Warning 101 | [PSS®E] | Thévenin source impedance abnormally high

Relevant software:

PSS®E

Emitted message:

Detected abnormally large infinite generator source impedance. (Generator at BUS=<bus> with Z=<pu>)

Description:

This warning indicates that the Thévenin equivalent source impedance in your PSS®E model is larger than recommended for SMIB studies.

danger

This warning indicates there may be an issue in your PSS®E model which may impact generator performance and/or benchmarking with other software packages.

Possible impact:

• If you are using a SET, LINE=, SCR=, XR= command, the fault level at the specified bus may be lower than specified by the command.
  • This can cause poor benchmarking with other software packages.
  • This occurs because the source impedance of the generator model (which represents the Thévenin equivalent source) is added in series with the specified impedance.
• Voltage playback may produce voltages which differ from those specified.
  • The voltage playback location in PSS®E is behind the generator's internal impedance, not at the terminals of the generator block in PSS®E.

Potential fixes:

To resolve this warning and fix the associated issues:

• Make the Internal Zsource of the Thévenin equivalent source as large as possible. Open your PSS®E model, double click on the generator representing your Thévenin equivalent source and complete the following (see image below).
  • Change the R source (pu) parameter from its current value to the minimum recommended resistance of 0 (note - this field defaults to 0 in PSS®E so it rarely needs to be changed).
  • Change the X Source (pu) parameter from its current value (likely the PSS®E default of 1.000) to the minimum recommended reactance of 0.0001 p.u.
  • Change the Mbase (MVA) value from 100 to a higher value such as 10000.

These changes will fix this warning by making the Thévenin source impedance very very small - effectively negligible compared to the rest of the system impedance. The effect of this impedance on the fault level is minimal and voltage playback will now work as expected.