Wah Hoon Siew

Surface charging and flashover on insulators in vacuum

Investigations have been carried out on right cylindrical teflon spacers in vacuum under DC stress to study the effect of cathode geometry on surface charging and flashover. Three cathode support arrangements were used. Type I involved a recess in the cathode into which the spacer was located. Type II was a simple flat plate cathode while type III had a raised insert on the cathode surface over which the spacer was located. In all cases the surface charge density and distribution is primarily dependent upon the magnitude of the electric area. Using the type I arrangement inception for charge deposition was always about 10 kV even when the spacer had been subjected to repeated flashovers and at applied voltages greater than this the density of the charge, which was fairly uniformly-distributed around the surface, was more or less proportional to the applied voltage. For the type II and III arrangements, a distinct, uniformly-distributed negative charge of up to 20 (mu) C/m2 was always detected at low values of applied stress and at a fairly well defined transition voltage this gave way to a distribution which was substantially uniform in the case of type II but quite filamentary in type III and both involved the deposition of positive charge.

Automatic fault location for underground low voltage distribution networks

This paper describes an automatic fault location technique for permanent faults in underground LV distribution networks (ULVDNs). It uses signals from an existing time domain reflectometry (TDR) instrument. It pre-processes TDR signals to eliminate reflections due to single-phase tee-offs, and to locate 3-phase open or short circuit faults and also uses adaptive filtering to compare the TDR signals to locate faults. In essence, the procedure minimizes the interpretation skill required from a user of a typical TDR based fault location instrument. The relative performance of the system is demonstrated using real field data.

A noise reduction technique for on-line detection and location of partial discharges in high voltage cable networks

Partial discharge (PD) measurements taken on-site are frequently affected by noise due to external disturbances. This paper describes a noise reduction method that may be used to isolate the PDs from the noise. The basic principles of wavelet de-noising analysis are presented. The PDs are then processed to allow accurate location of the PD. This method of location will also be presented.

Measurement of electromagnetic emissions from FACTS equipment operational within substations - part I

Within substations installed with flexible ac transmission systems (FACTS) equipment [such as static var compensator (SVC) or static synchronous compensators (STATCOMs)], transient electromagnetic emissions are generated during the switching of thyristor valves. These emissions may cause malfunction or damage to electronic equipment positioned nearby if appropriate immunity measures are not taken. An electromagnetic-compatibility measurement system was devised, and four SVC substations and one STATCOM substation were selected for electromagnetic-interference (EMI) measurement, analysis, and evaluation. The aim of the research project was to characterize both the conducted and radiated emissions produced by FACTS equipment. The probable impact of emissions from FACTS equipment is discussed. The measurement system used is described together with the measurement methodology. A simple circuit analysis was also carried out on the causes of FACTS-based EMI to provide some insight into the interference mechanism.

Application of K-Means method to pattern recognition in on-line cable partial discharge monitoring

On-line Partial Discharge (PD) monitoring is being increasingly adopted in an effort to improve asset management of the vast network of MV and HV power cables. This paper presents a novel method for autonomous recognition of PD patterns recorded under conditions in which a phase-reference voltage waveform from the HV conductors is not available, as is often the case in on-line PD based insulation condition monitoring. The paper begins with an analysis of two significant challenges for automatic PD pattern recognition. A methodology is then proposed for applying the K-Means method to the task of recognizing PD patterns without phase reference information. Results are presented to show that the proposed methodology is capable of recognising patterns of PD activity in on-line monitoring applications for both single-phase and three-phase cables and is also effective technique for rejecting interference signals.

Partial discharge location in power cables using a double ended method based on time triggering with GPS

Partial discharge (PD) diagnostics is the most widely used tool to assess the insulation condition of insulated power cables which facilitates informed maintenance planning leading to extended service life of ageing assets. Time domain reflectometry (TDR) using a single ended or double ended approach is the most widely used method for locating PD sources. The success of the single ended method is dependent upon cable network design. However, by monitoring PDs at both ends of the cable, i.e. double-ended PD monitoring, higher accuracy of PD location can be achieved with a higher success rate. The double ended method is not widely used due to its complex system design, time synchronization and communication requirement between measurement units. This paper proposes a double ended PD location system which triggers on the predefined one pulse per second (1PPS) signal obtained from a global positioning systems (GPS) using novel time based triggering logic (TBTL) implemented in field programmable gate arrays (FPGA). This system ignores non-useable (not synchronized) data caused by flywheel 1PPS from GPS receiver due to any short-term loss of satellite signals which eventually reduces the PD location accuracy. Furthermore TBTL also ignores spurious triggering pulses radiated from noise sources within the substation. With the use of a communication link between two ends of the cable provided via mobile broadband together with TBTL, eliminated the acquisition of non-useable(not synchronized) data. Based on laboratory tests and on-site measurements PD location accuracy of less than ± 10 m can be achieved. The system design, laboratory tests and on-site measurements are discussed.

Characterizing the Sensitivity of UHF Partial Discharge Sensors Using FDTD Modeling

Ultra high frequency (UHF) partial discharge sensors are widely used for condition monitoring and defect location in the insulation systems of high voltage equipment. Designing sensors for specific applications often requires an iterative process of manufacture, test, and mechanical modifications. This paper demonstrates the use of finite-difference time-domain (FDTD) simulation as a tool to predict the frequency response of a UHF sensor design. Using this approach, the design process can be simplified and parametric studies can be conducted in order to assess the influence of component dimensions and material properties on the sensor response. The modeling approach is validated using a broadband UHF sensor calibration system, which uses the step response of the sensor to determine its frequency-domain transfer function. The use of a transient excitation source is particularly suitable for modeling using FDTD, which is able to simulate the step response output voltage of the sensor, from which the frequency response is obtained using the same post-processing applied to the physical measurement. Comparisons between simulation and measurement are made for three different sensors, demonstrating sensitivity agreement to within about 10%. Some examples of simple parametric studies carried out using the FDTD model are also presented.

Computation of Transient Electromagnetic Fields Due to Switching in High-Voltage Substations

Switching operations of circuit breakers and disconnect switches radiate transient electromagnetic fields within high-voltage substations. The generated fields may interfere and disrupt normal operations of electronic equipment. Hence, the electromagnetic compatibility (EMC) of this electronic equipment has to be considered as early as the design stage of substation planning and operation. Also, microelectronics are being introduced into the substation environment and are located close to the switching devices in the switchyards more than ever before, often referred to as distributed electronics. Hence, there is the need to re-evaluate the substation environment for EMC assessment, accounting for these issues. This paper deals with the computation of transient electromagnetic fields due to switching within a typical high-voltage air-insulated substation (AIS) using the finite-difference time-domain (FDTD) method.

Fault location and diagnosis in a medium voltage EPR power cable

This paper presents a case study on fault location, characterization and diagnosis in a length of shielded 11 kV medium voltage ethylene-propylene rubber (EPR) power cable. The defect was identified on-site as a low resistance fault occurring between the sheath and the core. A 43 m section was removed for further analysis. The fault resistance was characterized and the location of the defect pinpointed to within a few cm using a combination of time-difference-of-arrival location and infra-red imaging. A combination of X-ray computed tomography, scanning electron microscopy and energy dispersive X-ray spectroscopy were then applied to characterize any abnormalities in the dielectric surrounding the breakdown region. A significant number of high density contaminants were found to be embedded in the dielectric layer, having an average diameter of the order of 100 μm, a maximum diameter of 310 μm and an average density of 1 particle per 2.28 mm3. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to determine the geometry and elemental composition of some initial contaminant samples. It was concluded that contamination of the EPR layer, combined with an observed eccentricity of the cable¿s core and sheath resulting in a reduced insulation gap, may have led to an electric field concentration in the region of the defect sufficient to initiate breakdown. Preventative strategies are discussed for similar families of cables, including more stringent dielectric testing requirements at the manufacturing stage and PD monitoring to detect incipient failure.

The use of power frequency current transformers as partial discharge sensors for underground cables

Partial discharge (PD) diagnostics can be used to diagnose insulation defects in medium voltage cables and thereby inform required maintenance planning to extend the lifespan of ageing assets. High frequency current transformers (HFCT) installed on the earth strap at the cable termination are widely used to detect the fast varying pulses resulting from PD. Due to constructional reasons earth strap accessibility in underground cables is often limited. Furthermore in three core cables, phase angle dependency of pulses resulting from PD sometimes fails to induce pulses in the earth screen. This paper investigates an alternative method of PD detection using conventional power frequency current transformers (PFCT) principally used for protection and measurement purposes. This investigation assesses the feasibility of this approach by conducting a magnetic study of the transformer core, frequency response analysis (FRA), and finally capturing and analyzing onsite PD measurements using conventional PFCTs and the developed PD data acquisition/interpretation system. Based on the laboratory and on-site measurement results, PFCTs can be considered for detecting PD. This approach does not require retrofitting additional PD sensors and can also be applied to older switchgear design without any outage. Hence this approach can be considered as a low cost and noninvasive method of PD detection in cables.