THE ZERO-FLUX CURRENT TRANSFORMER FOR IMPROVING ACCURACY, WITH APPLICATIONS FOR DISTORTED WAVEFORMS AND DC BIAS

摘要

Current measurements on electrical power systems very often concern nonsinusoidal conditions, due to the presence of distorting loads connected to the network (typically because of the presence of power electronic components) and the consequent harmonic pollution. In addition the presence of DC components, superimposed to alternating ones can make it impossible to correctly use the current transformers (CT's) and gives rise to the burden and composed (namely due to the distorted waveforms) errors, that are arising from the presence of the core flux in CT. Moreover saturation, hysteresis, and polarization are supported by the core flux. The presence of the core flux is the difference between a real CT and an ideal one. A method for compensating the core flux makes a real CT operating as an ideal one. Imperious to the, burden and composed error and to the presence of DC components, superimposed to alternating ones. A compensation method (CM) using as processing unit (PU) an operational amplifier (OA) that processes the analog compensating signal as been investigated in developing the denoted, "zero-flux current transformer" (ZFCT). The compensating analog signal is the voltage drop at the terminals of the secondary of CT. PU applies the compensating signal in series opposition with the burden's voltage drop. The compensation may be performed on every CT. An experimental CM-compensated ZFCT has shown, the reduction of the ratio error to at least one fifth and its imperiousness to the burden up to 10 ppm.