How to calculate the short circuit current in a correct way?
Calculation of short circuit current is essential for the design, dimensioning and protection of electric systems. Such system may be for example a substation, a transmission line, a generator or a variable speed drive system.
In fact, the calculation of a short circuit current is a fairly straight-forward routine. Nevertheless, there are few aspects to be considered. In our more than 12 years of practical engineering and design we have seen many calculation approaches. So how to calculate the short circuit current in a correct way? What is the best practice to apply? And what do the standards say?
Parameters and factors impacting the calculation of short circuit current
Besides the system parameters there are several factors impacting the calculation of short circuit current. Although multiple people may calculate correctly, they may end up with somewhat different results. It depends on the specific selection of several parameters. What are these parameters and how to set them reasonably?
Even when calcuting the short circuit current by using a professional software the user typically has multiple options. Depending on which reference standard is chosen the result is slightly different.
Contribution of VFD to the short circuit current
Specifically for drive applications the question is if and how much does the VFD contribute to the total short circuit current?
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Instantaneous versus quasi steady state value
When talking about short circuit current level we need to be more precise: Do we talk about the first peak? Or the RMS value after the decay of DC component? Depending on the usage we may need one or the other value. For example, a calculation of dynamic forces will use different value than a thermal withstand calculation.
How to calculate the short circuit current: Examples
In this section several practical examples illustrate the topic of short circuit current calculation.
Example 1:
3-phase fault between transformer and rectifier section of the VFD
- Grid nominal voltage 11 kV, frequency 50 Hz
- Grid min. short circuit power 180 MVA, maximum short circuit power 265 MVA
- 5-winding transformer with rated power of 6 MVA / 4 x 1.5 MVA
- Transformer secondary voltages 4 x 1.9 kV
- Transformer short circuit impedance 6.5% referred to 1.5 MVA
- 3-phase fault in one of the secondary system (other three secondary systems healthy)
- Diode Front End (DFE) type of VFD
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Example 2:
Phase-to-phase fault between two motor phases
- Squirrel cage asynchronous motor type
- Motor rated shaft power 7.4 MW
- Motor rated voltage 6.6 kV
- Total machine stray reactance of 19%
- Machine supplied from a voltage source inverter (VSI)
- Negligible impedance of motor cables
Is such information sufficient to calculate the short circuit current? Or any input missing? How does the VFD react in the fault case? Does the result depend on the actual operating point at the time when the fault happens? Figure out the answers as premium subscriber!
Example 3:
Same rating as in example 2 above, but considering synchronous machine…
- Electrically excited synchronous machine
- Brushless AC excitation
- xd’ = 0.272 pu, xd” = 0.16 pu, Td’ = 2.46 s, Td” = 0.18 s
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References
[1] Short circuit currents in variable speed drive systems, https://mb-drive-services.com/short-circuit-currents-in-variable-speed-drive-systems/
[2] What does the X/R ratio tell us? https://mb-drive-services.com/what-does-the-xr-ratio-tell-us/
