Kexiong Tan

The study of acoustic emission method for detection of partial discharge in power transformer

Six kinds of typical oil gaps were designed and tested to simulate partial discharges (PD) in the insulation of power transformers. The frequency spectra of acoustic emission (AE) of PD in those gaps were presented and investigated with a computer-aided measuring system. The attenuation of AE and the variation of its frequency spectrum in oil and in oil-impregnated paper were studied. Peak frequencies in the frequency spectra of AE of PD in these gaps were distributed in the 70-150-kHz range. Based on test results, the frequency bandwidth of 70-180 kHz was recommended for the AE measuring system of PD detection in power transformers.<<ETX>>

Simulation of transformer PD pulse propagation and monitoring for a 500 kV substation

This paper discusses the development of a simulation model to study the propagation of partial discharge (PD) pulses of transformers in a 500 kV substation. The results will be used to design an on-line PD monitoring system for transformers. The impedance matrix of a 500 kV, single-phase transformer is computed based on geometry data and its external coupling network (often referred to as coupling impedance) is calculated with electromagnetic transients program (EMTP). It was found that the frequency characteristic of the coupling network is complex and that the traditional lumped coupling capacitance representation is only feasible at <60 kHz. The entire substation network was included to calculate the PD propagation of the transformer. PD responses are obtained at the tank grounding and 500 kV bushing tap grounding with a simulated PD pulse injected at the 500 kV terminal and 220 kV terminal of the transformer, respectively. Simulation shows that, using the resonant frequency of the substation network as monitoring frequency, one effectively can increase the monitoring sensitivity. The off-line and on-line PD calibration can also be made based on the developed simulation model.

Measurement of dynamic potential distribution during the propagation of a local arc along a polluted surface

A new method for measuring the dynamic potential distribution during the propagation of a local arc along a polluted surface is described. In order to study the role of electric stress during the pollution flashover processes, a flat polluted glass plate model was designed. The measuring electrodes made with a strip of aluminum foil were stuck on the bottom of the plate, opposite to the side on which the local arc propagated. The capacitance between the polluted surface and the measuring probe, which is in series with an additional capacitor, was used to construct a potential divider. By means of the dividers and a magnetic tape data recorder, the dynamic potential distribution, the position of local arc, and its propagation velocity were measured. The results show that the velocity of arc propagation along a polluted surface is 0.2 to 1 m/s and that the maximum average potential gradient in the pollution layer is approximately=0.5 kV/cm. >

Deformation analysis of transformer winding by structure parameter

Frequency Response Analysis can be used to detect the transformer winding deformation. Its reliability and standard are studied. A simulation model of transformer winding is developed. The transformer winding is equivalent to a network circuit, and electrical components in the circuit can describe distributed elements of the winding. The components in the model are determined by the geometric and electric data of the winding. If there is any deformation in the winding, circuit parameters in the model will be changed and thus the character of the whole circuit will be influenced. According to the common situations of winding deformation caused by short circuit, the variation of the circuit parameters are analyzed and simulation calculated. Transfer function is used to describe the character of the model. The original frequency spectrum is taken as reference and that after deformation is analyzed. Correlative Coefficient value is used to judge whether and how bad the deformation happens. Different kinds and levels of the deformation are simulated and the results can match the data from field experiments. Some guide criteria for the detection of winding deformation is presented.

Locating partial discharge in power transformer by electrical method

To locale partial discharge (PD) source in power transformer by electrical method, the propagating principle of PD pulse along transformer windings and the response of PD pulse at detecting terminals outside the transformer are studied. Simulating analysis through numerical model and imitating experiment on transformer winding are carried out. A simulation model of transformer winding is established. The equivalent circuit is made-up in order that the transfer property of PD pulse along the winding could be calculated and analyzed. There are differences in the properties according to the PD source positions so that the relationship between the difference and PD positions can provide some information for PD locating. Based on numerical treatment of the relationship, the electrical method of PD locating is presented. The energy of the measured signal is used to locale PD source. The device for PD pulse imitating is developed for the experiments on transformer windings. The terminal signals caused by PDs at different positions are measured during the experiments, and the results indicate that the locating accuracy of these methods is satisfied for engineering practice use.

Decision on maintenance period for power transformer based on Monte Carlo simulation of reliability

The problem of selecting a suitable maintenance period is vital for preventive maintenance of power transformer. Traditionally, maintenance period can be calculated by model with an objective function, such as availability, maintenance cost, or reliability of the equipment. However, it is difficult to directly apply these models for power transformer, because its reliability is high, and there is no enough reliability data to calculate the objective functions. Therefore, a simulated maintenance period model is developed in this paper. Firstly, Monte Carlo method and Fault Tree method are applied to generate simulated fault times of the transformer system. Secondly, Mission Reliability is calculated based on these simulated data and acts as the objective function in the model. Finally, this model is applied to investigate the maintenance period of power transformer.

Internet based virtual hospital architecture for power equipment

Equipment diagnosis and maintenance plays a key role in reliable power system operations. Information technology opens new doors in this area with powerful and convenient information sharing capabilities. The concepts and prototype development of an Internet based virtual hospital (VH) for power equipment are introduced in this paper. The VH will serve as an information center for the power equipment diagnosis and maintenance community. The virtual hospital will feature a diagnosis/maintenance knowledge and education center, a diagnostic tools center, a worldwide maintenance expertise pool, forums and user groups, links to equipment manufacturers, links to standards, and many other relevant information. VH provides a convenient platform for experience/information sharing, discussions, and worldwide collaborations.

PD monitor system for power generators

This paper introduces a computerized PD monitoring system for generators and presents the experimental and numerical study of discharge pattern recognition methods. The system has specially designed transducers, data acquisition unit and software, and can obtain statistical as well as individual discharge information. In order to validate the performance of the system, experiments were done in the laboratory, using elaborately designed models that can generate various types of discharges. Feature extraction of the gathered data and neural network (NN) classification of the acquired discharge patterns were studied. The results showed that the surface fitting method is able to extract features from statistical data of discharges, and that NN is a potential classifier in practical applications.

A new stationary on-line PD monitoring system for power transformers

This on-line PD monitoring system can monitor partial discharge in two three-phase transformers continuously. It has the functions with fault reminiscence, fault warning and fault location. By means of using wide-band, multi-channel, high sampling rate and large capacity data acquisition system makes system has a good basis, it can use various kinds of digital signal processing method. Furthermore various kinds of data processing method can be added into the system flexibly by taking the Component Object Model as the basis of the whole software. This system has good expansibility and flexibility and can fit the requisition for different equipment and monitoring parameters.

PD pattern recognition of power capacitors model based on Combinational neural network

Five types of partial discharge (PD) models are designed to represent typical PD phenomena in power capacitors. A computer-based acoustic emission signal detecting instrument that is developed for PD is used to collect a lot of acoustic signals of model discharge. The acoustic signal duration of gas cavity discharge is very short, therefore, it is easy to be differentiated from other discharge patterns. According to the time domain and frequency domain graph of acoustic signals, the characters of oil gap discharge is close to that of discharge along oil impregnated paper surface. However, the character of discharge of metallic impurity in oil impregnated paper insulation is close to that of surface discharge of bushing. Combinational neural network (CNN) is used to recognize the five kinds of representative discharge patterns. The result shows that CNN is effective and the characteristics of acoustic signals could be used to recognize PD patterns in power capacitors.