Utility transformer primary fuse opening time

[kentirwin]

We have a 1500kva utility transformer that feeds the facility where I work. I have the transformer data supplied by the utility. My problem is that I can find no data for the primary fusing as far as opening time should there be a bolted fault on the secondary. Below is the data from the utility, Dominion Virginia Power.

The transformer is a 1500kVA, Deadfront, connected Grounded Wye / Grounded Wye. The primary voltage is
34.5kV, (Phase to Phase) and the secondary voltage is 277/480V, (Phase to Phase).

The transformer is fused on the primary side with a Load Sensing fuse (50 AMP COOPER 4000358C12B) in series
with a BCL fuse (COOPER 2-3545080M51M)

Re: Request for Impedance Information
R1 = 0.0486 per unit @ 100 MVA
R0 = 0.1170 per unit @ 100 MVA
X1 = 0.2996 per unit @ 100 MVA
X0 = 0.2612 per unit @ 100 MVA

The current source impedance on the source side of the transformer is:
The transformer impedances are:
Max Z = 6.18% Max X = 5.95% Max R = 0.71%
Min Z = 5.37% Min X = 5.34% Min R = 0.58%
This information is based on Dominion's records for the transformer mapped to this service and does not include any
secondary impedance.

Max available fault current is 37517 amperes.

Maybe some utility veteran could weigh in on the possible primary fuse opening time should a bolted fault occur on the secondary ahead of the main in the service switchgear which is 150' from the transformer. I myself have no experience with this kind of thing. I'd appreciate any kind of insight.

 

[iwire]

I am really a novice on this subject but I thought in this case it is assumed the utility fuse will never open.

 

[mayanees]

Opening time

Opening time

You have all the information you need in what they gave you.
Plot the fuse curves on a Time Current Characteristic (TCC) curve. Current on x-axis, time on the y-axis. Then draw a vertical line that corresponds to the max available fault current, and follow that line up through the fuse curves. As soon as it gets past the fuse's max-opening time, read the time value on the y-axis.
It's a simple exercise in a power system modeling software like SKM or Easy Power.
If you don't have the package, I'll find a few minutes to plot it out and will post the TCC.

 

[mayanees]

.. actually, you would fault the secondary with the 37517 amps given, then look at the resultant 34.5 kV primary current flow that the fuse would see, and plot that vertical line to cross the fuse curves.

 

[mayanees]

TCC

TCC

Attached is a TCC that models the system you described.
A fault at the secondary of the transformer is around 37,500 amps. A fault 150 feet from the secondary is around 31064 amps.
The vertical line shown at 31064 amps intersects the short-circuit protection fuse at approximately 0.248 seconds.

 

[kentirwin]

Attached is a TCC that models the system you described.
A fault at the secondary of the transformer is around 37,500 amps. A fault 150 feet from the secondary is around 31064 amps.
The vertical line shown at 31064 amps intersects the short-circuit protection fuse at approximately 0.248 seconds.

Thanks very much mayanees! Just out of curiosity, was the fuse data already in the device library of the software that you used to generate the TCC? Unfortunately my employer won't pony up the $$$ for SKM or etap.

 

[zog]

Thanks very much mayanees! Just out of curiosity, was the fuse data already in the device library of the software that you used to generate the TCC? Unfortunately my employer won't pony up the $$$ for SKM or etap.

Yes, they are built into the software. The scenario you are asking about here is often referred to the blind spot in a typical MV/LV substation, it is often where you find your highest arc flash hazard and it can be difficult to mitigate. A common solution is remote racking for the main.

If your main does not have INST trip functions you will see these high Ie's all the way to the line side of the feeder breakers so racking them out can also be a high risk task, remote racking works there too or you can install a "maintenance switch" on the main that inserts INST protection while working on the gear and can be turned off after the task is complete to re-establish your coordination.

 

[mayanees]

Thanks very much mayanees! Just out of curiosity, was the fuse data already in the device library of the software that you used to generate the TCC? Unfortunately my employer won't pony up the $$$ for SKM or etap.

You're welcome Kent. I actually enjoy this stuff, so no problem at all.
Yes as Zog states the fuse files are provided with the SKM software package. Their device library is extensive, and as long as one's maintenance fees are current, they will make digital files for most any devices they don't have, provided you give them a paper copy.
You could do this manually by acquiring the fuse curves from Cooper and plotting them out, then placing the bolted 3-phase fault current over top of the fuse curves. But you'd have to calculate the fault currents, and that's not a real simple process. And then you'd need to handle the cumbersome equations in IEEE 1584 for the incident energy calculation using the arcing fault current to establish the opening time for the fuse, which will be much slower for the arcing fault.
Most Power Study specifications mandate the use of a software package, so as to minimize errors in the analysis.
An often recommended approach for clients is to have the Study preformed by an engineering firm, then use an SKM Viewer software package (small = $1000) that enables interaction with the model for the purpose of generating arc-flash labels, EEWPs, one-lines, etc. An entry-level SKM combo pack sells for about $6k and gives full functionality for small systems (50 nodes), with pricing increased as you add more nodes. You can also upgrade a small package for the cost difference at any time.

The opening time for an arcing fault on the system you described is shown on the TCC below. The time is on the order of 3 seconds, for which the industry-standard timeout value of 2 seconds would be used to develop the incident energy. The actual time could be used by adjusting the parameters of the Study. I know one large corporation (P&G) that uses 3 seconds as the timeout value. Most devices trip before 2 seconds, so it doesn't often come into play.

Good luck with it. PM me if I can assist.

 

[Bugman1400]

You're welcome Kent. I actually enjoy this stuff, so no problem at all.
Yes as Zog states the fuse files are provided with the SKM software package. Their device library is extensive, and as long as one's maintenance fees are current, they will make digital files for most any devices they don't have, provided you give them a paper copy.
You could do this manually by acquiring the fuse curves from Cooper and plotting them out, then placing the bolted 3-phase fault current over top of the fuse curves. But you'd have to calculate the fault currents, and that's not a real simple process. And then you'd need to handle the cumbersome equations in IEEE 1584 for the incident energy calculation using the arcing fault current to establish the opening time for the fuse, which will be much slower for the arcing fault.
Most Power Study specifications mandate the use of a software package, so as to minimize errors in the analysis.
An often recommended approach for clients is to have the Study preformed by an engineering firm, then use an SKM Viewer software package (small = $1000) that enables interaction with the model for the purpose of generating arc-flash labels, EEWPs, one-lines, etc. An entry-level SKM combo pack sells for about $6k and gives full functionality for small systems (50 nodes), with pricing increased as you add more nodes. You can also upgrade a small package for the cost difference at any time.

The opening time for an arcing fault on the system you described is shown on the TCC below. The time is on the order of 3 seconds, for which the industry-standard timeout value of 2 seconds would be used to develop the incident energy. The actual time could be used by adjusting the parameters of the Study. I know one large corporation (P&G) that uses 3 seconds as the timeout value. Most devices trip before 2 seconds, so it doesn't often come into play.

Good luck with it. PM me if I can assist.

What size wire did you use for the 150' cable?

Also, I'm skeptible that the xfmr was a g-wye/g-wye. Does the OP have a photo of the nameplate?

 

[kentirwin]

You're welcome Kent. I actually enjoy this stuff, so no problem at all.
Yes as Zog states the fuse files are provided with the SKM software package. Their device library is extensive, and as long as one's maintenance fees are current, they will make digital files for most any devices they don't have, provided you give them a paper copy.
You could do this manually by acquiring the fuse curves from Cooper and plotting them out, then placing the bolted 3-phase fault current over top of the fuse curves. But you'd have to calculate the fault currents, and that's not a real simple process. And then you'd need to handle the cumbersome equations in IEEE 1584 for the incident energy calculation using the arcing fault current to establish the opening time for the fuse, which will be much slower for the arcing fault.
Most Power Study specifications mandate the use of a software package, so as to minimize errors in the analysis.
An often recommended approach for clients is to have the Study preformed by an engineering firm, then use an SKM Viewer software package (small = $1000) that enables interaction with the model for the purpose of generating arc-flash labels, EEWPs, one-lines, etc. An entry-level SKM combo pack sells for about $6k and gives full functionality for small systems (50 nodes), with pricing increased as you add more nodes. You can also upgrade a small package for the cost difference at any time.

The opening time for an arcing fault on the system you described is shown on the TCC below. The time is on the order of 3 seconds, for which the industry-standard timeout value of 2 seconds would be used to develop the incident energy. The actual time could be used by adjusting the parameters of the Study. I know one large corporation (P&G) that uses 3 seconds as the timeout value. Most devices trip before 2 seconds, so it doesn't often come into play.

Good luck with it. PM me if I can assist.

Thanks again. I've had demo versions of both SKM and etap but the device libraries in the demos are extremely limited. I did try to find the fuse curves online but never could. My last resort would have been to email someone at Cooper to get the data. You've saved me the trouble.

 

[kentirwin]

What size wire did you use for the 150' cable?

Also, I'm skeptible that the xfmr was a g-wye/g-wye. Does the OP have a photo of the nameplate?

12 sets of 4-500 KCMIL. When I get a chance I'll look for a nameplate and take a pic but I don't recall seeing one on the xfmr. The data in my original post was straight from a letter I requested from Dom VA Pwr.

 

[mayanees]

What size wire did you use for the 150' cable?

Also, I'm skeptible that the xfmr was a g-wye/g-wye. Does the OP have a photo of the nameplate?

I used (5) 500 kCMs for an ampacity of 1900 amps.

We're seeing more and more Utility grounded wye primaries. I think it's driven by the fact that transformers can be built with a lesser insulation rating, and transient voltages are minimized.

 

[mbrooke]

I used (5) 500 kCMs for an ampacity of 1900 amps.

We're seeing more and more Utility grounded wye primaries. I think it's driven by the fact that transformers can be built with a lesser insulation rating, and transient voltages are minimized.

A lot of it is actually driven by ferroresonance as utilities go for ever higher distribution voltages. Single phase switching is very common as well as single phase fault clearing (fuses).

In terms of transient over voltages its actually worse. If a primary phase falls into the MGN under the right conditions the MGN impedance between the fault point and substation will create a neutral shift causing the neutral to rise in voltage with respect to the other 2 phases which in term gets transferred into the secondary.

In none POCO applications the norm is indeed delta wye with a 3 phase primary switch.

 

[mayanees]

A lot of it is actually driven by ferroresonance as utilities go for ever higher distribution voltages. Single phase switching is very common as well as single phase fault clearing (fuses).

In terms of transient over voltages its actually worse. If a primary phase falls into the MGN under the right conditions the MGN impedance between the fault point and substation will create a neutral shift causing the neutral to rise in voltage with respect to the other 2 phases which in term gets transferred into the secondary.

In none POCO applications the norm is indeed delta wye with a 3 phase primary switch.

Thanks for that update mbrooke. I went back through the NYSEG SP-1099 Customer specs for 2.4-34.5 kV and read that in fact, as you stated, ferroresonance was specifically mentioned on page 36 as a reason to avoid delta primaries, along with single-phase switching light loading, u'g cabling, and for smaller-sized transformers (<300 kVA).

But in your last sentence I think you're saying most POCO installations are Delta/wye, though your last sentence was auto-corrected to none.
Shown below is a page from NYSEG's specification that mandates G-Y:G-Y for 13 kV systems. Do you represent another Utility that would allow a 13 kV delta primary with G-Y secondary? That's the configuration I grew up with, but recent dealings with NYSEG necessitated the 13.8 kV G-Y : 480/277V G-Y configuration.
Thanks for any additional information.

 

[mbrooke]

Thanks for that update mbrooke. I went back through the NYSEG SP-1099 Customer specs for 2.4-34.5 kV and read that in fact, as you stated, ferroresonance was specifically mentioned on page 36 as a reason to avoid delta primaries, along with single-phase switching light loading, u'g cabling, and for smaller-sized transformers (<300 kVA).

But in your last sentence I think you're saying most POCO installations are Delta/wye, though your last sentence was auto-corrected to none.
Shown below is a page from NYSEG's specification that mandates G-Y:G-Y for 13 kV systems. Do you represent another Utility that would allow a 13 kV delta primary with G-Y secondary? That's the configuration I grew up with, but recent dealings with NYSEG necessitated the 13.8 kV G-Y : 480/277V G-Y configuration.
Thanks for any additional information.

My mistake, I meant applications outside of utilities like industrial and oil refineries. Delta-wye is the norm and used 99% of the time in these applications.


Some utilities do use delta-wye. California utilities use them frequently as well as those in Europe where its exclusive.


Ferroresonance can be mitigated 2 ways:

1. 3 phase switching and protection via recloser. When POCOs are forced to or choose to use a delta primary and Ferroresonance risk is high they will often take this route. There are industrial facilities around here fed from 3 wire sub transmission and as a result all pad mounts have a delta primary. A SCADA recloser is placed at the riser to provide 3 phase switching and 3 phase protection. Newer versions have built in VTs where the controller is used to trip the recloser on an open phase condition.

2. Keeping enough load on the secondary. If the secondary is loaded at least over 20% Ferroresonance is unlikely.

Of note, Ferroresonance is far more likely at 25 and 35kv then 15kv, so in your situation 13kv will help your case.

 

[mbrooke]

BTW, can you show me the caution on the previous page regarding ferroresonance?

 

[mayanees]

sure

sure

BTW, can you show me the caution on the previous page regarding ferroresonance?

...follows

 

[Bugman1400]

My mistake, I meant applications outside of utilities like industrial and oil refineries. Delta-wye is the norm and used 99% of the time in these applications.


Some utilities do use delta-wye. California utilities use them frequently as well as those in Europe where its exclusive.


Ferroresonance can be mitigated 2 ways:

1. 3 phase switching and protection via recloser. When POCOs are forced to or choose to use a delta primary and Ferroresonance risk is high they will often take this route. There are industrial facilities around here fed from 3 wire sub transmission and as a result all pad mounts have a delta primary. A SCADA recloser is placed at the riser to provide 3 phase switching and 3 phase protection. Newer versions have built in VTs where the controller is used to trip the recloser on an open phase condition.

2. Keeping enough load on the secondary. If the secondary is loaded at least over 20% Ferroresonance is unlikely.

Of note, Ferroresonance is far more likely at 25 and 35kv then 15kv, so in your situation 13kv will help your case.

I agree with #2, the more load the less likely of a ferro problem.

I'm not sure I agree with #1, I've never heard of using a SCADA recloser to mitigate a ferro issue. The SCADA part has nothing to do with it, its just used for remote control......perhaps switching the load to another source or to retrieve load data. Using a 3PH recloser is common on distribution whether it has a ferro issue or not. I always thought the use of VTs in a recloser just verifies that the load side is dead if the recloser is open before it closes in case there is distributed generation on the load side.

Another method for controlling a ferro issue is to install TRV caps at the substation.

The use of delta primary xfmrs has the advantage of not passing zero seq current to the low side for a ground fault on the low side and vice versa. Many industrial customers prefer to have a resistive type ground source.

 

[mbrooke]

I agree with #2, the more load the less likely of a ferro problem.

I'm not sure I agree with #1, I've never heard of using a SCADA recloser to mitigate a ferro issue. The SCADA part has nothing to do with it, its just used for remote control......perhaps switching the load to another source or to retrieve load data. Using a 3PH recloser is common on distribution whether it has a ferro issue or not. I always thought the use of VTs in a recloser just verifies that the load side is dead if the recloser is open before it closes in case there is distributed generation on the load side.

Another method for controlling a ferro issue is to install TRV caps at the substation.

The use of delta primary xfmrs has the advantage of not passing zero seq current to the low side for a ground fault on the low side and vice versa. Many industrial customers prefer to have a resistive type ground source.

I only mentioned SCADA because that's what gets put up by most utilities involving new reclsoers. The SCADA part itself isn't important, however what mitigates the risk is the 3 phase switching and 3 phase protection provided by the recloser itself.


The recloser is responsible for protecting the transformer in addition to being a 3 phase disconnect. The micro processor controller can be configured with just about any time current curve including ones that mimic fuse links, like a 140T or 200K for example. In fact because the controller is so versatile curves can be chosen that will protect the transformer under any fault condition from sustained chronic over load to an asymmetrical bolted fault on the secondary without exceeding the transformer's damage curve. Incident and arc flash energy can also be reduced as well on the secondary.

Primary VTs allow the recloser to be programmed for loss of phase (an open phase) response during a broken conductor event (out on the line) which could put the transformer into ferroresonance. Sometimes its done by enabling the loop scheme mode in the controller and setting a trip lockout if phase voltages are imbalanced beyond 85% exceeding 10 seconds. Even if ferroresonance never takes place the facility will not be subjected to costly single phasing.


The cost may not be worth it for a small 30kva padmount, but its routinely done around here for large facilities like malls and factories working as intended.

I would definitely go with a delta primary for dozens of reasons involving power quality including an inability to pass zero sequence currents like mentioned and the ability to use any type of grounding configuration as desired on the secondary.

At 1,500 kva this is definitely an option worth considering.

What type of facility is this btw?

 

[mbrooke]

Attached is a TCC that models the system you described.
A fault at the secondary of the transformer is around 37,500 amps. A fault 150 feet from the secondary is around 31064 amps.
The vertical line shown at 31064 amps intersects the short-circuit protection fuse at approximately 0.248 seconds.

Im sure its obvious, but is this for a 3 phase symmetrical fault or a single line to ground fault? Or it doesn't matter in the case of wye-wye being a winding current transfer ratio of 1:1 so to speak.