Analysis of Measured Transmission Line Constants

Authors: Jeon Myeong-ryeal, Oh Sei-ill, Lee Hee,Shin Chang-gyun, Electric Power Research Institute, Korea

Analysis and Comparison

Analysis and Comparison of Calculated and Measured Values
The results for overhead and complex transmission lines are summarized in Figure 4.

For positive sequence impedance (Z1) transmission lines with error rates exceeding 5% included 4 out of 40 circuits with a maximum error rate of18.4%.
Statistics by error range are shown in row 1 of Table 1 with an average X1/R1 value equal to 6.17.

As protection relay settings assume about 5% of error rate for transmission line constants values, the use of present calculated values seems not problematic.

For zero sequence impedance (Z0) transmission lines with error rates exceeding 5% included 14 out of 40 circuits with a maximum error rate of 18.9%.

Statistics by error range are shown in row 2 of Table 1 with an average X0/R0 value equal to 4.88.

For the positive sequence admittance (Y1) statistics by error range are shown in row 1 of Table 2.

The error seems to arise from calculation error as well as from change of the fringing field underneath transmission lines, such as change of ground altitudes (growth of bush and trees, etc.).

Positive effects from improving the accuracy of admittance measurements are that when formulating reactive power compensation plan, investment cost for phase modifying equipment may be reduced.

For the ratio of zero sequence (Z0) to positive sequence impedance (Z1) the following results were obtained:

  • Average value of calculated Z0/Z1 = 2.69
  • Average value of measured Z0/Z1 = 2.66

The results for underground transmission lines are summarized in Figure 5.

For positive sequence impedance (Z1) transmission lines with error rates exceeding 5% included 23 out of 46 circuits with a maximum error rate of18.4%.

Statistics by error range are shown in row 3 of Table 1 with an average X1/R1 value equal to 6.21.

The calculated values are not suitable for application to protection relay settings as error rates are high (about 10%) and error ranges vary widely depending on the installation condition of underground transmission lines.

For zero sequence impedance (Z0) transmission lines with error rates exceeding 5% included all 46 circuits.

Statistics by error range are shown in row 4 of Table 1 with an average X0/R0 value equal to 1.90. 2) Zero sequence impedance (Z0)

The transmission district offices having substantial length of underground transmission lines require procurement of new test equipment for physical measurement of transmission lines. Research needs to be tasked to raise accuracy of calculation program.

For the positive sequence admittance (Y1) statistics by error range are shown in row 2 of Table 2. The ratio of average to calculated values is 131.9±61.3%.

The measured values of admittance of underground transmission lines were found to be lower than those of overhead transmission lines.

For the ratio of zero sequence (Z0) to positive sequence impedance (Z1) the following results were obtained:

  • Average value of calculated Z0/Z1 = 0.48
  • Average value of measured Z0/Z1 = 1.87

Transmission line constants per unit length for different conductors for admittance are shown in Figure 7 and for positive and zero sequence impedance - in Figure 8.

The analysis of the measured data obtained in this research clearly shows that:

  • The constants of overhead transmission lines are excellent as they stay well within acceptable error range.
  • The constants of underground transmission lines are remarkably high, particularly in zero sequence impedance due to underground cable grounding system, such as whether the close bond grounding is provided at one end or at both ends of cable spans. Further research must be conducted to review the accuracy and application problem with the calculation program for constants of underground transmission lines.

Also, as the importance of transmission line constants is expected to be emphasized with the innovation of power network operations and techniques, measurement equipment will be broadly introduced to enable extensive measurement and analysis of transmission line constants, so that an expanded data base can be effectively utilized.

Building an accurate data base of transmission line constants will greatly improve the quality of interpretation of electric power networks, and will further contribute to stabilization and optimum operation of electric power systems.

PDF Version

Power. Flexible. Easergy.
Let?s start with organization in protection testing