D.J. Swaffield

Discrimination of multiple PD sources using wavelet decomposition and principal component analysis

Partial discharge (PD) signals generated within electrical power equipment can be used to assess the condition of the insulation. In practice, testing often results in multiple PD sources. In order to assess the impact of individual PD sources, signals must first be discriminated from one another. This paper presents a procedure for the identification of PD signals generated by multiple sources. Starting with the assumption that different PD sources will display unique signal profiles this will be manifested in the distribution of energies with respect to frequency and time. Therefore the technique presented is based on the comparison of signal energies associated with particular wavelet-decomposition levels. Principal component analysis is adopted to reduce the dimensionality of the data, whilst minimizing lost information in the data concentration step. Physical parameters are extracted from individual PD pulses and projected into 3-dimensional space to allow clustering of data from specific PD sources. The density-based spatial clustering of applications with noise (DBSCAN) clustering algorithm is chosen for its ability to discover clusters of arbitrary shape in n-dimension space. PD data from individual clusters can then be further analyzed by projecting the clustered data with respect to the original phase relationship. Results and analysis of the technique are compared for experimentally measured PD data from a range of sources commonly found in three different types of high voltage (HV) equipment; ac synchronous generators, induction motors and distribution cables. These experiments collect data using varied test arrangements including sensors with different bandwidths to demonstrate the robustness and indicate the potential for wide applicability of the technique to PD analysis for a range of insulation systems.

Zero-Phase Filtering for Lightning Impulse Evaluation: A K-factor Filter for the Revision of IEC60060-1 and -2

The next revision of the international standard for high-voltage measurement techniques, IEC 60060-1, has been planned to include a new method for evaluating the parameters associated with lightning impulse voltages. This would be a significant improvement on the loosely defined existing method which is, in part, reliant on operator judgment and would ensure that a single approach is adopted worldwide to determine peak voltage, front, and tail times, realizing standardization in measured parameters across all laboratories. Central to the proposed method is the use of a K-factor to attenuate oscillations and overshoots that can occur with practical generation of impulse voltages for testing on high-voltage equipment. It is proposed that a digital filter that matches the K-factor gain characteristic be implemented and used for this purpose. To date, causal filter designs have been implemented and assessed. This paper is concerned with the potential application of a noncausal digital filter design to emulate the K-factor. The approach has several advantages; the resulting design is only second order, it can be designed without using optimization algorithms, it is a zero-phase design and it matches the K-factor almost perfectly. Parameter estimation using waveforms from the IEC 61083-2 test data generator and experimental impulse voltages has been undertaken and obtained results show that the zero-phase filter is the ideal digital representation of the proposed K-factor. The effect of evaluating parameters by the proposed method is compared to mean-curve fitting and the challenge of effective front-time evaluation is discussed.

Partial discharge characterization of streamers in liquid nitrogen under applied AC voltages

Streamer growth has been examined with the aim to further understand the issues related to using liquid nitrogen as a part of a dielectric system within power apparatus using high temperature superconducting conductors (HTSC). An experiment has been designed to image streamer events in liquid nitrogen for applied electric fields with alternating current using a technique that allows time correlated partial discharge signal and image capture of the density-change streamers. This method employs high speed digital imaging at 5,000 and 30,000 frames per second with stroboscopic backlighting of samples. Samples are point-plane with divergent electric field geometry and consists of liquid nitrogen and a composite solid barrier preventing total field collapse during discharge. Magnitude of apparent charge and phase relationship of partial electrical discharge in liquid nitrogen have been correlated to simultaneously captured images of the resulting density-change streamer. Images of streamers at different points on the applied wave are shown to highlight that different physical mechanisms of propagation occur, phi-q-n plots of partial discharge activity are presented for a range of applied voltages, bulk liquid temperatures and pressures. Analysis of obtained results support the hypothesis that positive filamentary streamer growth is caused by ionization processes occurring in the liquid phase.

Variable pressure and temperature liquid nitrogen cryostat for optical measurements with applied electric fields

A cryostat with a high-voltage bushing, optical observation ports, variable controlled temperature and pressure has been designed to further the study of liquid nitrogen as a dielectric medium. The novelty in this design lies in the simultaneous achievement of these functions in a single design with a sufficiently large vessel to accommodate realistic geometries for high temperature superconducting cable termination prototypes. In addition, a commercial single-stage helium expander cryo-cooler is integrated into the apparatus to achieve steady state temperatures down to 63.5 K, without the need for sacrificial loss of liquid nitrogen to maintain vessel temperature. The cryostat inner vessel is certified for operating up to 2 MPa. A custom-made filled-resin bushing provides an electrical feed-through rated to 76.2 kV ac. For optical measurements with a range of sample geometries four optical ports are incorporated into the vessel utilizing sapphire windows and indium seals to form the inner pressure vessel. A technique employing a copper-vapour laser light source and high-speed digital camera for stroboscopic image capture of density change streamers and bubble dynamics with synchronized collection of electrical discharge data has been developed. This design has been used to study pre-breakdown phenomena, bubble dynamics with applied electrical fields and electrical breakdown. General construction, mode of operation and initial results are presented.

Partial discharge in medium voltage three-phase cables

Cable distribution networks are inherently complex and inaccessible systems; many of them are coming to the end of their original design life. As assets, they represent a dynamic and challenging issue with regard to the tasks of maintenance and management. Partial discharge (PD) has long been recognized as both a cause and symptom of the degradation of dielectric materials that protect high voltage plant. Utilities use the analysis of PD activity to make pre-fault decisions in areas such as maintenance, supply continuity and asset management. On-line PD monitoring systems are still in their relative infancy. An EDF Energy Networks funded research is investigating and identifying trends in PD activity associated with specific faults that commonly occur in distribution networks. In this paper an experiment to mimic the conditions experienced by on-line cable sections in the field is described. PD measurement has been obtained using conventional techniques covered in IEC 60270 in parallel with a commercially available substation monitor that is employed in distribution networks worldwide. Later work will involve using this experiment to PD test cable samples that contain a range of defects. It is hoped that each defect mechanism will produce an unique trend in PD activity as it degrades towards failure.

Characterisation of partial discharge behaviour in liquid nitrogen

Interest in superconductors for use in the transmission and distribution of power has grown since the advent of high temperature superconductors (HTS). The benefits of reduced ohmic losses leading to greater operational efficiency appear attractive to power transmission and distribution companies. Potentially this technology will increase power transfer for the same or lower voltage and for the same or smaller physical dimensions. For the successful employment of this technology in the field, power utilities must be convinced of the operational reliability of designs. Many of these designs make use of liquid nitrogen as the system coolant and also as a dielectric fluid. It is therefore vital that liquid nitrogen be well characterised as a dielectric medium. This work describes an experiment to characterise partial discharge behaviour in a nonuniform field geometry. A technique is described which allows synchronised capture and measurement of apparent charge and images of density change streamer activity. A /spl phi/qn analysis of AC results has been completed and characterisation of partial discharge behaviour is presented and discussed.

Thermal bubble motion in liquid nitrogen under nonuniform electric fields

This paper describes an experimental study into the influence of a non-uniform electric field on bubble motion and behavior in liquid nitrogen. The electric field effect on bubble motion as it rises due to buoyancy within applied dc electric fields is quantitatively investigated using a rod-plane gap. Thermal bubble motion and bubble collision with the plane electrode processes were observed in these experiments. The experimental results show that dc non-uniform electric fields have an obvious effect on bubble behavior; bubbles move closer to the plane electrode away from a higher field region to a lower one, irrespective of electric field direction. A model based on analysis of the forces acting on the bubble has been developed. This set of differential equations describes the motion of a spherical bubble in the rod-plane gap with voltage and can be solved numerically to determine bubble trajectory. Compared to experimental data, the theoretical prediction is in very good agreement.

Application of Finite Element Analysis to externally forced water cooled cable circuit ratings

In order to increase the available current carrying capacity of high voltage cables, utilities may employ forced water cooling within power transmission networks to remove the heat from cable groups. For example the transmission network of England and Wales includes externally forced cooled circuits, i.e. cooled by pumping water through pipes buried in the vicinity of high voltage transmission cables. At the present time a simple and expedient computer program is used to rate these circuits. This model is similar to that of Electra 66. To attribute confidence limits to cable ratings attained using this method an extended 2-D heat-transfer FEA model has been constructed to allow the calculation of the cable core temperature for a typical water-cooled three-phase circuit of three single-core cables buried in flat horizontal arrangement within a backfilled trough. A sensitivity analysis of the model to changes in ac resistance, burial depth, dielectric loss, soil thermal resistivity and surface boundary condition has been performed and is presented.

Rating Independent Cable Circuits in Forced-Ventilated Cable Tunnels

Over the last decade, there has been a notable rise in the number of forced ventilated cable tunnel schemes in the U.K., with new construction at transmission and distribution levels. The ability to accurately calculate continuous and emergency circuit ratings for these installations is vital in ensuring that their full operational benefit can be realized. While the Electra 143 calculation method in present use is fast and easy to use, it relies on several simplifying assumptions which make it unsuitable for application to tunnels with multiple independent cable circuits. This paper details a series of modifications to the present method which allow the direct calculation of ratings for tunnels containing multiple independent cable circuits. Significant benefits can be obtained from using this approach to calculate emergency ratings in these circumstances, as demonstrated by the example calculations provided. Implementation of an axially varying ac resistance also improves the accuracy of loss calculations. A number of key tunnel design considerations are illustrated through the results of the example calculations.

Cryogenic dielectrics and hts power apparatus: research at the university of southampton

This article presented two areas of research poignant to the progression of HTS power apparatus design: electromagnetic design and dielectric phenomena. To be accepted into service, HTS power apparatus must be economically competitive and technically viable, which includes the need for reliability. To maximize the potential savings of HTS power equipment, the view taken at Southampton has been to design apparatus to operate at LN2 temperatures, thus greatly reducing the refrigeration load, cost, and complexity. Several demonstrator projects have been undertaken, including the design, manufacture, and testing of a 10 kVA transformer demonstrating the performance of HTS tapes and the satisfactory use of electromagnetic models to calculate losses, a key design parameter. Strategies for controlling partial discharge and resulting damage to insulation include increasing the operating pressure or reducing temperature. This decision has an impact on refrigeration load and operational cost. Therefore, it will depend on the application. Future research effort is focused on solids for use at cryogenic temperatures, and the interaction of electric field and bubble dynamics affording a greater understanding or the performance of cryogenic dielectrics.