Universal Current Transformer for Accurate Measurement of Short-circuit Currents

Przybysz, Jerzy; Olak, Jan; Piątek, Zbigniew

doi:10.12736/issn.2300-3022.2017109

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“…Moreover, in the case of the accidental opening of the secondary terminals of the current transformer, a very high voltage would be induced in the secondary winding [19,20]. With the use of numerical methods [21][22][23][24][25][26][27][28] and space-time analysis of 3D electromagnetic fields, it is possible to check the operation of a short-circuit current transformer during short circuits before testing a prototype at the design change, i.e., the currents at an asymmetrical short circuit [29][30][31][32].…”

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confidence: 99%

Analysis of the Operation of Cascade Current Transformers for Measurements of Short-Circuit Currents with a Non-Periodic Component with a Large Time Constant of Its Decay

Leśniewska

Olak

2022

Energies

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Simulations of emergency states and tests of resistance of electrical devices to emergencies are performed in specialized high-power laboratories, the so-called short-circuit laboratories. For most electrical power devices, such measurements are required by international standards. The basic equipment of short-circuit testing laboratories consists of current transformers for measuring short-circuit currents. These transformers should not only enable the accurate conversion of sinusoidal currents—which is typical for conventional current transformers, but also asymmetrical short-circuit currents containing an aperiodic component, which classic current transformers cannot reproduce. Therefore, manufacturers and designers try to meet the market demands and design these special class 0.2 current transformers. To meet the high technical requirements, the field-circuit method with three-dimensional space–time analysis of electromagnetic fields was used during design, considering physical phenomena in ferromagnetic cores (i.e., hysteresis and eddy current losses) and the load of the secondary winding of the current transformer by the measurement system. The article presents simulations of secondary currents for the short-circuit current transformer model, and the results were confirmed by measurements and oscillograms of the currents flowing in the windings of the real model. The prototype of the designed short-circuit transformer meets the IEC/EN standard requirements. When measuring harmonic currents, the transformation errors meet the requirements of class 0.2. During the short-circuit current waveforms, the maximum instantaneous peak error does not exceed 1% of the error for all the subsequent maxima of the current waveform during the specified transient switching cycle. In comparison, the standard allows this error to be 10%.

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