**8. Failure distribution over diagnostic parameters**

Concerning a failed cable group from the same generation the Weibull distribution has been calculated. The Weibull plots in figure 4 show the distributions of cable failures over the normalized dissipation factor for variable test conditions. It can be noticed that the test voltage does not influence the Weibull curves significantly. Also, there is no strong curve deviation comparing the Weibull plot of one cable generation to all failed cables. The dominating influence factor is the temperature, showing more suitable test (diagnostic) conditions in higher temperature regions due to a better allocation of the values.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 277

Finally, the difficulty of the data analyzing of such a complex data collection is only indicated. In principal, in order to fit the most suitable distribution function and to determine the most probable remaining lifetime of the sample, the data must be carefully and precisely selected. Thereafter, it is possible to evaluate parameters of the ageing models, ageing factor and as a final point to apply these results in field measurements. In this way the cables condition can be estimated and life-consumption prognoses can be developed.

In a long-lasting ageing experiment, the ageing characteristics and diagnostic possibilities of the insulation system of MV PILC cables have been investigated. The study could be divided in several phases: developing, realization and verification of the entire ageing system (ICAAS) and its hardware and software components; selection of the cable samples and installation in ICAAS; selection of the ageing parameters; ageing experiment with regular measurements of the diagnostic parameters; regular parametric studies on the tan*δ*, PD, RVM, PDC in different stages of the ageing process; correlation between electrical, environmental and condition parameters; chemical analyses of the mass and paper samples;

In the presented chapter dependencies of diagnostic parameters, like the PD and the tan*δ* characteristics are presented and discussed. The temperature region 20°C to 40°C is indicated as mainly inappropriate for measurements of cable's electrical properties and therefore diagnostic studies. PD diagnostics deliver a unique information with a mostly local character and should be executed and interpreted independently to other diagnostic measurements since considerable correlations between PD activity and tan*δ* profiles cannot

Additionally, Weibull plots for identical ageing-groupsof cables on different temperatures

Finally, one of the major future research objectives is and will be the further interpretation and development of theoretical models describing the correlations between the data from the determined sophisticated knowledge databank. Moreover, the developed models and

The authors would like to thank the following cooperating companies for the financial and organizational support of the entire project: N-ERGIE AG (Germany), N-ERGIE Netz GmbH (Germany), N-ERGIE Service GmbH (Germany), Bayerische Kabelwerke AG (Germany).

(20°C and 80°C) are shown, and the characteristic tan*δ* values are pointed out.

correlations will be verified through already started diagnostic field studies.

*University of Erlangen-Nuremberg, Institute of Electrical Power Systems, Germany* 

developing of the ageing/life models; reliability analyses, etc.

**9. Conclusion** 

be stated.

**Author details** 

I. Mladenovic and Ch. Weindl

**Acknowledgement** 

**Figure 18.** Weibull plots for variable tests conditions shown for all failed cables and one cable generation

Nevertheless, experimental results have also shown that temperatures lower than 20 °C are suitable for diagnostic measurements, too. Anyway, regarding to Figure 18 and Figure 19, the critical *<sup>n</sup>* tan tan -interval measured at 20°C is in the region of 1,8-2,4, and at 80°C it is within 13-14.

**Figure 19.** Weibull plot for 20°C, variable test-voltage and for all failed cables or one cable generation

Finally, the difficulty of the data analyzing of such a complex data collection is only indicated. In principal, in order to fit the most suitable distribution function and to determine the most probable remaining lifetime of the sample, the data must be carefully and precisely selected. Thereafter, it is possible to evaluate parameters of the ageing models, ageing factor and as a final point to apply these results in field measurements. In this way the cables condition can be estimated and life-consumption prognoses can be developed.
