Estimation of the inductive current transformer derating for operation with distorted currents

Abstract

Abstract. The aim of the paper is to present a new method for estimation of the CT (Current Transformer) rated primary current and / or load of the secondary winding derating levels for transformation of distorted current in relation to the rated values fixed for transformation of sinusoidal current. This ensures the values of the CT composite errors/transformer correction factors as resulting from its IEC 60044-1/IEEE C57.13 accuracy class in condition of distorted currents transformation.

A practical approach to evaluation of accuracy of inductive current transformer for transformation of distorted current higher harmonics

“…No increase of this component of determined composite error for such conditions, in relation to the situation, when sinusoidal current is transformed, indicate that there is no magnetic core saturation and there is no need to decrease tested CT secondary winding load. However, taking into consideration, that the percentage composite error determined for distorted current exceed 0.3%, to ensure high accuracy of harmonics transformation, rated load active power of tested CT secondary winding should be reduced to 10 W [11,12]. This is due to the fact, that in such non-sinusoidal conditions for the same maximum magnetic field strength in the magnetic core, higher maximum magnetic flux density is obtained.…”

“…Therefore, such approach for inductive CT characterization in the presents of harmonic distortion does not allow, for high current distortion, even approximate determination of its accuracy. Moreover, transformation of a signal with significant values of a multiple successive harmonics, due to its shape similar to rectangular waveform, cause a significant increase of the magnetic flux density in inductive CT magnetic core, which for rated load of the secondary winding and rated primary current may lead to its saturation and a rapid decrease of the accuracy for transformation of harmonics [11,12]. Magnetic core saturation may be detected from the increase of value of main component of the composite error in relation to the value initially determined for sinusoidal current of frequency 50 Hz (60 Hz) for the same load of the secondary winding and primary current rms value.…”

“…This involves the use of a test signal consisting of multiple harmonics from 2nd to 13th, which rms values are equal corresponding limits for harmonic currents produced by equipment connected to public low-voltage systems presented in the IEC 61000-3-12 standard for a given THD I level [10]. The composite error is measured for transformation of such a distorted current and from FFT analysis the rms values of its harmonic components are calculated to determined composite errors of each harmonic transformation by tested CT [11][12][13]. In accordance with the definition presented in the IEC 61869-2 standard, under steady-state conditions, the composite error is the rms value of the difference between the instantaneous value of the primary current and the instantaneous value of the actual secondary current multiplied by the rated CT current ratio [6].…”

Operation of inductive protective current transformer in condition of distorted current transformation

“…Due to increased level of the conductive disturbances in currents and voltages of the power network proper operation of protective CTs for such conditions is required. Inductive CTs for transformation of distorted current are often characterized by lower accuracy than for sinusoidal signal [1][2][3][4][5][6][7][8][9][10][11]. The shape of magnetization characteristic determined for same magnetic core for sinusoidal primary winding supply voltage and distorted voltage is different.…”

“…Depending on the content of higher harmonics in CT distorted primary current for the same maximum values of the magnetic field strength in the magnetic core higher maximum values of the magnetic flux density may be achieved. Distorted primary current rms value equal to 100% of rated sinusoidal primary current rms value with rated load apparent power of measuring CT secondary winding may cause saturation of the magnetic core [6][7][8][9][10][11].…”