Capacitance, attenuation and characteristic impedance of a 132 kV power cable at various frequencies

The basic transmission characteristics of power cables, such as attenuation and surge impedance as functions of frequency, are required as data for transient analysis and for the interpretation of h.v.-surge recordings. Measurements of capacitance, inductance and associated dielectric and I2R losses for sample lengths of 132 kV power cable have been carried out over a wide frequency range from 50 Hz to 800 kHz. The results concerning the main cable insulation at ambient temperature are shown in graphs of tan ¿ and the percentage variation in capacitance against the logarithm of frequency. The influences of skin effect are shown in graphs of inductance and series resistance against frequency for a short-circuited cable; they are, at least for a range of ultra-audio frequencies, broadly as might be predicted on the basis of conventional `skin-depth¿ theory. From the measured quantities, characteristic impedances and attenuation coefficients have been calculated and presented graphically; a step-voltage test yielded a surge impedance in agreement with that calculated for a frequency of 1 MHz. The cable samples were wrapped with metal tape, and the capacitance and tan ¿ measurements were carried out between the cable (outer) screen and the metal tape. Results concerning the neoprene and hessian serving (required for idealised studies concerning propagation in the sheath-earth mode) are presented in graphs similar to those for the main insulation. The variation with temperature of the effective permittivity and tan ¿ of the impregnated-paper insulation, obtained by heating the core of the cable to simulate the cable loss at full rating, is also given with the relevant thermal measurements.