Analysis of Measured Transmission Line Constants

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

Analysis of Measured Transmission Line Constants

The accuracy of line impedance data has great impact on system analysis.

The transmission line constants are the most important element of data needed for the operation of an electric power network. It comprises positive-sequence impedance (Z1), zero-sequence impedance (Z0) and admittance.

The parameters of line constants are conventionally computed by calculation programs, and the measured values of transmission line constants have been utilized as reference data when a newly built generating plant or substation undergoes a commissioning test.

Notwithstanding, it is known that the conventional method of reading voltage drop after applying voltage to the transmission would not work effectively in energized substations due to the influence of induction voltage.

However, the new type of measurement equipment introduced hereon is unique in terms of injecting electric current through the circuits and measuring the voltage raised from the loaded test current. Because this new measurement device is equipped with an additional feature for selecting variable frequency for the source current, it could advantageously perform measurement of transmission constants without receiving any interference from induction voltage of the frequency in use.

The measurement of transmission constants as described below has been conducted with the help of the new sophisticated measurement equipment to verify and analyze the deviation between calculated and measured values of transmission constants.

Representation of Line Constants Measurement
The transmission line constants are defined as the constants showing electrical impedance values between busbars of transmission lines in electric power networks. These data are crucial in the electrical interpretation of power networks. The transmission constants are also utilized in the construction or expansion of power facilities as basic data for areas such as the simulation review of load flow and fault current, voltage stability and the protection relay settings.

While the calculated values have been conventionally used in specifying transmission constants for the reasons of physical obstruction to the field measurement of transmission line constants, there has been growing support for adopting the new measuring equipment featured with a frequency-dependent device.

Therefore, we have responded by demonstrating measurement of transmission line constants with the new equipment as described in this article.
The new data obtained by this measurement will be utilized as basic materials for future management of and policies for transmission line constants by analyzing and comparing calculated and values measured.

Measurement of Transmission Line Constants
There are three methods of applying test voltage and current to transmission lines, i.e., phase-to- ground, line-to-line, and 3-phase combined-to-ground. The positive sequence impedance was measured by the line-to-line method, while zero sequence impedance was measured by 3-phase combined-to-ground method.

The impedance was computed by measuring the results of test voltage applied by the test equipment, and the admittance by measuring the transmission charging current at the time of initial energization.

Measurement Equipment
The measurement equipment used is as follows:

  • A compact multifunctional primary test set capable of applying up to 2000V and 800 A, with a frequency range of 10-400 Hz
  • Coupling unit

We need to highlight that measurement is impossible with induction voltage exceeding 500V (as is the case with 345kV overhead transmission lines.) See Figure 3.

Measurement Schematic for Line Constants
The transmission constants were measured, as shown in Figure 3, by connecting the measurement equipment to the EDS terminal behind the line DS at measuring end of  Substation "A", and grounding 3-phase combined via EDS at the other end of Substation "B". See Figure 2.

Computation of Transmission Line Constants
The transmission line constants were computed, as shown in Figure 4, by using a frequency-dependent method at frequencies of 20, 40, 80 and 100 Hz, respectively to find corresponding R and X values, which were averaged to produce mean values.

However, the 60-Hz setting was excluded taking into account noticeable errors due to the surrounding electro-magnetic induction. Figure 6 clearly shows that the resistance component remains almost constant with variation of frequency, while the reactance linearly increases as frequency rises.

Measurement of Transmission Line Constants
Approximately 5% of all transmission lines have been selected for measurement of transmission constants for analysis of deviation between calculated- and measured values. These data will be used as basic material when management and policies are established for overall transmission constants in the future.

The transmission line circuits were tested during a 12 weeks period between 18 September and 15 December 2006 are as follows:

  • A total of 86 circuits of 154 kV transmission lines under jurisdiction of 11 KEPCO Power Transmission District Offices and Jeju Branch Office. (10 out of 96 circuits are not considered due to suspended power supply)
  • 34 circuits of overhead transmission lines (cable types: ACSR 330, ACSR 410, and ACSR 410B)
  • 46 circuits of underground transmission lines (Cable types: XLPE, OF, CV, and CNCV)
  • 6 complex circuits

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Power. Flexible. Easergy.
BeijingSifang June 2016