Closed Transition and AIC ratings

[Mike01]

This springs from the M-T-M post below. If you operate with an ATO (automatic throw over) scheme you temporally parallel both sides when transferring back to normal operation. For example double ended substation with a M-T-M configuration (tie normally open / closed transition) upon loss of L1 the tie breaker closes and L1 main opens, upon restoration of power to L1 and a 15 min. time delay to insure the utility distribution has stabilized the ATO transfers back to normal operation (L1 closed / Tie open / L2 closed) the ATO is set up for closed transition so for a incremeintal amount of time the substation is in closed transition. Should the bracing of the switchgear be reflected to accommodate the 2xSC for the duration of the closed transition? If the facility in question will be servicing the Xfmt1 so they closed transition the tie to de-energize X1 via closure of the tie and the opening of L1 when normal operation is restored via the ATO closed transition must the AIC account for 2xSC for the duration of the SC? We see installations all the time of ATS?s with closed trasition where under the closed transition (100ms or less) the available fault current may exceed the 35Kaic the switch is rated at? (the typically any c.b. rating of a closed transition overlapping neutral ATS 35Kaic in this example). Is there any standard to address this issue in a low voltage distribution or even medium voltage distribution or is it the responsibility of the Engineer sealing the drawings to make that decision? Your thoughts.:-?

 

[zog]

There are some guys here that can answer this better, I am sure they will. But basically during that closed transition time your fault current may exceed the AIC ratings of your equipment. This is sometimes accepted as a "calculated risk".

 

[jim dungar]

The AIC and Withstand ratings of devices are based on a bolted fault current. If you are paralleling one live source with another live source, what are the odds of a bolted fault occurring during a 1-2sec transfer period?

A discussion about the probability of acing faults caused by failure of the device to operate properly may be a more valid discussion instead.

 

[don_resqcapt19]

This has been discussed a number of times in the forum. I think the most recent is here.

 

[don_resqcapt19]

1-150 Log #2969 NEC-P01(2005 code cycle)
(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

 

[don_resqcapt19]

(Log #4007)(2002 code cycle)
1- 211 - (110-9): Reject​

SUBMITTER:

Robert B. Alexander, Fluor Daniel​

RECOMMENDATION: ​

Add text after the first sentence to read:​

The available short-circuit current shall be permitted to exceed the

interrupting rating of the equipment for not more than two seconds

during an automatically controlled, closed transition, power
transfer.
SUBSTANTIATION: ​

References:​

Proposal 1-174 NFPA 70-A98 ROP.

Proposal 1-175 NFPA 70-A98 ROP.

Comment 1-215 NFPA 70-A98 ROC.
Comment 1-216 NFPA 70-A98 ROC.
Comment 1-217 NFPA 70-A98 ROC.
A reading of the above proposals and comments has left several
users in confusion as to the intent of the Panel. Since I was an
original submitter, I have been questioned several times as to the
Panel's intent. Currently I recommend rating the equipment based
on calculated contributions from both sources. I do not believe it is
technically necessary, but I do not believe that the current wording
can be interpreted otherwise.
I am personally opposed to any manual system of closed transfer;
the potential to leave equipment in a dangerous condition is too
great.
I take some encouragement that the Panel obviously reread my
Proposal 1-175 when rejecting Comment 1-215. The Panel
statement simply challenged the two-second interval, but otherwise
seemed to accept it. I will attempt to justify it now. The systems that
would most commonly use the proposed wording are normallyopened,
automatic, secondary selective, distribution equipment.
The actual interval of parallel operation during an automatically
controlled transition is typically, on the order of 8-16 cycles. This is
obviously considerably less than the two seconds suggested.
However, the submitter is aware of a few automatic transfer switches
that use molded case circuit breakers as the switching devices. The
mechanical linkages that control the transfer have enough "slop"
that the actual time of the closed transition is a bit less than two
seconds.
As pointed out in the original proposal, the transfer itself will not
precipitate a downstream fault where the danger is downstream
devices being over stressed. The devices used for switching cannot
actually see contributions from both sources, whereas downstream
devices potentially could. The probability of the concurrent failure
required to actually create a dangerous condition is infinitesimal.
Finally, as pointed out in several of the proposals and comments,
these are commonly applied configurations in industrial facilities,
especially continuous process ones, with a long history of success.
Note: Supporting material is available for review at NFPA
Headquarters.
PANEL ACTION: ​

Reject.​

PANEL STATEMENT: ​

Complex systems design criteria such as​

closed transition are inappropriate for specific inclusion in the NEC.

Existing sections, such as 90-4 may be an appropriate avenue to deal

with such issues.​

NUMBER OF PANEL MEMBERS ELIGIBLE TO VOTE: 13​

VOTE ON PANEL ACTION:​

AFFIRMATIVE: 11

NEGATIVE: 1
NOT RETURNED: 1 Macias
EXPLANATION OF NEGATIVE:
FLOYD: Proposal 1-211 (Log #4007) brings attention to a design
and application issue that is not adequately addressed by product
standards and the NEC. Some manufacturers have addressed this
performance rating in their product literature, while others have
not. From a user's perspective, this proposal will help assure a safe ​

application and installation as noted in the proposal substantiation.​

 

[zog]

The AIC and Withstand ratings of devices are based on a bolted fault current. If you are paralleling one live source with another live source, what are the odds of a bolted fault occurring during a 1-2sec transfer period?.

I would say the possibility of that hapening during switching is much more likely than it is during a static state.

 

[mayanees]

I would say the possibility of that hapening during switching is much more likely than it is during a static state.

Zog,
I think Jim's point is that the live bus you're switching into will not be a bolted fault, since it's energized.
But I'm of the opinion that there still could be an issue with the transfer means, breaker or ATS.

If I'm designing a new system today, the equipment will be rated for both sources. Granted I haven't come across a system yet where equipment just isn't available to meet the system duties - at which point I may have to make an exception, and take the argument to the AHJ.
But I would only support the temporary overduty as a last resort, and would publish documents making everyone aware of the danger, and I'd locate the equipment somehwere where it could be remoteley operated in a safe manner.

We've just finished a design that had a network system consisting of (3) 2500 kVA transformers with 65KA capacity and it all paralleled on a 200kA bus.. only for overlap periods of a few cycles, but even for that period, 200kA equipment was adequate.

John M

 

[jim dungar]

I would say the possibility of that hapening during switching is much more likely than it is during a static state.

Under what conditions do you feel that a 3-phase bolted fault is much more likely to occur during live line-live line switching? The only one I can think of, is when the two sources are 'out of phase'.

I believe the typical failure mode during switching are: series arcing, line-line arcing, and line to ground arcing. All of these faults are likely to result in a three phase arcing fault.

 

[zog]

Under what conditions do you feel that a 3-phase bolted fault is much more likely to occur during live line-live line switching? The only one I can think of, is when the two sources are 'out of phase'.

I believe the typical failure mode during switching are: series arcing, line-line arcing, and line to ground arcing. All of these faults are likely to result in a three phase arcing fault.

2 sources out of phase is one, the other is a fault in the tie breaker itself.

 

[zog]

We've just finished a design that had a network system consisting of (3) 2500 kVA transformers with 65KA capacity and it all paralleled on a 200kA bus.. only for overlap periods of a few cycles, but even for that period, 200kA equipment was adequate.

John M

I have thousands of breakers with 200kA AIC's, not that hard to do. In fact I am rebuilding 37 of them in my shop right now.

 

[jim dungar]

2 sources out of phase is one, the other is a fault in the tie breaker itself.

I believe a fault in the tie device would typically result in one of the arcing faults I described and not in a 3-phase bolted fault. Of course any fault in the tie device will see fault current from both sources. I just don't believe it will be bolted fault currents.

The last MTM system I was asked about, had a combined 3-phase bolted fault current of 105kA so the gear was specified with 200kAIC breakers. The combined arcing fault current was <100kA. The instantaneous override protection of the 200kAIC breakers made selective coordination with the existing downstream equipment impossible while the use of 100kAIC allowed the coordination.

 

[elec_eng]

M-T-M with normally closed tie

M-T-M with normally closed tie

This springs from the M-T-M post below. If you operate with an ATO (automatic throw over) scheme you temporally parallel both sides when transferring back to normal operation. For example double ended substation with a M-T-M configuration (tie normally open / closed transition) upon loss of L1 the tie breaker closes and L1 main opens, upon restoration of power to L1 and a 15 min. time delay to insure the utility distribution has stabilized the ATO transfers back to normal operation (L1 closed / Tie open / L2 closed) the ATO is set up for closed transition so for a incremeintal amount of time the substation is in closed transition. Should the bracing of the switchgear be reflected to accommodate the 2xSC for the duration of the closed transition? If the facility in question will be servicing the Xfmt1 so they closed transition the tie to de-energize X1 via closure of the tie and the opening of L1 when normal operation is restored via the ATO closed transition must the AIC account for 2xSC for the duration of the SC? We see installations all the time of ATS’s with closed trasition where under the closed transition (100ms or less) the available fault current may exceed the 35Kaic the switch is rated at? (the typically any c.b. rating of a closed transition overlapping neutral ATS 35Kaic in this example). Is there any standard to address this issue in a low voltage distribution or even medium voltage distribution or is it the responsibility of the Engineer sealing the drawings to make that decision? Your thoughts.:-?

I am working on a hospital project where they have a multiple of M-T-M double ended substations with a normally closed tie. Now, they want to have two-more double ended substations for new buildings and they would like to have the same normally closed tie configuration..13.8KV to 480V and 2500KVA...Non of circuit breakers are electrically operated..all manual operated circuit breaker...I was told that they have a Kirk key interlocking on some of unit substations but they didn't like to interlocking operation.....so, all new installation, they requires the closed tie...I don't have the AIC ratings.

By reading the previous threads, this seems odd configuration.. questions:

1) Is this common configuration as well?

2) Is there any issues with this configuration?

3) I don't believe there is any sync check relay..How likey is it that two xfmrs will be out of the phase? what will cause the out of the phase?

4) Assuming the AIC rating will be adequate for the available fault current, is this recommended?

 

[zog]

I am working on a hospital project where they have a multiple of M-T-M double ended substations with a normally closed tie. Now, they want to have two-more double ended substations for new buildings and they would like to have the same normally closed tie configuration..13.8KV to 480V and 2500KVA...Non of circuit breakers are electrically operated..all manual operated circuit breaker...I was told that they have a Kirk key interlocking on some of unit substations but they didn't like to interlocking operation.....so, all new installation, they requires the closed tie...I don't have the AIC ratings. [/quuote]

So they have the kirk keys system, but dont use them because they dont "like" them?? And they dont know the fault current? Sounds like a disaster waiting to happen. Without knowing the available fault current and AIC ratings for this syetem your questions #1 and #2 cannot be answered.

3) I don't believe there is any sync check relay..How likey is it that two xfmrs will be out of the phase? what will cause the out of the phase?

Phase checks are done as part of the commisiong process, so unless some cables or connections are removed and re-installed they should be in phase. Now a phase shift could be a problem in some rare conditions with loading, but usually not a big enough shift to be a problem.

4) Assuming the AIC rating will be adequate for the available fault current, is this recommended?

That is one big assumption my friend. But it is an common practice with the proper equipment for critical loads, there are advanatages and disadvantages to a system like this, as with all designs.

 

[WastefulMiser]

Per UL 891 and UL 1558 (2005), if paralleling occurs a 25 is required. While the two (or more) sources are paralleled the ability to open the breaker is required if one of the sources loses a phase or a source drops out. From the switchgear, I am not too sure how you'd be able to tell if a phase is lost or a source drops out all together while paralleling ... some say a 32 is all you need, while others say a 32 isn't adequate.