Shaoyu Liu

Lab investigation of PD development in transformer winding

Two winding insulation breakdown failures in Extra High Voltage transformers are investigated. The discovered cause, partial discharge development between winding turns, which is normally considered rare, is studied through lab simulation. Simplified winding samples are tested under a combination of various environmental conditions, including the electric field, temperature and humidity. Studies revealed that the deterioration of oil-paper insulation in transformer winding can be defined in four stages from the initiation of partial discharge to the final breakdown according to the detected partial discharge phenomena. Each stage is identifiable by the trending of partial discharge pulse magnitude, the repetition rate, the power spectrum, the phase-count in 2-D models, and the accumulation of the dissolved gas components, etc. The above partial discharge deterioration stage can be identified by all signals including the IEC current pulse signal, the ultra wide band current pulse signal, the ultra high frequency signal, though each with a different sensitivity. Such results, although not applied in field investigation yet, has the potential to be employed in early warning of partial discharge development and the local insulation degradation.

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

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.

Disequilibrium of moisture in oil-pressboard insulation under transient temperature condition

Moisture has strong influence on insulation performance of oil-pressboard system in power transformer. Moisture in oil and pressboard can be obtained from Oommen paper oil equilibrium curves under constant temperature condition. Temperature of power transformer is related to its load and the atmospheric temperature. If atmospheric temperature or load changed, oil-pressboard insulation will work under transient temperature condition. In present, there are few attentions to the disequilibrium of moisture in oil-pressboard insulation under transient temperature condition. In this paper, a test system has been set up which consists of programmable oven, Karl Fischer moisture meter and oil-pressboard samples. Two kinds of transient temperature conditions have been simulated which were 80°C to 20°C in 5 hours, 20°C to 80°C in 5 hours. Moisture in oil and various thick pressboards from 0.5mm to 5mm have been measured in these two conditions. The results have been presented and analyzed.

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

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.

Distribution of moisture in oil immersed pressboard under transient temperature condition

Moisture has strong influence on insulation performance of oil pressboard system in power transformer. Moisture in oil immersed pressboard is uniform under constant temperature condition. Moisture in pressboard can be calculated according Oommen paper oil equilibrium curves. Temperature of power transformer is related to the atmospheric temperature and the load. If atmospheric temperature or load changed, oil and pressboard insulation will work under transient temperature condition. If temperature increased, moisture will move from the pressboard to the oil; if temperature decreased, moisture will move from the oil to the pressboard. In this case, the distribution of moisture in oil immersed pressboard is un-uniform. The ununiform distribution of moisture can be ignored in very thin pressboard. However, it is important to understand the distribution in thick pressboard for assessment of insulation condition. In this paper, a test system has been set up which consists of programmable oven, Karl Fischer moisture meter and oil-pressboard samples. Transient condition of temperature decrease from 80 °C to 20°C in 5 hours has been simulated. Moisture in various pressboards from 0.5mm to 5mm has been measured. The results have been presented and analyzed.

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

GIS partial discharge severity assessment: A digital image approach

Gas insulated substation (GIS) has been widely used in the field of high voltage power transmission. However, GIS equipments are witnessing more and more internal faults and failures as the voltage class of power network and capacity of power system keep upgrading. Therefore, an effective way in assessing the internal insulation conditions of GIS equipments is needed more urgently than ever. The severity of partial discharge used to be mainly judged through comparing partial discharge spectra on the basis of visual comparison and individual experience, which have served as the most important measurement in the field. Nevertheless, this subjective method falls short of accuracy and efficiency. To better measure the severity of partial discharge in GIS, this paper adopts digital image processing techniques to analyze the correlation between the spectra captured and the severity of partial discharge. Verified by laboratory experiments, the digital image approach demonstrates effectiveness in accurately assessing the GIS partial discharge severity.

Characteristics of partial discharge during its evolution in transformer winding

Following the investigation of two recent transformer failures in winding insulation, researchers have carried out experiments to identify the cause of the failure, which was found to be the partial discharge initiation between transformer winding turns under harsh environment and its development to breakdown under normal operation condition over a long period. Further studies were carried out to simulate the entire ‘life’ of inter-turn partial discharges, i.e., from the discharge initiation to the final breakdown of oil-paper insulation. A lab test platform was built to simulate the evolution of such discharge, and to record all the necessary information during the course. This platform is equipped with the sensors and instruments to record the traditional IEC discharge current, the ultra wide band discharge current, the ultra high frequency signal, and the ultrasonic signal, plus the dissolved gas components. Experimental data were analyzed along the partial discharge development. It was found out that such course could be divided into several stages of different severity according to the combination of discharge characteristics. The characteristics discussed in this paper include the discharge magnitude, phase, repetition rate, and the frequency spectrum of discharge waveform. Possible application and associated limits are discussed for field investigation.