Ignoring transmission impedance in network models

[reynal74]

Why do some utilities simulation tools ignore the transmission impedance in network models for its analysis? I understand that it will increase the voltage drop and it would not allow proper sizing of conductors. But, it is necessary for sizing of breakers at substations and other devices, also for faults at the station buses.

Regards,

Reynaldo Salcedo

 

[jghrist]

I don't understand. What kind of network model would ignore the transmission impedance? How could a model of the transmission network ignore the transmission network elements?

 

[davidbeach]

Yeah, what jghrist said. No "solution" tool that would ignore something as critical as transmission impedance is worth using.

 

[reynal74]

That is exactly the same thing that I was thinking. I believe that only loadflow could (might) be done under those conditions, and that is only assuming that the voltages are considered from head of feeders and completely ignoring the substation.

Regards,

Reynaldo Salcedo

 

[davidbeach]

When dealing with radial circuits you can always lump everything upstream into the source impedance. That isn't ignoring the transmission impedance, it is just lumping it with everything else upstream of the substation.

 

[reynal74]

Thanks a lot for your answers, I thought for a second that the way I was thinking about modeling a distribution system was wrong.

Regards,

Reynaldo Salcedo

 

[cuky2000]

It is common in MV and HV neglect the resistance and consider the impedance as pure reactance. Probably this may be the issue.

Occasionally a source could be consider as an infinite bus (zero impedance) just for simplification. However modeling network with negligible impedance does not sound right without additional explanation.