An overview of acoustic emission inspection and monitoring technology in the key components of renewable energy systems

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 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

The invention discloses a method and a device for evaluating the discharge severity of gas insulating combined electric equipment. The method comprises the following steps of: detecting partial discharge of GIS equipment of the gas insulating combined electric equipment; converting the data of detected partial discharge into a map; comparing the map with a preset map database; and judging the severity of partial discharge of the GIS equipment. By adopting the invention, the reliability and the accuracy for evaluating the discharge severity of GIS equipment can be improved.

Method and device for evaluating discharge severity of gas insulating combined electric equipment

The widespread application of SF6 gas insulated substations (GIS) in high voltage power transmission requires reliable fault detection and diagnosis techniques. Partial discharge (PD) detection has so far been proven to be the most effective tool for GIS insulation diagnosis. Both ultra high frequency (UHF) technique and acoustic means have been popularly used in PD detection thanks to their well-noted anti-interference performance. In spite of numerous researches on either UHF or acoustic techniques, little has been done to draw a comparison between the two. To address this research gap, the present research simulates five major types of PD discharges caused by the immobilized particles on insulator surface, free metallic particles on insulator surface, protrusion on inner shell, high-voltage electrode faults and floating conductor discharge on a well-established 220kV test assembly, where PD signals were simultaneously detected by both UHF and acoustic methods. The two detection means are compared among the five PD models and different applied voltages in terms of inception voltage for discharge, average apparent discharge magnitude, discharge pulse count and average discharge amplitude. Based on the test results, it is suggested that the UHF method can be used as the major measurement for the diagnosis of insulation degradation in GIS with other necessary means serving as supplementation.

Partial discharge detection for GIS: A comparison between UHF and acoustic methods

In order to research the change of partial discharge (PD) in degradation process of the inter-turn insulation of transformer thoroughly and to explore effective characteristic parameters of the partial discharge, and to use them to diagnose severity of inter-turn insulation deterioration. A discharge test platform is established to simulate the partial discharge of inter-turn insulation. Four voltage levels, 42.5kv, 40kv, 37.5kv, 35kv, are selected through the previous exploratory experiments. By using constant AC voltage, the partial discharge is detected by conventional pulse current signals and experiments are done at least five times under each voltage level., The PD frequency, discharge capacity per second, average discharge capacity, maximum discharge capacity, cumulative discharge capacity and pulse phase are obtained. Some characteristic parameters, such as cumulative discharge capacity, discharge capacity per second, and pulse phase, change more obvious and have certain rules, and these characteristic parameters can provide certain theory basis for detection and diagnosis of the actual transformer partial discharge.

The developing rules of partial discharge in the inter-turn insulation of transformers under constant AC voltage

In order to research the difference of the partial discharge in the inter-turn insulation of transformers under different voltage forms, a discharge test platform is established to simulate the inter-turn discharge By using constant AC voltage method and increasing voltage method, the partial discharge is detected by the conventional pulse current signals and the experiments are done at least five times at each voltage form Based on the comparison and analysis of partial discharge signal, the pulse phase, cumulative discharge capacity and discharge capacity per second reflect better stable similarity in different voltage forms, however, the PD frequency, average discharge capacity and the maximum discharge capacity show relatively large difference Conclusions of the experiments have certain guiding significance to the promotion of laboratory research results and estimation of the development of the actual transformers PD defect

Chun Deng

Papers

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

Method and device for evaluating discharge severity of gas insulating combined electric equipment

Partial discharge detection for GIS: A comparison between UHF and acoustic methods

The developing rules of partial discharge in the inter-turn insulation of transformers under constant AC voltage

The comparative analysis of the partial discharge in the inter-turn insulation of transformers between constant voltage and increasing voltage