Faisal Peer Mohamed

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.

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.

A novel approach in the measurement of high frequency disturbances from very large electrical systems

This paper demonstrates the feasibility of an interactive distributed measurement system having the capability of wireless triggering and time synchronisation for the purpose of detection, measurement and location of disturbances from very large electrical systems. System design, laboratory testing and preliminary site trials are discussed. Examples of how the system might be used are included. The first example relates to railway track circuit monitoring; the 2nd example relates to multi ended PD monitoring for insulated power cables; third example relates to radiometric partial discharge monitoring in substations and the last example relates to power quality monitoring and phasor measurements.

Synchronised power quality monitoring system using global positioning system (GPS)

Restructuring in network industries, high penetration of market players and increased demand, all of these have made it a necessary requirement to provide highly reliable mains supply with controlled quality parameters such as harmonic distortion and voltage limits. To achieve this power systems owner needs to put restrictions on the connected loads which make the power quality or power angle unacceptable. This can be achieved using synchronised monitoring across various substations in the power systems. Proposed in this paper is a potential power quality monitoring system which can be viewed as a combination of power quality and phasor measurement. Synchronization is done using global positioning systems and the events are time stamped using global time reference.

MV cable lifetime improvement analysis through transformer tap changes

Cable life depends mainly on the thermal stress, which relates to the current applied on the cable. Voltage changes in medium voltage (MV) cables due to transformer tap changes will also change the current flowing through the cable, which will change the cable temperature. In order to extend the cable life, this paper aims to simulate and analyse the potential thermal lifetime improvement of cables through long-term tap changes within the statutory levels. Firstly, the IEC standard (60287) method for rating and modelling cables is applied to evaluate the cable temperature under different voltages and relative currents. Different cable configurations will also be considered in simulations as temperature is dependent on the cable dimensions. Then, typical thermal lifetime analytical expressions will be used to evaluate the long-term influence of voltage changes. Lastly, the obtained thermal lifetime assessments under different transformer tap changes and different cable configurations will provide a potential understanding of cable lifetime changes through implementation of permitted regulatory voltage changes.

Effect of voltage reduction in minimising partial discharge activity in cables — Experimental study

A common cause of insulation degradation in high voltage (HV) cables is due to partial discharges (PD). PD may be initiated through imperfections and contaminants present in the insulation. It can be hypothesized that a reduction in system voltage can potentially reduce PD, which will correspondingly extend the service life of the cable. Currently, industry voltage statutory requirements permit ±6% tolerance setting on nominal voltage on distribution networks. Such ±6% voltage reduction on may have little or adverse effect on PD magnitude which depends on the nature of defect. Hence the accuracy and type of measuring PD while reducing system voltage is more critical for this application. Power cable as a low pass filter which attenuates and disperse the PD pulses. In this paper effect of voltage reduction on PD is investigated using standard electrode geometries such as point-plane, point-point and point- rod using IEC60270 measurement system together with frequency domain measurements. It has been observed that PD frequency characteristics at different voltages varies in the wideband frequency spectrum.

Online partial discharge detection in HV cables using conventional current transformers

This paper investigates the performance of conventional current transformer for online partial discharge detection in high voltage cables. It is based on the frequency response of such current transformers. The frequency behaviour was investigated using a simulation package (Powersim) by varying parameters including core permeability, stray capacitance, core material etc. Experimental investigation was also conducted on a 33 KV 200/5A current transformer. Simulation and experimental results are presented and compared in this paper.