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Contactless capacitive sensors are applied to monitor(over)voltages near the overhead line terminations of asubstation or at cable to line transitions. The sensor response isthe signal time derivative when loaded with a resistiveimpedance and the waveform is restored by integration. As partof this differentiating/integrating (D/I) measuring concept, theuse of open-air sensors results in excellent EMC characteristicsbut the inherent cross-coupling to other phases has to be dealtwith. Three applications are presented: 1) For a 150 kV cabletermination the partial discharge activity needs to be related tomomentary phase voltages; 2) Measured slow front overvoltageat a 380 kV cable termination from line energizationare compared with predictions from numerical simulation; 3)The perspectives of employing the D/I method for (very) fastfront overvoltages near a 380 kV circuit breaker are examined.

The low-voltage distribution grid is an essential link for the integration of distributed renewable sources. It is expected to experience stronger loading and load variation in the near future. At the same time, part of the low-voltage network is an aging infrastructure. Distribution system operators are therefore becoming increasingly interested in being able to diagnose their low-voltage assets. This paper describes corrosion phenomena that are observed in aluminum conductors after a damage is inflicted to a cable section. Such a damage is suspected to initiate long-Term degradation. Short-duration intermittent current peaks are observed in laboratory experiments and similar peaks are now also recorded in the field. The perspectives for online monitoring, by registering these events, are investigated by having a large number of measurement devices installed on connections in service. The research is directed to gain a better understanding of the relation between the current peaks and cable circuit failures as well as the underlying degradation mechanisms.

Temperature variation from dynamic cable loading affects the propagation characteristics of transient signals. The distortion of modal signal components as a function of temperature in a three-phase medium-voltage cable is investigated. The temperature influence arises mainly through the complex insulation permittivity, which has a non-linear relationship with temperature. Near the maximum operating temperature of the cross-linked polyethylene insulation, the propagation velocity increases by 0.56% per degree centigrade but is an order of magnitude less sensitive at ambient temperature. The paper presents modeling results based on cable impedance and admittance matrices obtained from electromagnetic field simulation, taking into account the time-varying temperature distribution in the cable cross-section. The results are verified by applying Rayleigh&ndash, Schr&ouml, dinger perturbation analysis. In the time domain, signal patterns shift when the modal propagation velocities change upon cable loading. Moreover, separation of degenerate modes is observed when the cable phase conductors carry an unbalanced current. The perspectives for exploiting the temperature dependency of signal propagation for pinpointing cable defects and for dynamic rating of underground power cables are discussed.

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.

Transient voltages from (dis)connecting compensation coils near high voltage transformers are studied. Non-invasive sensors are needed in order to exclude any influence by the measurement equipment on the overvoltage magnitude. This study describes results from open-air capacitive sensors as part of a differentiating/integrating measurement system. Its bandwidth (up to 10 MHz) allows to record large detail in the switching events. However, the coupling strengths and the cross-couplings inherent to the use of air capacitive sensors need to be determined. Calibration based on symmetric phase voltages during normal operation supplies part of the information. Three methods are compared to supplement this knowledge for establishing the complete coupling matrix between phase conductors and sensors. One method employs estimates from the system configuration, in particular, its symmetry. The other techniques extract information from fast transient events within the responses for both switching on and off. The decoupled waveforms from different methodologies align within their confidence bounds and predict the maximum overvoltage magnitudes within an accuracy of 5% when employing information from the fast transients. Based on measurements performed on two compensation coils, both tested three times, larger overvoltages occurred during energisation compared to de-energisation with maximum amplitude exceeding three per unit.

The propagation modes of travelling wave in a threecore power cable (XLPE or PILC) with common earth screen can be uncoupled into three modes: one shield to phase (SP) mode and two phase to phase (PP) modes. A generic model approach is applied in this paper to estimate the transmission line parameters of three-core cables with common earth screen, which includes the effect of different layers on series impedance and admittance. The characteristic impedance, the propagation velocity and the attenuation for SP and PP modes of two cable types, PILC and XLPE are modeled and compared with test results. Calculated parameters match within 5% for the characteristic impedances of different modes and 5% for the velocity for the PILC cable. Larger deviations occurred for XLPE cable, due to difficulties in appropriate modeling the semiconducting insulation screens around each conductor. The attenuation for both PILC and XLPE is hard to predict because data on complex permittivity, being the dominant factor, is hardly known.

Design of cables for operation in a low pressure environment is challenging, when applications also demand lightweight and flexibility, and material choice is restricted. The operating pressures in combination with relevant distances, can be situated near gas breakdown and cause partial discharges, making long-term damage conceivable. An experimental setup is developed, based on partial discharge detection in a vacuum system, to characterize the partial discharge inception voltage of three coaxial cable designs in an argon and nitrogen environment at controllable pressure. In order to cope with outgassing behavior, a lumped element model is presented to simulate the internal pressure distribution along the cable as a function of settling time. Microscopic cross sections of the cables are analysed with electrostatic voltage simulations. Using scaled Paschen curves, expected partial discharge inception is determined and compared with measurements. Quantifying cable performance, in terms of minimum PD inception voltage and pressure, in relation to its design is feasible, but also deviations occur which are discussed.

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) power cables is increasing due to environmental, political and operational aspects. To study the impact of HVAC power cables on temporary overvoltages and slow front transients, a simulation model of the large-scale network is required. Guidelines concerning the modeling detail for the surrounding network need to be established to ensure sufficient accurate results. Existing guidelines for purely OHL transmission networks may not be adequate since the integration of HVAC power cables shift the harmonic impedance of the network to lower frequencies and that can affect the extent of the needed simulation model. Much effort can be saved from obtaining detailed data for an extensive but proper model, but a less detailed model might not describe the network behavior with sufficient accuracy. In this study, simplification options are considered, for which the time to construct and analyze a simulation model can be reduced. The simplification options are compared with the reference model (full detail), and judged based on three indicators for the level of accuracy: frequency and amplitude of the main resonance, and maximum over-voltage. As a result, guidelines on the extent of the simulated network and in which detail the model has to be built are proposed. The inverse approach, i.e. increasing level of detail until simulation results hardly change leads to similar results, but requires equivalent short-circuit networks for the rest of the grid. The method is tested for the Dutch 380 kV grid where an underground cable connection of 10.8 km length is introduced in 2013 and a second connection is planned for 2017.

The increasing utilization of HVAC underground power cables gives rise to technical challenges related to the occurrence of electromagnetic transients. This paper discusses measurements with sensors based on open air capacitances sensing the electric fields from the line having a wide frequency bandwidth and excellent electromagnetic compatibility performance. However, since the sensors are open air capacitors, they also intercept field lines from other phases and measurement decoupling is necessary. Results from measurements, using the differentiating/Integrating technique, at a 380 kV mixed overhead line underground power cable connection obtained at a transition point and at a substation are presented and compared with PSCAD/EMTDC simulations. The measurement system gives accurate results for low and high frequency transients, while the effect of uncertainties in the decoupling is minor. For switching transients, the simulation model was validated in terms of measured and simulated voltage waveforms resulting in similar maximum overvoltage and frequency content.

Contactless capacitive (open-air) sensors are applied to monitor overvoltages near overhead line terminations at a substation or at the transition from underground cables to overhead lines. It is shown that these sensors, applied in a differentiating/integrating measuring concept, can result in excellent characteristics in terms of electromagnetic compatibility. The inherent cross-coupling from open-air sensors to other phases is dealt with. The paper describes a method to calibrate the sensor to line coupling matrix based on assumed 50 Hz symmetric phase voltages and in particular focuses on uncertainty analysis of assumptions made. Network simulation shows that predicted maximum overvoltages agree within typically 7% compared to reconstructed values from measurement, also with significant cross-coupling. Transient voltages from energization of an (extra-)high voltage connection can cause large and steep rising ground currents near the line terminations. Comparison with results obtained by a capacitive divider confirms the intrinsic capability in interference rejection by the differentiating/integrating measurement methodology.

Application of open-air sensors is an attractive option for detecting switching over-voltages as their installation does not interfere with operation of the lines. Drawback is that coupling strengths are not known beforehand and cross-coupling between a sensor near one phase and other phase conductors or even to other circuits affects the recordings. Calibration of the coupling matrix between phase voltages and sensor signals can be realized by relating the power frequency component in the recorded signals to a priori assumed base of symmetric phase voltages. This paper establishes a method to quantify the different causes for measurement uncertainties. The coupling matrix cannot be completely reconstructed from measurements, enforcing model assumptions which introduce uncertainties. Uncertainties arising from not perfect symmetric phase voltages and measurement equipment are investigated as well, but play a minor role when judging over-voltages on a per unit base. The methodology is presented for measurements at an overhead line to underground cable transition point and to a substation. At transition points, the sensors could be positioned such that the cross-coupling remains low and the resulting uncertainty in the determination of maximum per unit over-voltages is typically 2%. Despite the installation at the substation resulting in relatively strong cross-coupling, per unit over-voltages could still be determined with an accuracy within 10%.

The performance of flexible cables for applications at the kilovolt level over a gas pressure range from below 1 Pa to ambient needs to be quantified. Braided cable designs provide the flexibility, but necessarily contain voids where at certain pressure values the Paschen minimum voltage can be exceeded. In our prior research a methodology was proposed based on partial discharge diagnostics. However, the reproducibility needs to be investigated in view of the stochastic occurrence of the discharges in cable voids differing in shape and size. From a cable with braided screen, the variation of the partial discharge inception voltage as function of pressure is characterized, both for tests on different samples and for consecutive tests on the same sample. Variation between different samples is significantly larger, but overall the performance is reproduced within typically 20%. Based on analysis of variance, results are found to be robust against experimental parameters defining the partial discharge inception criteria. In addition, the influence of external parameters such as temperature, magnetic field and radiation exposure, which can be part of the operational conditions of equipment, is investigated. It is found that temperature rise resulted in a performance increase of the cable, which is not in line with expectations from the change in gas density but may be attributed to reduced secondary electron emission. Exposure to magnetic field and γ-radiation did not show noticeable effect.

On-line partial discharge (PD) detection and location has proven to be a valuable tool for assessing the condition of medium-voltage (MV) power cables in service. A few advances are discussed in this paper. It is of importance to know all factors affecting PD activity in order to judge its severity. Cable temperature, more specifically its variation due to load cycling, is one key parameter. From the variation of the signal propagation velocity along the cable it is shown that the temperature can be monitored within one degree centigrade accuracy. The PD activity was found to respond upon temperature cycling caused by load variation. Moreover, data gathered from years of field experience has led to more insight in the predictability of upcoming faults based on (trends in) PD activity. It is possible now to identify the time between the moment partial discharges appear (or reach a certain level) and a final breakdown for different types of cable insulation. Occasionally, cable faults occur without being preceded by PD activity. For distribution network operators, a quick location of a MV cable fault, either a direct complete fault or a self-healing (or intermittent) fault, will contribute to reduce the outages both in duration and frequency. The earlier applied PD location technique is extended to capture also faults based on first arrival of the associated transients. This fault pinpointing is possible for any MV power cable type within 1% of the cable length, independent of the network grounding and whether a fault is a permanent one or a self-healing one. This paper presents the various techniques and results from test cases and a real captured field fault.

The increasing integration of underground power cables in the transmission network gives rise to technical challenges, especially concerning electromagnetic transients (e.g. resonances). In order to study transient phenomena, accurate simulation models should be built where the chosen modeling detail of the cable configuration is of utmost importance. In some cases the well-established Universal Line Model (ULM) shows stability problems due to passivity violations. In this paper the ULM is compared with a Finite-Difference Time Domain (FDTD) model in terms of accuracy, computational performance and stability. Simulation results show that with comparable accuracy the FDTD model present improved stability but with higher computational burden. Moreover, a sensitivity analysis for the FDTD model is performed to check how the chosen time-step affects the resulting waveforms and it is evident that for larger time step deviations in both the maximum overvoltage and the oscillation frequency are present.

Summary High frequency signal components from partial discharge in an underground power cable propagate according to available modes provided by the cable structure. Which mode or combination of modes is detected depends on type and installation of sensors. Without the ground return impedance, the earth screen voltage is assumed to be equal to the ground potential and all current will flow through phase conductor(s) and earth screen. However the soil may contribute to the transmission line propagation modes and affect the detected partial discharge signals. This paper investigates the effect of the ground return path on partial discharge detection at cable ends. Existing theory and modeling (modal analysis, ground impedance) are customized for the analysis. Transmission line parameters measurements approve the modelings. Time domain signal propagation simulation shows that the ground mode has a minor contribution to the signal propagation. The implication for partial discharge detection is discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Transients from lightning strikes can enter underground cables at overhead line to power cable transitions. Possible overvoltages on these surges at cross-bonding connections of the cable screens are of major concern. A model is developed for modelling overvoltages from transient signal propagation through a combined cross-bonding cable and box. This model is applied to the first Dutch 400 kV cable connection. Such model incorporates model parameters whose values depend on design details of the cross-bonding box. The values for these model parameters are extracted from the measured transmission and reflection signal on steep pulses injected into the actual cross-bonding box configuration. The model combines transmission line description for the cross-bonding cables with mainly inductive behaviour of the cross-bonding box. The obtained results are verified by measurements. The model is applied to investigate overvoltages induced at the cross-bonding cable and box on 1.2/50 µs impulse voltage injection representing a lightning impulse voltage applied to the core conductor of the cross-bonding cable. Furthermore, the effectiveness of surge arresters to reduce overvoltages at the cross-bonding cable screen is demonstrated by simulations with this model in PSCAD.

Underground low voltage (LV) power cables have a long life span, and so far minimum effort was put in methods for condition assessment. However, a slow but steady increase in the amount of faults is being observed and future de-central power generation may further put pressure on the reliability of the LV grid. Therefore, condition monitoring for LV cable systems is under investigation. With the occurrence of intermittent faults, fast transients from discharges may be observed before an outage occurs. To estimate if such signals are measurable, a model of the cable valid up to 50 MHz was derived. Numerical simulations have been used to obtain the parameters of the multi-conductor transmission line model, with special consideration of the copper screen configuration. Measurements on a 60 m cable segment have been performed to verify the model, showing good agreement. This model will be used to investigate signal propagation through a LV grid with focus on the feasibility to base LV condition monitoring on signals which may originate from upcoming cable faults.

Future generation lithography machines require a significant increase in actuation power of the moving stages, in order to ensure the continuous demand for improved productivity. Higher current levels to the actuators could achieve this, but it requires thicker conductors which increases mass and hampers flexibility. Operation of modern extreme ultraviolet (EUV) lithography machines demand a high flexibility of the cables to ensure a lifespan of 109 movements. The option pursued here, therefore, is higher voltage, i.e. a value possibly exceeding the Paschen minimum, whereas the operating pressure may go down to 0.1 Pa.

Higher actuator voltages are required for future generation lithography machines to increase productivity. The operating pressures in combination with relevant insulation distances, can be situated near gas breakdown and cause partial discharge activity, making long-term damage conceivable. High voltage cone-shape connectors with pliable insulation can be applied for the low and medium vacuum range. However, materials and space are restricted making a cone shaped connector not ideal. In this paper, a flat connector concept is presented with a pliable insulation material allowed to be used for the aforementioned application. The permittivity of this material is measured and simulation results of the connector reveal critical design points. A prototype connector is made and its partial discharge inception voltage is characterized for the low and medium vacuum range. The results are discussed and compared with simulations.

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.

Developments in the electricity sector, related to integration of sustainable energy, not only affects the medium voltage but also the low voltage infrastructure. Condition monitoring of Low Voltage (LV) underground power cables becomes of increasing interest to safeguard reliable power delivery. Since the start of phasing out oil-paper insulated cables, polymeric insulation is most widely applied. As phenomena associated with high electric field stresses are not expected, little is known about the degradation mechanism of LV cables with polymeric insulation. External damage is known to be a major cause of defects, which directly causes a failure or initiates degradation resulting in a failure later on. To study the degradation mechanisms, short cable sections with artificial damage have been subjected to periodic water exposure in the laboratory. This shows that dry-band arcing is a major mode of degradation, with accumulation of pollutants due to chemical decomposition. Several factors such as water conductivity, presence of oxygen, soil type and insulation material have been investigated. Strong dependencies upon these parameters are observed. However, it may be hard to quantify the degradation process in practical situations. The presence of a water film that can sustain dry-band arcing will occur only in certain situations and for limited amount of time. However, the accumulation of these events, mainly caused by periods of precipitation, will eventually lead to breakdown at the damaged spot.

Faults in Low-Voltage (LV) cables commonly develop from earlier inflicted damage. Outages can be preceded by high-current transients of duration too short to activate protective devices like fuses. This paper explores such early signatures of possible upcoming faults both by laboratory experiments and in a field study.Upon exposure of moisture the breakdowns and their voltage and current characteristics are investigated for two LV cable types (OIP and PVC). In laboratory tests, a clear intermittent behaviour is observed. Typical breakdown signals show a current transient igniting around the voltage peak and its extinction is usually close to the voltage zero crossing. LV grid faults should be distinguished from events occurring in connected households. Though signal patterns can be comparable, a distinction can usually be made, mainly based on current peak values because of the interruption characteristics of household miniature circuit breakers. A pilot study involving 23 connections in the LV grid has shown similar signals as obtained in the laboratory tests. Typical cable fault related transients were observed beforehand. This indicates a correlation of such transients and the likelihood of an outage. A large variation of the time to an outage was observed ranging from several hours to several months.

Higher actuator voltages are required for future generation lithography machines to increase productivity. The operating pressures in combination with relevant insulation distances, can be situated near gas breakdown and cause partial discharge activity, making long-term damage conceivable. High voltage cone-shape connectors with pliable insulation can be applied for the low and medium vacuum range. However, materials and space are restricted making a cone shaped connector not ideal. In this paper, a flat connector concept is presented with a pliable insulation material allowed to be used for the aforementioned application. The permittivity of this material is measured and simulation results of the connector reveal critical design points. A prototype connector is made and its partial discharge inception voltage is characterized for the low and medium vacuum range. The results are discussed and compared with simulations.

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.

Models in high frequency range for underground power cable connections are essential for the interpretation of partial discharge (PD) signals arising e.g. diagnostic techniques. This paper focuses on modeling of power cable joints. A lumped parameter odel and a cascaded transmission line model are proposed based on scattering parameters (S -parameters) measurement on a 10 kV oil-filled PILC-PILC straight cable joint in the frequency range of 300 kHz-800 MHz. It is shown that the lumped model is suitable for up to 10 MHz while the transmission line model can cover the whole frequency range. The cascaded transmission line model is applied to simulate the reflection on a 150 kV single core XLPE straight joint. Comparison between measurement and simulation indicates that the model parameters (characteristic impedance and propagation coefficient) can be matched to predict the joint’s propagation characteristics.

In countries with a high ambient temperature and strong solar irradiation, transformer winding hot-spot temperature may increase over its maximum permissible limit. This can considerably reduce the insulation life of the transformer by enhanced degradation of the paper insulation. According to current loading guides, for each 6 K increase in working temperature, the ageing rate increases with approximately a factor two. Therefore, it is important to take into consideration the impact of the sun on the power transformer thermal behavior. In this paper, a modified hot-spot temperature model is presented to account for the effect of transformer winding temperature rise by solar irradiation. The effects of solar irradiation on transformer winding paper insulation are shown by comparing the degree of polymerisation (DP), the fault probability and the remaining life. Here, the fault probability is defined as the probability that the estimated DP-value at a certain moment in time is below a certain end-of-life criterion (threshold value). An additional winding hot-spot temperature rise of 9 K during the summer and a temperature rise of 6 K during the winter may occur in countries with strong solar irradiation. This may result in a reduction of the remaining lifetime by up to 40%.

The age of the majority of power transformers installed in the western electricity network reaches 30 to 60 years and replacement on short term seems eminent. A technically sound policy concerning the replacement of these assets requires a model that estimates the life expectancies of individual components and from that calculates parameters related to the behavior of a population of assets as a whole. A probabilistic approach is adopted and is applied to thermal degradation of the transformer paper insulation. In this paper, we will focus on the determination of the population reliability from individual reliabilities. These individual reliabilities are based on Arrhenius modeling of paper insulation degradation, including the inherent uncertainty in the parameters involved. A statistical failure model is used to obtain the population reliability figures. The modeling method is demonstrated on two populations of power transformers in The Netherlands to evaluate the different replacement alternatives. Using the model, strategies can be defined to maximize transformer utilization and postpone replacement. The downside is the need to replace the complete fleet in a relatively short time afterwards.

Matched filtering can be applied to recorded signals for maximum detection sensitivity during online partial discharge (PD) measurements in medium-voltage cable circuits. PD waveform predictions are combined with a noise spectrum estimation to construct matched filters. The PD waveform is specific for each cable circuit under test and the predictions are based on online system identification measurements and a standard cable model. For some cable circuits, for instance with a ring-main-unit along the cable, the predicted waveforms only approximate the real waveforms due to signal distortion not accounted for in the model. This results in suboptimal detection sensitivity. This paper describes an automated procedure for updating the matched filters based on detected PD signals. The algorithm analyzes detected pulses to judge whether the current set of matched filters perform optimally. If not, the current set of matched filters is updated, increasing the PD detection sensitivity and potentially also the location accuracy. At the same time, recognition of recurring disturbances is improved and better rejection criteria can be defined. The method is tested on signals measured on live cable connections.

Uncertainty analysis is an essential part of models that estimate remaining lifetimes of power transformers. Uncertainty can be included in modeling the insulation aging process. If the uncertainty becomes too large, the insulation condition can, in principle, be determined by measuring the DP value, and more accurate projections can then be made. A drawback of the model presented here is the required input data, i.e., average values of dynamic load and ambient temperature, and maximum values, because the degradation rate is not a linear function of temperature. This information is not available for most transformers. However, even if the loading data are available only over times when the load approaches the rated value, a reliable estimate of the paper degradation can still be made. The observation that tap-changer failure is still the major transformer failure mode supports the simulated results presented above, which show that a transformer failure peak due to paper insulation failure is expected only after several decades of service. Although prediction of transformer performance may be considered speculative because of uncertainties in the model parameters and incomplete service data, it can serve as a tool to compare different maintenance and replacement strategies. Thus annual replacement strategies appear superior to equal loading because the latter provides no guidance on the order of replacement. It has also been shown that transformers should be replaced before the beginning of the failure wave and that variation in annual load growth between transformers leads to spreading of the replacement wave. A disadvantage of a distributed transformer loading is the reduction of the average remaining lifetime due to relatively fast degradation of the more heavily loaded transformers. Comparison of these strategies requires a technical reliability model, which is the foundation of risk-based asset management.

Online partial-discharge (PD) monitoring of medium-voltage power cables is traditionally performed on a cable between two consecutive ring main units (RMUs). PD location is achieved using a sensor at each cable end. Monitoring multiple consecutive cables, with RMUs in between, using a single monitoring system is more efficient. In addition, substations and RMUs without possibilities for sensor installation can be circumvented by installing the sensor at the next RMU. This paper studies how RMUs and substations along the cable under test affect online monitoring, including their influence on detection sensitivity, location accuracy, and charge estimate accuracy. Single-sided PD measurement, including PD location, with an RMU or substation at the far end is also considered. Models for RMUs and substations are proposed and verified by measurements. The performance of online PD monitoring is studied for a number of network configurations.

Air-break disconnectors (DS) in a high-voltage network can interrupt small capacitive currents (e.g., occurring when disconnecting short unloaded transmission lines, current transformers, and so forth). The performance of a device with high-velocity opening auxiliary contacts attached to the DS enhancing the interrupting capability is presented. A series of laboratory experiments is carried out with the interrupted current range between 5 and 27 A. The experimental observations include voltage and current characteristics of the DS, and synchronous optical recording of the arc with a high-speed camera. Electrical and optical data are studied for fail and successful interruptions. The results show: 1) The short arcing time by the fast opening contacts limits the energy input into the arc, enhancing the probability of arc extinction; 2) the contribution to the arc energy mainly comes from the oscillations upon restrike. The energy input to the arc increases with higher breakdown voltage, larger air gap, and higher interrupted current; 3) the arc brightness is synchronous with arc current. With higher interrupted current, the overall arc is brighter and its remnants decay slower. Based on the results, potential successful approaches to increase the interruption capability are discussed.

Capacitive current interruption with air-break disconnectors in a high-voltage network is an interactive event between the circuit and arc with a variety of interruptions and reignitions. In this contribution, first, a theoretical analysis related to this interaction is presented. The effect of capacitances at the source side (Cs ) and load side (Cl ) is investigated. Three distinct frequencies are identified as contributing to the voltage and current events in the circuit. Besides the power frequency quantities, a medium frequency transient arises related to the excursion of voltage across capacitances to the applied voltage, and a high-frequency transient arises due to charge redistribution between load- and source-side capacitance at reignition. Second, experimental results from an interruption measurement are studied in detail. Typical waveshapes of voltages across the capacitances, disconnector, and currents through the disconnector show that the transients during interrupted are in agreement with the theoretical analysis. Reignition voltage of the air gap and energy input to the arc on reignition are also studied. It is concluded that besides a higher interruption current and a higher power supply level, a lower Cs /Cl ratio leads to more severe interruption and longer arc duration. Finally, the actual status of IEC recommendations on testing, that has taken into account this arc-circuit interaction, is summarized.

In power cables fast transient signals arise because of partial discharges. These signals propagate to the cable ends where they can be detected for diagnostic purposes. To enable optimal detection sensitivity and to judge their severity the propagation parameters Z c (characteristic impedance) and ÿ¿ (propagation coefficient) need to be known. A three-core power cable with a single metallic earth screen around the assembly of the cores has multiple, coupled propagation modes with corresponding characteristic impedances and propagation coefficients. This paper presents a practical method to measure and analyse the cable parameters. The propagation modes are decoupled into a modal solution. The modal solution is interpreted in terms of convenient propagation modes: a shield-to-phase (SP) propagation mode between conductors and earth screen and two identical phase-to-phase (PP) modes between conductors. The measurement method, based on a pulse response measurement, to determine all transmission line parameters of the SP and PP modes is proposed and tested on a cable sample. The model is validated by predicting the time, shape and amplitude of multiple reflections in all modes resulting from an injected pulse.

A transmission line model of a power cable is required for the analysis of the behavior of high-frequency phenomena, such as partial discharges, lightning impulses and switching transients, in cables. A transmission line is characterized by its characteristic impedance, attenuation coefficient and propagation velocity. The semiconducting layers in an XLPE cable have a significant influence on these parameters. Unfortunately, the dielectric properties of these layers are usually unknown and can differ between similar types of cables. In this paper it is shown that nevertheless the characteristic impedance and propagation velocity of single-core and three-core XLPE cables can be estimated using available information from the cable specifications. The estimated values are validated using pulse response measurements on cable samples.

Accurate location of the origins of partial discharges in power cable systems, based on arrival times, is imperative 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 trigger level, Akaike Information Criterion, energy criterion, Gabor's signal centroid 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. Further, the methods are tested on a medium-voltage cable system by injecting PD pulses in a cable with one joint. Each algorithm is applied to the measured pulses and the resulting location is compared with the known joint location. From the results the energy criterion method and the phase method show the best performance.

Traditionally when studying harmonics at transmission system level the medium and low voltage grid is modeled with either an equivalent short-circuit impedance or a fixed PQ load (based on the powerflow) directly at the secondary side of the transmission transformer. However the recent replacement of conventional loads with electronic loads as well as the extended use of power electronics in LV level will result in reduction of damping and increase of household capacitance which may change the harmonic impedance seen from the upstream network (MV or HV) both in the dominant frequency and magnitude. In this paper a HV feeder is modeled in detail starting from the transmission transformer up to the household equipment to investigate whether changes in the type of load affect the impedance seen from the HV. The simulations are performed using software package PSCAD/EMTDC.

Sustainable developments impose increasing pressure on Low Voltage (LV) underground power cables, leading to an increasing interest in condition assessment for LV systems. LV cables have been designed mainly on mechanical considerations taking insulation of low voltage levels for granted. However, concerns on degradation aspects is now growing. To make a comparison of insulation materials, water cycling tests on artificially damaged cable sections have been performed. The results show that the material has a major role in degradation rate due to local decomposition upon dry-band arcing, as can also be expected from chemical point of view. Chemical compounds add pollution to the damaged spot, enhancing degradation. PVC shows fastest degradation rate compared to polyurethane and XLPE respectively. These results can be considered in optimal cable design. In addition, aspects like water tightness should also be considered.

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) power cables is increasing due to environmental, political and operational aspects. For transmission networks consisting of purely OHLs there are proposed guidelines concerning the modeling depth needed for acquiring accurate results when simulating transients. However, these guidelines might not be adequate when mixed OHL-Cable-OHL configurations are present in the studied network. This paper presents two methods to evaluate the required modeling depth of a transmission network consisting of both cables and OHLs when studying slow front transients and more specifically line energization. The first method evaluates the maximum overvoltage caused by the energization while the second method evaluates the harmonic content of the energization. The simulations are performed using software package PSCAD/EMTDC. It was found that the modeling depth for both objectives exceeded the advised depth proposed in the guidelines for purely OHL connections.

Transmission system limits, originating from right-of-way permission processes for new corridors, can be surpassed by erecting new circuits in close proximity, or even on common structures. Especially for long distance bulk power transfer, high voltage direct current technology promises superior features. Adding a circuit to a transmission corridor is one method of significantly increasing power transfer capability. Converting an existing AC circuit for DC operation is another method that additionally decreases investment in new lines and towers. Consequences of such application are various AC/DC interaction phenomena on either AC or DC lines for both steady and transient conditions. This paper presents a study on AC/DC transmission system interactions for a hybrid tower configuration. In normal operation DC voltage showed no significant fundamental AC frequency component, while induced transient overvoltages displayed notable levels. QC 20160829

In medium- and high-voltage cables with a helical structure of the earth screen current pulses from partial discharges travelling along the cable are observed to follow the spiralling structure. The resulting tangential current produces an axial magnetic field which can be detected by an inductive sensor without having to make any change to the cable. A specially developed inductive sensor placed around the outer sheath of an intact cable is made selectively sensitive to the high-frequency signals (± 100 MHz) arising from the partial discharges. The discharge pulses are recorded by a fast digitizer (bandwidth of 600 MHz). In laboratory experiments partial discharges with a minimum magnitude of about 5 pCfrom electrical treeing in short, medium-voltage cables could be detected. Location by means of transit time measurements was performed with an accuracy of about 5 cm. In one case, the polarity of the discharge signal was successfully used to locate the discharge to within 1 cm.

The area of pulsed-power technology covers a broad range of powers (up to 1014 W), voltages (up to 106-107 V), currents (as high as 106 A or higher) and pulse durations (below 10-8 s). 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 present a novel compact TLT (transmission line transformer). The TLT consists of 10 coaxial transmission lines which are connected in parallel at the input and in series at the output. To verify the theoretical analysis a model of the transmission line transformer has been simulated by a time domain electromagnetic field solver. An optimized transmission line transformer has been built and first measurements have been performed. The equivalent circuit model and the simulations agree with the experimental results. The effect of secondary mode suppression by means of ferrite has been investigated. Both simulations and experiments showed that ferromagnetic cores around each individual transmissions line in our design can be omitted.

Coupling of carrier wave frequencies up to 95 kHz (within the European CENELEC A-band) for online diagnostic data transfer in 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. The effectiveness of coupling schemes depends on the impedances of substation equipment at the cable terminals. The frequency response of a 10-kV, 400-kVA three-phase cast resin distribution transformer is investigated. In the frequency range of interest, the behavior is well described by 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 within the CENELEC A-band may be sufficient, except for substations at the end of a grid. Transferring signals containing frequencies up to several megahertz, which is already required for synchronization of partial discharge detection and location equipment, is feasible under all conditions. Measurements on life substations indicate that up to these frequencies substation components can still be accurately modeled as lumped circuit impedances.

On-line partial discharge (PD) detection of three-phase belted medium voltage cable connections results in a number of interpretation differences as compared to off-line measurements where only one phase is energized. The induced currents and charges in the phase conductors and earth screen upon a PD not only depend on the discharge site, but also become phase angle dependent. Furthermore, simulations show that the PD distribution itself varies with the amount of eccentricity of the rotating electric field and may differ from the off-line distributions obtained with a linear field. Finally, the PD propagation in a multi-conductor cable also alters the signals measured at the cable terminals. In this paper, induced charges and PD distributions are studied by means of computer simulation. The cable propagation characteristics are verified by measurements.

Underground power cables are increasingly employed to transfer electricity. In the Netherlands, all medium-voltage (MV) and low-voltage (LV) networks are realized with underground power cables. Consequently, power cable diagnostic has became a relevant subject. An online defect positioning technique is proposed based on characteristic impedance variations, which usually occur in the power cable at the defect position. To this end, the cable impedance is monitored at a ring-main-unit (RMU) with the help of online impedance measurement technique. The measurement utilizes a bulk current injection probe (BCIP) clamped around the earth wire inside the RMU. The frequency domain impedance is transferred into the time domain impulse response by applying the inverse discrete Fourier transform (IDFT). Thus, the frequency domain impedance variation can be observed in time domain which includes the defect position information.

Increased pressure on condition monitoring of Low Voltage (LV) underground power cables can be expected due to rising penetration of de-central generation. Digging damage plays a large role in LV cable failure statistics while those damages do not necessarily lead to an outage directly. Artificially damaged cable sections of two commonly used LV cable types have been tested on water exposure. Upon water evaporation, partial discharges occur causing degradation of the insulation material. In one case this was found to lead to breakdown. Several dependencies can be identified that need further investigation. These results enhances the understanding of failure behavior as it may take place underground for cables in service, and can be used in the development of condition monitoring techniques and lifetime predictions.

Wear and tear, electrification and aging of (low voltage) distribution grids require maintenance in order to assure continuous provision of electricity. Situational awareness is required to find out when parts of the grid need maintenance and which parts should be prioritized. The paper proposes a conceptual value model which relates sensor data from the condition of the asset to the potential value of information (i.e. interpreted data) for the distribution system operators. Distribution density functions account for uncertainties in model parameter values. They can be narrowed down by monitoring relevant degradation mechanisms. To this end, degradation processes are simulated in laboratory scale setups, emulating low voltage cables in moist and submerged environments. Understanding fundamental aspects of degradation, failure and value determination of sensor data allows prioritization of maintenance actions that lead to best results in terms of customers-minutes-lost, remaining useful lifetime of assets and cost-effectiveness. © 2015 IEEE.

Insulation temperature is the critical parameter for dynamic rating of underground power cables. Power cables on medium voltage (MV) level are too numerous and widespread to economically justify solutions like optical fibers as temperature sensors, often employed for high voltage power cables. This paper proposes to utilize the propagation velocity of high frequency signals as indicator for MV cable dynamic rating. Laboratory scale tests are performed for both PILC and XLPE cables. Test results show that the high frequency signal propagation velocity for XLPE insulation increases with the temperature rise while PILC cable shows opposite behavior. The variation of propagation velocity with temperature is consistent with the variation of permittivity measured on test samples of both materials. The variation of propagation velocity of XLPE is confirmed by data of a power cable in service subjected to strong load cycling monitored over a week. Field data on propagation velocity also matches load variation observed for a PILC cable for load cycling recorded over one week. From laboratory tests and field measurements, it seems feasible to extract thermal information from MV cables for dynamic rating by means of high frequency propagation velocity.

Time domain reflectometry (TDR) applied to live cables is hampered by the absence of a clear reflection from the far end. Instead, location of partial discharge (PD) can be extracted from small structures in recorded patterns arising from any impedance variation along the signal propagation channel. This paper explores two approaches for a single-sided PD location system. The first method compares similar signal sequences occurring in PD pattern and pattern obtained from an injected signal. The second method exploits the change of impedance at a far end Ring Main Unit (RMU) by introducing ferrite material, whose effect can be switched on and off. Experiment on medium voltage (MV) cable with about 560 m length shows that both methods can locate the PD within 2% uncertainty.