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Jim Phillips (brainfiller)

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: 1525 Location: Scottsdale, Arizona

Making Sense of the NumbersUtility Company Shortcircuit 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 shortcircuit 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 shortcircuit 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 numbers—now what?Unfortunately, a single standardized format for shortcircuit data does not exist. Instead, depending on the individual utility, data may be provided in one of several different formats such as the following: • Shortcircuit amperes (A)• Shortcircuit megavoltamperes (MVA)• Perunit and symmetrical components Of course, with multiple formats, confusion could (and often does) result. I will compare the different formats using a threephase shortcircuit current of 6,000A at the 23kilovolt (kV) level. Since arc flash calculations are based on a threephase model, only the threephase shortcircuit calculations are used. Some of the values are slightly rounded. Shortcircuit ampere formatThis is the simplest format because it defines the shortcircuit current in terms of amperes at a specified location. As an example, the utility has provided the following information: Shortcircuit amperes threephase = 6,000AVoltage = 23 kV linetoline Since the data is already in terms of amperes, no additional calculations are necessary. Shortcircuit MVA formatUtility companies often provide short circuit data in terms of shortcircuit MVA. This format combines the shortcircuit current with the voltage and the square root of 3 (for a threephase representation) to provide the data in terms of shortcircuit power. Below is an example of the MVA format. Threephase shortcircuit MVA = 240 MVAVoltage = 23 kV linetoline To convert threephase shortcircuit MVA to shortcircuit current in amperes, use the following equations: Shortcircuit amperes = [MVA x 1,000] / [kV linetoline x the square root of 3] where 1,000 is the conversion from MVA to kVA Shortcircuit amperes = [240 MVA x 1,000] / [23 kV linetoline x 1.732]Shortcircuit amperes = 6,000A Perunit and symmetrical components formatThe perunit and symmetrical component format can appear to be the most complex of all. The term “perunit” is simply the decimal equivalent of percent, i.e., 50 percent is equal to 0.5 per unit. In general, the perunit 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 shortcircuit current using this approach is not that difficult. The example below illustrates shortcircuit data using the perunit system and symmetrical components: MVA base = 100 MVAkV base = 23 kV linetolineZ1 = 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 “base values” necessary for the calculations. For the threephase shortcircuit current, only three steps are needed to convert the perunit and symmetrical component values to shortcircuit 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 perunit threephase shortcircuit current (I p.u.) with the following equation: I p.u. = V p.u. / Z1 V p.u. in the equation above is the perunit voltage. In the absence of being provided the perunit 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. = 1.0I p.u. = 1.0 / 0.418 = 2.39 p.u. Step 3: Convert perunit shortcircuit 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 resultsAlthough 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 Learn more about Power System Engineering Class
_________________ Jim Phillips, P.E. Brainfiller.com


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K. Engholm

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

Joined: Sat May 17, 2008 5:00 pm Posts: 86

This is great article! Thanks Jim.


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arcad

Post subject: Re: Short Circuit Data  Per Unit, Amps, Symmetrical Compone Posted: Thu Jul 10, 2014 12:40 pm 

Sparks Level 

Joined: Wed Jun 10, 2009 5:00 pm Posts: 263 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 derate the effective rating of the gear. You may consider ShortCircuitAnalytic 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 undervalued incident energy and arc flash boundary.



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PaulEngr

Post subject: Re: Short Circuit Data  Per Unit, Amps, Symmetrical Compone Posted: Thu Jul 17, 2014 2:01 pm 

Plasma Level 

Joined: Tue Oct 26, 2010 9:08 am Posts: 2174 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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JBD

Post subject: Re: Short Circuit Data  Per Unit, Amps, Symmetrical Compone Posted: Thu Jul 17, 2014 2:55 pm 

Joined: Mon Jan 18, 2010 11:35 am Posts: 524 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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arcad

Post subject: Re: Short Circuit Data  Per Unit, Amps, Symmetrical Compone Posted: Fri Jul 18, 2014 8:25 am 

Sparks Level 

Joined: Wed Jun 10, 2009 5:00 pm Posts: 263 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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