Rogier Jongen

Application of statistical methods for making maintenance decisions within power utilities

In this article, the theoretical considerations that have to be taken into account when using statistical failure analyses of service-aged components are discussed. Furthermore, two case studies in which statistical failure analysis is used to predict future failures and to see in what way strategies can influence this prediction are included. Then, conclusions are drawn based on the analysis of the two case studies

Statistical approach in power cables diagnostic data analysis

This paper describes the application of statistical analysis to the on-site diagnostic data of HV power cables to set-up knowledge rules and measuring criteria for particular cable insulations and accessory (joints, terminations). Such approaches can successfully be employed later in Asset Management decision processes. In this paper based on different cable data populations of two diagnostic parameters like: partial discharges and tan delta, statistical analysis has been applied. Besides this, several practical examples also have been presented.

Identification of partial discharge defects in transformer oil

Partial discharges in (power) transformers are often a predecessor of a serious fault. For this reason partial discharge measurements are an important diagnostic tool to monitor the insulation condition of a transformer. For on-line measurements, a detection method based on the high-frequency (UHF) signals emitted by the discharging source is used and can be detected. Moreover, the UHF technique offers the possibility to locate and identify the PD source. To investigate the ability to accurately identify and discriminate between different sources, a research with modeled defects has been recently started. Up to now, three different partial discharge sources using three different defect models were tested in transformer oil: HV corona, floating part and surface discharge. Simultaneous measurements on these defects have been performed using 1) an detection circuit based on the IEC 60270 recommendations and 2) wide-band UHF system. With the IEC 60270 based measuring system the PD magnitude and phase resolved PD pattern is acquired. With the UHF system measurements are performed in frequency domain and in time domain. By using the spectrum analyzer as a band pass filter, a phase resolved PD pattern can be obtained as well. The results of the two measurement systems are compared with each other and conclusions are drawn.

Application of statistical analysis in the asset management decision process

One of the tools to answer some typical asset management (AM) questions is the use of statistical analysis of life time data of high voltage components. When life time data of components is available, statistical analysis can be performed. The analysis can give information about the failure rate of components, expected failures in the future, conditional remaining life estimation and influence of maintenance- and/or replacement strategies. In this contribution the general approach of the analysis is shown together with some examples of typical AM questions for typical high voltage components. Statistical analysis is used to find answers to these questions.

On-site testing and diagnosis of transmission power cables up to 230 kV using damped AC voltages

This article presents an overview of on-site testing and diagnosis of new and service-aged transmission HV power cables up to 230 kV using damped AC voltages.

Failure analysis of in service failed resin cable joints by means of a statistical approach

Statistical failure analysis can be a powerful tool for asset replacement strategies. In parts of the Netherlands, a specific type of resin joint for 10 kV paper-oil insulated cables was applied substantially in the 1970s. These joints have now a considerable contribution in outage time because of breakdown of the resin insulation. The reported failures over the last six years, together with the population still in service are used as input for statistical analysis with Weibull and Normal distributions. It can be concluded that the joints are in their wear-out life period and that ageing is the main reason for failures. The total number of expected failures, obtained from the analysis, shows an increasing trend in coming years. A testing program, started a few years ago, influences the failure behavior. An active replacement strategy can also influence the number of expected failures. A comparison is made between the influence of testing and replacement strategy.

Statistical analysis of in service failed epoxy resin bushings in a 50 kV switchgear assembly

In the 1960s up to the 1980s a very compact oil-filled switchgear assembly was installed in the 50 kV networks of different utilities in the Netherlands. Per bay the switchgear consists of different oil-filled compartments where the high voltage parts are supported and connected from one compartment to the other by epoxy resin bushings. The population of this type of bushing is relative large. The number of failures however is low, and most of them occurred in the early nineties. This initiated a concern about the possibility of an oncoming re-investment wave due to replacement of this type of circuit breaker. In this contribution a statistical analysis of failure data was performed for failure prediction in the future. From the statistical analysis it can be concluded that the bushings are in the wear-out life period of the bathtub curve. However, the relative low value of the shape parameter indicates that the ageing process is in its early stage. If the failure rate function obtained from the analysis is compared with the population in service, than the total population can be stated as young. With the analysis an estimate is made of the expected number of failures in coming years, which can help the asset manager to determine if action has to be taken to meet the business policy

New developments in on-site testing of long lengths of (E)HV power cable

This contribution presents new developments in the field of on-site (E)HV cable testing, in particular testing of long lengths of power cable. A modern solution is presented for on-site testing and diagnosis of transmission power cables by means of sinusoidal damped AC voltages. With this testing technique on-site voltage testing is performed with additional non-destructive measuring techniques e.g. partial discharges or dissipation factor measurements. To improve the PD detection and particularly the PD location possibilities on long cable lengths an additional range extension measurement system has been developed for PD detection on longer cable lengths. This distributed measurement system can be installed at cable accessories to detect PD signals.

On-site testing and diagnosis of long medium voltage cables

Many parameters can influence the quality of the PD measurement and various issues are important during the execution of a PD measurement. In particular when testing long lengths of MV cables the PD detection sensitivity and particularly the PD site location is affected. To increase the measurement sensitivity the measurement system can be extended with an additional PD measuring unit at the far end of the cable. This unit measures PD activity with the conventional standardized PD detection synchronized with the damped AC test voltage. This two sided measurement provides higher detection sensitivity in the case of long cable lengths. Furthermore, when the measurement data of the 2 units at both ends of the cable are combined with each other, further analyses provides the PD site location in case of measured PD activity. In this way it is possible to test and diagnose longer lengths of MV cables.

Influence of ambient temperature on the failure behavior of cable joints

The failure behavior of resin cable joints in the medium voltage underground network in the Netherlands shows a correlation with the ambient temperature of the air above the ground. This behavior is especially seen when the failures occurred during the summer months are considered. In the last years the months July or August were very hot and the number of failures increased significantly in these months. The ambient temperature has an influence on the soil temperature through the year even 0.5 -1 meter below surface level where most cables are installed. It can also be seen that the failures follow the rise and fall of the average air temperature during the different days of the month with increasing and decreasing number of failures with some time delay. The change of the soil temperature around the joint results in a higher temperature within the joint, thus contributing to the failure of a joint, which is already in its ageing stage. The presence of a hotspot in combination with a rise of the soil temperature can result into breakdown. In this paper the failure behavior of resin cable joints and the possible effect of ambient temperature are discussed.