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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.

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.

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.

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.

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.

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.

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.

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

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.

The detection of the high frequency signal propagating along underground power cables is part of many monitoring techniques, e.g. partial discharge (PD) based diagnostics. On one hand, higher frequency corresponds to better spatial resolution, which means more accurate PD location. On the other hand, signal attenuation increases with frequency. Apart from the signal itself, noise level and detection equipment also play a role in the signal detection process. This paper focuses on the detection limitation of high frequency components in PD signals travelling along an underground power cable considering effects of signal attenuation, noise level and applied equipment. The attenuation coefficient is based on measurements from 10kV three-core XLPE cables. Though the attenuation coefficients for other types of cables differ, the measured value for this particular cable provides a practical parameter value, and it can be altered to match other cable types. The detected analog signal is digitized through an analog-to-digital converter (ADC) and may be averaged before being digitally stored. In addition, an amplifier and/or filter can be applied before the analog to digital (AD) conversion. The vertical resolution and the vertical sensitivity of the ADC are crucial for signal detection. Effect of noise is considered in this paper by analyzing Gaussian noise and typical noise characteristics obtained from field measured. Sinusoidal wave and Gaussian pulse shapes are applied as input signals for the cable. Firstly, the relationship between maximum cable length and detectable frequency components for a specific set of detection equipment conditions is analyzed without averaging. This is the limitation from ADC. Secondly, the merits and limits of averaging are studied. The required averaging time for different frequencies as a function of PD signal propagation length is studied. Finally, the effect of averaging and analog filtering is demonstrated with test measurements.

In this paper a model for cables with a double earth screen layer consisting of unequal conducting materials is developed. The earth screen is modeled by a single-layer with properties such that the transient behavior of the actual cable is reproduced. The first approach aims to equate the impedances and admittances of both models by varying parameters of the single-layer sheath model to optimize the match with the double-layer cable sheath. In the second approach the differences between the ABCD matrices of both models are minimized. By analyzing the ABCD matrices a method is developed to obtain a close match for transient behavior between single- and double-layer models. Analysis of the accuracy of the results shows a good match for the investigated cable.

Models in high frequency range for underground power cabling system are the basis for the analysis of many condition diagnostic techniques, for instance, partial discharge (PD) monitoring. This paper focuses on modeling of power cable joints. A cascaded transmission line model is proposed for 10kV oil-filled PILC-PILC straight power cable joint in thefrequency range of 300 kHz-800 MHz. It can be applied to interpret PD signal transmission and reflection behavior in power cable connections. The model is verified by S-parameter measurement.

The introduction of 25 kV a.c. traction in the Dutch railway system required a careful study of the compatibility with existing 1500 V d.c. traction and with other systems in the vicinity of the railway. Over the last decade, a number of studies have been carried out, using specially developed measurement systems and methods of analysis. The present paper describes a newly developed Poynting sensor that builds on the earlier developed differentiating/integrating measurements system. Measurements on the railway power system with current injection at e.g. 60 and 48 Hz will be discussed as well.

In Extra/High-Voltage (EHV/HV) power systems, power cables are chosen instead of over-head lines (OHL) at some places. In the Randstad region in the west of the Netherlands, the cable connection consists of a double circuit where each phase is connected via two parallel cables. The cables are cross-bonded every 0.9 km. A complete model is required to efficiently analyze interaction with the remainder of the connection consisting of overhead lines. This paper presents a method to build a symbolic model of the parallel connection of multiple cables per phase in a (EHV/HV) cable system, which can be the basis of transient analysis in the frequency domain of a large transmission system containing multiple parallel conductors. The model is analyzed and compared by means of a frequency scan of the system with PSCAD/EMTDC simulation.

Partial discharge analysis has a long record as reliable diagnostic tool to assess the integrity and the quality of electrical insulation of power systems. On-line partial discharge monitoring systems for live medium voltage cable connections have recently been introduced in Dutch grids and also a few worldwide. To enhance the applicability of on-line cable diagnostics there is a need to develop a PD interpretation approach suitable for on-line monitoring. This paper discusses various PD parameters, including PD charge magnitude, PD occurrence rate and PD charge density and their related statistical values such as mean and maximum, used to interpret the PD activity. Trend watching of these parameters is employed to study the degradation state of cable connections and their components. The approach is applied to field data obtained over a year through monitoring several live circuits. In this paper, three examples are presented. Weak spots were detected and failures were prevented by early warning.

In Extra/High-Voltage (EHV/HV) power systems, over-voltages occur e.g. due to transients or resonances. At places where no conventional voltage measurement devices can be installed, on-site measurement of these occurrences requires preferably non intrusive sensors, which can be installed with little effort under all circumstances. This paper presents three methods, which involve different algorithms based on a capacitive measuring system using simple design and easy-to-install sensors which couple with air-capacitors to the lines. Each capacitive sensor couples in principle to all phases. The presented methods differ in the pre-assumptions needed to determine the coupling matrix to reconstruct the phase-voltages, either power frequency, transient or resonant over-voltage on each of the three phases. The feasibility and the performance of the methods are discussed.

PD monitoring on-line with location (PD-OL) is one of the successful diagnostic tools for MV cable networks which has been recently introduced to the main utilities in the Netherlands and also a few worldwide. Data acquired by PD-OL need to be correctly interpreted in order to identify the upcoming faults in MV cable networks and possibly forecast the remaining life of the component. Interpretation of the measured signals by means of statistical analysis of various PD patterns as well as trend watching in the patterns, which is considered as the prominent advantage of the on-line monitoring, enables us to estimate the degradation stage in insulation systems. In this paper, we present the result of such analysis applied on two life circuits. The data was obtained over a period of over two years continuous monitoring of the circuits including PILC/ XLPE cables and oil-filled / grease / polymer joints. The results of analyses on several weak spots which were detected before failure are presented. Besides, results of failed accessories are discussed in relation to their prior PD behavior as diagnosed by PD-OL.

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. We will illustrate the approach by applying it to a well-known degradation process: thermal degradation of the transformer paper insulation. In this paper we focus on the determination of the population reliability from individual reliabilities. These individual reliabilities are based on Arrhenius modelling of paper insulation degradation including its inherent uncertainties. A statistical failure model is used to obtain the population reliability figures. We demonstrate the modelling method by applying it to an existing population of power transformers in the Netherlands, to evaluate the expected replacement wave. Further, the model is applied to analyse alternative replacement scenarios.

High-speed opening auxiliary contacts ("whip type device") is used to enhance the capacitive current interruption capability of air-break disconnectors. Firstly the paper introduces the basic working principle of this device. Secondly a series of laboratory tests are described. The voltages at both sides of the disconnector and the current through the disconnector are measured. The arc images are recorded by a high-speed camera with 1000 frames/s as well. The test data and arc images are studied for both failed- and successful interruptions. Finally, conclusions and remarks are given.

Partial discharge (PD) location in online diagnostics on medium voltage cables is achieved using a sensor at each cable end. Monitoring consecutive cables with a single monitoring system would, however, be more efficient. Moreover, substations and ring main units (RMU) along a cable connection without possibilities for sensor installation can be circumvented by installing the sensor at the next RMU. This paper studies the influence of RMUs and substations along the cable under test on online PD monitoring, including their influence on detection sensitivity, location accuracy and charge estimate accuracy. Models for RMUs and substation are proposed and verified by measurements. The performance of online PD monitoring is studied for a number of network configurations.

On-line partial discharge detection and location equipment for medium voltage cables have become available recently. Efficient implementation can be realised by monitoring cable connections containing several cable sections with ring main units or even a complete substation in between. This paper discusses the consequences related to diagnoses of complex cable connections by a single partial discharge detection system. The feasibility of monitoring several power cable sections at once is illustrated with a practical example obtained with recently installed systems in the Dutch grid. It is shown that detection and location of partial discharges is indeed possible for signals travelling over long connections including ring main units.

Online partial discharge (PD) detection and location in cable systems is a valuable tool for the estimation of the condition of the system. Different types of cables are encountered in medium-voltage (MV) connections and appropriate models must be constructed to predict PD signal propagation. For three-core power cables with a single common earth screen a multiconductor transmission line (MTL) model is required. This paper presents an MTL model of a such a cable, including interpretation of the decoupled solution as convenient propagation modes. A complicating factor in the estimation of the transmission line parameters of these propagation modes is the presence of semiconducting layers. These layers have a significant influence on the transmission line parameters. Unfortunately, the dielectric properties of these layers are usually unknown for the frequency range of interest for PD diagnostics. It is shown that nonetheless the characteristic impedance and propagation velocity can be estimated using information available in most cable specifications. the estimated values are validated using pulse response measurements on a cable sample. ©2009 IEEE.

A power cable model for partial discharge (PD) pulse propagation requires knowledge of all factors influencing the transmission line parameters of the cable (characteristic impedance, attenuation coefficient and propagation velocity). These frequency dependent quantities depend, amongst others, on the dielectric properties of the semi-conducting layers. For existing power cable connections, but often also for newly manufactured cables, these quantities, which vary between different cable types, are not known with acceptable accuracy. This paper discusses methods to estimate the characteristic impedance and propagation velocity using the cable geometry and the dielectric properties of XLPE, but without using the dielectric properties of the semiconducting layers. Measurements on cable samples show that these approximations are valid with sufficient accuracy. The attenuation of PD signals can only be accurately predicted with detailed knowledge of the dielectric properties of the semi-conducting screens.

Partial discharge (PD) analysis is a reliable tool to assess the integrity of electrical insulation. Representation and interpretation of the data, obtained from e.g. online PD monitoring, are key issues to reveal the discharge source, i.e. defect type, as well as the physical phenomena behind the occurrence. Analyses of various PD patterns such as discharge height distribution presented in this work provide useful statistical parameters to identify the discharge source. Research shows that the 2-parameter Weibull distribution is a reliable model to quantify the characteristics of the patterns of the defect. The model fits well to the charge-height distribution. In addition, trends in the discharge density pattern that occur over long times, can be used as complementary information to discover the discharge nature. It alerts for a possible failure and therefore assists in taking corrective measures to prevent failure. This paper presents the application of such statistical modeling to the area of on-line power cable diagnostics. Data obtained from laboratory experiments as well as field data have been studied.

The age of the majority of power transformers applied in the western electricity network varies between 25 and 50 years. Depending on the load history and time of operation, replacement on short term is imminent. A technically sound policy concerning the replacement of these assets must be based on knowledge of (i) the life expectancy or reliability of individual components, (ii) how these failure probabilities cumulate to a replacement wave, and (iii) how to manage an expected replacement wave. The population reliability is obtained from individual transformer reliabilities using Arrhenius based modelling of paper insulation degradation. This modelling technique includes measures to cope with inherent uncertainties in available data. Population reliability figures are obtained using an adapted k-out-of-N failure model. The modelling method is applied to existing populations of power transformers in The Netherlands, to evaluate their expected replacement wave.

An accurate model of the high frequency behavior of a power cable is required for the precise simulation of the propagation of partial discharges in such a system. A cable model consists of the transmission line parameters: characteristic impedance, attenuation coefficient and propagation velocity. The semi-conducting screens of an XLPE power cable have a significant influence on the pulse propagation along the cable. Unfortunately, the dielectric properties of these layers are usually unknown and can vary hugely between cable types. This paper shows how the characteristic impedance and propagation velocity can be estimated using the cable geometry and the dielectric properties of XLPE. Typical uncertainties in the input parameters result in an uncertainty of a few percent in the approximation of the characteristic impedance and propagation velocity.