**2. Insulation system of PILC cables**

252 Dielectric Material

the dissipation factor tan(

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

Unfortunately, there are still no well-established and physically - founded substantial

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

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.


) at different test-voltage levels.

criteria which define e.g. the probability of the next failure versus the PD or tan

the equipment's temperature and its gradients on diagnostic parameters.

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.

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

The length of the cellulose chains defines the degree of polymerization (DP), the number of glucose units that make up one polymer molecule:

$$DP = \frac{molecular\ weight}{molecurl\ weight\ of\ base\ unit} \tag{1}$$

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 255

and break the glycosidic bonds. Finally, it results in free glucose molecules (decrease of DP) and release of moisture, carbon monoxide and carbon dioxide, as shown in Figure 2. Additional presence of moisture again accelerates the process of cellulose decomposition. If the temperature would exceed 200°C, other reactions would occur, including even

Moreover, gasses formed through the thermal degradation would fill the cavities (if there), presenting in this way an electrical weakness within the insulation and a potential PD source. An increase of the electrical stress would intensify PD activity producing additional gases in the locally heated area, leading further to the expanding of weak region. However, this chain process can also be interrupted by voids migration or by the voids refilling with the mass, as soon as the mass viscosity reaches a necessary value and it moves under the pressure caused by a temperature increase. The process of "moving" or "disappearing" PD sources makes the diagnostic of the PILC cables based on PD very complex, unreliable and

The presence of moisture leads to hydrolysis, the most dominant degradation process, where chemical connections are divided through the influence of water. Moisture could penetrate the insulation due to mechanical damages in the lead sheath, or it is a self-product of cellulose degradation through the other degradation processes, initiating in this way a chain reaction. This process could be additionally accelerated by higher temperatures. According to (Glaubitz, Postler, Rittinghaus, Seel, Sengewald, & Winkler, 1989) the maximal allowed short circuit temperature of three core PILC cables is 155°C. Therefore, the temperatures that could lead to the pyrolysis of the cellulose are not common in the field. In

destruction of the solid components.

**Figure 2.** Degradation of cellulose (Unsworth & Mitchell, 1990)

incomplete.

Properties of the cellulose and therefore paper are strongly dependent on DP.

For example, a new insulation paper has a DP of 1100-1300, which decrease with the operation time or ageing, reducing as a consequence the mechanical strength of the paper. A DP of less than 500 indicates significant thermal degradation, and finally, a DP of 200 is concerned as the limit of the mechanical strength and end of paper life, (Küchler, 2009), since weakening of mechanical properties could lead to e.g. cable failure during short circuits.

The structure of the impregnating compound was changed over the decades of manufacturing PILC-cables. At the beginning of 20th century, mineral oils have been used, followed by oil-rosin up to lastly non-draining (non-migratory) compounds in the time before World War II, (Bennett, October 1957). This poly-isobutylene compound – MIND (mass-impregnation with non-draining compound) held up to today. It presents the differently proportional mixtures of natural or synthetic resin, paraffin, bitumen and oil, (www.wikieduc.ch, 2010). Beside it is much more practical for handling; process of oxidation in MIND compound is much more slowly than in insulating oils. Also its dielectric loss angle is less dependent on the temperature. One of its disadvantages, as given in (Bennett, October 1957), is that brand new cables could contain numerous voids, due to the high expansion coefficient of the material. Also, the compound retains its nondraining properties up to 70°C, nowadays improved to 90°C, or even 100°C, (Kock & Strauss, 2004).
