Paper Insulated Lead Covered Research Articles

Lifetime Assessment of PILC Cables with Regard to Thermal Aging Based on a Medium Voltage Distribution Network Benchmark and Representative Load Scenarios in the Course of the Expansion of Distributed Energy Resources

The decentralized feed-ins from distributed energy resources (DER) represent a significant change in the manner in which the power grid is used. If this leads to high loads on electrical equipment, its aging can be accelerated. This applies in particular with regard to the thermal aging of older generations of power cables, namely paper insulated lead covered (PILC) cables. This type of power cable can still be found frequently in medium voltage (MV) networks. If aging of these cables is significantly accelerated in the presence of DER, distribution system operators (DSO) could face unplanned premature cable failures and a high replacement demand and costs. Therefore, this paper investigates the thermal aging of PILC cables in a MV distribution network benchmark for different load scenarios, using standardized load profiles and representative expansion scenarios for wind power and photovoltaics plants in particularly affected network areas in Germany. A main objective of this paper is to present a methodology for estimating the thermal degradation of PILC cables. An approach is used to draw simplified conclusions from the loading of cables to their conductor or insulation temperature. For this purpose, mainly Joule losses are considered. In addition, thermal time constants are used for the heating and cooling processes. Based on the insulation temperature, thermal aging is determined using the Arrhenius law or the Montsinger rule. However, it is important to note that there is an urgent need for research on reference data in this area. For this reason, the results of the lifetime estimation presented in this paper should only be considered as an approximation if the selected reference data from the literature for the aging model are actually applicable. The lifetime assessment is performed for a highly utilized line segment of the network benchmark. Accordingly, extreme values are examined. Different operational control strategies of DSO to limit cable utilization are investigated. The results show that the expansion of DER can lead to a short but high cable utilization, although the average utilization does not increase or increases only slightly. This can lead to significantly lower cable lifetimes. The possible influence of these temporarily high loads is shown by comparing the resulting cable lifetime with previous situations without DER. It is also shown that DSO could already reduce excessive aging of PILC cables by preventing overloads in a few hours of a year. In addition to these specific results, general findings on the network load due to the influence of DER are obtained, which are of interest for congestion management.

Analysis and evaluation of dielectric parameters for design verification and calibration of a newly developed diagnostic system for MV power cables

For the purpose of a condition-based asset management, in-depth knowledge of the equipment condition is needed. In medium-voltage power networks, paper-insulated lead-covered (PILC) cables are to see as special critical assets. In order to evaluate the most appropriate diagnostic parameters and their characteristic values, multiple PILC cables have been artificially aged at power frequency during a long-lasting laboratory ageing experiment. Over this process, the relevant parameters like dissipation factor and partial discharge behaviour have been monitored and recorded and a fundamental database was built up. As a next step, field measurements of these values need to follow. Therefore, a mobile measurement system has been furtherly developed and integrated in a cable test van. In order to get improved diagnostic criteria the mobile test system facilitates, that the dielectric properties of the PILC cables are determined at 0.1 and 50 Hz test voltage frequency. Here, the development, the calibration and the adjustment of the measurement system as well as influences on field measurements will be described and discussed.

Off-Line Field Diagnostics for MV and HV Oil-Paper Insulated Cables

Focusing on off-line electrical diagnostics this paper presents a technical approach, including acceptance criteria options, for in-situ field condition assessment of MV paper insulated lead covered (PILC) cable and HV/EHV self-contained fluid-filled (SCFF) cables. The proposed approach consists of a combination of off-line AC Hipot, partial discharge (PD) measurement and frequency domain dielectric spectroscopy. Case studies illustrate the successful practical application of thistechnical approach for diagnostics and condition assessment, including replacement prioritization or ‘return to service’ decisions on critical cables.

Extending the Functionality of On-line PD Monitoring Equipment for MV Power Cables

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.

Modeling partial discharge in a three-phase cable joint experiment with minimal adjustable parameters

A general physical model of partial discharge (PD) has been developed and used to simulate discharges within a void at the tip of a metallic spike defect within a three-phase 11 kV paper insulated lead covered (PILC) cable joint. Discharges are modeled by altering surface charge density at the void boundary using a logistic function distribution. The model was validated against experimental data, and a good agreement was observed with minimal free parameters. The model was then used to investigate the impact of single phase energization on PD activity in three-phase PILC cable joints. It was concluded that PD testing of three-phase PILC cable joints should be performed at raised temperatures with the cable fully energized as this results in a higher frequency of PD activity, and reduces the level of background PD from cable terminations. This research represents a further step towards developing PD models that can describe measurements taken from operational high voltage plant.

AC and impulse performance of medium voltage ethylene propylene- rubber cables with over 25 years of in-service aging in a wet underground environment

Many electric utilities, including Memphis Light, Gas & Water (MLGW), began installing underground residential electric (URD) facilities in the late 1960s. The earliest URD customers at MLGW were served by paper-insulated lead-covered (PILC) type cable which has a very good service history but was expensive and time-consuming to install. In an effort to install URD facilities at a comparable cost to traditional overhead facilities, direct buried high-molecular weight polyethylene (HMWPE) insulated cables were utilized. These unjacketed cables were constructed with a #2 AWG copper conductor, 6.1 mm (240 mils) of HMWPE insulation, and copper concentric neutrals. Unjacketed feeder cables were constructed with a 380 mm2 (750 kcmil) aluminum conductor, 6.1 mm (240 mils) of cross-linked polyethylene (XLPE) insulation, and copper concentric neutrals. Initially, feeder cables were direct buried but by the mid-1970s were installed in conduit. All cables were operated at either 12 kV or 23 kV phase-to-phase system voltages.

Autonomous classification of PD sources within three-phase 11 kV PILC cables

To allow utilities to fulfill self-imposed and regulative performance targets that apply to them, the demand for new tools to help judge the health of modern power distribution networks has increased. The analysis of partial discharge (PD) signals has been identified as a potential diagnostic tool for the condition monitoring of HV plant. In order to investigate the PD activity produced by a range of defects within three-phase paper insulated lead covered (PILC) distribution cable under rated conditions, an experiment has been developed. The experiment incorporates a commercially available on-line PD measurement system employing a high frequency current transformer (HFCT) to record PD data in a manner that is currently in operation in the UK. By replicating field conditions and using realistic hardware to collect experiment data, that any findings or analysis tools developed during this investigation are directly transferable to use in the field. Four defective cable samples, each containing different imperfections that are known to reduce in-service plant life have been fabricated and extensively PD tested. The raw experiment data was processed to produce a dataset containing a range of features from individual PD pulses including time, frequency and time-frequency information. This data was used to optimize and train several support vector machine (SVM) models to perform automated pulse classification. Four SVM models were tested using different combinations of pulse features to identify which characteristics were most effective at transferring source dependent information for classification. The results of the automated algorithm validated the approach returning a classification accuracy of 91.1%.

Advanced signal processing and modeling for partial discharge diagnosis on mixed power cable systems

Power cable grids have been growing enormously over the years; due to their aging, there is a strong need for Partial Discharge (PD) measurement techniques in order to identify incorrectly installed or damaged cable joints or cavities in cross-linked polyethylene (XLPE) insulated and paper-insulated lead-covered (PILC) cables. Mixed power cable systems consist of a mixture of interconnected cables of various types. The Time Domain Reflectometry (TDR) is a standard technique for the location of PD in power cables. However, the localization process on-the-field is impeded by various interferences and noise. Additionally, propagation velocities of PD pulses traveling on the line differ depending on the cable type (XLPE or PILC cables), which leads to significant location errors. This paper provides automatable signal processing tools for increasing the detection ratio as well as location accuracy in the field of mixed cable systems. A mixed cable system model provides the possibility of analyzing PD data of even complex cable interconnections.

On-line cable diagnostic posibilities in an artificial aging environment

Since more than a decade the development of reliable and accurate diagnostic tools for power cables is in the centre of interest of scientists and power utilities. With the de- velopments in our power systems, like the increasing feed-in of renewable energy sources, several issues could be additionally generated, such as extreme concurrence factors, short- and long- duration variations, voltage imbalances, waveform distortions, voltage fluctuations and power frequency variations. These impacts affect the aging rapidity of the electrical equipment and finally of the entire the electrical distribution network. Using appropriate diagnostic methods, it is possible to determine the electrical equipment's condition. The significance of the results depends on the quality of the measurement system and of the data interpretation. Based on these results, a reliable mainte- nance and investment strategy could be made, and the reliability of the grid could be improved. With this goal, an accelerated aging project for MV PILC (paper insulated lead covered) cables was started. A specially designed aging system for the acceler- ated aging of MV cables has been developed to point out the most relevant aging parameters, their limits, their development and to upgrade the accuracy of the MV cable diagnostics in this way. In this paper, the diagnostic possibilities, mainly the dielec- tric loss factor measurement and its dependence on environ- mental influences are presented.

Life extension and condition assessment: techniques for an aging utility infrastructure

Many utilities are focusing their efforts on developing methods to assess the condition of equipment to extend the life of existing infrastructure. In many cases, these utilities focus their efforts on extending the life of power transformers and paper-insulated lead-covered (PILC) cables because these components offer the greatest payback. Extending the life of aging infrastructure requires the use of noninvasive condition assessment and online preventive techniques that can easily be implemented in the field. Such condition monitoring, assessment, and life extension techniques will help utilities decide whether an aging transformer or underground PILC distribution cable can be operated reliably beyond its design life.

Condition assessment of distribution PILC cables from electrical, chemical, and dielectric measurements

A study was conducted to evaluate some new diagnostic techniques enabling users to effectively manage their paper-insulated, lead-covered (PILC) cable assets. The objective is to present the results obtained with four diagnostic techniques designed for use on in-service cables, but used only to a limited extent on PILC cables. These techniques are LIpATEST leakage current test method, isothermal relaxation current (IRC), return voltage method (RVM), and dielectric spectroscopy. In addition, the AC breakdown strength of the cable was also measured. The differences between the titration measurements and the predictions made by AC breakdown and the non-destructive techniques, such as RVM and dielectric spectroscopy, were noted with specific reference to moisture content in the cables. The different diagnostic techniques used in the study gave a reasonably consistent and coherent evaluation of the insulation condition of several field-aged PILC cables. The moisture content deduced from RVM and dielectric spectroscopy does not agree with Karl-Fischer titration of the same samples. It is suggested that aging induces more detrimental polarization processes in the cable oil, whose impact on electrical properties is still unknown but appears to be much more influential than moisture for the cables tested. The results confirm a previous suggestion that PILC cables kept under load are not significantly affected by water ingress.

Problems and Experiences with Polyethylene Jackets on Metal-Sheathed Cables

Objectives, test methods, results, and conclusions are detailed for four projects involving polyethylene protective jackets for underground high-voltage metal-sheathed cables. These projects involve: (1) evaluation of jacket materials, (2) the effect of fault currents on cable jackets, (3) the effect of switching surges on cable jackets, and (4) protection of joint housings in jacketed PILC (paper-insulated lead-covered) cable.