Abstract
In recent years, the zero-mode inrush current of high-impedance transformer with built-in high-voltage winding (T-Hin), which has large amplitude and decays slowly, causes the misoperation of zero-sequence overcurrent protection. Compared with magnetizing inrush current, the waveform of zero-mode inrush current is inconsistent and irregular, and few researches have proposed the mathematical analysis as well as the improved protection using waveform characteristics. In this paper, the mathematical expression of transformer zero-mode inrush current is derived. Further considering the parameter differences, the zero-mode inrush current of T-Hin is larger, which tends to cause the misoperation. The mathematical waveforms fit well with the recorded waveforms. Both recorded waveforms and mathematical waveforms in various conditions prove that the second harmonic ratio (the ratio between the second harmonic and first harmonic) of zero-mode inrush current is significant. Based on the above analysis, a criterion based on the second harmonic ratio restraint of zero-mode inrush current is proposed. If the second harmonic ratio exceeds the setting value, it is considered that the inrush current is generated and sends a signal to restrain the protection. The theoretical setting value of the proposed criterion and the practical engineering method for determining the setting value are obtained.
Highlights
IntroductionIn the process of transformer energization or the voltage recovery after the removal of external fault, a large magnetizing inrush current (including sympathetic inrush current) will occur [1,2,3]
In the process of transformer energization or the voltage recovery after the removal of external fault, a large magnetizing inrush current will occur [1,2,3]
According to the above mathematical analysis and recorded waveforms, it is found that the According to the above mathematical analysis and recorded waveforms, it is found that the second harmonic ratio of the zero-mode inrush current is high, which may be used as a second harmonic ratio of the zero-mode inrush current is high, which may be used as a found that the second harmonic ratio of the zero-mode current is high, which is an important waveform characteristic
Summary
In the process of transformer energization or the voltage recovery after the removal of external fault, a large magnetizing inrush current (including sympathetic inrush current) will occur [1,2,3]. The magnitude of inrush current may be as high as ten or more times that of transformer rated current. Though the magnitude of inrush current may be less compared to short circuit current, the frequency and its duration can lead to more adverse effects compared to other faults. An inrush current is a transient current with high amplitude and contains a significant and slowly decaying DC component. The zero-mode inrush current, which is the sum of the instantaneous values of the three-phase magnetizing inrush current (3i0 = iA + iB + iC ), has hardly been
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