Line impedance measurement

Line impedance measurement

Why should be measured line parameters?

The impedances of overhead transmission lines and power cables are1 needed for distance protection, fault locators and power system calculation. The calculation of line impedance is based on ideal geometrical assumptions like homogeneous ground resistivity and continuous line geometry. Varying ground resistivity, buried conductors and a missing earth conductor between two towers are not considered in the idealized impedance calculation, although they have a strong impact on the real line impedance, that means calculated values are inaccurate. For these reasons, we recommend to measure the impedance of the line, instead of calculated. The measurement with the OMICRON CPC 100 + CP CU1 system highlighted deviations of 48% between the imprecise, mathematically determined values and the zero-sequence impedance values actually measured.


  • Distance protection – Inaccurate line impedance can cause malfunctions of distance protection, wrong displayed fault locations and faulty calculation result. A protective device shuts down the system if a short circuit occurs in the section it is assigned to monitor. However, if a protective device cannot be set up correctly, it may also react improperly to faults outside the protected section. This can cause lines to be shut down unnecessarily, which in turn can lead to power cuts.
  • Fault locator – Imprecisely adjusted fault locators also give inaccurate readings of the distance to the respective fault. In these instances, repair crews cannot be sent to precise locations, and this increases both the time required to carry out the maintenance work and the associated costs.

The k factor is an important setting of distance protection relays. The precision of this setting affects the accuracy of distance protection relays dramatically. The k factor can be calculated, but the calculation results give only a rough estimate of the actual value. As a wrong k factor setting can cause worse power quality, higher risk to lose the system stability and loss of power supply, k factor measurements are essential for fast, selective and reliable distance protection.
Mutual coupling of power lines
When two or more lines are running parallel to each other, mutual impedance between the lines modify the voltage and current profile measured in the protective relays protecting each line.



Measurement of Coupling into Signal Cables
Inductive coupling between a power system and adjacent cables is a common phenomenon. Where the adjacent cable is used for carrying relatively low-level signals, the level of coupling can lead to problems in the associated equipment. This coupling impedance Zk can be measured using the CPC 100 and CP CU1 test system. The principle of frequency selective measurement of the test system allows the identification of interference from a specific power line to the signal cable under test or any other conducting material that might be affected by interference caused by induction from electrical power lines.