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The insulation of HVDC extruded cable system joints. Part 2: Proposal of a new AC voltage PD measurement protocol for quality control during routine tests
Abstract
The review of materials, design and testing of joints for HVDC extruded cable systems provided in previous Part 1 paved the way to this Part 2 position paper by the DEIS HVDC Cable Systems Technical Committee, whose aim is to remedy the scarcity of existing standardized tests on joints. After a sound analysis, here routine tests are identified as the first practical target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers' experience for quality control of joints during routine tests and VHF/UHF wireless sensors as the best tool for such measurements in the noisy environment of factories. Thereby, a novel protocol for PD measurement using AC voltages and VHF/UHF electromagnetic sensors, for quality control during routine tests on HVDC extruded joints, is proposed.
... Moreover, they suggest the use of AC stress in order to test components employed in HVDC transmission systems, such as joints. For testing the dielectric of polymeric extruded HVDC cable joints, Mazzanti et al. published two interesting articles on tests for extruded HVDC cable joints [7,8]. In Reference [7], a review focused on typing, the design and testing techniques (especially for PD and space charge measurements) of extruded cable joints are reported in detail. ...
... In Reference [8], instead, a novel protocol for PD measurements during routine tests in HVDC extruded cable joints carried out by means of AC voltages and VHF/UHF electromagnetic wireless sensors was proposed. In summary, the protocol set the calibration of the VHF/UHF sensor, the measurement setup, the measurement procedure and the acceptance criteria. ...
Partial discharge (PD) detection can be considered one of the most useful tools for assessing the insulation conditions of the power apparatus in high-voltage systems. Under AC conditions, this analysis is widely employed in online and offline tests, such as type tests or commissioning, and can be carried out by applying the phase-resolved PD (PRPD) method, since the patterns can give information about the defect classification. Under DC voltages, the classic pattern recognition method cannot be performed, and the measurements show complexities related to the nature of the phenomena. For this reason, to date, a standard for PD measurements under DC does not exist. In previous papers, a new method for PD detection under DC stress voltages has been proposed by the authors. It is based on the application of a direct current periodic (DCP) waveform useful in obtaining PRPD patterns. The dependence of partial discharge inception voltage (PDIV) and PD repetition rate (PDRR) on the δ shape parameter of the DCP for different materials, as well as the capability to recognize different discharge phenomena, provided valid indications on the behavior of PD in the transition from AC to DC. The aim of this paper is to evaluate the time dependence of PD occurring in a dielectric by applying the DCP waveform. In our previous studies, the investigations were focused on the PD behavior under different values of the DC voltage periodic part. In another work, the DCP waveform with both positive and negative polarity was applied to several dielectric materials. In the proposed work, instead, the DCP waveform is applied for a long time in order to observe its effect on the PD behavior for 72 h. In this way, due to the space charge accumulation phenomenon, the aging effect, also due to the space charge accumulation phenomenon, is evaluated. The methodological approach was to acquire PRPD patterns over time and evaluate their trends in comparison with the sinusoidal case. The experimental results show that, with a DCP waveform having δ = 0.6, the aging effect similar to that provided by pure DC stress is observed, while the acquired PRPD patterns are easily interpretated, as in the AC case.
... EM approaches to detect PD activity have interesting characteristics because they allow noninvasive, continuous, and low-cost monitoring of PD activity; however, some of them have a large size, which makes them bulky when used inside equipment such as power transformers and gas-insulated substations (GIS). Several types of sensors can be used for EM wave detection, such as UHF/VHF antennas [12], VHF capacitive couplers [13], or inductive sensors [14]. Capacitive coupler sensors consist of a pair of electrodes separated by a known distance that detect the electric field created by PDs in shielded power cables. ...
Partial discharge (PD) diagnosis tests, including detecting, locating, and identifying, are used to trace defects or faults and assess the degree of aging in order to monitor the insulation condition of medium-and high-voltage power cables. In this context, an experimental evaluation of three different printed circuit board (PCB)-based inductive sensor topologies, with spiral, non-spiral, and meander shapes, is performed. The aim is to assess their capabilities for PD detection along a transmission power cable. First, simulation and experimental characterization are carried out to determine the equivalent electrical circuit and the quality factor of the three sensors. PD activity was studied in the lab on a 10-m-long defective MVAC cable. The three PCB-based sensors were tested in three different positions: directly on the defective cable (P1), at a separation distance of 10 cm to 3 m (P2), and on the ground line (P3). For the three positions, all sensors' outputs present a damped sine wave signal with similar frequencies and durations. Experimental results showed that the best sensitivity was given by the non-spiral inductor, with a peak voltage of around 500 mV in P1, 428 mV in P2, and 45 mV in P3, while the meander sensor had the lowest values, which were approximately 80 mV in P1. The frequency spectrum bandwidth of all sensors was between 10 MHz and 45 MHz. The high sensitivity of the non-spiral inductor could be associated with its interesting properties in terms of quality factor and SFR, which are due to its very low resistivity. To benchmark the performance of the designed three-loop sensors, a comparison with a commercial high-frequency current transformer (HFCT) is also made.
... As illustrated in [11], [12], the TC singled out a lack of standardized tests on joints in general, thereby focusing on the development of new techniques to characterize joints. Two innovative electrical techniques were identified: ...
The first time that the feasibility of space charge measurements on full sized HVDC cable joints has been explicitly considered and whether the PEA and TSM techniques could be utilized.
... As illustrated in [11], [12], the TC singled out a lack of standardized tests on joints in general, thereby focusing on the development of new techniques to characterize joints. Two innovative electrical techniques were identified: ...
This is the first time that the feasibility of space charge measurements on full-sized HVDC cable joints has been explicitly considered. In this article we also consider whether the PEA and TSM techniques could be used.
... These issues have also been treated in the framework of a long-lasting cooperation between the Roma La Sapienza University and the University of Bologna, which then became a partnership between TERNA and the University of Bologna. This partnership gave rise in the last fifteen years to several papers [6]- [18], one Wiley-IEEE book [19] and two IEEE Standards [20]- [22], where various theoretical and experimental tools were developed to estimate the end of life of HVDC cable systems accounting for the influence of laying environment, operational duty and cable technologies. In recent years, such a partnership has also involved the EnSiEl consortium [23], giving rise to two dedicated research agreements [24], [25] -one of which still ongoing at the time of writing of this paper [25]. ...
... This cannot be said for HVDC cable systems. Nowadays, the scientific community is interested in finding applicable procedures that can objectively evaluate possible ageing activities in HVDC cable systems [8], [9]. ...
... This leads the electric stress applied to the surface of a cavity to change over time as the load [10]. The probability for cavities to be entrapped within the insulation is higher in the case of on-site manufactured joints [11,12]. To limit the probability of service failure of cable lines due to the presence of defects, several standards and recommendations shall be considered by the manufacturers of High Voltage (HV) cables and accessories [13][14][15][16]. ...
The paper investigates the role of the space charge accumulation phenomena in the inception of Partial Discharges (PD) in a defect within the bulk of the dielectrics of a High Voltage Direct Current (HVDC) cable. It is widespread accepted that the number of PD occurring in an HVDC cable during its lifetime is largely less than that under AC voltage stress. This is essentially due to higher values of the voltage between the conductor and the screen needed to trigger PD under a steady DC voltage than the same value under AC stress. Nevertheless, due to the dependency of the electrical conductivity of the insulation on the electric field and temperature, PD in HVDC cables is strongly influenced by the temperature distribution and, consequently, on the load current. Thus, the main goal of this research is to evaluate the main parameters of the PD occurring in an HVDC cable during a load transient. Numerical simulations by using Comsol Multlphysics® are validated by the results obtained with an ad hoc experimental setup. A model DC cable, subjected to transient phenomena due to a current step and with an air void defect in the insulation material, is simulated and setup. On the other hand, an experimental setup is used to carry out PD measurements on the same case study. The results of this research show the strong dependency of the PD parameters on the temperature distribution varying over time during a load transient.
... Recently, many researchers have proposed new techniques useful to assess the quality of HVDC joints based on the detection of partial discharges [14][15][16][17][18][19]. ...
... While polymeric insulation materials for HVDC cables, such as XLPE or PP (polypropylen), have been investigated in a wide number of scientific works, further research work on materials for HVDC cable accessories is still underrepresented. Evaluations focusing on ethylen-propylene-dienmonomer (EPDM) and ethylene-propylene-rubber (EPR) compounds [6], [14]. However, silicone rubber (SIR) has been used in HVAC applications up to U m = 550 kV due to its thermal stability, mechanical characteristics, and good dielectric properties. ...
HVDC cable accessories play a significant role for the reliable operation of polymeric HVDC cable systems. The majority of materials used for HVDC cable accessories are based on EPDM or EPR compounds. However, the development and qualification tests conducted on HVDC cable systems show positive insights on the general application of silicone rubber compounds for HVDC components. For this purpose, a deeper investigation of the conduction mechanisms of silicone rubber compounds is required. Within this paper, the apparent conductivity of silicone rubber compounds under variation of electric field and temperature is studied and novel modelling aspects are presented. Besides the general conductivity dependence on electric field and temperature, the time dependent long-term behaviour under application of a dynamic temperature profile is considered. The measurement results show a positive agreement to empirical model descriptions based on thermal activated conduction extended by parameters for the modelling of time-dependent aspects. A minor and temperature-dependent electric field dependency is found for both material configurations.
The high temperature superconducting (HTS) AC and DC cables are gradually finding their way towards commercialization due to their ability to transfer bulk electric power with negligible dielectric loss. In this context, this paper critically reviews the different aspects of HTS cables used in various applications and related accessories. The operational, lab-scale, and prototype HTS cable projects are discussed in detail. Different dielectrics, electrical insulations, superconductors, cooling systems, cryogens, and upcoming projects of HTS cables are reviewed. The underlying challenges and future directions in AC and DC cables, electric aircraft and ships, associated costs, cryogenic insulation materials, cooling technologies, superconductors, and electrical insulations of HTS cables are discussed in detail. The space charge performance of presently-used polypropylene laminated paper (PPLP) insulation and future insulation, i.e., Kapton material, is experimentally analyzed, to show the future challenges in the insulation design. The technology readiness level, market trends, and relevant test standards are explored. The research and market trends suggest that HTS cables can replace conventional transmission methods. However, their consistent growth and adaptation of HTS cables will depend on design of cryocoolers, raw material cost of superconductors, and large-scale production.
High Voltage Direct Current (HVDC) underground and submarine cables constitute an essential technology for the long‐distance transmission of renewable electrical power with low losses, thereby fostering decarbonisation. In the past 2 decades, all this yielded a quasi‐exponential rise of HVDC cable systems installed worldwide, but has also made the working conditions of HVDC cables harsher. In this paper, the contribution of HVDC cables to decarbonisation is tackled, with particular reference to the situation in Europe, and a few simple calculations showing in general terms the quantitative contribution provided by HVDC cable systems to decarbonisation are given. Then, the major issues towards long‐lasting and reliable HVDC cable systems are summarised briefly, focussing—for the sake of brevity—on the influence of the main HVDC cable technologies, of cable laying environment and of duty service. Some hints are also provided at the chances given by multiterminal HVDC systems.
High voltage direct current (HVDC) cable systems with extruded insulation are becoming increasingly attractive versus the traditional mass impregnated non-draining (MIND) HVDC cable systems [1]. Voltage and power ratings of HVDC extruded cable systems commercially available-after being qualified according to CIGRE Technical Brochure (TB) 496 [2]-are exceeding 500 to 600 kV and 1 GW, respectively [3]-[6]. Of course, HVDC extruded cable systems still have issues.
Usability of a new Direct Current Periodic
waveform for partial discharge qualification
in HVDC systems is exemplified by tests
performed on different materials.
A partial discharge detection device for detecting and measuring partial discharges in electric systems or components, which delivers signals having a form much resembling that of the radiated pulse, for improved identification and analysis. The device is of small size, totally insulated and self-powered, and allows measurements to be performed with the highest safety with no need for direct connection, thereby allowing operators to stand at a distance and avoid any system shutdown while making measurements. Furthermore, it can also detect and deliver the sync signal, which is obtained by picking up the supply voltage of the discharge generating components.
In this paper, the features of a compact spherical antenna, already efficiently used for ultra wide band applications, are investigated by exploiting the possibility of detecting electromagnetic transients with frequency content up to the very high frequency range. Transient phenomena with the above mentioned frequency content are those encountered, for example, in partial discharge detection. The behaviour of the adopted compact spherical antenna enables to successfully detect the waveform of typical partial discharge transient radiated electromagnetic field, so preserving the information necessary for the subsequent pattern analysis, typically carried out by using classical conducted measures results. The compact antenna is firstly modelled by using two different moments method approaches in frequency domain: surface and thin-wire models are adopted in order to investigate different case studies so obtaining useful information about the behaviour of the antenna both in frequency and in time domain. Some comparisons with measured transient electromagnetic field data obtained with a real spherical antenna probe enable to successfully validate the use of the compact spherical antenna in detecting electromagnetic transients with frequency content up to the very high frequency range. A recent patent has been obtained also by one of the author by using this type of sensor.
A concise review is given of the progress made in the field of partial discharges (PD) at DC voltage. Although ample reference will be made to work of other authors in this field, the paper will concentrate on the progress that was made at Delft University of Technology over a period of 14 years in three PhD projects. In the first project, a start was made with the analysis of the physics of partial discharges at DC voltage and different types of PD were characterized based on parameters like time interval between PD and PD magnitude. In a second project, PD analysis was applied to HVDC apparatus and different means of classification of PD at DC voltage were proposed. In the third project, PD analysis was applied to HVDC mass-impregnated cables and test specifications were proposed. In this paper the work performed in the above three Ph.D. projects is summarized with ample reference to papers of other workers in this field. Attention is given to the mechanism of PD at DC voltage as compared to AC voltage and techniques for measurement and analysis of DC PD patterns. Examples of practical application of DC PD testing are given. Finally, some thoughts on future work are presented.
The fundamental theory of the UHF method for detecting partial
discharge (PD) in gas insulated substations (GIS) is presented. The
effects of position, size, current amplitude and pulse shape of the PD
source on the UHF signal can be predicted using this theory. Excitation
of propagating electromagnetic waves by a PD current pulse within the
coaxial waveguides formed by GIS components is explained by making use
of dyadic Green's functions for the electric fields of propagating
modes. Experiments with a coaxial test chamber are used to verify the
theoretical predictions, and comparisons are made between measured and
simulated UHF signals. Some implications for the UHF measurement of PD
are discussed, together with positioning and sensitivity requirements
for UHF couplers. A scheme is proposed for standardizing PD measurements
made using the UHF technique
This position paper by the DEIS HVDC Cable Systems Technical Committee provides a review of existing diagnostic electrical and dielectric techniques for testing the insulation of polymeric extruded HVDC cable joints in the present Part 1. Here, the state of the art on the insulation of HVDC extruded cable system joints is covered with reference to types, design and testing techniques. This helps to identify routine tests as the first target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers' practices for quality control of the insulation of accessories during routine tests and VHF/UHF wireless sensors as the best tool for performing such measurements on joints in the noisy factory environment. Thereby, a novel protocol for the measurement of partial discharges using AC voltages and VHF/UHF sensors, for quality control during routine tests on such joints, is derived in the next Part 2. This protocol is the main novelty of this investigation.
This position paper by the DEIS HVDC Cable Systems Technical Committee provides a review of existing diagnostic electrical and dielectric techniques for testing the insulation of polymeric extruded HVDC cable joints in the present Part 1. Here, the state of the art on the insulation of HVDC extruded cable system joints is covered with reference to types, design and testing techniques. This helps to identify routine tests as the first target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers’ practices for quality control of the insulation of accessories during routine tests and VHF/UHF wireless sensors as the best tool for performing such measurements on joints in the noisy factory environment. Thereby, a novel protocol for the measurement of partial discharges using AC voltages and VHF/UHF sensors, for quality control during routine tests on such joints, is derived in the next Part 2. This protocol is the main novelty of this investigation.
Medium voltage (MV)/high voltage (HV) cable lines are high-value assets and require substantial costs for their installation or replacement. For this reason, the capability to assess the condition of cable lengths and accessories on site is of great importance. Cross-linked polyethylene (XLPE) insulation is extensively used for MV, HV, and extra-high voltage (EHV) class cables [1] thanks to its excellent voltage-endurance and thermomechanical properties. In particular, XLPE-insulated cables feature substantially lower losses, easier manufacturing and jointing procedures, better environmental compatibility, and higher operating temperature compared with impregnated paper cables, which lead to lower costs in procurement and operation [2], [3].
The only book on the market that provides current, necessary, and comprehensive technical knowledge of extruded cables and high-voltage direct-current transmission. This is the first book to fully address the technical aspects of high-voltage direct-current (HVDC) link projects with extruded cables. It covers design and engineering techniques for cable lines, insulation materials, and accessories, as well as cable performance and life span and reliability issues. Beginning with a discussion on the fundamentals of HVDC cable transmission theory, Extruded Cables for High-Voltage Direct-Current Transmission: Advances in Research and Development covers: Both the cable and the accessories (joints and terminations), each of which affects cable line performance. The basic designs of HVDC cables-including a comparison of mass insulated non-draining cables with extruded HVDC cables. The theoretical elements on which the design of HVDC cables is based-highlighting the differences between HVAC and HVDC cables. Space charge-related problems that have a critical impact on extruded insulation for HVDC application. Recent advances in extruded compounds for HVDC cables such as additives and nano-fillers. The improved design of extruded HVDC cable systems-with emphasis on design aspects relevant to accessories. Cable line reliability problems and the impact on cable system design. Including more than 200 illustrations, Extruded Cables for High-Voltage Direct-Current Transmission fills a gap in the field, providing power cable engineers with complete, up-to-date guidance on HVDC cable lines with extruded insulation. © 2013 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.
Recently, the number of installed high voltage DC (HVDC) links using cables with extruded insulation has dramatically increased, since extruded insulation has several advantages over traditional oil-paper insulation for cables: easier splicing, no oil use, and higher service temperature [1], [2]. Conversely, a disadvantage of HVDC extruded cables is space charge (SC) buildup with HVDC. This is a critical issue because it further complicates the nontrivial DC electric field distribution within the insulation, which is also affected by temperature [1]. It is for this reason that considerable R&D of HVDC extruded cables has been devoted to the measurement of SC, and this has contributed to attaining the present day voltage and power ratings of 320 to 400 kV and 1,000 MW per bipole, respectively [3].
It is the intent of this book to combine high-voltage (HV) engineering with HV testing technique and HV measuring technique. Based on long-term experience gained by the authors as lecturer and researcher as well as member in international organizations, such as IEC and CIGRE, the book will reflect the state of the art as well as the future trends in testing and diagnostics of HV equipment to ensure a reliable generation, transmission and distribution of electrical energy. The book is intended not only for experts but also for students in electrical engineering and high-voltage engineering. © Springer-Verlag Berlin Heidelberg 2014. All rights are reserved.
A portable instrument based on an antenna probe and suitable for partial discharge detection, have been characterized through both laboratory and on-site experiments carried out on coils and stator windings of ac rotating machines. The antenna probe allows the capture of electromagnetic waves in a bandwidth of 0.1-100 MHz. Pulse signals and ac supply voltage reference are obtained by means of high-pass and low-pass filters, respectively, so avoiding the need of a direct connection with the objects under test. By this way, real-time phase resolved partial discharge patterns can be derived. Results of a comparison of the diagnostic information provided by PD patterns due to conducted and irradiated PD signals, are reported in the paper. It is shown that the diagnostic information remains the same when the sensor is moved along the tested object thus allowing the defect location and its identification.
A modern, reliable and ecological energy transfer from power producers, such as power plants or wind parks, to the sometimes far-off energy consumers, like industrial plants, large cities or off-shore oil platforms, can be realized efficiently by HV and EHV AC as well as DC cable systems or through gas- insulated transmission systems (GIL). These cable systems replace traditional overhead transmission lines more and more. One can experience that the general tendency for the manufacturing of cable systems at factories goes towards longer manufacturing units, especially with regard to the production of submarine cables. However, not only at the manufacturing of cable systems but also at the installation of these cable systems on site, the system lengths increase continuously. For the quality assurance after the manufacturing at the factories as well as after the installation on site, or also for the off-line monitoring of installed systems, a high voltage test is essential. Hereby, modern insulations can be tested sufficiently with alternating voltage only. This requires some 10 MVA testing power for the testing of super- long cables (SLC) and GIL. The generation of such enormous testing power through independent and mobile test systems at a justifiable technical effort can only be guaranteed by resonant test systems. This effort is optimized by the physical reasonably chosen resonant frequency. Therefore, frequency-tuned HV AC resonant test systems (ACRF) are the best solution for these special test applications. Based on elementary physical principles the right choice of test voltage, test frequency and testing power is demonstrated and clarified on some examples. The compliance of the test procedure with international test standards and recommendations is presented. Finally it is considered, what reserves can be raised for a further increase of the testing power to be expected in future.
High Voltage Direct Current (HVDC) Power Transmission Cables with Extruded Insulation and Their Accessories for Rated Voltages up to 320 kV for Land Applications -Test Methods and Requirements
High Voltage Direct Current (HVDC) Power Transmission Cables with
Extruded Insulation and Their Accessories for Rated Voltages up to 320
kV for Land Applications -Test Methods and Requirements, IEC
Standard 62895, 2017-05-11.
IEEE Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field Environment
IEEE Guide for Partial Discharge Testing of Shielded Power Cable
Systems in a Field Environment, IEEE Standard 400.3, 2006-02-05.
Test Methods for Partial Discharge Measurements on Lengths of Extruded Power Cable
Electrical Test Methods for Electric Cables Part 3: Test Methods for
Partial Discharge Measurements on Lengths of Extruded Power Cable,
IEC Standard 60885-3, 2015-04-09.
Bushings for DC Application
Bushings for DC Application, IEC/IEEE Standard 65700, 2014-03-19.