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 Post subject: Short Circuit Data - Per Unit, Amps, Symmetrical Components,Posted: Sun Apr 20, 2014 12:08 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1682
Location: Scottsdale, Arizona
Making Sense of the Numbers
Utility Company Short-circuit Data For Arc Flash Studies

Electrical Contractor Magazine - November 2012
Jim Phillips, P.E.

One of the first steps in performing an arc flash hazard calculation study is to request the short-circuit data from the electric utility company. This information is critical because it defines the magnitude of current that could flow from the utility and is used as a starting point for arc flash calculations.

In addition to requesting this data for normal operating conditions, it should also be requested based on minimum short-circuit current conditions, if available. The minimum condition could be for a utility transformer or transmission line out of service or similar scenario. The minimum value can then be used to determine if the lower current could result in a protective device operating more slowly, which may increase the total incident energy during an arc flash.

Too many numbersânow what?
Unfortunately, a single standardized format for short-circuit data does not exist. Instead, depending on the individual utility, data may be provided in one of several different formats such as the following:

â˘ Short-circuit amperes (A)
â˘ Short-circuit megavolt-amperes (MVA)
â˘ Per-unit and symmetrical components

Of course, with multiple formats, confusion could (and often does) result. I will compare the different formats using a three-phase short-circuit current of 6,000A at the 23-kilovolt (kV) level. Since arc flash calculations are based on a three-phase model, only the three-phase short-circuit calculations are used. Some of the values are slightly rounded.

Short-circuit ampere format
This is the simplest format because it defines the short-circuit current in terms of amperes at a specified location. As an example, the utility has provided the following information:

Short-circuit amperes three-phase = 6,000A
Voltage = 23 kV line-to-line

Since the data is already in terms of amperes, no additional calculations are necessary.

Short-circuit MVA format
Utility companies often provide short- circuit data in terms of short-circuit MVA. This format combines the short-circuit current with the voltage and the square root of 3 (for a three-phase representation) to provide the data in terms of short-circuit power. Below is an example of the MVA format.

Three-phase short-circuit MVA = 240 MVA
Voltage = 23 kV line-to-line

To convert three-phase short-circuit MVA to short-circuit current in amperes, use the following equations:

Short-circuit amperes = [MVA x 1,000] / [kV line-to-line x the square root of 3]

where 1,000 is the conversion from MVA to kVA

Short-circuit amperes = [240 MVA x 1,000] / [23 kV line-to-line x 1.732]
Short-circuit amperes = 6,000A

Per-unit and symmetrical components format
The per-unit and symmetrical component format can appear to be the most complex of all. The term âper-unitâ is simply the decimal equivalent of percent, i.e., 50 percent is equal to 0.5 per unit. In general, the per-unit method takes every electrical quantity and scales it by a reference value known as a base quantity. The utility derives the base values from two numbers: the MVA base and kV base

Symmetrical components is a method used for solving complex unbalanced power system problems. Such terms as positive, zero and negative sequence are part of the vocabulary of this method, and although the actual theory can be quite complex, calculating the short-circuit current using this approach is not that difficult.

The example below illustrates short-circuit data using the per-unit system and symmetrical components:

MVA base = 100 MVA
kV base = 23 kV line-to-line
Z1 = 0.418 p.u.

Z1 is referred to as the positive sequence impedance and represents the equivalent impedance of the utility in this case. One hundred MVA and 23 kV are the base power and voltage used to determine the âbase valuesâ necessary for the calculations.

For the three-phase short-circuit current, only three steps are needed to convert the per-unit and symmetrical component values to short-circuit current in amperes:

Step 1: Calculate the base current (I base) using the following equation:
I base= [MVA base 1,000] / [kV base x the square root of 3]
I base= [100 MVA x 1,000] / [ 23kV x the square root of 3]
I base= 2,510A

Step 2: Calculate the per-unit three-phase short-circuit current (I p.u.) with the following equation:

I p.u. = V p.u. / Z1

V p.u. in the equation above is the per-unit voltage. In the absence of being provided the per-unit voltage, which is usually the case, it is common to assume it is 1.0 p.u. This means the actual voltage is 100 percent of the base voltage, so for this example:

V p.u. = 1.0
I p.u. = 1.0 / 0.418 = 2.39 p.u.

Step 3: Convert per-unit short-circuit current to amperes with the following equation:

I amperes= I p.u. x I base
I amperes= 2.39 p.u. x 2,510A
I amperes= 6,000A

Different methods = same results
Although the three methods seem quite different from each other and some are more complicated, they all produce the same result, which can be used as the starting point for arc flash calculations.

Originally published in the November 2012 issue of Electrical Contractor Magazine

_________________
Jim Phillips, P.E.
Brainfiller.com

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 Post subject: Posted: Sun Apr 20, 2014 6:16 pm
 Sparks Level

Joined: Sat May 17, 2008 5:00 pm
Posts: 88
This is great article! Thanks Jim.

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 Post subject: Re: Short Circuit Data - Per Unit, Amps, Symmetrical ComponePosted: Thu Jul 10, 2014 12:40 pm
 Sparks Level

Joined: Wed Jun 10, 2009 5:00 pm
Posts: 267
Location: Toronto
Do not forget to request from utility company the X/R ratio at service entrance point along with the short circuit data. Furthermore, make sure the methodology utilized for short circuit analysis takes into account both active and reactive parts of equipment impedance. A lot of industry accepted calculation methods and software programs ignore the equipment X/R ratios in short circuit analysis and apply correction factors to presumably compensate for the error. Such approach introduces up to 15% uncertainty in a single step of adding two impedances alone while applying different correction factors does not quantifies or corrects the error but only amplifies it.

The issue becomes more complicated based on the fact that all protective devices are tested at predetermined X/R ratios. If the calculated X/R ratio at any given point in the electrical distribution system exceeds the tested X/R ratio of the overcurrent protective device, then you must de-rate the effective rating of the gear.

You may consider Short-Circuit-Analytic version 1.0 (SCA V1.0) software program for comprehensive short circuit analysis factoring in the equipment X/R ratios, contributions from motors and generators, etc.

Also, oftentimes the short circuit data provided by utility companies at service entrance point assume infinite bus. Namely, utility company may provide with the short circuit current data based on service entrance transformer admittance value only. The admittance is the maximum current at unit voltage which would flow through the circuit or component to a short circuit or fault when supplied from a source of infinite capacity. Such data is unacceptable in arc flash analysis as it produces unrealistically high available short circuit current and predicted arcing current values through the protective device, and hence a shorter clearance time resulting in under-valued incident energy and arc flash boundary.

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 Post subject: Re: Short Circuit Data - Per Unit, Amps, Symmetrical ComponePosted: Thu Jul 17, 2014 2:01 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
Its getting better. At least utilities are now providing data compared to previously.

The issue though is that normally larger industrial customers are fed from a distribution system or maybe even a transmission system. The exact X/R ratio is unknown because it depends on the state of the bus ties at the time since it is a network and not something simple like a typical industrial site radial feed system. So the utility engineer is going to have to provide either whatever they typically model or some sort of guess at best.

Getting hung up on X/R is even worse because it heavily depends on the loads on the system. If you are in an insolated, mostly residential area then most likely the major reactance on the network will consist of the reactance of the lines/cables. But if its in a heavily congested industrial park type of environment then most likely it involves lots and lots of highly inductive customer loads that the utility themselves can only estimate.

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 Post subject: Re: Short Circuit Data - Per Unit, Amps, Symmetrical ComponePosted: Thu Jul 17, 2014 2:55 pm
 Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 554
Location: Wisconsin
PaulEngr wrote:
The issue though is that normally larger industrial customers are fed from a distribution system or maybe even a transmission system.

By far and away, most of the customers I have worked with, purchase secondary power at 480V or less. If the facility is fed from a dedicated transformer several of the utility system uncertainties are mitigated.
The NFPA70E fear mongers have done a good job of equating the study of a 150kVA 240V facility to that of a 15MVA 24kV one, to the point where smaller companies are afraid to even begin the process.

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 Post subject: Re: Short Circuit Data - Per Unit, Amps, Symmetrical ComponePosted: Fri Jul 18, 2014 8:25 am
 Sparks Level

Joined: Wed Jun 10, 2009 5:00 pm
Posts: 267
Location: Toronto
PaulEngr wrote:
Its getting better. At least utilities are now providing data compared to previously.

The issue though is that normally larger industrial customers are fed from a distribution system or maybe even a transmission system. The exact X/R ratio is unknown because it depends on the state of the bus ties at the time since it is a network and not something simple like a typical industrial site radial feed system. So the utility engineer is going to have to provide either whatever they typically model or some sort of guess at best.

Getting hung up on X/R is even worse because it heavily depends on the loads on the system. If you are in an insolated, mostly residential area then most likely the major reactance on the network will consist of the reactance of the lines/cables. But if its in a heavily congested industrial park type of environment then most likely it involves lots and lots of highly inductive customer loads that the utility themselves can only estimate.

The system loads will have an impact both on X/R AND short circuit MVA coming from utility. Using both the minimum (all loads being turned off) and maximum available utility MVA (all loads are up and running) and corresponding X/R in short circuit analysis may solve the dilemma.

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