Paralleling Switchgear Short Circuit Rating

[JoeStillman]

My question is about UL 1558 switchgear designed for momentary paralleling between utility sources (across a main-tie-main) or generator-utility (closed transition transfer.) A manufacturer told me they don't usually count the fault contribution from both sources when choosing a short-circuit rating for the gear. I use both sources in my SC study because I feel like a sync-check malfunction or similar could have me closing in out of phase and let the smoke out. I specified a higher than normal transformer impedance to avoid using fused breakers and 200kA gear.

Was that necessary?

 

[Sahib]

I feel like a sync-check malfunction or similar could have me closing in out of phase and let the smoke out. I specified a higher than normal transformer impedance to avoid using fused breakers and 200kA gear.

Was that necessary?

You may specify to the manufacturer the rated out of phase making/breaking current for the swichgear to have it supplied to you.

 

[mayanees]

My question is about UL 1558 switchgear designed for momentary paralleling between utility sources (across a main-tie-main) or generator-utility (closed transition transfer.) A manufacturer told me they don't usually count the fault contribution from both sources when choosing a short-circuit rating for the gear. I use both sources in my SC study because I feel like a sync-check malfunction or similar could have me closing in out of phase and let the smoke out. I specified a higher than normal transformer impedance to avoid using fused breakers and 200kA gear.

Was that necessary?

I think yes.
The CMP has turned down repeated proposals that were directed at establishing a tolerance for short-duration transitions.

 

[GoldDigger]

My question is about UL 1558 switchgear designed for momentary paralleling between utility sources (across a main-tie-main) or generator-utility (closed transition transfer.) A manufacturer told me they don't usually count the fault contribution from both sources when choosing a short-circuit rating for the gear. I use both sources in my SC study because I feel like a sync-check malfunction or similar could have me closing in out of phase and let the smoke out. I specified a higher than normal transformer impedance to avoid using fused breakers and 200kA gear.

Was that necessary?

Depends in part on the relative short circuit current capabilities of the two sources. If the potential fault currents from both are equal, then the power during an out of phase condition would double, but not the current. And if, for example, the generator utility has only 1/10 the fault current potential (higher impedance) it does not make sense to add the two power or current values. One source will not deliver higher current or even the same current because of the other source's voltage if the other source has a higher impedance.
Power for arc flash might add if there were a second fault during an in-phase closed transition.

The possibilities really need to be formally addressed in regulations, but from the physics point of view the manufacturer is correct.

Executive summary: You will not let more smoke out in the switchgear if the two sources are out of sync than if one source presented a bolted fault to ground to the other. There may be more smoke from the two sources though, and it might affect success of OCPD operation at either or both since they would see twice the voltage to interrupt.

 

[Sahib]

Executive summary: You will not let more smoke out in the switchgear if the two sources are out of sync than if one source presented a bolted fault to ground to the other. There may be more smoke from the two sources though, and it might affect success of OCPD operation at either or both since they would see twice the voltage to interrupt.

The double voltage appears across the switchgear terminals during interrupting out of phase currents and the standards require the circuit breaker to break a current equal to 25% of the fault current across the terminals, at a voltage equal to twice the voltage relative to earth.

 

[Sahib]

Was that necessary?

It depends.

If the paralleling of power sources does not last more than 100ms or within time specified by the POCO, single source contribution only need be taken into account for SC study per IEEE.

If you plan for prolonged parallel operation, contribution from all sources to be taken into account and bigger switchgear would be necessary.

 

[ron]

It depends.

If the paralleling of power sources does not last more than 100ms or within time specified by the POCO, single source contribution only need be taken into account for SC study per IEEE.

If you plan for prolonged parallel operation, contribution from all sources to be taken into account and bigger switchgear would be necessary.

Which IEEE standard are you referring to? I am not aware of this reference existing.

 

[ron]

Depends in part on the relative short circuit current capabilities of the two sources. If the potential fault currents from both are equal, then the power during an out of phase condition would double, but not the current. And if, for example, the generator utility has only 1/10 the fault current potential (higher impedance) it does not make sense to add the two power or current values. One source will not deliver higher current or even the same current because of the other source's voltage if the other source has a higher impedance.
Power for arc flash might add if there were a second fault during an in-phase closed transition.

The possibilities really need to be formally addressed in regulations, but from the physics point of view the manufacturer is correct.

Executive summary: You will not let more smoke out in the switchgear if the two sources are out of sync than if one source presented a bolted fault to ground to the other. There may be more smoke from the two sources though, and it might affect success of OCPD operation at either or both since they would see twice the voltage to interrupt.

I don't understand the rational posed above, but when I calculate available fault current for (2) out of sync sources, the resulting fault current is the addition of the two. Just like you add the contribution of any source that can be out of sync like a motor, generator, etc.

 

[Sahib]

Which IEEE standard are you referring to? I am not aware of this reference existing.

IEEE 666.

 

[GoldDigger]

I don't understand the rational posed above, but when I calculate available fault current for (2) out of sync sources, the resulting fault current is the addition of the two. Just like you add the contribution of any source that can be out of sync like a motor, generator, etc.

Thought experiment: Take two 1.5 volt batteries to make the phasing simple.
Let's say the short circuit current from one battery is 2A.
Now,
A. if I connect the - of both batteries to ground and short the + together no current will flow. The voltages are the same.
B. If I now short the combined + to ground, the current will be 4A. But only 2A of that is actually going from
C. If I connect the batteries with one + to ground and the other - to ground (out of sync) and connect the free terminals together, 2A will flow between them, not 4A.
D. If while they are connected I short the common point to ground, no additional current will flow. Just the original 2A from battery to battery.
E. When I break the connection in C, the voltage across the contacts will be 3V during the opening process.

The application to the OP's situation:
1. The maximum fault current that will ever flow across the contacts of the paralleling relay will be the greater of the two individual sources, whether they are in sync or not. But the fault current from the smaller source will be more than it could normally deliver to ground, so it's OCPD might have problems.
2. The maximum current through the paralleling relay in the case of a bolted fault to ground at one source will be the short circuit current of the other source.
3. If the relay contacts try to open under an out of sync condition, they will see twice the nominal system voltage.
Therefore the SC rating of the contacts only needs to be the greater of the two sources and the interrupting voltage rating needs to be twice the system voltage.
4. And the SC rating of any OCPD on either supply will have to be that of the greater of the two fault currents (to be conservative) and the voltage rating of each OCPD needs to be twice what it otherwise would be.
5. Changing the situation from a fault to ground to a phase-to-phase fault does not make things any worse.

 

[rbalex]

This is a history of Proposals/Comments on Section 110.9 to avoid counting short circuits from both sources in a main-tie-main arraignment. (Sorry, the “basic” PDF was too large for one download.) I obviously disagree with CMP1.

BTW, IEEE - 666 was specifically rejected (May 2001 ROC, page 38, Comment 1-120.) EDIT ADD: Should have said "...rejected as a basis for acceptance in the NEC..."

 

[GoldDigger]

This is a history of Proposals/Comments on Section 110.9 to avoid counting short circuits from both sources in a main-tie-main arraignment. (Sorry, the ?basic? PDF was too large for one download.) I obviously disagree with CMP1.

BTW, IEEE - 666 was specifically rejected (May 2001 ROC, page 38, Comment 1-120.) EDIT ADD: Should have said "...rejected as a basis for acceptance in the NEC..."

The comment on page 69 recognizes that the paralleling gear itself does not need to be rated for twice the fault current, but any downstream devices on the load side may need to be, both in terms of interrupting capacity and arc fault evaluation. (With the worst case being in-sync, not out of sync.) I do not think this is what the OP was concerned about, just the rating of the switchgear itself. Maybe we can get a clarification on that?

 

[Sahib]

IEEE - 666 was specifically rejected (May 2001 ROC, page 38, Comment 1-120.) EDIT ADD: Should have said "...rejected as a basis for acceptance in the NEC..."

The NEC Code Making Panel reviewed proposed changes to the wording of Article 110.9 in 2002 and again in 2005 to specifically allow the short-circuit rating to be exceeded in cases of a momentary closed transition. Both proposals were defeated. However, it is instructive to review the statement of the Code Making Panel in its rejection of the 2002 proposal:
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.

Therefore, the NEC has specifically taken no position on the proper way to conduct a short-circuit study, but merely is stating that the results of the study will influence the selection of equipment with regard to short-circuit withstand and interrupting ratings.

 

[rbalex]

The NEC Code Making Panel reviewed proposed changes to the wording of Article 110.9 in 2002 and again in 2005 to specifically allow the short-circuit rating to be exceeded in cases of a momentary closed transition. Both proposals were defeated. However, it is instructive to review the statement of the Code Making Panel in its rejection of the 2002 proposal:
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.

Therefore, the NEC has specifically taken no position on the proper way to conduct a short-circuit study, but merely is stating that the results of the study will influence the selection of equipment with regard to short-circuit withstand and interrupting ratings.

You obviously didn't read the Proposals or Comments carefully - or Section 110-9 for that matter.

Code Making Panel 1 (CMP1) didn’t take a position on how to conduct a short circuit study since that isn’t the subject of Section 110.9. But Section 110.9 (first paragraph) damn sure tells you how to select the short-circuit interrupting rating; i.e.,“…not less than the nominal circuit voltage and the current that is available at the line terminals of the equipment.” This is significantly different than the second paragraph requirements for “equipment intended to interrupt current at other than fault levels”; i.e., they “… shall have an interrupting rating at nominal circuit voltage not less than the current that must be interrupted.”

The first paragraph requires that some currents that won’t be interrupted must still be included in the interrupting rating of the main-tie-main. EDIT ADD: BTW Section 90-4 means ask the AHJ and I dealt with that in Comment 1-116, May 2001 ROC, page 37.

 

[GoldDigger]

The first paragraph requires that some currents that won’t be interrupted must still be included in the interrupting rating of the main-tie-main.

Which is not justified by the laws of physics, but at least gives very clear guidance and puts the question to rest for now.

 

[Sahib]

.

Code Making Panel 1 (CMP1) didn?t take a position on how to conduct a short circuit study since that isn?t the subject of Section 110.9. But Section 110.9 (first paragraph) damn sure tells you how to select the short-circuit interrupting rating; i.e.,??not less than the nominal circuit voltage and the current that is available at the line terminals of the equipment.? This is significantly different than the second paragraph requirements for ?equipment intended to interrupt current at other than fault levels?; i.e., they ?? shall have an interrupting rating at nominal circuit voltage not less than the current that must be interrupted.?

Taken at face value, it seems to imply that the equipment specified?whether closed-transition switchgear or closed transition transfer switches and all of the equipment located downstream of these closed-transition devices?should be rated to interrupt the full available fault current of all utility and/or generator sources that may be connected during a closed-transition switching procedure. However, digging deeper reveals a picture that is not as clear cut.
In reviewing the NEC Handbook, Articles 110.9 and 110.10 are a matched pair of requirements. The Code Commentary for Article 110.10 states that ?Literature on how to calculate short-circuit currents at each point in any distribution system generally can be obtained by contacting the manufacturers of overcurrent protective devices or by referring to IEEE 141-1993 (R1999): IEEE Recommended Practice for Electrical Power Distribution for Industrial Plants (Red Book).? Furthermore, the Code Commentary for Article 705.1 states that ?Article 705 sets forth basic safety requirements for the installation of generators and other types of power production sources that are interconnected and operate in parallel as distributed generation.? Clearly, the use of closed-transition equipment does not equate to distributed generation.

.
Section 90-4 means ask the AHJ and I dealt with that in Comment 1-116, May 2001 ROC, page 37.

What is the result from CMP?

 

[rbalex]

Taken at face value, it seems to imply that the equipment specified?whether closed-transition switchgear or closed transition transfer switches and all of the equipment located downstream of these closed-transition devices?should be rated to interrupt the full available fault current of all utility and/or generator sources that may be connected during a closed-transition switching procedure. However, digging deeper reveals a picture that is not as clear cut.
In reviewing the NEC Handbook, Articles 110.9 and 110.10 are a matched pair of requirements. The Code Commentary for Article 110.10 states that ?Literature on how to calculate short-circuit currents at each point in any distribution system generally can be obtained by contacting the manufacturers of overcurrent protective devices or by referring to IEEE 141-1993 (R1999): IEEE Recommended Practice for Electrical Power Distribution for Industrial Plants (Red Book).? Furthermore, the Code Commentary for Article 705.1 states that ?Article 705 sets forth basic safety requirements for the installation of generators and other types of power production sources that are interconnected and operate in parallel as distributed generation.? Clearly, the use of closed-transition equipment does not equate to distributed generation.

Take it at face value. Section 100.9 doesn?t ?imply? that ?[t]he first paragraph requires that some currents that won?t be interrupted must still be included in the interrupting rating of the main-tie-main? - that?s the only reasonable interpretation for ?? the short-circuit interrupting rating ? shall not [be] less than? the current that is available at the line terminals of the equipment.? The rest of your "review" is irrelevant to the OP.

What is the result from CMP?

I guess you still aren't reading too carefully - it's in the documentation for the history I included above if you look as carefully as you did for of your irrelevant "review".

 

[JoeStillman]

The comment on page 69 recognizes that the paralleling gear itself does not need to be rated for twice the fault current, but any downstream devices on the load side may need to be, both in terms of interrupting capacity and arc fault evaluation. (With the worst case being in-sync, not out of sync.) I do not think this is what the OP was concerned about, just the rating of the switchgear itself. Maybe we can get a clarification on that?

I believe the most likely failure mode is out-of-sync transfer. The likelihood of a downstream fault during transfer is, as Mr. Alexander eloquently said, "infinitessimal". My concern is with the rating of the gear overall. The whole substation - mains, ties and branches - gets the same rating. Although least likely, the downstream fault IS the worst possible failure mode.

 

[GoldDigger]

The whole substation - mains, ties and branches - gets the same rating.

That pretty well nails it down.
The manufacturer's recommendation on counting only one source is well motivated by physics and will also insure that their equipment alone will not be subjected to overstress, but it is not enough to meet the Code.
I agree that the out-of-sync will be the greatest stress on the tie itself. And it will be exactly equal to the stress of closing the tie from an operating system onto a bolted fault on the other side.

 

[rbalex]

ANALYSING RELIABILITY, A SIMPLE YET RIGOROUS APPROACH (PCIC-2003-25) wasn't my best IEEE paper IMO, but it was the only one that has been elevated to IEEE Transactions status.

One of the things revealed in the underlying FMEA study was that Figure "g" was both the safest and most reliable configuration if proper load auto-starts/restarts are included in the control system. Since auto-starts/restarts are almost always in the control system in some form or another, eliminating the tie altogether is often the best solution since it eliminates several potential failure modes and it doesn’t really provide the additional perceived “flexibility”.

In fact, while I was on the NFPA 70E TC, I said I could design a distribution system that virtually never needed to be worked “live” if dual feeds were provided without a tie. It has all the redundancy necessary for a continuous process.


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