Measurement of overhead line and cable impedance values, earth impedance of large substations and electromagnetic interference between overhead lines and signal cables, using a new technology

This paper describes a measuring principle for overhead line and cable impedance values, earth impedance of large substations and electromagnetic interference between overhead lines and signal cables. A digital signal processor (DSP) generates sinusoidal signals in a frequency range of 15 to 400 Hz. This signal is fed to a switched mode power amplifier. A transformer at the output matches the internal amplifier impedance with the test object impedance. A multiplication of the measured signals with the generated signal sin(ωt) and the 90° shifted signal cos(wt) enables an excellent filtering of the noise and a measurement of the real and the imaginary parts with a high precision. Complex impedances Z=R+jX can be measured. When the measurements are made at frequencies different from the line frequency, they also can be made under difficult conditions without any interference problems. The impedance value at line frequency can be determined.