Q12592 : Comparisons between Direct Measurement of Transmission Line Impedances and Computations
Question:
The soils in our state range from silt in the south to gravely clays in the north. We use 100 ohm-meter soil resistivity for our modeling and calculation of transmission line impedances. Recently we have begun a program of direct measurement of our line impedances. The difference between the measured and calculated values are less than 2% for the positive sequence impedance and between 18% and 32% for the zero sequence impedance. We can reduce the calculated zero sequence impedance by approximately 10% by assuming 10 ohm-meter soil resistivity but this will still result in a 8% to 22% difference in the calculated value. This difference could be greater as we have only measured a few of our lines.
Can the measured values be trusted for modeling and protective relay work?
Is anyone aware of an explanation for the differences in the zero sequence values?
Are there improved methods for calculating the zero sequence line impedance?
Answer:
You have to relate zero sequence impedances as a parameter of metallic paths that reflects that some of the transmission line energy (at least during steady state conditions) can be dissipated in the path and environment without the owner being able to recover it...
Any unbalance in your system will result in some of this energy to be captured by soil and also, by foreign metallic objects or conductors that may be located in the vicinity of your line, including other transmission lines that you did not account for in your calculation model, gas or petroleum pipelines, aquaducts, electrified or non electrified railways, communication lines, etc.
Soil is part of the problem because soil is not uniform. Multilayer soils could provide a partial explanation. You can use Multilines or Multifields to set up multilayer soils and study the sensitivity of zero sequence impedances to soil structure.
You can use, Right-Of-Way or multiFields to study the effects of long metallic conductors on zero sequence impedances. You will be surprised to see how much a long pipeline can capture energy from the line.
As long as your zero sequence is lower than the calculated value, you should feel confident that by examining carefully the corridor, you will find the explanation.
If you measure a higher value than computed and you have the correct soil model, then you may have a problem!
