Lijian Ding

Influence of sintering temperature on the flashover performance of alumina insulators in vacuum

This paper reports on the influence of sintering conditions on the flashover/pre-flashover characteristics of alumina insulators in vacuum under an impulse voltage. Four kinds of alumina insulator were studied, which were prepared using the same formula with different sintering conditions. The surface pre-flashover and flashover phenomena of different samples in vacuum have been observed under impulse voltage. Based on the experimental results, the characteristics of pre-flashover and flashover vary with different kinds of test samples. It was found that the insulators with a higher sintering temperature had a lower flashover voltage, and more active preflashover. From a scanning electron microscope (SEM) analysis of alumina insulators, we conclude that the methods of preparing alumina samples can affect their microstructure resulting in a change in the pre-flashover and flashover performance as alumina insulators.

Partial discharge recognition of stator winding insulation based on artificial neural network

Partial discharge (PD) reflects insulation condition of high voltage electrical apparatus. Many large electrical machines tend to be subjected to large numbers of load cycles which often shorten the service time of the stator winding. PD accelerates insulation deterioration and there will be more serious PD phenomena at the continual development of insulation fault and before final breakdown of insulation than that at the beginning. PD contains characteristic quantities that can be used to inspect the insulation condition to avoid sudden failure especially for on-line monitoring. In this paper, five types of different physical simultaneous insulation models, which reflect PD in stator windings of large generators, were made. Simulated PD types included surface discharge at endwinding, slot discharge, delamination discharge in three different positions of ground wall insulation. Different levels of voltage were applied to models to obtain different extent level of PD. An artificial neural network (ANN) with backpropagation algorithm was designed to identify the types. The levels of PD extent have also been judged by their distribution. The recognition ability of the ANN was studied. Different types and levels of PD within the winding insulation of large generators were identified with a satisfactory recognition rate.

Experimental investigation of influence of surface roughness on flashover performance of ceramics in vacuum

This paper provides some experimental investigation on the influence of the surface roughness on the flashover performance of ceramics in vacuum. The test samples were prepared with four kinds of ceramics (99.9% Alumina) which were polished by four different SiC abrasives. The flashover performances of 20 kinds of test samples have been tested with an impulse voltage in vacuum. According to the experimental results, it was found that the roughness of the test sample was quite different among different ceramics although the same grinding procedure was used with the same abrasive. It was also found that the flashover performance was influenced by the surface roughness, and there is a trend that the samples with a lower surface roughness have a higher breakdown voltage.

A method to detect the deterioration of HTV silicone rubber under corona discharge

This paper has investigated the thermally stimulated current (TSC) characteristics of the deteriorated HTV silicone rubber under varied corona discharge duration, the hydrophobicity, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) of the corresponding test samples have also been measured. The experimental results show that the TSC characteristics of HTV silicone rubber gradually varied with the corona deterioration process, and it has relationship with the change of its hydrophobicity, SEM, and also FTIR. As a result, it could be suggested that the TSC characteristics might be used as an effective index for assessing the corona deterioration of HTV silicone rubber.

The relation of trap distribution of alumina with surface flashover performance in vacuum

Alumina ceramic samples prepared under different sintering temperatures and varied additives were measured to indicate the trap density and trap energy located in alumina materials by using thermally stimulated current (TSC). The surface charges on alumina in vacuum after applying a negative pulse voltage (0.7/4 µs), and flashover performances of the materials in vacuum also were measured. We found that the trap distribution in alumina has a correlation with surface charges and flashover performances in vacuum. It is shown that the higher is the trap density in the material, the higher is the surface charge density, and the lower is the flashover voltage on alumina surface. It is believed that the trapping and de-trapping mechanisms of carriers could play an important role during the development of the discharge processes, together with the secondary electron emission mechanism.

VHF PD detection of 110 kV XLPE cable accessories

Partial discharge (PD) can indicate defects in HV XLPE cable accessories. A new land of split-core PD sensor is designed that can be easy installed on-site and used safely, and has a bandwidth from 3 MHz to 60 MHz. A model of a cable accessory was made and some typical PD phenomena in this accessory were simulated, such as sliding discharge, floating discharge and cavity discharge. PD tests were made to find the features of the defects. The data from the tests were analyzed and the spectra of different types of PD were plotted using statistical parameters to reveal the feature of the different patterns of PD. The split-core PD sensor has been used on-site after sample testing in the laboratory. More than twenty cases of on-line PD detection for 110 kV XLPE cables have been obtained.

Hydrophobicity Loss of HTV Silicone Rubber after Corona Discharge

This paper presented the results of a study on the hydrophobicity loss of HTV silicone rubber under the different corona discharge conditions. The experimental results show that the speed and grade of hydrophobicity loss vary with the corona discharge time and the corona intensity. Moreover, it is noticed that the hydrophobicity would not become worse as long as the corona intensity keeps below a certain level.

The TSC characteristics of HTV composite insulators

This paper presents the thermally stimulated current (TSC) results obtained from an experiment which was carried out in order to identify the feasibility to evaluate the aging state of field HTV composite insulators by TSC technique. The test samples were prepared from four HTV composite insulators, which were randomly picked in-situ. The TSC experimental results show that the rough TSC shapes of four samples were similar despite different serving time, while the peak at high temperature area was only observed in one sample (insulator C). Further calculated results indicate that insulator C had the deepest trap level in our investigation. The performance difference between insulator C and the other three insulators can be obviously observed from the SEM morphologies, but is difficult to discern from their corresponding HC levels. Based on the above results, we can infer that although there are certain inner relations between the TSC characteristics of HTV composite insulators and their hydrophobicity, comparing to the later, the former is more sensitive to the aging performance under some conditions. Also the aging states of field HTV composite insulators have no direct relations with the servicing time in our investigation, when the TSC characteristics are taken into account

Effect of Corona on the Hydrophobicity Recovery Characteristics of HTV Silicone Rubber

This paper has investigated the hydrophobicity recovery characteristics of HTV silicone rubber after different corona deterioration time using the static contact angle method. The experimental results show that the hydrophobicity of HTV silicone rubber can get some recovery after the corona deterioration, and the procedure of hydrophibicity recovery would be varied with the corona deterioration condition, moreover, the hydrophobicity would not recover back to the initial value after a certain condition of corona deterioration, even if the storage time is long enough.

Experimental study on the TSC of alumina ceramics

This paper has investigated the thermally stimulated current (TSC) of Alumina ceramics using a uniform electrode system. Experimental results with different Alumina ceramics showed that TSC characteristics of the samples prepared with. varied metal oxide additives was different. The trap distribution parameters were calculated from TSC curves, the calculation results show that the additives significantly influence the trap distribution.