Showing papers by "Trevor Blackburn published in 2014"

Influence of temperature and moisture content on frequency response analysis of transformer winding

Abstract: Frequency Response Analysis (FRA) has been in use since the last decade as a sensitive method for detecting transformer internal defects. This study is focused on the influence of temperature and moisture migration on the FRA response of transformer winding. It also discusses the feasibility of FRA capability in moisture diffusion recognition in transformer paper insulation. To conduct this study, a single phase model transformer involving concentric LV and HV windings and a 20/0.4 kV, 1.6 MVA threephase two windings transformer are taken as test objects. Experiments are carried out at different temperature and moisture conditions. FRA spectra are then recorded and analyzed. FRA spectra deviations as well as total capacitance variations due to the temperature and moisture changes in the test objects are calculated. Karl-Fischer Titration (KFT) is utilized to monitor the moisture migration within oil and paper insulations. Furthermore, a mathematical model is used to simulate one of the test object windings and verify the experimental result, and also clarify the main reason of FRA spectrum deviation in this circumstance. Finally, statistical indices in FRA evaluation are calculated to explore their capability in FRA spectrum interpretation once the moisture content of paper insulation is changed.

Recognition of single and multiple partial discharge sources in transformers based on ultra‐high frequency signals

Abstract: Partial discharge (PD) is a symptom of insulation defect or degradation in high-voltage equipment. Thus, PD detection is an important diagnostic tool. Furthermore in practical situations, the PD can be generated from a single or multiple sources. Being able to detect and classify such PD events will help to determine the necessary corrective action to prevent insulation breakdown. To demonstrate, three different simulated discharge conditions in transformers were investigated: void, floating metal and their combination. The PD signals were captured using an ultra-high frequency (UHF) sensor and denoised using wavelet transform method by application of Matlab wavelet multi-variate denoising tool. Two types of mother wavelet, that is, db and sym, were applied to decompose the signals and extract the signal features in terms of their skewness, kurtosis and energy. These features were then used as input to train a neural network to analyse and determine the PD source type. Results show this technique is able to classify and recognise single and multiple PD source types with a high degree of success.

Detection of High Impedance Faults using wavelet transform

Abstract: High Impedance Fault (HIF) has long been a challenging problem in network protection. This is because of its difficulty to be detected and discriminated accurately and reliably. In real cases, arcing is usually associated with HIFs. In this paper, arcing currents are produced by setting up a HV experiment. Normal switching transients are also produced by simulation or experiment. They are then analysed based on wavelet transform. It was found by previous researches that db4 is sensitive in detecting high frequency transient. Different features for the fault currents are found and a detection criterion is developed. The simple detection criterion is used to discriminate the high impedance fault from switching transients and normal load current.

Characterization of Partial Discharge With Polyimide Film in $\hbox{LN}_{2}$ Considering High Temperature Superconducting Cable Insulation

Abstract: High temperature superconducting (HTS) cable has now reached the stage where their application in power systems is in the process of being implemented. Liquid Nitrogen (LN 2 ) and polyimide film have been used as a coolant and insulation in HTS cable systems. Partial discharge (PD) events may lead to material degradation and total breakdown. PD testing is an important quality check for the insulation of HTS cable. Thus it is necessary to consider PD for the insulation testing in LN 2 which is generated in void defects of the insulation layers. The PD characterizations of polyimide film in different defects were investigated under 50 Hz AC voltages in LN 2 and at room temperature (298 K). The results show that the number of discharges and the discharge quantity increase with the increasing of the applied voltage and the defect size. The PD inception voltage decreases when the void defect diameter enlarged and it is higher in LN 2 than that at room temperature. The maximum field strength for HTS tape insulation increases with the defect close to the tape boundary and the addition of contaminant relative permittivity. Obtained results at cryogenic temperature provide necessary information for insulation design and pre-shipment inspections of HTS cable system.

A comparative study of partial discharges under power and very low frequency voltage excitation

Abstract: Very low frequency (VLF) testing refers to AC testing in the frequency range of 0.01 Hz up to 1 Hz. VLF testing is increasingly becoming popular as an alternative approach for test objects which require significant reactive power such as power cables. However, reliable diagnosis of partial discharge characteristics under VLF voltage excitation is still lacking. The paper presents a comparative study between testing at normal power frequency and at VLF. Corona and surface discharges were produced using a needle/bowl and a short section of 11kV XLPE cable. Experiments were conducted over a range of frequencies from 50 Hz down to 0.08 Hz and the results were analyzed. They showed some similarity of PD characteristics in terms of their phase-resolved patterns. The discrepancies of PD results are described in details.

Arcing current features extraction using wavelet transform

Abstract: High Impedance Fault (HIF) has long been a challenging problem in network protection. This is because of its difficulty to be detected and discriminated accurately and reliably. In real cases, arcing is usually associated with HIFs. In this paper, arcing currents produced by using different metals are analysed based on wavelet transform. The wavelet db4 is used and found to be sensitive in detecting high frequency transient. Test results show that the features for different current magnitudes are different. These features are studied to develop a simple detection criterion to distinguish arcing fault currents from the normal load.

Effects of ambient conditions on partial discharges at very low frequency (VLF) sinusoidal voltage excitation

Abstract: Effects of temperatures on corona discharges developed by needle/bowl electrode configuration at different frequencies are investigated. PD measurements are carried out under enclosed controlled air temperature range between 20°C and 40°C. The test chamber is airtight so the humidity is constant. Sinusoidal voltage waveforms in the frequency range of 0.05Hz to 50Hz are used to stress samples. There are evidences that PD behaviors at VLF and power frequency are dependent on ambient temperature in a similar manner. Nevertheless, there are still noticeable differences in PD patterns at VLF at various temperatures, especially in the phase position of PD events. These differences are discussed in details.

Influence of moisture content variation on Frequency Response Analysis of transformer winding

Abstract: Frequency Response Analysis (FRA) has been used as a diagnostic method for transformers since the last decade and is emerging as a popular technique. Recent studies on this technique reveal that FRA data are not only influenced by transformer winding deformation but could be also affected by transformer insulation parameters. Hence, this study has concentrated on the influence of moisture content variation on the winding frequency response spectrum. To this end, transformer water dynamic is briefly discussed. A model transformer is used as the test object. Frequency response spectra of `wet' and `dry' conditions of the test object are examined for different moisture contents and different temperatures. The results of both examinations are then compared, and finally the common trend in FRA spectrum deviation under such circumstances is explored.

Paper moisture variation vs. mechanical deformation impacts on transformer frequency response spectrum

Abstract: Frequency Response Analysis (FRA) has been in use as a diagnostic method for transformers since the last decade and is emerging as a popular technique. However, accurate interpretation of FRA measurements is still under development. Recent studies reveal that apart from winding deformation, FRA data can also be influenced by moisture variation in transformer paper insulation. A better understating of this issue can in turn improve the frequency response interpretation Hence, a specific model transformer was fabricated for this study and the experiment was concentrated on the paper moisture variation and mechanical deformation influences on FRA spectrum deviation. Different physical changes to the test object are investigated and the results are recorded and discussed. These enable comparison of insulation characteristic and mechanical defect impacts on FRA data, and reveal factors influencing the FRA spectrum.

The influence of dielectric dissipation factor on transformer Frequency Response Analysis

Abstract: Frequency Response Analysis (FRA) is regarded as a robust technique to recognize transformer winding deformation. It has been implemented and used since the last decade. However, its interpretation and measurement sensitivity are still under development. Recent studies reveal that apart from winding deformation, FRA data can also be influenced by moisture variation in transformer paper insulation. Technically speaking, moisture variation will certainly influence the Dielectric Dissipation Factor (DDF) in transformer. Therefore, it is of interest to investigate the correlation between DDF value and FRA spectrum. A model transformer was fabricated for this study and the experiment was concentrated on the temperature changes and paper moisture variation and eventually DDF alteration on the FRA spectrum deviation. DDF was measured for the “wet” and “dry” model transformer at different temperatures. In parallel, the FRA spectrum was also recorded. The correlation between the transformer DDF value and FRA spectrum is highlighted and discussed.