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 Post subject: Arc Flash Model and Issues for Wind Farm Collection Circuits
PostPosted: Wed Apr 06, 2016 9:50 am 
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I am performing an AF Hazard analysis, primarily using ETAP, for a legacy Wind Farm project my company owns and operates. The collection circuits consist of a 34.5kV substation main breaker, 34.5kV sectionalizing outdoor switchgear breakers, each breaker feeding a 34.5kV collection cable daisy chained to feed multiple 1MVA pad-mounted transformers in a string (34.5kV/600VAC, wye-g/wye-g), fused at incoming 34.5kV), and each transformer (acting primarily as a generator step-up) connected at 600VAC to a 900kW wind turbine (WTG) incoming feeder breaker controller enclosed in a regular NEMA rated panel.
This configuration is quite normal in the industry. I am finding several AFH issues specific to this project which are probably very generic for this configuration:
1. At 600V: The clearing time on the transformer secondary is very slow based on the ‘standard’ fuses used on the HV side of the pad mount transformers, which makes the AF IE on the 600V secondary >40 cal. There is no remote operating capability for the WTG incoming breaker which must be manually opened and closed (only in non-generating standby mode).
a. Concern: I have followed the treads discussing normal breaker operation with the door closed and ‘less than’ PPE while operating. I have a fuse change-out solution which can get us to Cat 3 PPE, but right now our operators have been instructed to operate only with Cat 4 PPE until any solution is finalized. Obviously they hate doing so, and on paper even the Cat 4 does not meet requirements if the door was opened.[/align]
2. At 34.5kV: IEEE 1584 empirical calcs do not apply above 15kV, so ETAP uses the Lee method for estimating IE at these busses and breakers. I have to model these components as switchgear or similar. I have a sense that the resulting AF IE is high and there may be a better solution for modeling this equipment.
a. Question: Any recommendations or thoughts on a better way to model 34.5kV switchgear or 34.5kV padmount HV incoming cubicle?
i. In ETAP
ii. Duke Heat Flux Program? Multiplier for ‘box’ configuration?
iii. ARCPRO? I looked into its’ use but didn’t see a real advantage.
I apologize for the length of this note, but would really like to hear some feedback. I find almost no discussions specific to AF studies for windfarms. Thanks!


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 Post subject: Re: Arc Flash Model and Issues for Wind Farm Collection Circ
PostPosted: Wed Apr 06, 2016 10:29 pm 
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resowers wrote:
I am performing an AF Hazard analysis, primarily using ETAP, for a legacy Wind Farm project my company owns and operates. The collection circuits consist of a 34.5kV substation main breaker, 34.5kV sectionalizing outdoor switchgear breakers, each breaker feeding a 34.5kV collection cable daisy chained to feed multiple 1MVA pad-mounted transformers in a string (34.5kV/600VAC, wye-g/wye-g), fused at incoming 34.5kV), and each transformer (acting primarily as a generator step-up) connected at 600VAC to a 900kW wind turbine (WTG) incoming feeder breaker controller enclosed in a regular NEMA rated panel.
This configuration is quite normal in the industry. I am finding several AFH issues specific to this project which are probably very generic for this configuration:
1. At 600V: The clearing time on the transformer secondary is very slow based on the ‘standard’ fuses used on the HV side of the pad mount transformers, which makes the AF IE on the 600V secondary >40 cal. There is no remote operating capability for the WTG incoming breaker which must be manually opened and closed (only in non-generating standby mode).
a. Concern: I have followed the treads discussing normal breaker operation with the door closed and ‘less than’ PPE while operating. I have a fuse change-out solution which can get us to Cat 3 PPE, but right now our operators have been instructed to operate only with Cat 4 PPE until any solution is finalized. Obviously they hate doing so, and on paper even the Cat 4 does not meet requirements if the door was opened.[/align]
2. At 34.5kV: IEEE 1584 empirical calcs do not apply above 15kV, so ETAP uses the Lee method for estimating IE at these busses and breakers. I have to model these components as switchgear or similar. I have a sense that the resulting AF IE is high and there may be a better solution for modeling this equipment.
a. Question: Any recommendations or thoughts on a better way to model 34.5kV switchgear or 34.5kV padmount HV incoming cubicle?
i. In ETAP
ii. Duke Heat Flux Program? Multiplier for ‘box’ configuration?
iii. ARCPRO? I looked into its’ use but didn’t see a real advantage.
I apologize for the length of this note, but would really like to hear some feedback. I find almost no discussions specific to AF studies for windfarms. Thanks!


1. There is little difference between PPE 3 and 4 from a practical point of view.
2. Duke heat flux results are less than experimental data at 15 kv. Lee is grossly too high (300% at 15 kv) and gets worse as voltage increases since arc voltage increases slowly with current and is almost independent of system voltage. Lee model is 0.5*V*I.
3. There are no experimental results above 15 kv so all models are theoretical. OSHA has pretty much mandated Arcpro and this is the program also used for NESC (IEEE C2) tables. So there is IS justification but not elsewhere.
4. Normal operation considers risk. Even though a car crash can be fatal, we do not require 5 point harnesses, neck protection, and FR suits except for racing because fatal car crashes under normal driving conditions are rare. Same thing applies to operating electrical equipment.


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 Post subject: Re: Arc Flash Model and Issues for Wind Farm Collection Circ
PostPosted: Thu Apr 07, 2016 8:36 am 
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Thanks for the reply.

I guess my ultimate issue with having to purchase the Arcpro software is that the OSHA tables from 1910.269 App E are ultimately based on Arcpro calculations for phase-ground in open air, and it seems you are purchasing the software simply to find out what the conversion factors are for 'in an enclosure'... 'as indicated in the program instructions'. My 34.5kV switchgear needs to be modeled as being in a box. Are these Arcpro conversion factors posted anywhere besides the product instruction? What is the science behind the Arcpro conversion factors?

Also, just for clarity, we are considering a fuse change-out solution on the 34.5kV pad mount transformer primary because it accomplishes the following:
Existing fuse: 30A DEFL Bayonet, Secondary AFB=19.90 ft, IE=211 cal/cm^2, FCT=301 cycles (Exceeds max)
New fuse: 25A Current Sensing Bayonet, Secondary AFB=6.21 ft, IE=20.56 cal/cm^2, FCT=29.42 cycles. (Level 3)
The fuse change-out solution is supported by the subject matter expert at Cooper.


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 Post subject: Re: Arc Flash Model and Issues for Wind Farm Collection Circ
PostPosted: Thu Apr 07, 2016 6:42 pm 
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resowers wrote:
Thanks for the reply.

I guess my ultimate issue with having to purchase the Arcpro software is that the OSHA tables from 1910.269 App E are ultimately based on Arcpro calculations for phase-ground in open air, and it seems you are purchasing the software simply to find out what the conversion factors are for 'in an enclosure'... 'as indicated in the program instructions'. My 34.5kV switchgear needs to be modeled as being in a box. Are these Arcpro conversion factors posted anywhere besides the product instruction? What is the science behind the Arcpro conversion factors?

Also, just for clarity, we are considering a fuse change-out solution on the 34.5kV pad mount transformer primary because it accomplishes the following:
Existing fuse: 30A DEFL Bayonet, Secondary AFB=19.90 ft, IE=211 cal/cm^2, FCT=301 cycles (Exceeds max)
New fuse: 25A Current Sensing Bayonet, Secondary AFB=6.21 ft, IE=20.56 cal/cm^2, FCT=29.42 cycles. (Level 3)
The fuse change-out solution is supported by the subject matter expert at Cooper.


Its hard to say anything about Arcpro other than comparing it to experimental data. The manual states that it is based on about 4 different papers and those papers are fairly incomplete but the theory that it is based on is sound and lines up well with what physicists have managed to develop in terms of knowledge of arcs in general. The source code or detailed algorithms are not published so beyond the claims of the two authors we have no way to verify much of anything. As to the multipliers, those have been published on this forum as well as elsewhere but for enclosures I think the recommended numbers were "3-6", a pretty wide range of values. Arcpro was developed at Kinetrics which does all of the PPE testing as well as lots of other arc flash related testing so it is most likely that the authors amassed a lot of proprietary data and did some kind of mathematical analysis on the data but since various clients paid for the data and did not release it, you sort of have to take everything on faith. I do agree that the tables in 1910.269 as well as NESC came from Arcpro and that for open air at least the data is readily available without paying for the software.

You could use view3d to calculate the view factors for your equipment and use the Arcpro values as "free burning arcs" which is what they represent. View3d is a free software program that has been widely used to calculate the radiosity equations for radiative heating for a variety of conditions other than arc flash and recently Mike Lang and others have pretty well proven that the difference between open air and enclosed arcs is the view factor. The view factors from the IEEE 1584 model have also been published and are fairly easy to derive (just apply the scaling equation to the Arcpro open air results) since a range of "3-6" is not a very satisfactory answer.

Although this sounds like "pulling numbers out of the air", keep in mind that that is exactly what anyone who is analyzing arc flash above 15 kV is doing. There is no experimental evidence and thus no way to validate whether or not the result is accurate in any way. All published work above 15 kV is based on conjecture at lower voltages and/or comparison of different theoretical results to each other.

In other words what OSHA did is that they sampled three arc flash models. Duke heat flux seemed too cold. Lee seemed too hot. So with Arcpro right in the middle, it is just right. OK so I'm making jokes about fairy tales but that's what it is.


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 Post subject: Re: Arc Flash Model and Issues for Wind Farm Collection Circ
PostPosted: Fri Apr 08, 2016 8:36 am 
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http://www.e-hazard.com/blog/osha-interpretation-on-using-table-410-1-for-ppe-selection/

Looks like version 3.0 is finally on the way.

I notice the concern above is regarding operation. Does operation involve an interaction with a circuit in a way likely to generate an arc?


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 Post subject: Re: Arc Flash Model and Issues for Wind Farm Collection Circ
PostPosted: Fri Apr 08, 2016 8:43 am 
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Regarding the 600V, 1200A MCCB: No. The only concern is failure of the breaker itself when operated. The 'operator' is a hand switch which is directly part of the breaker


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 Post subject: Re: Arc Flash Model and Issues for Wind Farm Collection Circ
PostPosted: Fri Apr 08, 2016 1:56 pm 
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resowers wrote:
Regarding the 600V, 1200A MCCB: No. The only concern is failure of the breaker itself when operated. The 'operator' is a hand switch which is directly part of the breaker


Data from the IEEE Gold book shows that with frame sizes starting at 800 A and larger the failure rates are 3 times higher for the breakers. So there is a difference as the mechanisms get larger and heavier but only just so. No way to dive into the data though to tell if for instance this is inherent in breakers using air as a switching medium in general or if for instance it is reflective of a particular design.

And as has been mentioned earlier there is nothing magical about 600 V. In terms of breaker design itself the UL breakers are generally built towards specifications which emphasize low maintenance (bolted) while the ANSI specifications emphasize designs which are designed for heavy preventative maintenance in exchange for hopefully better reliability (which is usually not the case in practice). Today usually the only difference between ANSI and UL breakers is whether it is in a UL-rated enclosure or an ANSI one.


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