Power Line Impedance Determination Using the "3 Voltmeter" Measurement Method

One of the problems associated with power line filter specification is adequate knowledge of the source impedance into which the filter should be designed. In order to establish an average impedance with high and low bounds, a number of measurements have been made at various Navy installations. Results at present indicate that the average source appears inductive and the impedance magnitude varies from 2.0 ohms at 4 KHz to 10 ohms at 90 KHz. These values are based on data taken at 9 locations at 33 discrete frequencies. The procedure used to determine source impedances utilizes an adaptation of the "three voltage" method. A known impedance is placed in series with the desired impedance and a source voltage tuned to the frequency of interest. The magnitude of the impedance is found by comparison of the voltages across the known and unknown impedances. Phase information is obtained by construction of a vector diagram made up of the voltages measured across the known and unknown impedances and the source voltage. Absolute values of impedance depend on the accuracy of the voltage determination. Noise voltages are held to a minimum by use of 0.1 octave bandwidth from 4 KHz to 25 KHz, 100 Hz bandwidth from 25 KHz to 160 KHz and 3.5 KHz bandwidths over the remainder of the range to 500 KHz. Further, 60 Hz rejection is augmented by use of a 43 db notch filter prior to the input of the tuned voltmeter.