Yangchun Cheng

Contamination and AC pollution flashover characteristics of insulators under fog-haze environment

With the acceleration of urbanization and the rapid development of industry and agriculture, fog-haze occurs frequently in most parts of China, which is a serious threat to the safe operation of the power system. The effects of fog-haze on the insulators represent in two aspects: (a) increasing contamination on the surface of insulators and (b) increasing the leakage current and decreasing the flashover voltage of insulators due to the wet deposition. Therefore, in this paper, naturally polluted insulators under fog-haze were taken as test objects, and the contamination and AC pollution flashover characteristics of different kinds of insulators were studied. According to the test results, contamination on the insulator surface is increased significantly under fog-haze in a short time. Compared with porcelain and glass insulator, the composite insulator shows more serious contamination under fog-haze environment. Saturated moisture time of two-shed type porcelain insulator is the shortest and saturated moisture leakage current of that is the largest, while composite insulator has the longest saturated moisture time and the smallest saturated moisture leakage current. Composite insulator is with the largest flashover voltage gradient, which is followed by bell type porcelain insulator and deep under-rib type glass insulator, and that of two-shed type porcelain insulator is the smallest among these insulators.

Relationship between carbonized traces and space charges in oil-paper insulation

Among all the types of transformer insulation deterioration, partial discharge is relatively a direct one that may lead to carbon traces which have negative effect on insulation. We believe that the relationship between the trace length and total appearance charge of partial discharges is a key to diagnose the oil paper insulations remained life, even though the carbon trace in paper or pressboard cannot be seen directly like the transparent material such as XLPE. In this paper, we put forward an electrical method based on the hypothesis that the space charges just stay where the partial discharge happened before. Based on a distributed potential measuring system, we pay attention to carbon traces developed in a single electrical tree. After a series of high voltage experiments, we calculate the position of the space charges by their electric field and then we get the trace length by the farthest space charges. Finally, we get the result of experiment consisted with that of the theory calculations.

Intelligent evaluation of fault status in power transformer based on fuzzy fault Petri net

In order to study the fault propagation characteristics of power transformer—one of the most important devices in the power system—more effectively, according to the logic relation of system faults and the modeling method of Petri net, the fuzzy fault Petri net model of the transformer is established. Taking the case of the deductive reasoning of winding fault model, after determining the initial network parameters of the model, D-S evidence theory is introduced to implement confidence reasoning, which offers a reliable basis for condition-based maintenance, and provides necessary conditions for fault reasoning. The forward deduction algorithm based on the intelligent ignition judgment matrix is used to describe the occurrence of fault phenomena and propagation of the transition, which realize the intelligent and quick evaluation of the fault state.

Analysis of the influence of outside equipments on the online deformation detection of transformers

The Frequency Response Analysis (FRA) method is increasingly used in the diagnostics of power transformers, especially for the identification of winding integrity. For online FRA the signal is injected from the bushing tap by a capacitor, and the response is measured at the neutral point. Recently, an online method was put forward, in which sweep frequency signal or impulse signal was injected into the windings from the neutral point and its response was measured at the bushing both by magnetic coupling. But the influence of the outside equipments that connect with the transformer is unknown. In this paper a method to eliminate the influence of outside devices is put forward. Through the analysis of the two-port network equivalent of transformer winding, in which the outside equipment is equivalent to a impedance, we find that the zeros of the transfer function is not affected by the influence of this impedance. In this paper, the detail of this method is introduced, and is proved on a 220kV test transformer. The experiment results showed that when the outside equipment changed, the amplitude frequency response curve of the winding was different, but the minimum point location of the curve almost did not change. So we can use the minimum point to eliminate the influence of outside equipments.

A failure probability model of the moistened oil-paper insulation of transformers based on strength-stress model

Its easier for the oil-immersed power transformer to fail when moistened. As is known to us all, moisture may cause the great decrease of the breakdown voltage of the oil-paper insulation system. So, the failure probability model of transformer was established in this paper by using the theory of strength-stress model which is successfully used in the mechanical reliability field. The model could be used to quantitatively calculate the failure probabilities of a single transformer or transformers in a certain region. The appropriate descriptions of stress and strength were introduced to fit the strength-stress model for the two kinds of failures respectively. The random amplitude and times of lightning in a certain period are described by double integral of their probability density functions. The moisture content of one transformer influences the failure probability by changing the strength of the oil gap.

A method for on-line monitoring of electric tree growth in pressboard of transformers

In this paper, we propose a new method, distributed potential measurement system for monitoring of electric tree growth in pressboard of transformers. Based on the distributed potential measurement system, the position and quantity of PD and the voltage waveform of measuring electrode were obtained on-line. Moreover, According to the voltage waveform of each measuring electrode, the length and depth of electrical tree can be predicted. Multiple experiments were conducted in laboratory, which confirm the space charge position, the prediction of trees length and the real tree shape in pressboard are accordant. Research shows that the distributed potential measurement system can monitor the growth information and the development of electrical trees online, which lays a foundation for further research on the growth rule of the electrical tree.

The field experience of a dynamic rating system on overhead power transmission lines

Considering the safe operation of power grid, the actual thermal capacity of overhead transmission lines is far above their designed thermal capacity. How to fully explore the potential capacity of the existing overhead transmission lines becomes one of the greatest concerns. We propose a fire-new model for calculating the real maximum thermal capacity of transmission lines, high temperature equivalent model (HTEM). This model does not need to measure meteorological parameters, such as ambient wind velocity, ambient temperature and sunlight intensity. Therefore, the computational accuracy of real maximum thermal capacity is greatly improved. An on-line monitoring system has been designed, not only to carry out the new model, but also to measure the wire temperature, current and sag. The system has already been trial running on 110kV∼500kV high voltage transmission lines for more than 30 months. The data obtained by the system has provided a strong evidence for this model.

The statistical distribution of the DGA data of transformers and its application

Dissolved gas analysis (DGA) is one of the most widely used diagnostic tools for detecting faults and evaluating the condition in oil-impregnated power transformers. However, interpretation of DGA results is often complex and should always be done with care, and it is claimed to get local criterion from local transformers. The concentration values of dissolved gases in 760 transformers, from Guangzhou area of China, were processed to get their statistic distribution, by hypotheses of normal distribution, logarithmic normal distribution and Weibull distribution and Kolmogorov-Smirnov Test. Furthermore, the best theoretical distribution functions were established for each type of dissolved gas in 110 kV transformers and 220 kV transformers respectively, and the local typical values and alarm values were obtained from them. Finally, the local typical values were contrasted with that in the IEC standard and the national standard. And three problems were discussed.

Experiments on the multi-functional current sensor for condition detection of transformer bushings

In recent years, time based maintenance of electrical of power equipment is gradually replaced by condition based maintenance. In China, smart grid acts as the great goal of the electric power industry, in which smart equipment such as smart transformer play important roles. As one part of the smart transformer, a smart multi-functional current sensor was designed to replace the leakage current sensor and the partial discharge sensor, the latter are usually installed on the grounding line of the bushing tap. This sensor has small volume, convenient interface and can measure the leakage current and partial discharge and overvoltage signals at the same time. In this paper the construction design of the sensor is displayed. And experiments were carried out in the laboratory to prove the feature of the sensor to measure the signals and the protection circuit under lightning overvoltage. Finally, the sensor was used on a 220kV transformer to prove its effectiveness.

Vapor-liquid equilibria of (CF 3 I + CO 2 ) at the temperature of 243.150K

It has become an imperative case on seeking the substitute of SF₆ for its severe greenhouse effect in recent years, not only for scientists but also the electricity sectors. CF₃I, as a substitute gas for SF₆ with lower Global Warming Potential (GWP), has been investigated. The use of pure CF₃I in gas insulated switchgear (GIS) and gas insulated metal enclosed transmission line (GIL) is difficult because CF₃I has a high dew temperature. We have therefore mixed CF₃I with CO₂. In this paper, an apparatus was used to get the vapor-liquid equilibrium data for (CF₃I + CO₂) system at 243.150 K. All of the experimental data were correlated with the Peng-Robinson (PR) EoS using the classical van der Waals (vdW) mixing rule (PR-vdW model). The results show the changing rule for the vapor pressure and its corresponding mole fraction of (CF₃I + CO₂) gas mixtures to suit the operation environment in GIS/GIL and supply an accurate basis for the application of (CF₃I + CO₂) gas mixtures in electric equipment.