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Underground cables represent one of the biggest assets and investment demands of power utilities. In the same time they are the major source of faults and outages in medium voltage (MV) power networks. The oldest cable type, still present in a high percentage in today's MV power networks, is the paper insulated lead covered (PILC) cable. It was mainly laid in the period from 1920 to 1980, (Tellier, 1983), hereafter it has been systematically replaced by most distribution companies with thermoplastic polyethylene (PE) and finally cross-linked polyethylene (XLPE) cable types. Nevertheless, almost 95% of the MV power cable networks of "NUON Infra Noord-Holland" are made up of PILC cables, (E. F. Steennis, R. Ross, N. van Schaik, W. Boone & D.M. van Aartrijk, 2001), 65% of the network of one of the biggest energy supplier in Belgium, 56% in the urban areas in Bavaria (Germany), and ca. 50% of entire MV cable network of Germany. At the end of 20th century in Germany, as reported in (FGH - Forschungsgemeinschaft für Elektrische Anlagen und Stromwirtschaft e. V., 2006), there were more than 30% of cables over 30 years in service, and more than 15% over 45 years in field operation – almost all are PILC cables. Furthermore, this all corresponds to an cable network length of 110.000 km that consist only of cables which already have or soon will exceed the expected cable service life time of 40 years, and nearly 3,2 billion Euro of investments.

Within the years of service operation and especially when the predicted service lifetime is exceeded, the failure rate is expected to increase significantly. Sudden and unexpected cable failures mostly cause many incidental issues, additional costs and penalty payments. In order to optimize costs and to keep up or improve the reliability of the power system, more

The1presented chapter is further discussed in Ph.D. thesis "Determination of the Remaining Lifetime of PILC Cables based on PD and tanδ Diagnosticis," Mladenovic, 2012

© 2012 Mladenovic and Weindl, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and more utilities and distribution companies decide for condition based asset management and maintenance strategies. A sophisticated knowledge of the components actual condition and an early detection and prediction of service failures are therefore the bases for an efficient planning of the maintenance strategy and the resulting investments. For this purpose various diagnostic systems are used, which are mostly based on the measurement of the partial discharge (PD) activity and/or other dielectric key values like e.g. the value of the dissipation factor tan() at different test-voltage levels.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 253

Once the databank is formed-up and before the statistical analysis is applied, there is a necessity to discuss the physical dependencies of the relevant electrical properties on the environmental and test conditions and the ageing process of the paper-mass insulation system. Herby, the structure and the chemical background of the insulation components of the PILC cables will be shortly presented, and the way it could influence the ageing rapidity, the development of partial discharges and the thermal breakdown. The discussion of the behavior of the relative permittivity (ߝ (and conductivity (ߢ (with varying temperature and humidity, voltage and frequency as well as the influence of the impurities, cavities and bubbles presence on the electrical properties and ageing of the insulation material will follow. Hence, the main dependencies of the diagnostic parameters and their development can be explained, (Mladenovic & Weindl, Determination of the Characteristic Life Time of Paper-insulated MV-Cables based on a Partial Discharge and tan(δ) Diagnosis, 2008) (Mladenovic & Weindl, Dependencies of the PD- and tan(δ)-Characteristics on the Temperature and Ageing Status of MV PILC Cables, 2011) (Mladenovic & Weindl, Dependency of the Dissipation Factor on the Test-Voltage and the Ageing Status of MV PILC Cables, 2011) (Mladenovic & Weindl, Development of the Partial Discharges Inception Voltage for Different Sets of Pre-Aged PILC Cable Samples, 2010) (Mladenovic & Weindl,

Influence of the thermal stress on the diagnostic parameters of PILC cables, 2010).

published).

**2. Insulation system of PILC cables** 

**Figure 1.** Cellulose molecule (C6H10O5)n, (Colebrook)

The approach presented here, the entire ageing experiment, the data analyses and conclusions are systematically elaborated in (Weindl, Verfahren zur Bestimmung des Alterungsverhaltens und zur Diagnose von Betriebsmitteln der elektrischen Energieversorgung, 2012) Mladenovic, Ph.D., Determination of the Remaining Lifetime of PILC cables based on PD and tan(δ) diagnosis, 2012, to be published) (Freitag, Ph.D., to be

The insulation system of PILC cables is a complex and inhomogeneous structure of mass impregnated paper layers. During the operation, the electrical field is distributed so that the thin mass layers overtake a bigger part of the electrical field strength. The paper will keep the separation distance and will be a barrier to the impurities from layer to layer. An insulating paper (e.g. kraft paper consists of about 90% of long-chained macromolecules, Figure 1) i.e. cellulose fibrils, is formed by the polymerization of the glucose molecules.

Cellulose molecules arranged in fibrils have an immense tensile strength.

Unfortunately, there are still no well-established and physically - founded substantial criteria which define e.g. the probability of the next failure versus the PD or tan-levels for defined test voltages and test conditions. Hence,for further improvements of diagnostic systems and the prediction of failure-times a correlation of the field measurement data and parameters acquired under well-known laboratory conditions is necessary together with a following reference setting and interpretation. This could further lead to the development of physically oriented ageing models correlating the cable's level of lifetime consumption and several measured diagnostic parameters, their dependencies and development. The complex mathematical models can only be derived on the basis of a fundamental databank including cable specification data, service operation profiles and numerous electrical and diagnostic parameters monitored during and representing the complete cable life cycle. In this way, the assumption of the remaining life time will be based on numerous diagnostic measurements and parameters. A restriction to regular measurements and failure-time data out of the field would last in a monitoring process over several decades and more or less undefined, unknown or less reliable measurement conditions caused e.g. by the various influences of the equipment's temperature and its gradients on diagnostic parameters.

On this background, a system for artificial and accelerated ageing of MV PILC cables has been developed and realized, (Mladenovic & Weindl, Determination of the Characteristic Life Time of Paper-insulated MV-Cables based on a Partial Discharge and tan(δ) Diagnosis, 2008) (Dr.-Ing. Weindl & Dipl.-Ing. Mladenovic, 2009) (Mladenovic & Weindl, ICAAS – Integrated System for lasting Accelerated Aging of MV Cables, Data Monitoring and Acquisition, 2009) (Mladenovic I., 2009). The ICAAS (Integrated Cable Accelerated Ageing System) facilitates a realistic (50Hz) but accelerated ageing by applying pre-defined and concurrent thermal and electrical stressconditions with a highly sensitive and selective PDdetection and tan*δ* measurement. By controlling the technical and environmental conditions of the artificial ageing processes the ageing rapidity can be modified and increased. During the ageing experiment, a daily monitoring of the cable samples was realized by measuring the diagnostic parameters under pre-defined conditions and selective for each individual cable sample, (Freitag, Weindl, & Mladenovic, On-Line Cable Diagnostic Possibilities in an Artificial Aging Environment, 2011) (Freitag, Mladenovic, & Weindl, Fully Automated MV Cable Monitoring and Measurement System for Multi-Sample Acquisition of Artificial Aging Parameters, 2010). Moreover, the entire accelerated ageing process, all systemparameters and internal signals are monitored in close-meshed time intervals. Using a suitable set of pre-aged cabled samples, an ageing database of over 800GB was formed up that enables statistical approaches to determine the actual and integral ageing factor, the characteristics of the ageing process, the key ageing parameters, as well as their limits.

Once the databank is formed-up and before the statistical analysis is applied, there is a necessity to discuss the physical dependencies of the relevant electrical properties on the environmental and test conditions and the ageing process of the paper-mass insulation system. Herby, the structure and the chemical background of the insulation components of the PILC cables will be shortly presented, and the way it could influence the ageing rapidity, the development of partial discharges and the thermal breakdown. The discussion of the behavior of the relative permittivity (ߝ (and conductivity (ߢ (with varying temperature and humidity, voltage and frequency as well as the influence of the impurities, cavities and bubbles presence on the electrical properties and ageing of the insulation material will follow. Hence, the main dependencies of the diagnostic parameters and their development can be explained, (Mladenovic & Weindl, Determination of the Characteristic Life Time of Paper-insulated MV-Cables based on a Partial Discharge and tan(δ) Diagnosis, 2008) (Mladenovic & Weindl, Dependencies of the PD- and tan(δ)-Characteristics on the Temperature and Ageing Status of MV PILC Cables, 2011) (Mladenovic & Weindl, Dependency of the Dissipation Factor on the Test-Voltage and the Ageing Status of MV PILC Cables, 2011) (Mladenovic & Weindl, Development of the Partial Discharges Inception Voltage for Different Sets of Pre-Aged PILC Cable Samples, 2010) (Mladenovic & Weindl, Influence of the thermal stress on the diagnostic parameters of PILC cables, 2010).

The approach presented here, the entire ageing experiment, the data analyses and conclusions are systematically elaborated in (Weindl, Verfahren zur Bestimmung des Alterungsverhaltens und zur Diagnose von Betriebsmitteln der elektrischen Energieversorgung, 2012) Mladenovic, Ph.D., Determination of the Remaining Lifetime of PILC cables based on PD and tan(δ) diagnosis, 2012, to be published) (Freitag, Ph.D., to be published).
