fault resiliency to survive the inevitable load faults and human error.

The two factors of field-proven critical bus MTBF in excess of one million hours and double-conversion technology ensure reliability and functionality. With reliability and functionality assured, let us look at how different UPS system configurations compare for maintainability and fault tolerance.