Operation Description
2.8Tie Circuit Breaker or Logic Faults
The Continuous Power-Tie system has been designed to anticipate circuit breaker and logic faults. Should a circuit breaker or logic fault occur, the Power-Tie system maintains the critical bus function under fault conditions
2.8.1What Happens If a Failure Occurs During a Power Transfer
During normal operation, the Power-Tie logic maintains system isolation. Upon power transfer initia- tion, the Power-Tie logic checks the circuit breaker (CB) positions. The Power-Tie logic inhibits the power transfer if the circuit breaker positions are incorrect.
For Continuous Tie, if all circuit breakers are in their correct positions, then all circuit breakers momentarily remain closed, tying both systems together. The power transfer sequence stops. The static bypass becomes the backup.
If the operator selects momentary Power-Tie, the isolation circuit breaker opens. If this circuit breaker fails to open, the system returns to its initial configuration, maintaining the critical bus.
2.8.2What Happens If a Logic Failure Occurs During Power Transfer
If a failure occurs before the power transfer, the Power-Tie logic inhibits the power transfer operation. If a failure during the power transfer, one of the following occurs:
•If the failure occurs before overlap period has timed out, the Power-Tie logic returns the system to its initial configuration.
•If failure occurs after overlap period has expired (e.g., the isolating breaker has already been com- manded to trip), the Power-Tie logic completes the transfer.
•If the power transfer does not complete (e.g., the isolating breaker fails to trip within an accept- able time period), the Power-Tie logic returns the system to its initial configuration.
•If the failure occurs after the transfer has occurred, the static bypass becomes the backup.
2.8.3Hardware Failures
Improper operation of the Power-Tie are caused primarily by tie circuit breaker failures. The Power-Tie logic monitors the “Close” and “Open” sent to the circuit breakers. If the breaker command is not completed by the breaker within an acceptable time period, the Power-Tie logic takes corrective action. The Power-Tie implements corrective action based on the operation being performed and the current system configuration.
The Power-Tie logic takes protective action if a fault occurs on a UPS system during a load transfer or while the UPS systems are in continuous parallel operation. The Power-Tie logic takes action based on the current system configuration and the active operation to minimize the affects of the fault.
The Power-Tie controls add additional protection to the individual UPS systems. And, the protective controls of the individual UPS become an integral part of the overall fault protection scheme used by the Power-Tie controls.
2.9Recommendations - Tie Circuit Breaker Selection
Consider using non-automatic circuit breakers for continuous Power-Tie applications. The circuit breaker function should not be the primary fault limiting function. Using non-automatic circuit breakers eliminates a primary circuit breaker failure mode, the trip unit components within the cir- cuit breaker.
