https://iopscience.iop.org/article/10.1088/2053-1591/ab3f0d/pdf

Aging of silicone rubber-based composite insulators under multi-stressed conditions: an overview

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.

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

With the increase in the voltage and capacitance of gas-insulated transmission lines (GILs), the insulation failure of GIL has attracted more and more attention. Further understanding of the partial discharge (PD) and flashover characteristics of metal particles on the insulator surface, as well as the flashover mechanism, is necessary to reduce the failure rate and improve the reliability of the equipment. In this study, an ultra-high voltage (UHV) AC insulator PD test and measurement system based on the cross-reference pulse current (PC) and ultra-high frequency (UHF) methods were established. The PD development and flashover characteristics of 5 mm-long metal particles at different positions on the surface of a UHV AC insulator, as well as the surface charge accumulation, were studied. The results show that the discharge of millimetre-scale metal particles on the insulator surface under PD test conditions is relatively low (generally lower than 2PC), and it is difficult to be detected by the conventional PC and UHF methods due to their insufficient sensitivity. Moreover, it is found that the weak PDs of the millimetre-scale metal particles will result in a charge accumulation on the insulator surface under AC voltage, which eventually will lead to the insulator flashover. The PDs of the UHV AC insulator before flashover are generally small, which will make it difficult for online monitoring systems to give an effective alarm before flashover.

Surface charge accumulation and pre-flashover characteristics induced by metal particles on the insulator surfaces of 1100 kV GILs under AC voltage

Operating experience shows that residual DC voltage remains in GIS bus bar for a long time after opening of disconnectors. This DC voltage will not only cause accumulation of charges and contaminants on the insulator surface, but will also easily lead to motion of free metallic particles in SF6 gas gap, weakening the insulation ability of GIS. At the time of disconnectors reclosing, impulse voltage could be generated in the bus bar and superimpose on the pre-existing DC voltage. As a result, GIS will be under combined voltage of DC and impulse. This paper experimentally studies the insulation characteristics of GIS under combined voltage of DC and lightning impulse. It is found that, for non-contaminated insulators, when DC and LI voltage are of the same polarity, flashover voltage under combined voltage is nearly the same as the flashover voltage under LI alone. However, when they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In cases there are metallic particle or powder around insulator, pre-stressed DC voltage will cause adhesion of contaminants on the insulator surface and lead to decrease of flashover voltage. Specially, for the particle-contaminated insulator, it is found that the surface charge accumulation caused by DC voltage will have big influence on the flashover voltage, and the most critical condition is when DC and LI voltage are of the opposite polarity. In addition, the breakdown characteristics of SF6 gas gap with free metallic particle are investigated. It is found that under pre-stressed DC voltage, particle will do standing motion or bouncing motion. And breakdown voltage becomes lowest when particle is doing standing motion. The results of the experiments show that residual DC voltage in GIS could deteriorate its insulation ability and the combined voltage of DC and impulse is found to be a critical condition for GIS insulation. Moreover, the combined voltage is shown to be more sensitive to detect some insulation defects in GIS than applying impulse voltage alone, especially for free metallic particles, which are commonly found in GIS but up to now no effective detection method is proposed. Therefore combined voltage of DC and impulse could be considered to serve as a supplement of field test for GIS.

Study on insulation characteristics of GIS under combined voltage of DC and lightning impulse

In this study, mechanical analysis of the air-gap breakdown with free spherical metal particle is conducted in a DC uniform field to address the problem of metal-particle pollution in DC gas insulation. The criterion and spacing of micro-discharge between the particle and electrode is calculated based on the analysis of electric-field distortion with a lifting particle by using the streamer theory. The gas-breakdown equation that considers the effects of free particle is modified with the new concept of equivalent radius, and the different breakdowns triggered by various sizes of aluminum or copper particles are analyzed in ramp voltages. Different states of particle lifting, direct breakdown, or moving breakdown are also considered. The concept of risky radius is also proposed. The breakdown process is observed using a high-speed camera, and the model is verified through experiments. The breakdown voltage of the moving particle is lower than the still particle's. Three breakdown regimes in the DC uniform field triggered by free metal particles were defined, among which the lifting-voltage breakdown occurs only when the particle is close to the upper electrode for the first time. The breakdown voltage showed a minimum value when the particle moved nearer to the risky radius. The proposed research provides fundamental basis in evaluating the dangerous level of free conducting particles within DC GIL as to improve the insulation design.

Theoretical and experimental studies of air gap breakdown triggered by free spherical conducting particles in DC uniform field

In order to study the developing process of surface discharges initiated by immobilized metallic particles attached to Gas Insulated Substation (GIS) insulators under gradually-raised applying voltages, long-term laboratory tests were conducted on a well-established 220 kV test assembly to observe the entire evaluation process of surface discharges from its inception to gradual expansion and the eventual occurrence of surface flashover Real-time detection was conducted to record conventional impulse current (CIC) signals, UHF signals, ultrasonic signals and light emission signals from PD Features of surface discharges at different developing stages were captured from the discharge trend curves, scatter diagrams, two-dimensional histograms, and grey-scale maps of PD The test findings indicate that i) surface discharge initiated by immobilized metallic particles attached to GIS insulators undergoes three developing stages from corona discharge domination in the incipient stage, to co-existence of corona discharge and surface discharge in the expansion stage, and finally the surface discharge domination in the final stage; ii) the shape and phase distribution shown in scatter diagram, histogram and grey-scale map could be used as effective criteria to infer different evolutionary stages of surface discharges produced by immobilized metallic particles attached to GIS insulators

Surface discharge initiated by immobilized metallic particles attached to gas insulated substation insulators: process and features

Grounding metallic protrusion defect is one of the most common and hazardous grounding electrode faults in GIS equipments. The present research is designed to explore the evolutionary process of the partial discharge (PD) provoked by grounding metallic protrusion defect in GIS through observing the light and electromagnetic emission phenomena alongside the PD development. On the 252 kV GIS experiment platform, long term tests were undertaken to observe the entire PD evolution process from its very initiation till the eventual flashover as the applied voltage gradually increased. Ultraviolet camera and photo multiplier tube (PMT) were employed to document light emission phenomena while ultra high frequency (UHF) sensor was engaged to monitor the electromagnetic emissions during the partial discharge evolution process. It is observed that light emission appears to be inhomogeneous throughout the PD evaluation process. As partial discharge exacerbates, the phase distribution is widening and corresponding changes show up in the PD statistical spectrums. The phenomena of light and electromagnetic emission indicate that the partial discharge triggered by grounding metallic protrusion evolutes from inception phase to the developing phase and the threatening phase till flashover. It is suggested that these observed phenomena and inferred PD evolution stages could serve as useful support for the diagnosis and assessment of severity levels of the partial discharge triggered by grounding metallic protrusion defect in GIS.

Bi Bo Geng

Papers

Surface discharge initiated by immobilized metallic particles attached to gas insulated substation insulators: process and features

Partial discharge initiated by grounding metallic protrusion in GIS: Light and electromagnetic emission phenomena and discharge evolution process