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Condition based assessment of OLTCs has become more popular in recent years, and the importance of reliable and accurate diagnostic techniques is being realized. The work described in this paper aims to improve existing diagnostic techniques, develop new ones and combine some techniques (hybrid) in order to realize a highly reliable set of diagnostics for tap changers. Topics addressed in this paper include motor power measurement, dynamic resistance measurement and vibro-acoustic measurement, the latter designed and tested as part of this project. Preliminary results show that in particular a combination of aforementioned techniques may lead to improved condition assessment and may lead towards automated and online monitoring of tap changer operation.

Aluminium is widely used as conductor material for power cables, but corrosion taking place under specific circumstances can have impact on its reliability. Aluminium corrosion under the influence of an alternating current was studied experimentally. Submerged cable segments with inflicted damage exposing the conductors resulted in a continuous corrosion of the conductor material. Effects of temperature and applied voltage were studied by observing corrosion of an aluminium wire. These parameters strongly affect the corrosion rate. Comparison with copper showed that aluminium is far more susceptible to corrosion. The impact of corrosion on the reliability of the cables is discussed.

For the study of switching transients in Extra High Voltage (EHV) networks it is common practice to model a relatively small part of the EHV network. The downstream network is either disregarded or modeled as a parallel RC or RL circuit, based on the powerflow, directly at the secondary side of the transmission transformer. In this paper an investigation for the extent and model type of the downstream network (below 380 kV) is performed. A detailed model for an example downstream network is developed. Different simplifications and equivalent models are compared in frequency domain and in time domain, during cable energization. The impact of the lower voltage network on both the maximum overvoltage and the harmonic content during energization is assessed. Moreover, components that are of major importance are identified and proper equivalent models for the downstream network are proposed. Simulation results show that in particular the type of the 150 kV network (underground cable or overhead line) affects greatly both the maximum overvoltage and the harmonic content during energization. On the other hand, the parameters of end customer loads have only minor effect.

Traditionally, Extra High Voltage (EHV) transmission systems consist exclusively of Overhead lines (OHL) but in the recent years the utilization of High Voltage AC (HVAC) underground power cables (UGC) is increasing due to environmental, political and operational aspects. The application of EHV cables will influence the operation of the transmission system in both steady and transient state. Therefore, formulation of accurate cable simulation models is of great importance in order to understand the new phenomena introduced. This paper presents a comparison between two cable models, a detailed and a more simplified one, as well as a sensitivity analysis on several cable parameters. In this way, guidelines can be extracted concerning the parameters that have larger impact on the cable response and must be modeled in detail and the ones where simplifications can be made for achieving better computational speed. The simulations are performed using software package PSCAD/EMTDC.

Transients upon faults can be analyzed by the time domain approach adopted e.g. in available EMTP based software. When the connection becomes complex and the effect of circuit parameters must be systematically evaluated, the efficiency drops and simplifications are required to limit computation time. For a new connection (Randstad380) the complexity is related to the embedded mutually coupling cables (2 circuits, 2 cables per phase) and the use of cable joints over varying distances. This paper applies an alternative approach that analyzes transients upon faults in frequency domain. The modeling procedure is presented in detail and it is applied to the Randstad380 south-ring connection. Comparison is made with PSCAD/EMTDC modeling based on a simplified transmission line configuration, which is suitable for time-domain simulation. The results show that the presented method is able to effectively and accurately analyze fault transients.

A power cable model for partial discharge (PD)pulse propagation requires knowledge of all factors influencingthe transmission line parameters of the cable (characteristicimpedance, attenuation coefficient and propagation velocity). These frequency dependent quantities are related, amongst others, to the dielectric properties of the semiconducting layers. This paper discusses methods to estimate the characteristicimpedance, attenuation coefficient and propagation velocity using the cable geometryand the material properties of conductor, conductor shield, XLPE and semi-conducting layers. Measurementson cable samples show that the cable characteristics can be approximated withsufficient accuracy for PD signal propagation modeling, e.g. estimating the PD magnitude at their origins.

This paper presents the remaining lifetime calculation of power transformers paper insulation and consequently of power transformers. The calculations are performed based on two models, which are related to the thermal degradation of the cellulose winding paper insulation: the common IEC loading guide and a paper degradation model. The paper insulation model's prediction can be improved by involving data from furfural analysis. The remaining lifetime is extracted from the fault probability (reliability) of the paper insulation. The two models are brought together, to aid the asset manager in the decision making process. A probabilistic approach is used, which can be coupled to analysis in terms of risks, benefits, costs, and availability by the asset manager.

A series of experiments on the capacitive current interruption using an air-break disconnector (DS) with an Auxiliary Interrupter (AI) is presented. The AI contacts open with much higher speed (average llm/s) than the main contacts of the DS (0.2-0.6m/s). This paper mainly focuses on optical observation of the arc recorded by a high-speed camera. The result shows: (i) The arc brightness is synchronous with the arc current; (ii) The arc brightness increases with the higher current level to be interrupted I d ; (iii) With higher I d , the afterglow of the arc remnants decay slower.

For specific circuits the accuracy of online partial discharge (PD) detection and location (PD-OL, [1]) systems is impaired by complications in the cable circuit configuration, such as ring-main-units (RMUs) along the cable under test. For maximum sensitivity matched filtering is applied for extracting PD pulses from the recorded signal. The matched filters are based on a series of prior online system identification measurements and a standard cable model. Especially, with an RMU along the cable, the matched filters only approximate the real waveforms due to signal distortion. This results in sub-optimal detection sensitivity. This paper describes automated procedure for updating the matched filter bank based on measured (large) PD signals. The algorithm analyzes detected pulses to judge whether the current templates perform optimally. If not, a new template is created increasing the PD detection sensitivity and the location accuracy, and also improving the recognition of recurring disturbances. The method is tested on signals measured on a live cable connection.

Online partial discharge (PD) detection and location systems for medium-voltage (MV) cables have been developed over the past years and are currently introduced by utilities on an increasingly large scale. This paper addresses a few of the new challenges related to the integration of these systems in various cable networks. The effect of ring-main-units (RMUs) and substations on the propagation of PD signals is studied to determine feasible options for PD monitoring on two consecutive cables connected through an RMU or a substation. Different algorithms for PD pulse arrival time estimation are analyzed for robustness and PD origin location accuracy. An algorithm based on the signal energy proved to be most suitable for complex circuits where the PD shape is not only distorted by the propagation through the cable, but also by complex impedances in the RMUs along the cable under test and each cable end. Matched filters for PD detection can be improved using measured PD signals. This results in improved detection sensitivity and location accuracy.

Online partial discharge (PD) monitoring systems are traditionally installed at a single mediumvoltage (MV) cable connection between two ring-mainunits (RMUs). It is more efficient to monitor two or more consecutive cables using a single monitoring system. Moreover, practical experience with the PD-OL system [1], shows that for substations, with many parallel MV cables, and RMUs installing the inductive sensor may be hampered or even impossible. in this paper the influence of RMUs and substations on the propagation of PDs is studied. An RMU or substation can be modeled as a combination of complex impedances representing switchgear, transformer and MV cables. A PD pulse from a cable encounters a load impedance that does not match the cable's characteristic impedance, resulting in partial reflection and partial transmission transmission to other cables. Models for RMUs and substations are proposed and verified by measurements. Feasible options for online PD monitoring through RMUs or substations are determined. .

Capacitive current interruption with air-break disconnectors in a high-voltage network is an interactive event between circuit and arc with a variety of interruptions and re- ignitions. In order to investigate this transient phenomenon, a series of interruption tests was performed at KEMA High Power Laboratory. In this paper, a brief analysis of the interruption process is presented and is compared with experimental data from the test. Typical wave shapes of voltages across the capacitances, disconnector and current through the disconnector are given. Re-ignition voltage and energy input to the arc on re- ignition are also investigated. Comparison shows that the test data are in good agreement with simulation. It is concluded that besides higher interruption current and higher power supply level, a lower ratio between source side and load side capacitance leads to more severe interruption and longer arc duration. In the end, the actual status of IEC recommendations on testing, that has taken into account this arc-circuit interaction, will be discussed.

Accurate location of partial discharges in power cable systems, based on arrival times, is critical for the identification and assessment of defects. This paper evaluates different time-of-arrival algorithms in order to determine which method yields most accurate location under different circumstances. These methods are based on threshold, Akaikepsilas information criterion, energy criterion, Gaborpsilas signal epoch and phase in frequency domain. Several criteria are defined by which the algorithms are evaluated. These criteria include the sensitivity to noise, pulse shape and effect of load impedance. The sensitivity of the methods upon varying these quantities is evaluated analytically and by means of simulations. From the results the energy criterion method and the phase method show the best performance.

In view of the coming major asset replacement wave of power grid components, new lifetime prediction tools are developed to assist asset managers in making technically and economically sound decisions. This paper discusses the modeling technique based on the concept of quality parameters applied to paper insulated systems. The concept of quality parameters was recently introduced for power transformers to obtain a measurable quantity which relates the actual degradation process to an observable quantity. The present paper discusses a general approach for modeling other power components along the same lines. In case of a power transformer, oil samples can be taken and analyzed in order to estimate the DP (Degree of Polymerization) value, which is linked to the mechanical strength of the insulation material. This quantity then serves as quality parameter. Using a probabilistic approach based on the physical degradation mechanism a remaining lifetime model was developed. This resulted in a scheme, which is flexible in the sense that any new available data can be included to enhance the prediction reliability. The question addressed here is whether a similar approach can be applied to other power components as well. In particular we have investigated the possible use of quality parameters which are not directly correlated to one degradation mechanism. We will define different kinds of quality parameters and propose a component reliability model that incorporates all of these.

In this paper the concept of an integral transformer lifetime model is presented. The model provides the best possible prediction of future behaviour given the data available. It treats remaining life in terms of future failure probability, thereby giving better support to the decision taking process than a mere remaining life estimate. The core of the model is a generic description of ageing processes, coupled to a probabilistic approach. The approach presented utilizes various techniques to reduce the uncertainty that is inherent to modelling processes with incomplete knowledge of the operational history. One technique couples the process model to externally measurable quantities; another technique involves a sensitivity analysis, which shows what additional input data gives the most efficient way to improve accuracy. We will illustrate the approach by applying it to a well-known degradation process: thermal degradation of the transformer winding insulation.

The area of pulsed-power technology covers a broad range of voltages and pulse durations. State of the art systems have voltages up to a few hundred kilovolts at pulse lengths down to the nanosecond range. Special applications require extreme field strengths in the range of 1 GV/m. The design of a compact source meeting the above requirement aims for 1 MV, 1 ns pulses. In this paper, we propose a concept for a novel compact transmission line transformer. To verify theoretical analyses and understanding, a model of the transmission line transformer has been simulated by time domain electromagnetic field solver. An optimized transmission line transformer has been built and first measurements have been performed. The equivalent simulation model agrees with the experimental results. The effect of secondary mode suppression by means of ferrite has been studied. Both simulations and experiments showed that ferromagnetic cores around each individual transmissions line in our design can be omitted

In this work coupling of communication frequencies within the CENELEC A-band to medium voltage cables is studied. Inductive and capacitive signal coupling is considered not only on basis of technical performance, but also on basis of practical aspects as installation. The effectiveness of coupling schemes depends on the impedances of installed equipment at the cable terminals. The frequency response of a 10 kV, 400 kVA cast resin distribution transformer is investigated. In the frequency range of interest, the behaviour is best described as a capacitance of typically 1 nF. The signal transfer over a 4 km paper cable terminated by various load impedances to mimic real equipment is studied. From the results it is concluded that for inductive coupling performance can be low within the CENELEC A-band, but can become competitive if frequencies up to 1 MHz are allowed. Further, the location of signal coupling has a strong effect on the noise spectrum.

The development of an on-line PD detection system has its main challenges in the area of sensors and signal processing techniques. If localization is included by means of simultaneous signal detection at both cable ends, the problem arises of synchronizing the measuring systems at both sites accurately. One method, already successfully applied in off-line PD measurements, is the use of GPS. Besides its high costs, this system has one main disadvantage: it requires an external antenna in the line of sight of several satellites. This requirement may be a problem for on-line measurements because of the long measuring time. Another option to perform the synchronization is the injection of high-frequency pulses into one cable end and detecting them at the other end. Taking the propagation time along the cable length into account, this results in accurate synchronization of the measuring sides, without being dependent on any other system. This paper discusses several aspects of the pulse injection technique. The obtained resolution realized during field experiments with the method of pulse injection is within the resolution obtainable by means of GPS (100 ns).

For railways systems the overhead contact wire is normally electrically insulated from the portals or poles by means of insulators. The size of these insulators adds to the required distance between the contact wire and the grounded portals. In tunnels and under viaducts this distance is kept to a minimum for economical reasons. A possible option to decrease the required space is to abandon the concept of insulators, but to use electrically insulating beams as catenary supports. A major point of concern in the application of fibreglass-reinforced synthetics is the electrical behaviour with respect to aging. In this paper DC conductivity measurements are presented on a synthetic fibre glass-reinforced beam, which is in service for about twenty years. The voltage was varied up to 25 kV, and the current could be measured with sensitivity better than 1 nA during the field measurements, corresponding to over 10/sup 13/ /spl Omega/. Reference measurement on a small sample of the same material showed a conductivity of five orders of magnitude lower. The surface conductivity can be attributed to deposited material, probably carbon and metal from the catenary, in conjunction with moisture.

Partial discharge activity is measured at AC voltages close to breakdown of a 5 mm thick spacer. Patterns as a function of the phase angle of the applied voltage are observed which can be ascribed to charging processes on the insulator. Besides a stable regime of discharges, sudden changes in behaviour may also occur. These patterns often emerge as bursts (few seconds or minutes) after which they either quench or lead to a full breakdown.