Tie Breaker Closing Make Before Break 4

[rockman7892]

I have a situation at my facility where we have two plants and their respective 4.16kV power systems operating off of two different utility transformers. On the main switchgear lineup for each distribution system there is a tie breaker to conntect one power system to the other in the event that we loose one of our utilty transformers this allows us to run both plants at reduced load. The tie breaker system is designed only to have on transformer operating when the tie breakers are closed, or in other words the system is not intended to run with the utility transformer paralleled. The 2 main breakers and tie-breakers are vaccum breakers that have a kirk key interlock system in place to ensure that the ties and both mains are never all closed at once putting transformers in parallel.

I am aware of all the hazards and percautions involved with putting transforers in parallel (tap changes, circulating currents, increased fault capacity etc, phase seqencing etc...) Most of these percautions with the exception of phase synchronizing are for transformers paralled for an extended duration. I am curious what options I may have to set these tie-breakers up in a "make before break" transfer scheme in order to keep the power supply seemless to the plant that is already running. If I have a phase synchronizing relay on both voltage sources is it possible to momentarily parallel both sources before interrupting one of the mains only as quickly as it would take for the breakers to open and close from a relay operation?

One of the issues we have is that if we need to shut down one of the utility transformers in order to switch the plant load to the other transformer we have to shut the entire plant down in order to close the tie breakers and then bring the plant back up again. This interrupts production and is generally not acceptable. I am looking for what options may exist for making this transfer seemless by closing both tie breaker first before opening the main and briefly paralleling the two transformers under a relay controlled operation.

Is this even an option without getting into a larger order of other issues with the transformers themsleves?

 

[PHovnanian]

Very intersting point about issues with the transfer on a system with small inertia loads. It sounds like part of the sucess for implementing an open transfer scheme depends on the motor loads holding the bus voltage to an appropriate level during the couple cycle transition?

Not just voltage. Phase angle as well. Once the primary source is opened, the motor's generated emf begins to slip behind that of the supply voltage. At some point, this phase difference reaches 180 degrees, making the subsequent inrush worse than locked rotor MVA. The system may not like that much. Motors even less so.

 

[rcw retired EE]

The Beckwith relay monitors the phase angle and voltage differences across the open breaker and sends the close signal anticipating the breaker closing time, similar to a good auto-synchronzier.

A regular synch check relay is too slow to use in this application.

By closing in phase, the excessive torques are avoided. If the motor voltage drops below a threshold (usually 30% nominal voltage) where the torque excursions would be within motor design parameters, a breaker close signal is given. If the in-phase close doesn't work, the minimum voltage or dead bus logic closes the breaker. There are many options that can be configured.

A big portion of the cost for fast transfer systems is the engineering investigation to determine all of the settings and logic. The relays themselves are not cheap either. I'd recommend calling a couple of suppliers and getting some estimates for both the investigation and the actual hardware.

 

[AusLee]

In the low voltage field, we've had this situation a few times before, and it was dropped every time except on one high profile building where the decision was to use the generators as an intermediate point of supply:

1. Installation is connected to TX1
2. Start-up gensets and synchronize with TX1.
3. Switch load to gensets by opening connection to TX1.
4. ATS is closed transition.
5. Synchronize with TX2 and transfer the load to TX2 by disconnecting the gensets.

Actually the transformers were the same, the changeover was taking place at the HV switchgear supplying these transformers which was fed from two incomers on two different rings.

In line with what waross said, waiting for two separately fed utility transformers to synchronize may never happen, and we deemed that fast acting open transition systems unnecessarily stress the installation, plus, 10 years down the track, who knows how fast these mechanisms will be.

Another option in LV we use static transfer switches (especially when switching the load from one UPS to another). By emulation, i think the best low/no-risk solution for your case is an HVDC link, you're paying big bucks anyway for other decent working option, and for that money you can probably enforce the case of gensets as a more value for money in backup.