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Time‐domain reflectometry applied for power cable diagnostics employs transmission line parameters to determine signal shape and delay. This paper proposes to utilize pulse reflection measurement to evaluate the symmetry of a three‐core power cable as part of production quality assessment. For a rotation‐symmetric three‐core power cable, transmission line parameters can be obtained analytically, and with modal analysis propagation channels can be decoupled. Asymmetry caused by inaccurate core positioning complicates the decoupling process. This paper utilizes the “field‐circuit” approach for the analysis. The finite element method is used to obtain the electromagnetic field distribution for two types of asymmetry introduced in a cable design. Modal analysis results are validated by considering asymmetry as a perturbation of the symmetric cable design. The influence of asymmetry on the time‐domain pulse response is simulated. Deviation from a symmetric configuration is observable, in particular in terms of propagation velocity, which can be employed to assess cable manufacturing accuracy.

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.

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.

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.

An on-line partial discharge (PD) monitoring and location system for medium voltage cable circuits was developed previously. This paper explores ways to extend its range of application. The extension includes PD location method by time domain reflectometry (TDR) when reflections are not easily identifiable. The functionality of the PD monitoring equipment can also be widened by sensing other quantities related to the condition of the cable insulation using the same device. Dynamic cable temperature monitoring can be achieved by recording variation in the high frequency signal propagation velocity along the cable. Water ingress in paper-insulated lead-covered (PILC) cable decreases the cable’s characteristic impedance while it increases the permittivity of the insulation. It can be observed by changes in the reflection pattern from the cable or by a lowered propagation velocity. Instead of recording reflection patterns in time domain, a frequency domain approach based on an impedance scan is investigated to be applied for cables in service.

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.

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.

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.

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.

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.

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.

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.

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.

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

Storing large amounts of data in an efficient way from the point of view of very fast retrieval is animportant requirement for many industries like KEMA, a company offering consulting, testing andcertification in the energy business. Because their relational database could not cope fast enough with thelarge amounts of data involved, an alternative way for data storage was proposed, called HierarchicalData Format 5 (HDF5). HDF5 is a data model, a library, and file format for storing and managing data.Four hierarchical designs for storing and retrieving the large amounts of data involved were investigated.A benchmark was carried out in order to know which hierarchical structure would perform best.Eventually, a benchmark between HDF5 and MS SQL Server was carried out. It could be shown thatHDF5 performs four (4) times better for inserting and even 200 times better for retrieving data than theMS. SQL Server.

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.

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

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

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

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.

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

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.

PD measurement is a proven diagnostic for MV cable system insulation. The PD charge is a relevant parameter for interpretation of test results. In case of off-line PD diagnostics, the measuring system is relatively easy to calibrate. For on-line measurements, the magnitude of signals from the cable under test is also dependent on the impedances in the connected substation. In order to obtain correct PD magnitudes for on-line measured signals, this impedance must be known. In this paper, a substation model is presented, as well a measuring method to obtain model parameter values. Experiments show good agreement with the model. The measuring method provides a solution to determine the substation impedance(s) on-line, so without any switching, disconnecting or making galvanic contact. This can be an important benefit to on-line PD measuring systems.