An original electronic device for compensating ratio and phase-displacement errors, of measuring current transformers and clamp-on probes, is proposed. It is specially intended for low-frequency applications (power frequency and its harmonic components), where the magnetizing current is the main cause of measuring errors. This system reduces the magnetic flux in the transformer core, reducing in this way the magnetizing current. The proposed system even reduces the influence of the error produced by the variation of the burden connected to the secondary winding. Large values of burden can be used without effects on the transformer measuring errors. This device can be directly applied to conventional current transformers, connecting the device to the secondary terminals. It is not necessary to use any auxiliary winding or auxiliary core, or any modification of those transformers.
A simple designing method far linearizing circuits, based on one resistor, is presented. The errors caused by this circuit are calculated, considering the temperature range and the thermistor parameters. These errors are used as a basis far comparing the quality of active linearizing circuits. In many cases, complex circuits recently proposed have similar errors to single resistor linearizing circuits. lt is shown that the exponential approximation is one of the most important error sources. We propase to use more precise models. The relationship between the parameters of the exponential model and those of other models is discussed.
A current transformer with nominal ratio 10 A to 10 mA, intended for low-frequency applications, was developed. It includes an electronic device to reduce the magnetizing current, and a continuous shield in the secondary winding (coaxial cable) in order to eliminate the effect of stray capacitances. No guard-source is connected to the shield. It is proposed in this paper to leave the cable-shield-potential floating. This leads to high-accuracy results (ratio errors and phase-displacements in the order of few parts in 10 6 from 50 Hz to 1 kHz).
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