On-line impedance measurement is a useful tool to monitor the state, health and characteristics of electrical networks in power systems. This paper proposes a method for on-line, wide-bandwidth, impedance analysis of devices used in power networks. This method uses a signal generator to inject other than mains frequency signals on-line and then measuring current and voltages to determine the impedance at the injected frequencies. Two measurement circuits based on different types of current and voltage measurement methods are discussed. Their experimental results of impedance measurement are shown. This paper also suggested some possible applications of this on-line impedance analysis method in condition monitoring of power distribution transformers and electrical machines.

This paper deals with the requirements for the design of digital companding techniques in either delta or pulse-code modulation. Both delta and pulse-code modulation convert analogue signals into binary signals and in both these systems the dynamic range is normally small. By the use of companding, the dynamic range can be extended. Since both delta and pulse-code modulation are digital methods, they are well suited to the use of digital companding techniques. The binary transmitted signal normally contains a measure of the system performance. By observing certain patterns in this binary signal and using the occurrence or nonoccurrence of these patterns to change the gain of the modulator and demodulator, syllabic companding can be obtained. The selection of the binary pattern and the rate of change of gain of the modulator and demodulator, determines both the point at which the companding operates and the attack and decay times. The ratio of the largest to the smallest value of the gain determines the dynamic range. By the use of digital circuitry, the gain can be controlled with sufficient accuracy over a large dynamic range. The paper deals with the principles involved in selecting the binary patterns to control the gain of the modulator and as examples a delta modulation system and a pulse-code modulation system with companding ratios of 60 dB are discussed.