Luo Yongfen

Research on extraction technique of transformer core fundamental frequency vibration based on OLCM

This paper presents a new technique based on vibration measurement to diagnose power transformers, which is called the onload current method (OLCM). It can acquire the fundamental frequency component of the core vibration signal without running the transformer at the open-circuit condition. The diagnostic method adopted and the experimental test results are reported. Tests have been performed at normal operating conditions in both the manufactory and the laboratory. With the tests performed in the manufactory, the vibration characteristics of transformer windings and core are described, and then the principle of OLCM is introduced. To verify the validity of OLCM, the laboratory tests are conducted which relates the transformer vibrations to the simulative fault. But the presented method was verified on a 5-kVA transformer which is a very far cry from an actual power transformer. It needs to be developed further before OLCM can be used effectively on the transformer in the field

Simulation of PD location method in oil based on UHF and ultrasonic phased array receiving theory

In this paper, a new partial discharge location method based on UHF and ultrasonic phased array receiving theory is presented. The plane sensor is consisted of two phased receiving arrays, which respectively receive UHF and ultrasonic signals emitted by the same partial discharge source. Regarding the time point of signal detected by the UHF phased receiving array as time benchmark, the time delay of ultrasonic signals received by acoustic phased receiving array can be obtained and the distance between PD source and the sensor can be calculated, then the PD dimensional position can be found out using the azimuth and elevation angle at which the phased receiving arrays scan. Because PD sources of different space positions lead to different space angles of beam combination, which represent corresponding maximum PD signals, and to different time benchmark compared to the same sampling initial point and different time delay, the locating of multiple PD sources can be reached. Several examples of PD in oil are simulated using this novel location method and the results indicate the method can exactly locate the PD in oil and solve the location of multiple PD sources.

Phased-ultrasonic receiving-planar array transducer for partial discharge location in transformer

Partial discharge (PD) location in transformers is very important, and many methods that have been brought forward in past decades have a limitation theoretically, namely, they cannot locate multiple PDs in electrical equipment. In this paper, a new PD location method based on UHF and ultrasonic-phased arrays receiving theory has been presented, which has a possibility to locate multiple PDs. According to the method, a phased-ultrasonic receiving-planar array sensor that possesses 16 x 16 elements is designed; and, based on the phased-array theory, the characteristics of the plane sensor are studied. The laboratory experimental tests on the plane sensor element indicates that it has a good performance within the frequency band of the main ultrasonic energy produced by PD in transformer oil. Location tests are conducted on one or two piezoelectric ultrasonic sources in oil, which are both simulated as PD sources and triggered by an electrical pulse whose front is considered as a time benchmark in the locating algorithm. The test results show locations to one and two PDs can be realized in a single measurement, which lays a foundation for locating PDs in a power transformer in service

Relocatable Ultrasonic Array and UHF Combined Sensor Applied to PD Location in Oil

A relocatable combined sensor is designed to locate partial discharge (PD) in a transformer. Based on the sensor, PD location experiments are conducted. The sensor consists of a conformal L-shaped ultrasonic array of 13 elements and includes an ultra-high-frequency electromagnetic sensor. The L-shaped array achieves the effect of 97 virtual elements expanded with high-order cumulants processing technology. It greatly improves the aperture and direction sharpness of the array. The PD location experiments were carried out on a PD experiment platform. The result shows that the combined sensor can distinguish and locate one or multiple PD sources with average relative locating error 2.1% and 4.8%, respectively. Virtual expansion of an array with fewer number elements helps to design a miniaturized and combined sensor. This work should provide as a foundation for application of array technology in power equipment.

Comparison of DOA Algorithms Applied to Ultrasonic Arrays for PD Location in Oil

To reduce the number of elements and the cost of follow-up circuit of a

Study of Improved RSS Algorithm for Ultrasonic Array Applied to PD Location in Transformer

16 \times 16

Simulation of cross-shaped ultrasonic array applied for partial discharge location in oil

ultrasonic array sensor, two ultrasonic array sensors are designed. One is L shaped with 13 elements and the other is cross shaped with 25 elements. Based on these two ultrasonic array sensors, this paper compares the directional performance of a two-sided correlation transformation algorithm, which is a broadband focusing algorithm, an array expanded algorithm based on fourth-order cumulants, and a combination of these two algorithms, namely, synthesis algorithm. The broadband focusing algorithm can adjust to the broadband ultrasonic signal emitted by partial discharge (PD) source. After virtual expansion, the L-shaped and cross-shaped ultrasonic arrays can achieve the effects of an array with 97 and 193 elements, respectively. The adaptive capacity of broadband ultrasonic signal emitted by the PD source, and the precision of direction to PD are compared for the three algorithms. Directing simulation and experiments with single or multiple PD sources are carried out. The results show that the directing precision is highest with the synthesis algorithm, the error is <;2°, the peaks of spatial spectrum are sharp, and the best broadband adaptive capacity and noise suppression ability are achieved. This paper should provide a foundation for the application of array technology in enhancing the reliability of power transformer operation.

Chaotic Characteristic of Time Series of Partial Discharge in Oil-Paper Insulation

An improved rotated sub-space (RSS) algorithm is proposed for the application of array technology in partial discharge (PD) location in transformer. The algorithm is based on classic RSS algorithm and high-order cumulants, and it is studied through numerical simulation and experimental tests. Based on an L-shaped and a cross-shaped array, the simulation of single and multiple PD source direction is carried out to compare the classic RSS algorithm with improved RSS algorithm. The results confirm the processing capacity in wideband focus, aperture expansion, enhance of directing, and noise suppression of improved RSS algorithm. Moreover, both algorithms are used for PD location experiments, showing that the error of improved RSS algorithm is ~3°. Improved RSS algorithm solves the problem of wideband focusing in PD location, enhances the performance of the array aperture and directing, achieves the effect of the array consists of much more elements with a virtual expansion, simplifying the follow-up circuit.

Simulation of Cross-shaped Ultrasonic Array Applied to Partial Discharge Location in Oil

A study on a combined sensor was made for partial discharge (PD) location in oil. The sensor includes a cross-shaped ultrasonic phased array of 13 elements and four ultra-high frequency (UHF) electromagnetic sensor. The ultrasonic array can achieve the performance of 61 elements with high-order cumulant processing technology. It can improve the array aperture and direction sharpness and greatly simplify the complexity of the following circuit and reduce the manufacturing cost. The method of PD location which use expanded ultrasonic array accompanied with UHF electromagnetic array has better performance. The spatial spectrum comparing with MUSIC (multiple signal classification) method shows that virtual expanding array of the fewer number array elements with high-order cumulant processing technology has strong ability to suppress colored Gaussian noise interference. This provides the possibility for the practical application of array technology in power equipment.

Cruciform ultrasound array sensor and method for positioning transformer partial discharge

The chaotic characteristics of time series of five partial discharge (PD) patterns in oil-paper insulation are studied. The results verify obvious chaotic characteristic of the time series of discharge signals and the fact that PD is a chaotic process. These time series have distinctive features, and the chaotic attractors obtained from time series differed greatly from each other by shapes in the phase space, so they could be used to qualitatively identify the PD patterns. The phase space parameters are selected, then the chaotic characteristic quantities can be extracted. These quantities could quantificationally characterize the PD patterns. The effects on pattern recognition of PRPD and CAPD are compared by using the neural network of radial basis function. The results show that both of the two recognition methods work well and have their respective advantages. Then, both the statistical operators under PRPD mode and the chaotic characteristic quantities under CAPD mode are selected comprehensively as the input vectors of neural network, and the PD pattern recognition accuracy is thereby greatly improved.