Abstract
At the time of the commissioning of an underground cable transmission line, it is a common practice to perform measurement of its electrical constants. In such measurement performed by a cable manufacturer, the following straightforward method is used for each circuit of the cable line. A three-phase voltage source is used to apply a balanced three-phase voltage at the power frequency to one end of the cable circuit with the other end short-circuited, and the positive-sequence impedance is obtained by the measured voltage and current. To obtain the zero-sequence impedance, a single-phase voltage at the power frequency is applied to the bonded three-phases of one end of the cable circuit, to the ground, with all phases at the other end grounded. If there is no parallel cable circuit in service, this method gives an accurate result. However, if there are one or more parallel cable circuits in service, the cable circuit under measurement gets electromagnetic induction from the magnetic flux generated by the currents on the parallel circuits. This induction generates superimposed currents and thus makes the measured result inaccurate. Considering this point, a method to measure a voltage and a current at a frequency different from the power frequency so as to separate the power-frequency current component generated by the electromagnetic induction and obtain the impedance value at that frequency is proposed in this paper. Using impedance values measured at two frequencies different from the power frequency, the power-frequency impedance value is then obtained by linear approximation. The proposed method is applied to obtain the impedances of a 132-kV underground cable line, and a reasonable result is obtained.
