Tech corner: Closed Transition Transfer Switch (CTTS)
A Closed Transition Transfer Switch (CTTS) is often used in applications where the load cannot tolerate a momentary loss of power or when operating requirements dictate zero interruption.
Peak shaving is a common example, where rate discounts are offered by the Utility Company to commercial and institutional customers who are willing to operate their standby generators during periods of peak demand. In this instance, both utility and standby power are briefly interconnected or paralleled (100ms or less) to allow for a seamless transition between sources.
When establishing a standby generation interconnect, most utility companies have specific requirements to ensure that safety, protection of equipment, and system reliability are not compromised. Some of the technical specifications often cited by the Utility company are as follows:
- Parallel Limit Timing Relay (IEEE device 62PL): set for a maximum delay of 100ms and trips one of the sources open if the source paralleling time exceeds this amount. This is a standard feature in an Eaton CTTS.
- Reverse Power Relay (IEEE device 32): trip opens, after a maximum delay period defined by the utility, to prevent reverse current flow onto the utility grid. This is available via COE.
- Lockout Relay (IEEE device 86): a mechanical locking device that is typically wired into a disconnect device (circuit breaker or switch) and when tripped will prevent a signal from closing the disconnect device. A person must manually reset the lockout relay before the disconnect device can be reclosed. This ensures that a de-energized system is not reenergized by automatic control action, and prevents a failed control from auto-reclosing an open breaker or switch. This is available via COE.
When specifying the withstand/close rating of a transfer switch in a closed transition application, care should be taken to consider the available fault current when both sources are momentarily paralleled. While faults can occur at any time, initiating a change to the system (moving contacts, vibration from switching, energizing previously de-energized lines, etc.) can indirectly result in movement of energized electrical conductors which can increase the probability of an arc flash event.
For more information on the interconnect or inter-tie requirements in your area, or transfer switches in general, consult the local utility company and/or Parts & Power as your Caribbean distributor for Eaton electrical products.