Abstract
Partial discharge (PD) behaviors of oil-paper insulation is distinctive in AC and DC combined electric fields in converter transformers from PD behaviors in pure AC or DC electric fields. The present study focuses on the PD developing processes and characteristics of oil-paper insulation systems with needle-plane defects under different AC/DC proportions. The degradation of oil-paper insulation can be accelerated by PD pulses incurred by needle-plane defects. AC-DC combined voltages are applied to the needle-plane defect model simultaneously in the established experimental platform, and the proportions of AC/DC voltages are decided according to the cases in actual converter transformers. The developing processes from the initiation of partial discharge until final breakdown were observed for each AC/DC proportion. PD parameters and patterns were acquired by a detector using the pulse current method. The test results indicate that the inception and breakdown voltages increase with the increase of the DC component in AC-DC combined voltages. However, pulse repetition rate and amplitude of PD shows a descending trend when AC/DC proportion decreases. Meanwhile, the PD recurrence rate in the phase between 180° and 360° becomes higher than that in the phase between 0° and 180° at the initial stage as the DC proportion increases; high-amplitude discharges mainly occur in the phase range between 180° and 360° when the pressboard is close to breakdown. The current study is useful in further research on fault diagnosis in converter transformers.
Highlights
For the converter transformers used in HVDC (High Voltage Direct Current) transmission systems, the valve-side windings withstand complicated voltage stresses, which consist of AC, DC and harmonic waveforms [1,2]
The needle-plane defect model for tests is shown in Figure 1a, which can simulate the existence of inhomogeneous electric fields in converter transformers
The results of this paper show that AC/DC proportion has an obvious influence on Partial discharge (PD) characteristics, drawing the following conclusions: (1)
Summary
For the converter transformers used in HVDC (High Voltage Direct Current) transmission systems, the valve-side windings withstand complicated voltage stresses, which consist of AC, DC and harmonic waveforms [1,2]. It is significant to interpret PD characteristics under AC-DC combined voltage with different proportions and provide the diagnosis basis distinguished with the insulation problem at different locations of converter transformers. Much research has been concentrated on partial discharge characteristics of oil-paper insulation under AC-DC combined voltages [6,7,8,9,10,11,12,13,14,15]. From the above literature review, little research concerns the effects of AC and DC voltage proportions on the partial discharge characteristic behavior of needle-plane defects in oil-pressboard insulations. The considerable knowledge gap motivated us to conduct research on the PD developing process and characteristics of needle-plane defects in oil-paper insulation under AC-DC combined voltages. The present study can provide a basis for the realization of fault diagnosis and condition monitoring of HVDC converter transformers
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