Main Tie Main Automatic closed transition

[elec_eng]

I am designing a 3000A main-tie-main system from (2) 2500kva, 25kV to 480V utility transformers for a hospital. Our typical design is to control CBs manually with kirk key and tie breaker normally open.

But in this project, the CBs are controlled automatically. Now the owner?s facility engineer is asking if we can close tie breaker before we open the one of the mains when we restore the system to normal after a single source failure. This will be automatic transfer and the main will be often without any delay right after tie breaker is closed so two source will be paralleled very short period of time.

The 2500kva utility transformers are identical and supplied from the different substations so, they might be supplied from the different sources.

I know I have to consider the higher AIC rating of the CBs and switchgear but considering the transformers are supplied from the different sources, can I make a closed transition?

 

[rcwilson]

There are many past discussions on this topic that you could search. Most have to do with the high short circuit currents during the transition as you already mentioned.

But your question is about momentary paralleling two sources that may not be fed from the same upstream source and could be out of phase. Put a synch check relay across each main and tie to prevent closing unless the sources are close in phase angle and voltage. Use a relay that has live-line, dead-bus function otherwise the synch check contact will not allow energizing a dead bus.

 

[iceworm]

... considering the transformers are supplied from the different sources, can I make a closed transition?

Adding to Bob's response:
Only you (and likely the facility engineer as well) can answer this:

1. Are the two sources in sync, locked for ever? Can one side be energized and the other down?
   
2. Can the two sources normally be in sync but under some conditions drift out of sync?
   
3. Are the two sources independent and only ocasionally drift into sync then immediately drift out?

Depending on your answers, here are some possible answers to your question:

1. Yes, probably want a sync check relay - something like a Basler BE1-700V.
   
2. Yes, probably want a sync check relay - something like a Basler BE1-700V.
   
3. Maybe, this one is tough. Sync check relay and fast CBs might work.

ice

 

[ghostbuster]

Our hospital had a new H.V. switchboard shipped for the expansion (main-tie-main).The utility would not allow automatic operation of the H.V. tie period.It had to be strictly manual operation by the utility personnel only.

 

[elec_eng]

Bob and Ice:

I was consdiering Sync Check Realy as well but my concern was if the closed transition makes sense or not considering the two sources are independent. What if they never sync, then it will never transfer??or they are bound to sync some point?

ghostbuster:

That is next thing what I am going to do. I will try too find out if the POCO will allow the closed transition or not.

 

[iceworm]

... my concern was if the closed transition makes sense or not considering the two sources are independent. What if they never sync, then it will never transfer??or they are bound to sync some point? ...

Yes, that is exactly what you have to figure out. You're right, if the two don't sync, it won't closed transfer.

If they drift in and out of sync, they have to stay in sync long enough for two CB to operate - maybe 30ms - 80ms. The first cb closes to parallel, the second opens to separate. Your tie will not stand paralleling the two once they drift even the slightest bit apart. For the short time the systems are paralleled, your tie is trying to hold the two in sync. With two stiff utilities on the two services, the two transformers and tie cb are essentially transfering power between the two services. The current will get very high very quickly. I'd guess the transformer secondary CBs will open first.

You are the engineer of record, so you have to figure it out. Looking at it from here, I can't tell which it is.

Your customer, the facility engineer, wants a closed transfer. Okay. Looking at this from my point, I often function as a facility engineer, there would be two cases:

1. I would already know it was possible and wanted the engineering package to provide for closed transfer, or;
   
2. I didn't know and wanted the research done to see if it was possible.

In either case I would tell the design firm what I wanted and why. It doesn't look like you got that information.

I'd likely be calling the facility engineer and asking and calling who ever is supplying the services and asking. You can't figure it out without knowing.

ice

 

[jim dungar]

FWIW

Maybe you will be lucky and the two utility feeds are actually the same source. I have seen many installations where there is a single utility substation with two different feeds to a facility, By a simliar same token, the utility may keep the two sources in sync through their own M-T-M configuration.

 

[iceworm]

Our hospital had a new H.V. switchboard shipped for the expansion (main-tie-main).The utility would not allow automatic operation of the H.V. tie period.It had to be strictly manual operation by the utility personnel only.

... I will try too find out if the POCO will allow the closed transition or not.

If you are contemplating a primary M-T-M, then scratch all I have said - ghost has it right.

However, from your first post, I am guessing your M-T-M switching is on the hospital xfms secondaries. If so, the utility rarely cares. They want to know, but the interest is making sure the protective relays are set up to disconnect the utility quickly should something go wrong. They don't want to upset the rest of their customers.

ice

 

[elec_eng]

I didn't know and wanted the research done to see if it was possible.

That is the case here. I will call the utility co. and try to find out. Thank you so much for the great advice.

 

[elec_eng]

Maybe you will be lucky and the two utility feeds are actually the same source. I have seen many installations where there is a single utility substation with two different feeds to a facility

What I was told was each substaion is fed from the different transmission line. That doesn't necessarily mean they are different source, but might have a good chance to be different source.

By a simliar same token, the utility may keep the two sources in sync through their own M-T-M configuration.

Good point. I will check that as well. Thanks.

 

[elec_eng]

If you are contemplating a primary M-T-M, then scratch all I have said - ghost has it right.

However, from your first post, I am guessing your M-T-M switching is on the hospital xfms secondaries. If so, the utility rarely cares. They want to know, but the interest is making sure the protective relays are set up to disconnect the utility quickly should something go wrong. They don't want to upset the rest of their customers.

ice

Yes...My M-T-Mis on the secondary of xfmers.

 

[SG-1]

Besides a 25 there are other types of controllers for this type of an application. Other manufactures probably make simular units.

https://docs.google.com/viewer?a=v&..._IEbhr&sig=AHIEtbREcvHNG4OonhNQ3mBJSiNftfiuGw

 

[weressl]

I am designing a 3000A main-tie-main system from (2) 2500kva, 25kV to 480V utility transformers for a hospital. Our typical design is to control CBs manually with kirk key and tie breaker normally open.

But in this project, the CBs are controlled automatically. Now the owner?s facility engineer is asking if we can close tie breaker before we open the one of the mains when we restore the system to normal after a single source failure. This will be automatic transfer and the main will be often without any delay right after tie breaker is closed so two source will be paralleled very short period of time.

The 2500kva utility transformers are identical and supplied from the different substations so, they might be supplied from the different sources.

I know I have to consider the higher AIC rating of the CBs and switchgear but considering the transformers are supplied from the different sources, can I make a closed transition?

Utility must agree to the control scheme. They may require time delay on transfer, eg. require you to pre-hold on synchronization and not to execute the transfer unless there is sync for X seconds or even minutes and that will shut your instantaneous transfer availability.

There were various submittals through the years to the NEC to allow for closed transition at fault level considered only for ONE transformer, but was repeatedly rejected. Utilities COMMONLY allow this type transfer.