Ben Quak

On-site testing and PD diagnosis of high voltage power cables

In addition to after-laying of new-installed high voltage (HV) power cables the use of on-site non-destructive on-site testing and diagnosis of service aged power cables is becoming an important issue to determine the actual condition of the cable systems and to determine the future performances. In this paper based on field experience an overview is presented on on-site testing and partial discharge diagnosis of HV power cables with regard to on-site testing methods: energizing, diagnostic aspects, possibilities and implications for new and service aged power cables.

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.

Estimation of dielectric loss using damped AC voltages

In this article some fundamental aspects of dielectric loss estimation using DAC voltage are described. Damped ac voltage testing is an alternative method of continuous ac voltage testing. It provides a method of assessing the condition of the insulation of capacitive test objects, e.g., power cables or stator windings of generators. The decay of damped ac voltages used for on-site energizing of power cables, and testing generator stator insulation, can be analyzed to estimate the dielectric loss (tan δ) of the insulation.

Diagnosis of Onload Tap Changer Contact Degradation by Dynamic Resistance Measurements

Due to different degradation mechanisms, onload tap changers (OLTCs) may contribute to power transformer failures. If offline diagnosis is undertaken during functional checks performed during the service life of OLTC devices, some of the degradation effects can be assessed. This paper addresses an OLTC diagnostic method based on the measurement of dynamic resistance. An experimental investigation of the degraded OLTC contacts was used to identify a number of defects revealed by dynamic resistance measurements.

Advanced on-site diagnosis of power transformers

This paper discusses the recently developed analysis method for service aged power transformers. This is a complete approach to assess the condition of a power transformer, both the on-load tap changer as well as the transformer windings. This paper also presents several case studies of tap changer defects that are discovered with the dynamic resistance measurement function.

Fundamental aspects of information processing and the decision process to support asset management

Nowadays asset management is regarded as the answer to the changing environment in which power utilities operate. Successful asset management heavily relies on the use of information and data to facilitate the asset management decision process. Decisions should be based on information from various sources, from within the organisation of the utility as well as from its environment, and involve a large amount of different information aspects. This paper describes the fundamental aspects of the asset management decision process, and the information sources necessary for this decision process. This information sources can be categorised into technical, economical and societal aspects. The decision process itself can be separated into three layers regarding component, network and corporate decisions.

On-load tap changer diagnosis: Interpretation of dynamic resistance deviations

This paper discusses the recently developed analysis method for service aged power transformers equipped with an OLTC: dynamic resistance measurements (DRM). Because the on-load tap changer contributes to many power transformer failures the emphasis is on OLTC diagnosis. Off-line diagnosis of on-load tap changers during regular maintenance can detect maintenance errors and assess the condition of OLTC parts not accessible for inspection. DRM can be valuable to on-load tap changer (OLTC) condition assessment because of the large variety of defects and OLTC degradation mechanisms that can be found. This paper deals with the results that can be measured with DRM. Besides undisturbed DRM-data, most common deviations originating from degraded and defect tap changers will be discussed. It is explained how these different types of defects appear in the DRM results. The location and shape of the deviation can be used to find the cause and importance of the defect. A subdivision is made between OLTCs in good condition, OLTCs with increased contact resistance, OLTCs with open contacts and OLTCs with deviating switch times. By doing so, an overview of possible DRM results is gained. The seriousness of the defects is also discussed in this paper. Differences between the DRM results of different types of OLTCs will be discussed by showing results of DRM tests. Most graphs presented in this paper are recorded on service-aged OLTCs in the Dutch utility grid. Some measurement results are performed during experiments in a high voltage laboratory.

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.

Effect of test parameters on dynamic resistance measurement results from on-load tap changers

Static resistance measurements on power transformers give an indication about increased contact resistances of the OLTC. Increased contact resistances can also be measured by means of a dynamic resistance measurement (DRM). This paper elaborates on condition diagnosis of electrical contacts of in-tank high-speed resistor tap changers using DRM. DRM uses a low test current through the on-load tap changer to detect increased contact resistances and current interruptions. The difference with static resistance measurements is that the tap changer is switched through all its tap positions during the test. The test current gets no change to fully stabilize due to the inductance of the transformer windings. DRM is therefore less accurate then static resistance measurements but contains more information about the type and location of tap changer defect. This contribution will show that more defects on OLTCs can be found with DRM compared to static resistance tests: the additional value of DRM compared to static winding resistance measurements is discussed. Secondly, important topics that should be considered when performing dynamic resistance measurements are discussed, like the effect of the test current amplitude, the secondary short-circuit and circuit resistance. Simulation results and experimental data are presented to make the reader familiar with the pros and cons of dynamic resistance measurements.