M. S. Abd Rahman

Construction of finite impulse wavelet filter for partial discharge localisation inside a transformer winding

In high voltage (H.V.) plant, ageing processes can occur in the insulation system which are totally unavoidable and ultimately limit the operational life of the plant. Ultimately, partial discharge (PD) activity can start to occur at particular points within the insulation system. Operational over stressing and defects introduced during manufacture may also cause PD activity and the presence of this activity if it remains untreated will lead to the development of accelerated degradation processes until eventually there may be catastrophic failure. Therefore, partial discharge condition monitoring of valuable HV plant such as a transformers and in particular along a transformer winding is an important research area as this may ultimately provide asset health information enabling the maintenance and replacement processes to be carried out effectively. Wavelet multi-resolution analysis consists of a series of quadrature filter banks which are associated with a high pass and low pass filter. The process is performed in order to decompose original signals into different levels that contain different time-frequency resolutions of the original waveform. Thus, the spread of signal energy over different time/frequency ranges can be determined. The use of system identification in the frequency domain using the Wavelet transform provides unique selections of the particular frequency range of interest of the measured PD signals that have propagated inside a transformer winding. Wavelet decomposition levels can be combined linearly with Principal Component Analysis (PCA) and this may provide useful information about the location of the discharge source within the winding and with further implementation using an infinite impulse response (IIR) filter approximation, it is possible to construct a standard filter based on the Wavelet

The use of three dimensional filters for on-line partial discharge localisation in large transformers

Partial discharge (PD) activity develops in high voltage transformers due to ageing processes in the insulation system which are totally unavoidable. Moreover, operational overstressing and defects introduced during manufacture can also cause the existence of PD activity. The presence of this activity needs to be identified and treated, otherwise it will lead to further ageing and degradation processes until eventually there may be catastrophic failure. Previous reported work by the authors used Wavelet Transforms and Principal Component Analysis (PCA) separately in order to develop an approach for PD localisation within a transformer winding which is more suitable for implementation in an off-line mode. In this paper a new technique has been developed for on-line application using filters designed based on wavelet functions in the frequency domain and PCA loading coefficients. The combination of wavelet and PCA produces a unique frequency spectrum for the three dimensional filters. A transfer function of corresponding frequency spectrum can be obtained using estimation based on an IIR filter approximation. This paper reports on selection of filter order and its influence on PD source localisation. Ultimately, this study can be used to determine the optimum selection of the three dimensional filters for a given discharging transformer that will facilitate on-line localisation of the PD source.

The study of the effect in variation of A.C. applied voltage to the partial discharge localization inside transformer windings

Ageing processes typically associated with the dielectric and insulation system occur inside high voltage plant. These unwanted processes may lead to generation of partial discharge (PD) activity and ultimately limit the operational life of the plant. The operational over stressing and defects introduced during manufacture also lead to the existence of PD activity and the source of this activity needs to be identified and remedied before further degradation processes cause catastrophic failure. Therefore, research into localization techniques of PD sources inside high voltage plant generally and particularly within transformer windings has become important in order to provide the asset health information that allows maintenance and replacement processes to be carried out effectively. Locating a PD source inside a transformer winding using on-line measurement data is vital in order to achieve these objectives. Methods have been proposed to achieve this and in this paper the distribution of PD signal energy in the time and frequency domains is used as a location tool. Analysis of experimental data reveals that this approach is independent of the applied voltage.

Autonomous localization of partial discharge sources within large transformer windings

Partial discharge (PD) condition monitoring inside a HV transformer generally and particularly along a transformer winding has become an important research area with the ultimate aim of providing asset health information that enables maintenance and replacement processes to be carried out effectively. As far as PD activity inside transformer windings is concerned, an electrical detection method has been developed based on the use of radio frequency current transducers and subsequent digital signal processing of captured measurement data. A localization approach based on the measurement of currents at the bushing tap point and neutral to earth connection has been developed, with the assumption that different PD source locations will generate unique signal profiles in terms of the distribution of measured current energies with respect to both frequency and time. Therefore the technique presented is based on analysis of measured current energies associated with different frequencies. Principal Component Analysis (PCA) is then applied to reduce the dimensionality of the data, whilst minimizing lost information in the original dataset. This non-linear analysis of captured current data is not practicable for the field but the process can be represented through the use of three finite impulse response filters that have the ability to perform PD source localization automatically and are straightforward to implement in monitoring hardware.

Comparison of laser ablation and inclined plane tracking tests as a means to rank materials for outdoor HV insulators

As the popularity of polymeric composites for outdoor high voltage (HV) use continues to increase, surface degradation through electrical discharge activity is becoming a more serious issue. In order to design composites offering improved resistance to degradation, and hence improved component lifetimes, tests need to be performed under controlled conditions in order to rank different materials. Inclined plane tracking tests are often used to determine the relative performance of various material formulations used in their construction. An alternative approach is to employ a high power laser to deposit known amounts of energy at a known rate to the sample surface. In this paper a variety of material formulations were subjected to inclined plane tracking tests as well as to laser ablation testing. The results indicate that the two test methods compare favorably and that laser ablation offers a quick and cost effective way of screening candidate material formulations, prior to more detailed study.

Partial discharge propagation inside a high voltage transformer winding: Comparison of measurement and simulation

Partial discharge (PD) analysis, based on measurement data, assumes that PD signals propagate inside a transformer winding from the PD source to the measurement points. In this paper, signals are injected via an external connection into specific points along the winding of a transformer experimental model. The measurement data obtained using radio frequency current transducers (RFCT) connected to the external bushing tap and the neutral to earth connections are illustrative of the propagation of signals inside a transformer winding. Thus, this paper reports on the development of a simulation model based on a lumped parameter network approach and compares simulation results with experimental measurements of PD signal propagation inside a high voltage transformer winding model. The physical model is an interleaved disc type winding which consists of eight sections and is modeled as having internal winding series resistances (R), inductances (L), series capacitances (K) and shunt capacitances (Cg), the model also includes the effect of their mutual inductances. The parameters are calculated using analytical calculations based on geometrical dimensions and the winding is modeled using a commercially available mathematics package. The simulation creates PD signals at both ends of the winding i.e. at terminal 1 and terminal 8, which are grounded through a small capacitive and resistive element to represent the bushing end and neutral to earth connection respectively. An experiment has been developed that can be used to validate the simulation data and hence determine the accuracy of the derived model and ultimately its use as a PD localisation approach.

Laser ablation of polymeric composites

Degradation of polymeric insulation due to surface electrical discharge activity will become a more serious issue, as these materials find more widespread use. In order to improve their resistance to degradation, tests need to be performed under controlled conditions in order to rank different materials. Whilst electrical discharge activity can be simulated in the laboratory this is difficult to localize and characterize. An alternative is to use a laser to impart a known amount of energy at a known rate to a sample surface. In this investigation, the resistance of various material formulations to laser induced surface damage was tested and the materials ranked. The results are discussed in terms of the material composition and final application.

Laser ablation of silicone composites

Silicone rubber based composites are widely used to produce outdoor insulators. In adverse weather conditions these can be damaged by surface discharge activity resulting in deterioration of their dielectric properties and an eventual need for replacement. Inclined plane tracking tests are frequently used to determine the relative performance of different material formulations used in their construction. An alternative approach to characterisation is to use an infra-red laser to deposit known amounts of energy at a known rate to the samples surface allowing comparative ranking of different materials. In this paper several silicone based composites have been ranked using a laser ablation technique and the results were then compared to those obtained from tracking tests on the same materials. The comparison indicates that laser ablation ranks the materials in the same order as the tracking tests and may therefore constitute a quick and cost effective method for the routine characterisation of outdoor insulation components.