**6. Artificial ageing experiment**

262 Dielectric Material

& Zaengl, 1986):

constant.

conductivity

have reference tan

(polarized) molecules, the charge carriers and the number of electrons having at least the necessary energy to overcome the potential barrier or in other words the activation energy *<sup>a</sup> E* , (Küchler, 2009). Therefore, there is a constant rise of conductivity, (Bayer, Boeck, Möller,

> con tan Ae

**Figure 6.** Structural dependencies of the dissipation factor tan

mobility. Finally, with higher temperatures *<sup>r</sup>*

where are: A - the pre-exponential factor, *T* is the absolute temperature and k Boltzmann

*<sup>a</sup> E T* /k

on temperature with three different polarization mechanisms (dashed blue line)

On the other hand, the polarization processes show different resonant phenomena over a wider temperature range. Appropriate temperatures drive different polarization processes. Even one polarization process like the orientational polarization can have e.g. two resonant temperatures due to the presence of different molecular structures, like e.g. moisture in the paper-mass insulation system. Therefore, with an increasing temperature the relative permittivity rises stepwise at discrete temperatures as a consequence of increased dipole

present, will develop faster leading to the complete breakdown of the insulation.

evaluated correctly. According to (Bayer, Boeck, Möller, & Zaengl, 1986) the tan

which results in a partial disorganization of the dipole arrangements caused by the field. The activity of PD, if any, will change according to ideal gas and Paschen's law, (Mladenovic, Determination of the Remaining Lifetime of PILC cables based on PD and tan(δ) diagnostics, 2012, to be published). Also, conductive channels within insulation, if

Since the temperature cannot be adjusted in field measurements, the temperature dependency of tanδ is inappropriate to be used as a diagnostic criteria directly. Although, it is very important to know its characteristic behavior for different ageing situations and to


(9)

, relative permittivity *<sup>r</sup>*

decreases again due to the thermal agitation

, and

> of MV

For a successful development of reliable ageing models, it is of prime importance to have different but constant ageing conditions and access to the regularly measured and monitored parameters up to the failure events. Therefore the characteristic key-values of the PD and tan were acquired selectively for each cable at least daily over the complete artificial ageing period of two years. Beside the main field of thermo-electrically aged cable samples, selected cables were set under thermal stress only, while another group cables was electrically aged. In this way it should be possible to determine the parameters in the ageing models and the influence of each stress type on the ageing rapidity. The thermal ageing can principally be modeled by Arrhenius law, the electrical ageing by e.g. the inverse power law. When concurrent stress conditions are applied combined and complex ageing models have to be developed and analyzed. In addition, the Weibull distribution function is fitted to the fault behavior or measurement data characteristics. Therefore, the most probable lifetime, i.e. the most probable time to the next failure and its dependency on the values of the diagnostic parameters, load conditions or cable temperatures can be evaluated.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 265

Since most of the system components are highly specialized, and therefore rather expensive or not available on the market, almost all of the ICAAS system was designed and built in the Institute's laboratories. Voltages of up to four times the nominal operating voltage, and currents large enough (up to 500 A rms) to heat the cable conductors to the desired temperatures above 100 °C, have to be generated by the developed ageing system. The voltage generation is bases on a resonant system (Figure 8). The series resonant circuit consists of the cable capacitance and a purpose-developed variable inductance coil with more than 3000 windings in more than 20 layers, and its inductance range up to 580 H,

Figure 9.

**Figure 8.** Structure of the voltage generating resonant circuit

**Figure 9.** The resonant coil - a key ICAAS component
