Abstract: Very low frequency (VLF) testing refers to AC testing in the frequency range of 0.01 Hz up to 1 Hz. VLF testing is increasingly becoming popular as an alternative approach for test objects which require significant reactive power such as power cables. However, reliable diagnosis of partial discharge characteristics under VLF voltage excitation is still lacking. The paper presents a comparative study between testing at normal power frequency and at VLF. Corona and surface discharges were produced using a needle/bowl and a short section of 11kV XLPE cable. Experiments were conducted over a range of frequencies from 50 Hz down to 0.08 Hz and the results were analyzed. They showed some similarity of PD characteristics in terms of their phase-resolved patterns. The discrepancies of PD results are described in details.

Abstract: Joule loss in conductor and eddy current loss in sheath are critical for calculating thermal distribution and ampacity of underground power cable. These two types of losses are usually well estimated in most cases by IEC-60287. However, the problem becomes complicated for the underground power cable system as analyzing these losses is not easy due to the electromagnetic coupling between a large number of cables in such system. Nevertheless, IEC-60287 has recommended a simplified method for calculating the ampacity of multi-circuit cable system by ignoring the proximate effect between circuits. In spite of inaccuracy, this simplified method is commonly used in practical applications for its ease of use. In this paper, an electromagnetic-thermal coupled model is proposed and numerically calculated by finite element method (FEM). A case study of 110 kV underground power cable system is given. Results show that the ampacity obtained from our proposed model agrees very well with that from IEC-60287 for one-circuit cable system, whereas it apparently deviates from what IEC-60287 recommends for multi-circuit cable system. Such deviation is primarily caused by the capability of our model to precisely calculate the losses.

Abstract: Modeling underground cables for application in harmonic calculations in frequency domain is discussed thoroughly in this paper. At first, equations and frequency-dependent parameters to describe the zero-, negative-, and positive-sequence cable impedances as well as the coupling between them are presented. Afterwards, the influence of cable arrangement on the zero-, negative-, and positive-sequence cable impedances is investigated over the frequency. In this regard, some representative cable systems with different arrangements are simulated. Physical interpretation of the simulation results for the different cable arrangements contributes to comprehend the behavior of underground cables over the frequency in zero, negative, and positive sequences. Subsequently, based on the findings, modeling underground cables for harmonic calculations in the frequency domain is discussed. Finally, the validity of the proposed model is proved by comparing the simulation results with measurements on a medium voltage cable.

Abstract: This paper concerns the effect of configuration of high voltage power cables on induced voltages in their metallic sheaths. One, two or three cables per phase and phase sequence of the cables are considered. Computing of the voltages was performed for a normal operation and in case of earth fault. Various ways of the sheaths bonding are considered as well. Shock hazard and voltage stress of non-metallic outer sheath of cables are evaluated. Favourable configurations of the power cable system are indicated.

Abstract: Since the early 1950s electric utilities have installed millions of miles of low-voltage underground cables to improve the reliability and resilience of secondary power distribution. Early cable technology included butyl rubber insulated conductors and paper insulated lead coated cables. Over the decades, age and heat have degraded the insulation, leading to catastrophic failures and underground fires that can pose a public and employee safety hazard, cause equipment damage, and result in prolonged power outages. Assessing the condition of underground cables is expensive. Most test techniques are invasive (power outages are required), intrusive (cables need to be disconnected), and destructive (high voltages are applied to create partial discharges). The novel active clamp assessment technology discussed in this paper accurately determines the condition of underground cable insulation. It is non-intrusive, non-invasive, and non-destructive. Results from simple tests provide accurate condition assessments used to prioritize cable replacement. The technology can be called a cable failure prediction system.

Abstract: An ever-increasing appetite for electrical energy by industrial, commercial, and household consumers has resulted in a continuously growing demand for transmission capacity and improved reliability of elecricity supply. The reliability of a HV power plant must be maintained at a high level over its lifetime, typically 50 years. All newly manufactured components of electricity networks undergo various types of tests to ensure their reliability and longevity. The tests follow specifications set out in national and international standards. These tests could be described in general terms as: design-verification tests,; production-quality tests, and ; tests after installation. Of all systems in a HV plant, cable systems must satisfy possibly the widest range of test requirements. So in this article we use electrical testing of cable systems to show the origins of the main test specifications. Although we refer throughout the paper mainly to cables with extruded insulation and their accessories, most of the discussion can be generalized to other HV equipment such as power transformers or insulators. We limit our discussion to insulated cable systems built to transmit electrical power in bulk at the subtransmission and transmission voltage levels, i.e., those rated above 30 kV.

Abstract: Underground cabling systems which includes cables and ducts play a vital role in the planning and assessment of electro-thermal coordination that will incorporate smart energy transmission and utilize the prevailing weather conditions in implementing future smart distribution systems. This paper investigates the effects of increased adequacy through real-time line thermal ratings on network losses and risk of ageing. The analysis is performed on a range of underground cable duct installation methods in a distribution network. The paper proposes a new technique of assessing the reliability of distribution systems when real-time line rating is implemented on power cables considering the ducting methods and accounting for the reliability risks of cable ageing. Probabilistic assessments on nine different ducting installations for a buried 22kV XLPE cable showed that an oil pressure type cable ducting has the lowest risk of asset ageing when real-time line rating is implemented on the network.

Abstract: Reliability of power cable insulation is the key to the security of power system. In this paper, A comprehensive method for the diagnosis on insulation condition of distribution Cross Linked Polyethylene cable is presented, which including on-line and off-line Partial Discharge test and Very Low Frequency dielectric dissipation factor test. The Level in addition to the location of Partial Discharge and the assessment for condition of cable can be done by respectively using Partial Discharge and Very Low Frequency dielectric dissipation factor test. Experimental results have proved that this method is useful to assess the condition of Cross Linked Polyethylene cable.

Abstract: Tan Delta testing is a diagnostic method of testing cables to determine the quality of the cable insulation. This helps create a systemized method to analyse aging of cables in the long run. It utilizes the phase shifts caused by the displacement current induced due to the existence of impurities in the insulation or stress on the cable. Current method for tangent delta measurement is performed at VLF (0.02 and 0.1 Hz) or power frequencies of 50 and 60 Hz. However the usage of high voltage at VLF or power frequency will require large cost and space. Alternate method is by using High frequency. The novel part of the HF experiment setting is that short samples of cables will be used (1m).A smaller testing environment is needed compared to LF experiment setting. Thus cost and space will be saved. However the utilization of short cables reduces the total capacitance of the cable. Therefore frequency (ω) has to be increased from power frequency to high frequency to obtain sufficient induced current. The utilization of HF will create high electric stress on the dielectric thus inducing dielectric current that is measureable which proves crucial in determining actual Tan delta values to indicate the real condition of the cable for diagnostic purpose.

Abstract: In order to address the detection and localization of partial discharge (PD) in cross-bonded (CB) high voltage (HV) cables, this paper investigates the characteristics of PD propagation in CB HV cables. On-line PD detection and location in power cables has been a research topic for many years. The issue has not been solved due to surrounding interference and complicated propagation modes in cable joints. It becomes even more complex with CB cables. The main challenge to PD monitoring of CB cables is as a result of the interconnectedness of the sheaths of the three single phase cables. The cross-bonding of the sheaths makes it difficult to localize which of the three phases a PD signal has emanated from. Co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. A second challenge is, therefore, the coupling characteristic when a PD pulse propagates in HV cable joints and the co-axial cables, making PD detection and localization more complex. The paper presents experimental investigations into PD pulse coupling characteristic between the cable center conductor and the sheath and the behavior of PD pulse propagation in CB HV cables, which can be used for PD localization in CB HV cable system.

Abstract: Cable terminations are known as the weak point of cross-linked polyethylene (XLPE) power cables. Therefore, power companies are constantly seeking new ways to create more effective cable termination designs. In order to improve cable termination design, it is required to understand the nature of weak points at the cable end. Partial discharge measurement is a non-destructive measurement method, which can be used to access insulation quality. Therefore, this work investigates partial discharge severity of unterminated cable ends under various conditions to observe the strongest factor that degrades the insulation quality at the cable end. Factors which determine good cable termination design may also be attained from the measurement results of this work.

Abstract: Underground residential distribution (URD) power cables are aged due to electrical, thermal, mechanical and environmental stress during their service. For utilities, the online monitoring and diagnostics of the dielectric conditions of the cables help in managing their cable asset. One of the cable online diagnostic methods is based on analysis of the capacitor switching surge transient voltages on cable. Switching transient voltages measured at both ends of cable carry significant information on the overall health condition of the cable insulation. An experimental MV power cable grid using field-aged and new cables was setup. Analysis of the switching transient voltages on the experimental grid using wavelet and principal component analysis will be presented. Significant-feature extraction and neural network based field-aged and new cable classification results will be shown.

Abstract: While the power cable industry throughout the world is moving towards using extruded cable systems for most new ac cable projects, some North American utilities continue the use of high-pressure pipe-type (HPPT) cable systems for specific projects because of the long history of reliable operation using oil-impregnated paper and because the systems afford favorable conditions where long pulling lengths are required. This paper discusses the use of a unique cable design and application of horizontal directional drilling (HDD) to install nitrogen gas-filled HPPT cable under a marine setting in order to minimize the impact on the environment. The HPPT cable system permitted very long cable pulling distances of more than 1.6km, and was able to meet the utility's power transfer needs. The paper summarizes challenges that were encountered during design and construction of the project.

Abstract: For decades DC (Direct Current) has been used to assess the condition of medium and high voltage power cables. There are several advantages using this DC test due to its low weight, portability, usability, and being cost-effective. Meanwhile, this DC method has its limitations including possible destructive consequences [1]. It has caused cable failures due to `charges' trapped in the insulation. This induced local field stress when AC power reenergized and accelerated the growth of water-trees. Therefore, recently the Very Low Frequency (VLF) test has been recommended to use as an installation test of XLPE insulated cables according to IEEE standards. In this paper the VLF cable test will be studied in detail for testing XLPE power cables using the Tan δ and Partial Discharge measurements. The focus is to detect any existing fault, defects, or irregularities at the inspection of the cables before any breakdowns occur. The test data on service aged XLPE cables used in hydro power generators will be analyzed.

Abstract: The prequalification test of 500 kV XLPE cables and accessories has been successfully finished at CEPRI(China Electric Power Research Institute)in partnership with two cable & accessory manufacturers, HANHE and VISCAS. The prequalification test program was aimed at assessing, for the first time in China, the long-term reliability of the 500 kV cable system and in particular the prefabricated joints. The cable system were installed in representative arrangement simulating actual installation conditions used in the underground cable network of China. During more than 8760 h long term test with daily thermal cycling to a maximum temperature of 95°C, key parameters like the voltage, heating current, surface temperature, and SF6 pressure were monitored. Following the long term test at 493 kV, lightning impulse tests were performed on the complete cable system. The final examination showed that no macroscopical electrical degradation, moisture ingress, leakage, corrosion or harmful shrinkage was found in the cable and accessories.

Abstract: “Nan'ao Island ±160kV multiterminal flexible DC transmission Project” developed by China Southern Power Grid is China first project that HVDC submarine cable and land power cable with XLPE insulation have been applied. This paper demonstrates the simulation verification design of the insulation thickness and electric stress of the ±160kV DC cables for this project and proves that the electric stress at any point of insulation complies with the performance of insulation material. And the type test and prequalification test of ±160kV DC cable system are also introduced to prove the long-term reliability of DC power cable and factory splice.

Abstract: On-site testing and diagnosis of distribution and transmission power cables is one of the issues of asset management within power utilities. Due to the importance of power cables in the transmission network it is of importance to test the initial condition during after-laying, to test the actual condition of HV power cables during operation and to test the repair work after a failure.

Abstract: The work presented in this paper is a summary of the technical brochure being prepared by CIGRE Working Group B123 that focuses on single conductor, high voltage, ac, land cable systems, excluding pipe type and GIL cables It considers extruded dielectric insulation, laminar dielectric insulation, single-point bonding or balanced cross-bonding, so that there is minimum ground return current Magnetic field management techniques are discussed that are currently used for underground transmission cable systems and the shielding effectiveness of practical methods is quantified The cable current de-rating aspects of the various field management methods are presented This paper does not cover any environmental or biological effects of EMF and does not discuss any specific levels of EMF

Abstract: Condition assessment of cable systems can today be performed at different voltage frequencies. This includes VLF testing at 0.1 Hz, 50 Hz (also on-line) and also at higher frequencies in the range of 200–500 Hz by using oscillating wave test techniques. An important parameter used during condition assessment is the inception voltage of partial discharges. If this voltage is well above the service stress the cable system is likely not aged by partial discharges during service. The underlying assumption during testing is that this voltage is independent of frequency.

Abstract: Polymeric insulated extruded power cables are extensively being used for power transmission and distribution in electrical systems Such cables are subjected to extensive high voltage testing for performance evaluation and quality control purposes During such tests, the cable ends have to be prepared carefully to make a proper end termination Usually de-ionized water terminations are used for this purpose Alternatively, cable ends are terminated using resistive or resistive–capacitive stress control methods This paper presents a method of calculating the voltage distribution across various terminations when subjected to ac as well as dc test voltages The proposed method is used to determine the effect of different parameters on the voltage and stress distribution on such cable ends

Abstract: For testing and partial discharge (PD) diagnostic purposes many different test voltage wave shapes and frequencies have been established over the past years. Their application is well proven and is guided by IEEE 400 norm. For testing purposes the voltage need to produce enough stress to lead failures to breakdown. The very low frequency (VLF) waveforms turn out to be very effective and economical for that purpose. For PD diagnostic voltages are needed, with waveforms close to power frequency and in its application non destructive for the test object. VLF voltage could cause in case of long excitation time during PD diagnosis unwanted breakdowns at weak spots, even if the applied voltage is not that high like it is used for withstand testing. Damped AC voltage (DAC), which is close to power frequency, is well proven to be very effective for partial discharge diagnosis and causes nearly no risk for breakdown due to the short excitation time even for critically aged cables. A new test and diagnostic system combining both, providing an effective test voltage for withstand testing and being non destructive for diagnostic measurements, is introduced recently. This paper describes the application and comparison of the new test and diagnostic system for partial discharge diagnosis by using DAC. Furthermore true VLF cosine rectangular withstand testing with accompanying PD monitoring is discussed. It is demonstrated that cosine rectangular VLF waveform delivers comparable results of PD parameter to judge the severity and to locate PD defects in MV cable systems.

Abstract: Partial Discharge (PD) is the most important issue that arise when testing service aged and non-aged cables. For long time, High Voltage Direct Current (HVDC) was used to detect the occurrence of PD activity in cables. Due to technical reasons, the HVDC was replaced by the Very Low Frequency (VLF). Wide range of frequencies and wave forms were used for this purpose. This paper presents an evaluation to the ability of VLF to detect successfully the presence of PD. The study was based on laboratory testing data collected from medium voltage cable having artificial defects. PD was measured on healthy and unhealthy samples of 12/20 kV cables with VLF as well as with power frequency. Different wave forms such as sinusoidal and rectangular with different frequencies in the range from 0.01Hz to 0.1 Hz were used.

Abstract: In this paper, a simple and efficient procedure to model a medium voltage cable directly in time domain is presented. The proposed methodology, considering the skin effects and dielectric losses, takes into account the frequency-dependent parameters of the line. Based on the estimated propagation speed of the presented model, the laboratory and field testing results show that the accuracy Partial Discharge location of a medium voltage cable can be achieved.

Abstract: The present invention relates to a mobile tester (2) for the VLF testing ("Very Low Frequency" testing) of a measurement object (3), in particular a high-voltage or medium-voltage cable, which has means for generating an AC voltage which is used as the test voltage and has an effective amplitude of greater than or equal to 1 kV and a frequency in the range of between 0.01 and 1 Hz, a connection element (9) for connecting the measurement object (3) to which the test voltage is to be applied, and means for measuring and evaluating the test voltage applied to the measurement object and the test current caused thereby. In a first operating mode, the tester (2) is set up to autonomously carry out a VLF test using the test voltage generated thereby. The tester (2) also has a communication interface (17) for emitting a synchronization signal (S) and/or for receiving an externally generated synchronization signal (S), and the tester is set up, in a second operating mode (parallel operating mode), to synchronize the generated test voltage with the test voltage generated by at least one further tester (2) of the same type on the basis of the synchronization signal (S).

Abstract: We present herein a compact voltage clamp for measuring electric field in monoconductor and multiconductors cables. The clamp exploits electro optic (EO) principle to diagnostic low or high voltage equipment. This device allows to measure electric field surrounding a multiwires power cable and permits to deduce amplitude, phase and location of each conductors in a cable. The field has been firstly theoretically investigated and numerically computed. The experiments validate the sensor performances such as dynamics and temporal resolution in an actual industrial environment.

Abstract: It is better than 20-years since the newer cable field-testing methods of Very Low Frequency (VLF), Tan Delta, and field partial discharge testing have been introduced to the field-testing arena. Over time these technologies have matured, spawned a new testing industry, IEEE standards, technical committees, and a plethora of technical papers. How has this affected the cable manufacturer and what is his perspective on this area of field-testing both as an acceptance and maintenance test method? This paper intends to discuss the guidelines established and some of the shortcomings that have been found. Although this paper may review the various standards and testing methods, its focus will be with the VLF test.

Abstract: In this paper the results of 20 years of type testing MV, HV and EHV cables, accessories and cable systems are presented. This survey is an update of a previous publication [1] and confirms the previously presented data. It shows that still 20 % to 50 % of all type tests on accessories result in a change in design or it results in stopping the type test. These results show the manufacturer the necessity of thoroughly testing new designs of cables and accessories. For the user of cable systems, these results indicate the value of purchasing type tested components or even systems. Interfacial problems show the importance of testing the combination of cable and accessories that will be used. Individually type tested components are not a guarantee that the combination will pass the type test. Especially for large cable projects, it is advisable to type test the desired combination of cable and accessories before installation commences.

Abstract: Measurement of switching transient voltages appearing at various locations of Underground residential distribution (URD) power cables are vital for the online monitoring and diagnostics of cable insulation condition. The cable elbow connectors are manufactured with capacitive voltage test points to indicate the power frequency voltage on the cable conductor. The cable elbow connector voltage test points are proposed to measure switching transient voltages in cables. The test points are also used to measure standard lightning impulse. Using this technique, less expensive cable elbow connectors can be calibrated and installed widespread in underground residential distribution power cables for surge voltage transient measurements.

Abstract: During the last 140 years, insulated cables have been applied for power transmission and distribution, as well as signaling and other purposes. While the basic functions of power cables remain unchanged — providing a conductor to carry current and a dielectric insulation to support the line-to-ground voltage — the materials and characteristics used in power cables have evolved. Cables experience faults, and some methods to find those faults have changed as technologies have developed. Classical bridge techniques are still used, often with improved null detectors, but modern methods such as time domain reflectometry and faulted circuit indicators provide novel methods for better accuracy and faster location. This paper summarizes recent work in both areas and provides background on cable failure modes and typical fault location methods.