Arc Flash Step by Step #5 Short Circuit Study

Perform short circuit current analysis

Once our model is ready we can create short circuit scenarios. This depends on system but most often in Europe we will use IEC 60909 or local harmonized versions. Since we know that our focus is on arc flash analysis, equipment evaluation and protective device coordination we need more than one scenario.For simple industrial system this will require creating at least two scenarios:

• Minimum short circuit current (Utility min short circuit power, no motors etc.)
• Maximum short circuit current (motors contribution, C=max etc.)

For complex systems multiple scenarios are needed due to possible system configuration. This involves:

• Parallel source operations (PV, Generator, Utility)
• Parallel transformers (closed or open tie breaker)
• MV ring operation (open or closed)
• Motor contribution (big motors separately modeled, VFD with bypass etc)
• Generator mode (emergency operation, normal operation)
• UPS mode (bypass or normal)
• System specific configurations (open, closed breakers, test lab modes etc)

Short circuit results depends on all of those factors. Additionally it makes difference if we calculate fault close or far from generator which is affected by AC decaying component. IEC 60909 representation gives clear overview of this behavior. 

Figure 1. Example of IEC 60909 short circuit current near and far from generator

Short circuit analysis will be used to identify worst cases. This means minimum and maximum short circuit currents. Results of this study will be used for further calculations so it is important to validate it.

How results can look like?

It can be summarized in table which helps to find right data. Additionally it is nice to show partial results on single line diagram (keep it readable). Some examples below:

Figure 2. Example of IEC 60909 short circuit results on SLD

Figure 3. Example of IEC 60909 short circuit current results list per scenario

As mentioned earlier it could be that scope of work contains part of the network and rest is not included. While this is fully understood from cost point of view it may have big influence on end results. Since we are looking for short circuit current which will lead to (usually) longest tripping times and highest incident energy, it is important to be not too conservative. Narrowing analysis to part of installation can end up with not realistic results and over/under sizing PPE, leading to wrong conclusions and costly recommendations. This is something we would like to avoid.

How we use short circuit results?

• Equipment evaluation will use maximum short circuit results to compare withstand rating of equipment like cables, switchgears, breakers, transformers etc
• Protective device coordination (selectivity analysis) require both maximum and minimum, sometimes scenario specific results for protection settings calculations
• Arc flash analysis requires worst case combination ( min, max etc) in relation to protection scheme

Coming back to partial short circuit calculation It is site specific and requires recommendation from power system engineer for each project to make it right(see scope of work kick-off discussion).

Common issues related to short circuit data

Typical situations:

• We have previous short circuit study but it lacks min max values and was too general missing short circuit sources especially generators and motors
• We don’t have study but our scope of work is limited so big part of the system won’t be included in short circuit study
• no main transformer data or hard to get (the same for utility data)
• ring topology with multiple short circuit paths (for large sites)
• parallel operations for utility and other sources (situation for partial modelling)

Prepared by Marcin Ruta

Power Systems Engineer at MR Power Tech

[email protected] 

 www.mrpowertech.com

M: +48 799 724 370