A nomally closed tie breaker at a medium voltage main substation is normal in some situations. Some considerations when contemplating a NC tie are:
- How many NO main-tie-main substations are cascaded downstream of the NC substation? Draw a timing diagram of a 480V main-tie-main transfer scheme that is fed from a 5 kV main-tie-main transfer scheme and you will see what I mean - the undervoltage relay pickup delays start to stack up. If you employ an auto-restart scheme on your motors this is important.
- Do you have large motors that will require both mains and the tie to be closed to start the motor? If so it is better to run with the tie closed than institute some sort of short term scheme where the motor start button closes the tie for 30 seconds to let the motor start.
- What is the available fault current in the open tie and closed tie cases? Do you want to buy breakers capable of interrupting the higher closed tie fault current, or will that fault current help with system stability, motor starting, etc?
- With a NC tie a breaker failure protection scheme may be required depending on available fault currents and the system configuration.
- If you want to have multiple closed tie substations then more complicated breaker failure, directional relaying, or line protection schemes may be required to clear large magnitude faults quickly enough to maintain system stability.
In summary, closed tie substations require more power system design and protection knowledge than most consulting engineers or facility maintenance/operations personnel have. For large plants that have the knowledge and sophisticated engineering staff a closed tie system can make sense, but for the average industrial plant it probably doesn't.
