Arc Flash Step by Step #4 System Modelling
by MR Power Tech

Arc Flash Step by Step #4 System Modelling

System Modelling

This step is beginning of engineering part. Nowadays it is common and reasonable to use power system software to perform analysis. Most common in use are SKM, ETAP, Easypower, DigSilent, CYME and others. It still can be done by hand or with use of excel however complexity of electrical systems will make it not efficient and costly. CAE software is a great and helpful tool but it is only as good as person that is using it . We need to remember that what comes IN goes OUT. Proper modelling is a must.

System modelling requires us to establish few areas:

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  • System topology and operations modes
  • System prospective short circuit current
  • Equipment data
  • Electrodes configurations (new from 2018)
  • Required scenarios 
  • Assumptions  

System topology and scope of work

Once we collected all the data or enough data, modelling can be started. It can vary from one software to another but common part is to create digital model as base for further modifications and adjustments. In Step 1 we focused on scope of work and here is why. We can evaluate :

  • complete system (in electrical terms)
  • part of the system

Complete system study allows to determine multiple scenarios and finding min and max short circuit current. If we evaluate part of the electrical system we need precise data about short circuit current in evaluated part. It is very common for short circuit studies to focus only on max values for equipment sizing. First tricky thing behind arc flash is that max current doesn’t always lead to highest incident energy.

Operation modes

When system model is ready,  attention is needed for scenarios, proper standards for calculations and adjusting analysis options. While software generally helps it is only tool than can be used good or bad. We are following mostly IEEE 1584 method for calculating arc flash in AC systems. This for European market requires some adjustments:

  • short circuit analysis method acc. IEC-60909
  • bus bar gaps to follow IEC-61439 and IEC-62271-1  (recommended)
  • working distance is another topic however IEEE-1584 standard options are reasonable
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Figure 1. Example of Single Line Diagram

Our base model is ready so it is time to create scenarios. We need to keep in mind that our arc flash analysis focus is to identify and provide recommendations for improvements. This means that scenarios need to take into account :

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  • Short circuit study results
  • Protective device coordination (selectivity)
  • Arc flash analysis results
  • *Load flow study (if included)


Optionally, depending on our system we can consider

  •  BESS system connected
  • PV/WIND etc system connected
  • UPS, RUPS, DRUPS systems
  •  CLR (Current Limiting Reactor) in operation
  • Specific switching arrangements (parallel transformers, multiple generators etc)
  • Protection specific functions modes (ARMS, arc fibre optic detection, arc quenching devices, differential protection etc)

Equipment and electrodes configuration

Data retrieved from site visit, documentation and provided sources need to be carefully implemented into digital model. Sometimes it requires verification of data base and validation of available models for errors. Those issues have different weight and it is related with installation size. Most recommended is to use data verified by site visit especially for settings of protective devices. Electrode configurations is new topic from latest 2018 release of IEEE-1584 and it will be discusses separately in next chapter.

Assumptions

From my experience so far assumptions are almost impossible to avoid but need to be minimized. This happens because often it takes too much time to retrieve data from :

  • Utility operator
  • Locations that can’t be de energized due to ongoing process (sometimes it can take months to get the data)
  • Restricted access to locations (eg. transformer rooms or utility switch rooms)
  • Old equipment (missing data sheets etc)

Usually some reasonable assumptions can be made based on experience. It is always project specific but some general assumption list can be made for:

  • transformer short circuit voltage/impedance rating
  • use types and tripping curves
  • low voltage breakers types and settings (similar model based)
  • system short circuit power (depends on configuration sometimes it has little effect for end results)
  • induction motors data
  • switch gears and panels ratings
  • cable lengths and type (this is tricky and requires results review)


Prepared by Marcin Ruta

Power System Engineer at MR Power Tech

 www.mrpowertech.com


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