Partial Discharge Location within a Transformer Winding using Principal Component Analysis

Partial discharge (PD) may occur in a transformer winding due to ageing processes, operational over stressing or defects introduced during manufacture. The presence of PD does not necessarily indicate imminent failure of the transformer but it will lead to serious degradation and ageing mechanisms which can be considered as a precursor of transformer failure. A necessary step is required in order to prevent degradation due to PD activity which may ultimately lead to failure. PD might occur anywhere along the transformer winding, the discharge signal can propagate along the winding to the bushing and neutral to earth connections. As far as maintenance and replacement processes are concerned, it is important to identify the location of PD activity so any repair or replace decision is assured to be cost effective. Therefore, identification of a PD source as well as its location along the transformer winding is of great interest to both manufacturers and system operators. The proposed method for locating PD sources in windings is based on wavelet filtering and principal component analysis. An experiment has been developed based on a high voltage winding section that has been used to produce PD measurement data and to investigate the feasibility of the proposed approach

Location of Partial Discharges within a Transformer Winding Using Principal Component Analysis

Partial discharge (PD) may occur in a transformer winding due to ageing processes or defects introduced during manufacture. A partial discharge is defined as a localised electric discharge that only partially bridges the dielectric insulator between conductors when the electric field exceeds a critical value. The presence of PD does not necessarily indicate imminent failure of the transformer but it is a serious degradation and ageing mechanism which can be considered as a precursor of transformer failure. PD might occur anywhere along the transformer winding and the discharge signal can propagate along the winding to the bushing and neutral to earth connections. As far as maintenance and replacement processes are concerned, it is important to identify the location of PD activity so any repair or replace decision is assured to be cost effective. Therefore, identification of a PD source as well as its location along the transformer winding is of great interest to both manufacturers and system operators. The wavelet transform is a mathematical function that can be used to decompose a PD signal into detail levels and an approximation. Wavelet filtering is often used to improve signal to noise ratio (SNR) of measured signals, but in this case it is used to identify the distribution of signal energies in both the time and frequency domains. This method produces a feature vector for each captured discharge signal. The use of principle component analysis (PCA) can compress this data into three dimensions, to aid visualisation. Data captured by sensors over hundreds of cycles of applied voltage can be analysed using this approach. An experiment (Figure 1) has been developed that can be used to create PD data in order to investigate the feasibility of using PCA analysis to identify PD source location

Improved Battery Models of an Aggregation of Thermostatically Controlled Loads for Frequency Regulation

Recently it has been shown that an aggregation of Thermostatically Controlled Loads (TCLs) can be utilized to provide fast regulating reserve service for power grids and the behavior of the aggregation can be captured by a stochastic battery with dissipation. In this paper, we address two practical issues associated with the proposed battery model. First, we address clustering of a heterogeneous collection and show that by finding the optimal dissipation parameter for a given collection, one can divide these units into few clusters and improve the overall battery model. Second, we analytically characterize the impact of imposing a no-short-cycling requirement on TCLs as constraints on the ramping rate of the regulation signal. We support our theorems by providing simulation results.Comment: to appear in the 2014 American Control Conference - AC

Broken-Symmetry States of Dirac Fermions in Graphene with A Partially Filled High Landau Level

We report on numerical study of the Dirac fermions in partially filled N=3 Landau level (LL) in graphene. At half-filling, the equal-time density-density correlation function displays sharp peaks at nonzero wavevectors $\pm {\bf q^{*}}$. Finite-size scaling shows that the peak value grows with electron number and diverges in the thermodynamic limit, which suggests an instability toward a charge density wave. A symmetry broken stripe phase is formed at large system size limit, which is robust against purturbation from disorder scattering. Such a quantum phase is experimentally observable through transport measurements. Associated with the special wavefunctions of the Dirac LL, both stripe and bubble phases become possible candidates for the ground state of the Dirac fermions in graphene with lower filling factors in the N=3 LL.Comment: Contains are slightly changed. Journal reference and DOI are adde

D- shallow donor near a semiconductor-metal and a semiconductor-dielectric interface

The ground state energy and the extend of the wavefunction of a negatively charged donor (D-) located near a semiconductor-metal or a semiconductor-dielectric interface is obtained. We apply the effective mass approximation and use a variational two-electron wavefunction that takes into account the influence of all image charges that arise due to the presence of the interface, as well as the correlation between the two electrons bound to the donor. For a semiconductor-metal interface, the D- binding energy is enhanced for donor positions d>1.5a_B (a_B is the effective Bohr radius) due to the additional attraction of the electrons with their images. When the donor approaches the interface (i.e. d<1.5a_B) the D- binding energy drops and eventually it becomes unbound. For a semiconductor-dielectric (or a semiconductor-vacuum) interface the D- binding energy is reduced for any donor position as compared to the bulk case and the system becomes rapidly unbound when the donor approaches the interface.Comment: Submitted to Phys. Rev. B on 19 November 200

Correlation of p-doping in CVD Graphene with Substrate Surface Charges

Correlations between the level of p-doping exhibited in large area chemical vapour deposition (CVD) graphene field effect transistor structures (gFETs) and residual charges created by a variety of surface treatments to the silicon dioxide (SiO(2)) substrates prior to CVD graphene transfer are measured. Beginning with graphene on untreated thermal oxidised silicon, a minimum conductivity (σ(min)) occurring at gate voltage V(g) = 15 V (Dirac Point) is measured. It was found that more aggressive treatments (O(2) plasma and UV Ozone treatments) further increase the gate voltage of the Dirac point up to 65 V, corresponding to a significant increase of the level of p-doping displayed in the graphene. An electrowetting model describing the measured relationship between the contact angle (θ) of a water droplet applied to the treated substrate/graphene surface and an effective gate voltage from a surface charge density is proposed to describe biasing of V(g) at σ(min) and was found to fit the measurements with multiplication of a correction factor, allowing effective non-destructive approximation of substrate added charge carrier density using contact angle measurements

Partial discharge testing of defective three-phase PILC cable under rated conditions

The ability to accurately monitor the health of power distribution plant is a very attractive prospect for utility companies. This capability would provide a system that engineers could use to assess the real-time state of the network. Analysis of the data produced could allow for more informed decisions to be made in the areas of asset replacement and maintenance scheduling amongst others. It is widely accepted that partial discharge activity is linked with the electrical ageing/degradation of high voltage equipment. Work at Southampton is focused on obtaining a better understanding of the characteristics and trends of partial discharge events associated with medium voltage cables under, 'real life' conditions. An experiment has been developed that allows for service conditions to be applied to defective paper insulated lead covered cable samples. The samples under investigation were exposed to mechanical damage designed to replicate typical problems found on an active circuit. Partial discharge measurement was undertaken during the stressing process

Partial discharge analysis of defective three-phase cable

Power distribution cable networks represent a dynamic and complex challenge with regard to the issues of maintenance and providing a reliable, high quality supply of electrical power. Utilities historically used regular off-line testing to investigate the health of their assets. This method of testing is reasonably effective for this purpose but does have certain drawbacks associated with it; customer supply can be interrupted during the testing process and the cables are generally not tested under normal operating conditions. Meaning that the test data is not representative of the Partial discharge (PD) activity that is apparent under on-line conditions and the testing activity itself could trigger previously dormant PD sources. The modern approach for understanding the health of medium voltage (MV) cable distribution networks is to continuously monitor the assets whilst on-line. Analysis if the field data is then used to inform decisions regarding asset replacement and maintenance strategies. PD activity is widely recognised as a symptom linked to the degradation of the dielectric properties of high voltage plant. UK Power Networks sponsored research is being undertaken to investigate the evolution of PD activity within three-phase paper insulated lead covered (PILC) cables containing introduced defects. An experiment has been designed to stress cable lengths in a manner that is representative of the conditions met by on-line circuits [1]. A cable section containing a defect that is known to lead to the premature failure of in-service cables has been PD tested over a range of operating temperatures. The experiment utilizes three-phase energization at rated voltage as well as thermal cycling of the cable to replicate the daily load pattern experienced by circuits in the field. The extension to this work involves PD testing cable samples containing a range of defects to produce a data set consisting of PD pulses produced by varied sources. Analysis of this data should lead to a better understanding of the signals produced by the premature ageing of these types of cable

A new method for automatic Multiple Partial Discharge Classification

A new wavelet based feature parameter have been developed to represent the characteristics of PD activities, i.e. the wavelet decomposition energy of PD pulses measured from non-conventional ultra wide bandwidth PD sensors such as capacitive couplers (CC) or high frequency current transformers (HFCT). The generated feature vectors can contain different dimensions depending on the length of recorded pulses. These high dimensional feature vectors can then be processed using Principal Component Analysis (PCA) to map the data into a three dimensional space whilst the first three most significant components representing the feature vector are preserved. In the three dimensional mapped space, an automatic Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm is then applied to classify the data cluster(s) produced by the PCA. As the procedure is undertaken in a three dimensional space, the obtained clustering results can be easily assessed. The classified PD sub-data sets are then reconstructed in the time domain as phase-resolved patterns to facilitate PD source type identification. The proposed approach has been successfully applied to PD data measured from electrical machines and power cables where measurements were undertaken in different laboratories