Paralleled Unit Substations

[msteiner]

I have an industrial client who we've designed two new unit substations for, with a Main-Tie-Main configuration on the 480V secondary. We originally had a traditional interlock arrangement on the M-T-M, where only two of the three breakers can be closed at a given time. The client wants the ability to close the tie before opening a main, so the downstream loads never see an interruption. I'd like to make sure we've thought through everything before proceeding with this.

1. We've calculated the fault current assuming paralleled sources, representing worst case.
2. The arc flash calc will look at all switching scenarios.
3. The transformers in the unit substations have identical ratings (voltage, turns ratio, kVA, impedance, delta-wye, etc.).
4. The 13.2kV primary consists of dual feeders (customer owned), but both are fed from the same utility source, so are always in sync.
5. Because we're not doing a closed transition with a utility source, there are no power company requirements to comply with.

What else do I need to consider?

 

[powerpete69]

Sounds like you are in pretty good shape per my opinion.
We even bus-tie unlike transformers, delta-wye to wye-delta transformers to avoid interruptions when opening mains for repair or servicing. Works well.
No explosions, fires or anything!!

 

[ron]

Seems like you have it covered, although it is surprising to me if the SCCR was originally selected for open transition, that it is also adequate for closed transition (both sources paralleled).

We even bus-tie unlike transformers, delta-wye to wye-delta transformers to avoid interruptions when opening mains for repair or servicing. Works well.
No explosions, fires or anything!!

This is also surprising to me as the phase shift from primary to secondary of a delta-wye and a wye-delta transformer are different and by paralleling the secondaries should not be synchronized even staring with sync'd primaries. They shift 30 degrees pos and neg relative to the primary.

 

[Hv&Lv]

Are you doing any voltage regulation after the individual transformers?
I would enable the 25 sync elements if available just cause...
same utility source, as in same feeder circuit or same utility substation but different circuits?

 

[msteiner]

I hadn't intended to do voltage regulation. Talk more about that.
The swgr is GE Entellisys, so it can definitely do sync check.
Same utility source meaning same feeder circuit. The utility feeds two separate circuits from the same substation into the customer's gear, which then sends only one out to the plant via a NO/NC switch arrangement. The one circuit goes out to the plant on two separate OH feeders. Hope that makes sense....

 

[Ainsley Whyte]

I have an industrial client who we've designed two new unit substations for, with a Main-Tie-Main configuration on the 480V secondary. We originally had a traditional interlock arrangement on the M-T-M, where only two of the three breakers can be closed at a given time. The client wants the ability to close the tie before opening a main, so the downstream loads never see an interruption. I'd like to make sure we've thought through everything before proceeding with this.

1. We've calculated the fault current assuming paralleled sources, representing worst case.
2. The arc flash calc will look at all switching scenarios.
3. The transformers in the unit substations have identical ratings (voltage, turns ratio, kVA, impedance, delta-wye, etc.).
4. The 13.2kV primary consists of dual feeders (customer owned), but both are fed from the same utility source, so are always in sync.
5. Because we're not doing a closed transition with a utility source, there are no power company requirements to comply with.

What else do I need to consider?

Could you share the type MCC that you are considering and size transformers ?

 

[msteiner]

Could you share the type MCC that you are considering and size transformers ?

It's GE (now ABB) Entellisys switchgear. Xfmrs are 2500kVA AA, 3333kVA FA.

 

[Ainsley Whyte]

It's GE (now ABB) Entellisys switchgear. Xfmrs are 2500kVA AA, 3333kVA FA.

What is the load requirement for those substation ?
Are you considering Automatic change over when one feeder goes down the other comes in ?

 

[msteiner]

What is the load requirement for those substation ?
Are you considering Automatic change over when one feeder goes down the other comes in ?

I'm not sure what you mean by "load requirement."
The switching would be initiated manually at the HMI station, but the actual opening/closing of breakers would happen through the gear's control system.

 

[powerpete69]

This is also surprising to me as the phase shift from primary to secondary of a delta-wye and a wye-delta transformer are different and by paralleling the secondaries should not be synchronized even staring with sync'd primaries. They shift 30 degrees pos and neg relative to the primary.

It is suprising to me as well, I even started a thread on it a few months ago. They've been bus tie-ing these different transformers for the last 20 years with no issues. If you simulate delta-wye to wye-delta bus tie in Easypower, nothing out of the ordinary shows up either. Business as usual.

 

[Hv&Lv]

It is suprising to me as well, I even started a thread on it a few months ago. They've been bus tie-ing these different transformers for the last 20 years with no issues. If you simulate delta-wye to wye-delta bus tie in Easypower, nothing out of the ordinary shows up either. Business as usual.

I can see that Being done by feeding a DY11 reverse rotation. The offset degrees will cancel out.

i have several stations that were connected years ago(decades) any ole way.
now that we are in the process of doing loop ties we are running into 60 degree shifts, and have to swap transmission lines to fix the displacement issues.

 

[Hv&Lv]

I hadn't intended to do voltage regulation. Talk more about that.
The swgr is GE Entellisys, so it can definitely do sync check.
Same utility source meaning same feeder circuit. The utility feeds two separate circuits from the same substation into the customer's gear, which then sends only one out to the plant via a NO/NC switch arrangement. The one circuit goes out to the plant on two separate OH feeders. Hope that makes sense....

If you aren’t doing regulation then it isn’t a problem. I was thinking if your going to do regulation after each transformer you would want some kind of check to get the voltages or steps the same.

with what your describing I think your ready to go also..

 

[Ainsley Whyte]

I'm not sure what you mean by "load requirement."
The switching would be initiated manually at the HMI station, but the actual opening/closing of breakers would happen through the gear's control system.

load requirement i mean how much VA EACH MCC will supply ?
your design is very normal as long as the control interlocks are set up correctly.

Do you have a Single line drawing ?

 

[xptpcrewx]

It’s hard to say without having the one-line and additional information but I would definitely add a 25 and possibly a 27 phase loss element. Blocking 52 closure on DLDB and DLLB is also necessary.

Assuming you are banking the transformers for indefinite periods of time, you should also consider interlocking with upstream breakers for proper energization sequence to prevent back-feeding incase one source is lost. Lastly, directional overcurrent may be a good idea to prevent tripping out both mains for upstream faults.


Sent from my iPhone using Tapatalk

 

[11bgrunt]

I can see that Being done by feeding a DY11 reverse rotation. The offset degrees will cancel out.

i have several stations that were connected years ago(decades) any ole way.
now that we are in the process of doing loop ties we are running into 60 degree shifts, and have to swap transmission lines to fix the displacement issues.

I don't understand how these can parallel if there is a phase shift.
I heard a story about two overhead primary circuits that were being tied together from two different substations. There was a phase shift. Wire stayed in the air for about five minutes after being tied together and then fell to the ground from overheating..

 

[Ainsley Whyte]

It is suprising to me as well, I even started a thread on it a few months ago. They've been bus tie-ing these different transformers for the last 20 years with no issues. If you simulate delta-wye to wye-delta bus tie in Easypower, nothing out of the ordinary shows up either. Business as usual.

Really are you sure i have hand a single line drawing of a plant that i was working and two transformer tie to a bus however they had to be strict interlock not allowing both to tie together i can share the drawing

 

[powerpete69]

Really are you sure i have hand a single line drawing of a plant that i was working and two transformer tie to a bus however they had to be strict interlock not allowing both to tie together i can share the drawing

The voltages between phases is always measured before making the tie to insure there is no phase change.

 

[Ainsley Whyte]

You are talking of synchronization of both transformer checking voltage, phase rotation and frequency ?

 

[Ainsley Whyte]

I hadn't intended to do voltage regulation. Talk more about that.
The swgr is GE Entellisys, so it can definitely do sync check.
Same utility source meaning same feeder circuit. The utility feeds two separate circuits from the same substation into the customer's gear, which then sends only one out to the plant via a NO/NC switch arrangement. The one circuit goes out to the plant on two separate OH feeders. Hope that makes sense....

I just read this again but a drawing would do better

 

[Hv&Lv]

I don't understand how these can parallel if there is a phase shift.
I heard a story about two overhead primary circuits that were being tied together from two different substations. There was a phase shift. Wire stayed in the air for about five minutes after being tied together and then fell to the ground from overheating..

you have a delta wye on one station, and a wye delta on another station.
whether the delta wye is leading or lagging 30 is determined by how the delta is closed.
the transmission line coming in can be fed “backwards” as in the leading “A” phase on H3, “B” on H2, and “C” on H1, will also cause a shift to lead rather than lag.

like I said earlier, we have several that won’t tie now because there is a 60 degree difference in the phase shift. We have to swap the ”A” and ”C” phases on the transmission side to fix this displacement issue.
now, this causes our secondary rotation to be backwards. We fix that by swapping A and C on the first vertical span out of the station, or swapping it at the UG risers.
we just did another one last fall. One circuit only had one bank on it. We left that one alone and fixed rotation at the meterbase.