Fault current switching

[mbrooke]

Is it mandated anywhere, or at least customary, to consider the fault current during the brief period when transformers are being switched in and out of service in parallel? That would mean a normally open bus tie is closed before transformer breakers are opened to prevent a momentary interruption of load. Main-tie-Main type deal. Argument is that a fault is not likely to happen while this is happening.

 

[mivey]

I wouldn't just for momentary switching like an automated swapover scheme. If they are going to stay in parallel for longer you would of course.

add: It would also depend on the added protection costs vs. the cost of equipment being protected and repair/replacement cost. Your backup protection should kick in if it all goes down the tube. If you don't have backup protection then you may want to err on the conservative side.

 

[mbrooke]

I wouldn't just for momentary switching like an automated swapover scheme. If they are going to stay in parallel for longer you would of course.

Thats what I often see. Though it sure would be awful for a fault to happen.

 

[mivey]

Thats what I often see. Though it sure would be awful for a fault to happen.

I added to my prior post. It is a risk assessment but usually with automated transfer schemes we don't worry about the additional fault current.

 

[mbrooke]

I added to my prior post. It is a risk assessment but usually with automated transfer schemes we don't worry about the additional fault current.

Makes sense- thank you

 

[Tony S]

It is a risk assessment but usually with automated transfer schemes we don't worry about the additional fault current.

We would have to do a formal risk assessment every time we carried out exactly this switching procedure. The outcome was always the same “minimal risk”.

 

[ron]

You need to consider the fault current from both sources during the momentary paralleling.

There was some posts about this a while back and a gent from the Chemical Industry who was involved with submitting proposals and comments had great references I'll try to find.

I believe it was the 1998 ROP / ROC in Section 110-9 and 2001 RP / ROC same section where it was proposed to ignore short term paralleling during MTM transfers, and the technical committee said they wouldn't add it to the code, ask the AHJ.

 

[ron]

Actually Don had found this one from the 2005
http://forums.mikeholt.com/showthread.php?t=98469&page=4&highlight=momentary

1-150 Log #2969 NEC-P01


(110-9)


Final Action: Reject



Submitter:
David Soffrin, American Petroleum Institute



Recommendation:



Add the following after the first sentence:

During the momentary paralleling of an automatically controlled power transfer between two sources, the single-source condition
interrupting rating shall be considered adequate provided there is no intentional time delay and the parallel condition cannot be
maintained. The initiation of the automatically-controlled transfer shall be permitted to be manual or automatic.


Substantiation:



The condition identified in the proposal is common in double ended substation arrangements where maintaining power to the loads is

critical for operation or system safety. It is also a common practice in generating stations where continuity of power flow may be
requried for public safety. These systems are designed for only a momentary parallel condition, typically only long enough for the
closure of the paralleling breaker to initialize the opening of the designated breaker returning the system to single source configuration.
During the brief parallel time (often only a few cycles), the short circuit rating of the switchgear feeder breakers may be exceeded. This
added provision in 110.9 would recognize this arrangement and establish limitations under which it can be applied. IEEE 666, Design
Guide for Electric Power Service Systems for Generating Stations, section 4.6.1 specifically allows this arrangement and equpiment
rating.


Panel Meeting Action: Reject


Panel Statement:

The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed

through evaluation of specific installations.


Number Eligible to Vote: 12


Ballot Results: Affirmative: 12

 

[mbrooke]

Actually Don had found this one from the 2005
http://forums.mikeholt.com/showthread.php?t=98469&page=4&highlight=momentary

1-150 Log #2969 NEC-P01


(110-9)


Final Action: Reject



Submitter:
David Soffrin, American Petroleum Institute



Recommendation:



Add the following after the first sentence:

During the momentary paralleling of an automatically controlled power transfer between two sources, the single-source condition
interrupting rating shall be considered adequate provided there is no intentional time delay and the parallel condition cannot be
maintained. The initiation of the automatically-controlled transfer shall be permitted to be manual or automatic.


Substantiation:



The condition identified in the proposal is common in double ended substation arrangements where maintaining power to the loads is

critical for operation or system safety. It is also a common practice in generating stations where continuity of power flow may be
requried for public safety. These systems are designed for only a momentary parallel condition, typically only long enough for the
closure of the paralleling breaker to initialize the opening of the designated breaker returning the system to single source configuration.
During the brief parallel time (often only a few cycles), the short circuit rating of the switchgear feeder breakers may be exceeded. This
added provision in 110.9 would recognize this arrangement and establish limitations under which it can be applied. IEEE 666, Design
Guide for Electric Power Service Systems for Generating Stations, section 4.6.1 specifically allows this arrangement and equpiment
rating.


Panel Meeting Action: Reject


Panel Statement:

The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed

through evaluation of specific installations.


Number Eligible to Vote: 12


Ballot Results: Affirmative: 12

Where can I send you a check? This is exactly what I was looking!

I don't entirely agree with their substantiation, but could also see why they think so.

 

[mbrooke]

You need to consider the fault current from both sources during the momentary paralleling.

There was some posts about this a while back and a gent from the Chemical Industry who was involved with submitting proposals and comments had great references I'll try to find.

I believe it was the 1998 ROP / ROC in Section 110-9 and 2001 RP / ROC same section where it was proposed to ignore short term paralleling during MTM transfers, and the technical committee said they wouldn't add it to the code, ask the AHJ.

Honestly, this is one of the few times I would argue that its not a design issue, but rather a safety issue. Perhaps not likely to happen, but what if some component does fail during the transfer procedure itself?

 

[ron]

I've heard the argument that it the transfer is less than 100ms, like many closed transition transfer switches, it is faster than a blink of an eye, I shouldn't have to consider it. But I don't want to lose sleep at night, so any closed transition, I just add all sources together.

 

[ron]

Where can I send you a check? This is exactly what I was looking!

I accept adult beverages only. I'm travelling to client sites in Louisiana and Utah before heading home for Thanksgiving, but I'll stop by soon

 

[mbrooke]

I accept adult beverages only. I'm travelling to client sites in Louisiana and Utah before heading home for Thanksgiving, but I'll stop by soon

All right, we have a deal. :thumbsup: