Bo Qi

Creepage discharge of oil-pressboard Insulation in AC-DC composite field: phenomenon and characteristics

Converter transformers use oil-pressboard as its main dielectric medium and are operated mainly in AC-DC composite electric field. Creepage discharge is one of the common defects for oil-pressboard insulation, especially in converter transformers. Oil-pressboard insulation shows distinctive flashover behaviors and surface electric strengths in AC-DC composite electric field from that in pure AC or DC electric field. Compared to conventional power transformers, there remain bigger challenges to realize insulation assessment and fault diagnosis for the converter transformers. Adequate knowledge of the phenomenon and characteristics of oil-pressboard insulation creepage discharge is critical for fault diagnosis of converter transformers. This paper attempts to fill the existing research gap by studying the dynamic process and phenomenon of creepage discharge in the composite field so as to provide criteria for insulation diagnosis and assessment. With the established experimental platform, the present paper observed the phenomenon of oil-pressboard insulation creepage discharge from its initiation till final flashover under composite AC-DC voltages, identified the trends of partial discharge and dissolved gas throughout the entire process, and summarized accordingly the criteria for insulation diagnosis and assessment. Results indicate that 1) for creepage discharge of oil-pressboard insulation under AC-DC superimposed voltages, abrupt changes were observed in terms of discharge repetition rate and magnitude. Discharge phases kept enlarging during the whole discharge evolution process; 2) based on the observed discharge characteristics, the evolution process of creepage discharge could be classified into three stages which imply three severity levels of the discharge; 3) compared to that in AC electric field, the creepage discharge in AC-DC composite field witnessed much less amount of and different composition of dissolved gas in oil. More C2H6 and C2H2 were observed in the AC-DC composite electric field whereas CH4 and C2H2 dominated the AC electric field. The research results indicate that as much as both PD-identification and DGA methods are recognized as effective means of fault diagnosis for converter transformers, the referential assessment criteria should be enriched and adjusted for diagnosis of creep discharge under AC-DC composite voltages, taking into full consideration of the distinctive characteristics of both discharge and gases dissolved in oil.

Influences of Different Ratios of AC-DC Combined Voltage on Internal Gas Cavity Discharge in Oil-Pressboard Insulation

The degradation of insulation would be accelerated by the partial discharge (PD) incurred by an internal air-gap defect in oil-pressboard insulation. At different bridge rectifier locations in the converter transformer, the oil-pressboard insulation typically endures various ratios of combined ac and positive dc voltages. Therefore, the dielectric status of the converter transformer is closely correlated to the ratios of ac to dc voltages. It is practically significant to conduct the study of the correlation between discharge characteristics and ac-dc ratios. At the same time, it enhances the diagnosis of faults in converter transformers. According to the actual internal operation conditions of converter transformers, the study established a test platform. Furthermore, the conventional pulse current method was adopted to compare the discharge characteristics at the initiation and close-to-breakdown stages. Under these ratios, test results display distinctive PD dynamics. The PD of the air gap generates surface charges. These charges, in turn, establish an inverse electric field. As a result, the proportion of dc in the ac-dc combined voltage increases, and total initial voltage and total breakdown voltage increase continuously, but the recurrence rate of discharge pulse and discharge magnitude decrease at the initiation and close-to-breakdown stages. As the positive dc proportion increases, the discharge recurrence rate in the negative half cycle gradually exceeds that in the positive one. These research findings provide useful criteria for the fault diagnosis of converter transformers.

Effect of TiO 2 nanoparticles on streamer propagation in transformer oil under lightning impulse voltage

Recent experiments have shown that some nanoparticles can influence the breakdown strength of transformer oil under lightning impulse voltage. To reveal the working mechanism, this paper presents an experimental study on the effect of TiO2 nanoparticles on the impulse breakdown strength and prebreakdown streamer propagation process in transformer oil-based nanofluid under both positive and negative lightning impulse voltage. The test results verify that the modification of nanoparticles on breakdown strength of transformer oil has a distinct polar effect: positive breakdown voltage of nanofluid is increased by up to 30.8%, whereas the negative one is decreased by 6.8%. Streamer shape, propagation length and velocity in both pure oil and nanofluid were investigated using the shadowgraph technique. It is revealed that the propagation characteristics of positive and negative streamers in nanofluid are markedly affected by the addition of TiO2 nanoparticles. The positive streamers in nanofluid form a bush-like structure with thicker and denser branches, developing much slower than tree-like streamers in pure oil. While negative streamers in nanofluid have a tree-like shape with much longer branches, propagating faster than the original bush-like streamer in pure oil. These differences in streamer propagation characteristics and breakdown strength in pure oil and nanofluid are closely related to the change of space charge distribution caused by shallow trap in nanofluid. More negative charges are formed through capturing fast electrons into slow electrons in shallow traps induced by the presence of TiO2 nanoparticles, which change the local electric field in front of the streamer tip. Thus, streamer propagation process in nanofluid is dramatically modified, leading to the change in breakdown strength.

Electric field distribution in oil-pressboard insulation under AC-DC combined voltages

Converter transformers use oil-pressboard as their main dielectric medium and are operated mainly in AC-DC combined electric field. For oil-pressboard insulation, the electric field under DC voltage shows distinctive characteristics from that in AC electric field. Compared to conventional AC power transformers, the electric filed distribution in the converter transformers is much more complicated and remains a big challenge for optimization of insulation design. Existing simulation research in this field falls short of experimental verification and shows limitations in the insulation design of the converter transformers. This paper attempts to fill the gap by measuring the dynamic process and distribution of electric filed in oil-pressboard insulation under the combined field so as to provide reference for insulation design. With the established Kerr-effect electric field measurement platform, the present paper recorded the dynamic process and distribution of AC and DC electric fields at different AC/DC ratios for the double-layer overlapping oil-pressboard insulation model. The research results indicate that 1) the AC electric field distribution in oil is homogeneous and symmetric. The field strengths form linear relation with the amplitudes of applied voltages; 2) the DC electric field distribution in oil is heterogeneous and asymmetric. Under the application of positive DC voltage, the field strength around the upper electrode is much smaller than that of the lower one. The DC electric field strength shows no linear growth with the increase of the applied voltages; 3) the accumulation of interface charges forms the major cause resulting in the asymmetric distribution of the DC electric field at different positions of the insulation. The research results imply that although RC model is recognized as effective simulation method for insulation design of conventional transformers, it shows its limitations and should be enriched and adjusted according to the electric field distribution under AC-DC combined voltage and taking into full consideration of the accumulation of interface charges.

Transient electric field characteristics in oil-pressboard composite insulation under voltage polarity reversal

Polarity reversal generally enhances the electric field intensity in oil-pressboard composite insulation, leading to partial discharge and even insulation flashover or breakdown of the HVDC converter transformer. In an attempt to study the transient electric field in oil at polarity reversal, the present research adopted the Kerr electro-optic effect technique to conduct real-time measurement of the oil electric fields at the polarity reversal time of 10, 60 and 120 s respectively. Dielectric interface conditions were also adopted to capture the dynamic development of interface space charges. Research results indicate that (i) negative charges, under the absorption of insulation pressboard, are more likely to accumulate at the oil-pressboard interface. As a result, the development of electric field in oil at polarity reversal demonstrates prominent polarity effect, with faster electric field attenuation under negative voltage; (ii) the development of electric field in oil shows asymmetric features under double polarity reversals. The first reversal witnesses a high magnitude of electric field in oil that is about 1.47 times of the initial capacitive electric field, whilst the second reversal sees only 85% of the initial capacitive electric field. Such asymmetric development is mainly attributed to different accumulation performances of the positive and negative charges; iii) the shorter the polarity reversal time is, the higher the oil electric field strength will be. The maximum magnitudes of the field strength after polarity reversals with different time, however, are not much different from one another.

Effect of Dispersion Method on Stability and Dielectric Strength of Transformer Oil-Based TiO2 Nanofluids

Dispersion stability of nanoparticles in the liquid media is of great importance to the utilization in practice. This study aims to investigate the effects of mechanical dispersion method on the dispersibility of functionalized TiO2 nanoparticles in the transformer oil. Dispersion methods, including stirring, ultrasonic bath, and probe processes, were systematically tested to verify their versatility for preparing stable nanofluid. The test results reveal that the combination of ultrasonic bath process and stirring method has the best dispersion efficiency and the obtained nanofluid possesses the highest AC breakdown strength. Specifically, after aging for 168xa0h, the size of nanoparticles in the nanofluid prepared by the combination method has no obvious change, while those obtained by the other three paths are increased obviously.

Effect of Fe3O4 nanoparticles on positive streamer propagation in transformer oil

Fe3O4 nanoparticles with an average diameter of 10 nm were prepared and used to modify streamer characteristic of transformer oil. It was found that positive streamer propagation velocity in transformer oil-based Fe3O4 nanofluid is greatly reduced by 51% in comparison with that in pure oil. The evolution of streamer shape is also dramatically affected by the presence of nanoparticles, changing from a tree-like shape with sharp branches in pure oil to a bush-like structure with thicker and denser branches in nanofluid. The TSC results reveal that the modification of Fe3O4 nanoparticle can greatly increase the density of shallow trap and change space charge distribution in nanofluid by converting fast electrons into slow electrons via trapping and de-trapping process in shallow traps. These negative space charges induced by nanoparticles greatly alleviate the electric field distortion in front of the positive streamer tip and significantly hinder the propagation of positive streamer.

Methods to reduce errors for DC electric field measurement in oil-pressboard insulation based on Kerr-effect

Kerr-effect based methods show great potential for the measurement of electric field in liquid dielectric. However, the accuracy of the Kerr-effect based measurement over the DC electric field in oil-pressboard insulation remains to be a challenge. With the establishment of a Kerr-effect based non-contact and real-time electric field measurement system, the present paper analyzes the factors causing the measurement errors and thus proposes the countermeasures accordingly. It is indicated that the measurement errors resulted from the edge effect generated by electrode and pressboard could be reduced to 1.3% by lengthening the electrode, shortening the spacing distance between the electrode and the test chamber, and providing the electrode covering. The errors caused by non-ideal characteristics of the optical devices could be adjusted through the calibration of its parameters. The errors caused by the noise from laser source could be reduced to 0.86% for DC electric field and 1.6% for AC electric field through suitable selection of AC modulation frequency as 1310 Hz, which could also improve the signal-to-noise ratio. The test results conclude that the measurement system established under the present research could accurately measure DC electric field of 20 V/mm with the error being smaller than 5.4%.

The electric field distribution in oil-paper insulation under combined AC-DC voltage

The valve winding in the converter transformer withstands several types of voltage, including the AC, DC, and the combined AC-DC voltage. It is important to consider the effects of voltage type to the electric field distribution. In this paper, a combined AC-DC source was constructed. The electric field distribution of oil paper insulation model was measured by Kerr electro-optic effect, under combined AC-DC voltage with different composite ratios of DC/AC. The results indicate that: 1) DC electric field distribution is asymmetric, which is caused by the difference of the charge accumulation status in different area. The electric field intensity in the left area accumulated negative charge is lower than that in the right area accumulated positive charge. 2) AC electric field distribution depends on the geometric structure and is symmetric. 3) The AC electric field intensity rises with the increase of AC component linearly. However, the relationship of the DC electric field and the DC component is complicated and non-linear in most area.

Simulation of ion mobility in transformer oil in different electrode systems

Accurately measurement of ion mobility behavior in transformer oil is critical for reasonable designing and safely operating HVDC transformers due to its electric field distribution determined by the ion migration in the oil. In this paper, we aim to investigate the effect of electrode configuration on the testing accuracy of ion mobility via the reversal polarity method. Both parallel plane electrode and cylindrical capacitor electrode models were simulated in two-dimension ion drift physical model based on Einsteins relation and Poissons equation. The results show that electric field distortion between cylindrical capacitor electrodes has no obvious influence on the ion mobility in comparison with that obtained at the parallel plane electrode configuration. So, cylindrical capacitor electrode configuration can accurately measure the ion mobility in the transformer oil by the reversal polarity method.