How to Present - Report on "Short-Circuit Study"

Short-Circuit - an undesired state of Power system wherein equipment's (such as Generator, transformer, transmission lines, cables, motors etc;) are stressed beyond their capability which may lead to equipment breakdown if the same is not cleared within device allowable withstand time.

This article is about necessary elements to be discussed while preparing a Report on Short-circuit study on any utility or Industrial power system network

To begin with, Presentation during project kick-off with client (Before start of study, with all available inputs from client):

• Define purpose of study (e.g: 1 - Preliminary study for switchgear specification, 2 - Final Energisation studies, 3 - Adequacy Study to add a new source of Power (mention: local generator or grid incomer), 4 - Adequacy Study for capacity enhancement (with various options considered for enhancement), 5 - Short-circuit study & Relay co-ordination study together)
• List available data for study (highlight if additional site surveys to be done for collecting existing/running system information), Vendor data & assumptions made for missing information should be clearly highlighted and impacts of assumption to be mentioned (like if the assumption is not valid then study needs to be redone, report accuracy slightly affected etc;).
• Define calculation standard (i.e; calculations as per IEC 60909 or relevant IEEE)
• Define Operating scenarios considered for study.
• Define criteria for calculating minimum short-circuit level & maximum short-circuit level.

Typical criteria for Maximum short-circuit study

• Utility/Grid incomer available maximum short-circuit level.
• Operation scenario where maximum sources are in operation & in parallel
• Voltage factor (maximum limits) ‘c’ as per IEC 60909.
• Short-circuit impedance (%age Impedance) of various power system elements shall be minimal (until & unless test reports are available, Power system engineer shall consider tolerance on lower side)
• Contribution from motors (typically applicable for Industrial power system where majority of load is rotating in nature)
• Resistances of overhead lines at minimum temperature at site (as per plant design basis)
• For Industry facility Cable impedance can be neglected

Typical criteria for Minimum short-circuit study

• Utility/Grid incomer available minimum short-circuit level.
• Operation scenario where minimum sources are in operation (Typically bus-tie are kept open).
• Voltage factor (minimum limits) ‘c’ as per IEC 60909.
• Short-circuit impedance (%age Impedance) of various power system elements shall be maximal (until & unless test reports are available, Power system engineer shall consider tolerance on higher side)
• Contribution from motors are neglected
• Resistances of overhead lines and cables at maximal permissible temperature at the end of the short-circuit

What is expected in report (as a minimum):

• Though analysis of the short-circuit currents for each operating scenario with a tabulation comprising fault levels of the system and device duty.
• Ensure while checking for switchgear adequacy by Indicating / highlighting areas of concern where device interrupting duty is not meeting the requirement (available fault level).
• While checking for Switchgear adequacy: for some cases / operating scenario device interrupting duty may not meet required specification, for such cases a clear recommendation should be suggested to avoid operation of such scenario.

Following information (attachments / tabulation) as a minimum to be made available in report for Each Operating Scenario

1. Brief description on "Operating Scenario".
2. List of assumptions for scenario.
3. Details of each Switchboard/Equipment tag & substation location.
4. System Voltage Level.
5. Manufacturer and type of equipment (with its specification / Vendor catalogs).
6. Device rating (e.g: CB's fault duty etc).
7. Results obtained from Software programs in a tabulated (results for 3-ph, 2-ph, 2-ph-G, 1ph-G faults & peak short-circuit current).
8. Observations or comments (e.g: system is adequate or not).
9. Furnishing inputs for preparing switchgear specification.
10. Detailed SLD of the power system under study (input form client)
11. System model print outs indicating through modelling of all power system elements clearly identified with tags as per mentioned in SLD.
12. Switchboard wise summary sheets for
    1. Transformer and Motor indicating various information as collected from datasheets
    2. Power sources (fault sources like utility incomer, generator incomer, and incoming transformer feeder from GRID) and their relevant system information like type of earthing, source X/R etc;
13. Further recommendations if any (for e.g: System engineer may be interested to know transformer impedance minimum & maximum limits to keep fault-duty of switchgear within a pre-requisite range)

Comments from Power system Study Experts are Welcome :)