STATIC TRANSFER SWITCH

IT managers will be surprised to learn that some medium and high power UPS systems on the market today (rated 50 kW and higher) use undersized static bypass switches despite their negative implications. By using a contactor or a circuit breaker in parallel with SCRs, these static bypass switches are able use smaller, less expensive SCRs that are rated to carry less than full load current continuously. This paper shows that the availability of the UPS system is compromised when undersized static bypass switches are employed in the system. The advantages of fully rated static bypass switches are discussed.

Fully and Partially Rated Static Bypass Switch The term “fully rated” or “partially rated” refers to the rating of the SCRs for continuous duty. A fully rated static bypass switch employs SCRs that are mounted on a heat sink and typically cooled by fans. By contrast, a partially rated static bypass switch employs SCRs that are typically mounted on a smaller heat sink without fans.

In the partially rated static bypass switch, both the SCR switch and the contactor relay (or electrically operated breakers) are simultaneously turned on when the UPS inverter is turned off. After approximately one second, the SCRs are also turned off. Thus, the SCRs are able to bridge the time after the UPS inverter turns off and the contactor or breaker completes the closing cycle. During the transfer from bypass operation to normal operation, the SCRs are turned on simultaneously when the contactor or breaker is turned off. After approximately one second, the SCRs are turned off while the UPS inverter is simultaneously turned on.

Rating of the Static Bypass Switch The fully rated static bypass switch is generally designed to carry 125% of the rated load. The partially rated static bypass switch is typically only designed to carry 100% of the rated load, although it can be designed to carry 125% rated load. If there is a continuous overload on the partially rated switch with the system in bypass mode, then either the load will be lost (because the breaker trips due to overload) or the contactor will drop (due to over current). By contrast, a fully rated static bypass switch that is designed to carry 125% rated load can sustain continuous overloads indefinitely, and that will naturally increase the reliability of the system with respect to overloads.