Utility refuses to provide X/R ratio 1

[wbd]

hello,

I'm working on performing a short circuit study in order to get the arc flash hazards. The utility servicing the customer will only provide a range of fault currents available but refuses to provide the X/R ratios.

This is what I received from the utility person I've been dealing with: (the utility's name has been replaced with XXXX)

___________________________________________________________

The following are directions I received from our corporate office:

The X/R ratios will not be provided

In order to assess the arc flash exposure both the fault current and clearing time are required to determine the level of protection needed. Instinctively it is easy to assume that the highest fault duty will provide the highest arc flash exposure but more often than not this is not true. For lower fault duties the clearing time can be quite large leading to a high calorie level. XXXX’s distribution system is very dynamic. The following factors can affect the fault duty:

1. Dependant on which circuit is supplying the transformer. XXXX’s system is dynamic and is continually being reconfigured. This can significantly change the fault duty level.

2. Transformer size (including any future replacement size)

3. Impedance of transformer (XXXX specifies a maximum and minimum impedance range)

4. Secondary impedance

The following factors can affect the fault clearing time:

1. Which protective device operates.

2. The transformer fuses may vary as there is more than one type and size that may be used.

3. The impedance of the fault.



Due to the many variables we are in the process of developing a generic table. Depending on the size and type of transformer a minimum and maximum fault current will be provided. The fault current will be calculated using the following assumptions:

1. For the minimum fault level:

Lowest system fault duty (i.e. 370A).
Use the maximum impedance for the smallest transformer able to be installed at that location.
2. For the maximum fault level:

100MVA system bus
Use the minimum impedance for the largest transformer able to be installed at that location.


Additionally it will be recommended that the customer use their own protective device to determine the fault clearing time.
____________________________________________________________

I believe I can not perform the required studies without the X/R ratio and the utility is preventing my client from providing a safe workplace in accordance with OSHA standards.

Any ideas on how to proceed from this point would be greatly appreciated.

 

[apowerengr]

Ask the utility for the primary supply system min and max fault values and the associated X/R ratios for three phase and slg faults and the existing service transformer sizes and impedances. Let them know you'll reference the study documents as accurate for current configurations only and caution the reader to verify existing conditions against the study results before proceeding.

 

[wareagle]

"the utility is preventing my client from providing a safe workplace in accordance with OSHA standards."
I don't believe that. You have all the information they are going to give you. The reason is that if they give you a fixed figure it may change the next day. What are you going to do with the X,R and transformer info and they change the
feed to a new sub further away? What happens when the transformer that is being used now is increased in size?
This is the problem in trying to make your caculations.

 

[wbd]

wareagle,
I don't agree. How do you perform a short circuit calculation without the X/R ratio?
They have the min and max s/c's so they could give me the associated X/R ratios along with a disclaimer that this is only a range of possible values and would change if the circuit configuration changes.

 

[rbulsara]

wbd:

Arc flash calcs are not a fixed one time calcs, nor are any of short circuit calcs. You have to make 'reasonable' assumptions for may variable factors. You have to use best information available and qualify you analysis by assumptions you have made.

The electrical distribution systems are dynamic and it is not going to change. Arc flash calculations methods are even not perfect and you will not be able to wait till perfection so do the best you can. Even if the utility compnay gives you X/R ratio the arc flash hazard varies by many factors and configurations of your own system within a facility.

So no utility compnay is not preventing you from the making your or your clients workplace safer. You will make it safer than before just by applying best possible warnings but never bullet-proof.

 

[wareagle]

WBD
Will they give you the fault current at the primary terminals of the transformer? If so you can convert it to
an X quanity. R is small when compared to the other components.

 

[DaveScott]

X:R = 10:1
Just a rule of thumb that I have often used.

 

[unclebob]

wbd,

I think the utility gives you enough informations. Even if the X/R ratio is kwown, in normal operation, the value of X during a fault goes up. So the X/R ratio during a fault is not known.

 

[Dumbo2929]

The X/R ratio determines the decay of the DC component that is present when the fault occurs. If the fault source was all reactive (high X/R) then it would never decay. If the fault source was all resistive (low X/R) it would decay immediately. A larger X/R ratio increases that first ½ cycle peak current value (referred to as asymmetrical peak) and makes the current magnitude when the breaker tries to clear the fault (usually within 3-5 cycles) that much higher due to the rate of decay. Extremely high X/R ratios (over 10) can force de-rating of circuit breakers, due to the stress of trying to clear these fault magnitudes.

 

[jghrist]

X/R for 3 ph fault on a 12 kV line of one of our clients:

1.58 at end of line
15.7 at substation

 

[mykh]

Check short circuit online calculator at

http://www.arcadvisor.com

It features input data analysis and hard coded error propagation rules ensuring that the resulting fault current values are not more precise than justified by the accuracy of input data including X/R.

 

[jghrist]

the resulting fault current values are not more precise than justified by the accuracy of input data including X/R.

Does the program flag the output as SWAG if the X/R is a scientific wild a** guess?

 

[Robert789]

I think the utility's response is appropriate. When approached with similar requests, I provide available fault current at the transformer secondary assuming infinite bus on the primary side. I also let them know that this is based on the existing transformer which could be replaced with a larger unit at any time if conditions merit doing so.

I have enough to worry about without having to notify countless customers every time that a distribution upgrade or feed change is made.

In addition to the X/R ratio, one needs to consider the L/E ratio of the area (look in the yellow pages to find the number of Lawyers vs the number of Engineers).

 

[itsmoked]

Robert789; you have hit the nail on the head.. The 'L' is why the utility won't say.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[wbd]

Thank you all for your responses. However, I am still stuck. I understand all the things about a system being dynamic, txf's can change, etc but that would also apply to just sizing the eqpt for the max. fault conditions.

Additionally, the following statementfrom IEEE 1584 justifies the importance of obtaining the X?R ratio:
1. IEEE 1584, Section 4.2 states "Get the available fault MVA and power angle or X/R ratio from the utility. Most utilities will readily supply information on the available fault level and X/R ratio at point of service. When information is not provided, public utility commissions can be requested to require utilities to furnish this information. Available fault data must be realistic; not conservatively high."

Here is a document that is the basis for the arc flash calcultions, that has been developed by an internationally recognized group, been peer reviewed and adopted by the industry. I am going to be persuing this with the utilities public utility commission. This is the first utility that I have encountered that is unwilling to provide the required information.

 

[mpparent]

Here's NEMA's take on it:

http://www.nema.org/prod/be/lvde/upload/LVDEARCFLASH.pdf

Mike

 

[stevenal]

In my opinion, the standards do not adaquately address the equipment sizing issue either. NEC for example applies at the time of instalation and has no maintenance requirement for meeting an increasing available fault current. "Conservatively high" can help in this case, but might not be conservative when looking at arc flash.

As far as "peer reviewed and adopted", 1584 was sponsored by the Petroleum and Chemical Industry Committee of the Industry Applications Society. They'll get more buy in from utilities if they get the Power Engineering Society involved. The PUC statement sounds like they actually anticipated the problem you have. When utilities are forced into outages rather than reconfiguring (which may affect fault current) the PUC will be getting a lot more calls.

 

[apowerengr]

Robert789 makes a typical mistake for utility engineers - giving a facility owner secondary faults based on an infinite bus will typically lower the arc-flash hazard levels as compared to actual conditions. This is due to the typically inverse characteristic of overcurrent protective devices. Although the practice of infinite bus calculations was widely used and applicable for recommending overcurrent device interrupting ratings, it's use as a basis for arc-flash hazard calculations is not recommended.

 

[wbd]

apowerengr-
I agree with you but I'm also a utility engr and do some consulting on the side. In my utility job, I provide the fault levels with the X/R ratio. I add some disclaimers such as that this is for the normal system configuration and that it is advisable to check with the utility periodically (yearly) to see if there have been any changes.