**3. Ageing mechanisms of mass-impregnated paper**

During the operation, the insulation of the PILC cables is exposed to numerous effects like temperature, electrical field, moisture presence (invasion), mechanical stress, which causes over the time steady insulation degradation. Herby, the paper presents the weaker component, and it's therefore the main cause of the insulation ageing. As presented in (Bennett, October 1957) the degradation of the mass is an oxidative and slow process. Moreover, oxygen presence in the cable insulation is not worth of consideration differing it from the oil insulation in transformer. There are several mechanisms of the impregnated paper degradation like hydrolytic, oxidation, thermal and electrical degradation (Emsley & Stevens, 1994). In Figure 2 the degradation of cellulose is shown.

During field operation, there is a regular thermal degradation of the components insulation. The maximal nominal operating temperature of the 12/20 kV three core PILC cables is 65°C, (Glaubitz, Postler, Rittinghaus, Seel, Sengewald, & Winkler, 1989). Temperatures in the range lower than 200°C, in the absence of oxygen and moisture tends to open glucose rings 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 destruction of the solid components.

254 Dielectric Material

circuits.

dielectric loss angle

Strauss, 2004).

**3. Ageing mechanisms of mass-impregnated paper** 

Stevens, 1994). In Figure 2 the degradation of cellulose is shown.

The length of the cellulose chains defines the degree of polymerization (DP), the number of

*molecular weight of base unit*

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

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

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 &

During the operation, the insulation of the PILC cables is exposed to numerous effects like temperature, electrical field, moisture presence (invasion), mechanical stress, which causes over the time steady insulation degradation. Herby, the paper presents the weaker component, and it's therefore the main cause of the insulation ageing. As presented in (Bennett, October 1957) the degradation of the mass is an oxidative and slow process. Moreover, oxygen presence in the cable insulation is not worth of consideration differing it from the oil insulation in transformer. There are several mechanisms of the impregnated paper degradation like hydrolytic, oxidation, thermal and electrical degradation (Emsley &

During field operation, there is a regular thermal degradation of the components insulation. The maximal nominal operating temperature of the 12/20 kV three core PILC cables is 65°C, (Glaubitz, Postler, Rittinghaus, Seel, Sengewald, & Winkler, 1989). Temperatures in the range lower than 200°C, in the absence of oxygen and moisture tends to open glucose rings

is less dependent on the temperature. One of its disadvantages, as

(1)

*molecular weight DP*

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

glucose units that make up one polymer molecule:

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 incomplete.

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

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 (Emsley & Stevens, 1994) the chemical mechanisms of low-temperature (<200°C) degradation of cellulose is thoroughly described.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 257

or less complementary to one other and summarized could deliver more complete and more

The dissipation factor is the tangent of the loss angle defined as the phase shift between

Losses in the dielectric are caused by the moving of the charges under the influence of

and the reactive power ( *Qc* ) through the component *<sup>c</sup> I* , Figure 4. Thus, dissipation factor

tan .

*c*

*P Q* 

) are determined through the current component *I*

(2)

**Figure 4.** Simplified equivalent circuit and vector diagram of a real dielectric

reliable information about insulation condition.

**Figure 3.** Principal overview of diagnostic systems for power cable

**4.1. Dielectric losses and dissipation factor** 

leakage current and the applied test voltage to 90°.

applied electrical field. The real losses (*P*

could be determined as:

tan

Moreover, through the transients, short circuits or load variations in field operation, the cable temperature could vary and change very suddenly. As it was shown in (Soares, Caminot, & Levchik, 1995) there is no influence of the temperature increase (heating rate) on activation energy; it was opined that thermal decomposition of the kraft paper mostly proceeds through steady depolymerisation. Anyway, hydrolytic degradation is the most powerful degradation in cellulose, and due to the moisture outcome it could be initiated by both, thermal or oxidative degradation.

Obviously, ageing is a complex chain cause-reaction-cause process. Summarized, it results in the decrease of DP, increase of moisture content, and appearance of different gasses, resulting in the changing of electrical and mechanical properties of the insulation system.
