How to set up a model of HV grid as an external grid for Short-Circuit calculation?
When entering the data for an external network you should firstly be aware of the meaning of Minimum and Maximum current calculations.
The Data for the external grid comprise:
- The fault level may be entered either in MVA or kA in the Max Values box on the VDE/IEC Short-Circuit tab. You will also need to have the R/X ratio (which may also be represented as X/R ratio if the Options button is pressed).
- You will also need to enter the various ratios between the sequence components in the Impedance Ratio box.
In the attached example you will find a representation example of a complete HV grid and its representation, for both IEC and complete calculation. The results are identical for both grid representations (detailed and representative grid) when you follow the guidelines as described:
- Isolate the distribution grid from the busbar were the HV transformer is feeding to
- Perform a Short-Circuit calculation and show Sk", R and X at the fed busbar
- You can substitute the detailed HV grid by a representative "ExtGrid" element entering the same values of Sk" and R/X in the VDE/IEC Short-Circuit tab.
- Isolate the distribution grid from the busbar were the HV transformer is feeding to (as for IEC)
- In the detailed HV grid you will have to de-activate automatic tap changing in the load flow command. Otherwise the impedance of the HV grid will vary, which is not reflected by the constant impedance of the representative "ExtGrid".
- Perform a Complete Short-Circuit calculation and show Sk", R and X at the fed busbar. Please make sure to select the Load Flow Initialization option in the advanced options of the Short-Circuit Calculation.
- You in this case also can substitute the detailed HV grid by a representative ExtGrid with the following settings in the Complete Short-Circuit tab:
- Sk" and R/X ratio just like calculated for the detailed HV grid
- In Advanced Options page factor c must be set to 1.0 (this makes sure that the calculated impedances are identical)
- The ExtGrid-Element will work as a Slack. You will have to adjust the set value for the slack as the bus voltage which you calculate for the detailed HV grid.