**6.3. Measurement techniques**

266 Dielectric Material

input/output values.

discharge up to approximately 50 kV.

**6.2. Selection of cable samples and ageing parameters** 

Other requirements of the resonance circuit are a duty cycle of 100%, exceeding the specifications of many of the standard test systems on the market, and absence of partial

The conductor current is generated using a custom-made pulse-width-modulated high current transformer, with an adequate electrical strength between the primary and the secondary side connected to the cable samples. In this way the ageing voltages up to 50 kV can be applied in parallel. Preselected voltage profiles and defined load patterns, as well as other technical parameters, are controlled by a custom-built measurement and control system. In Figure 7 some of the main analog values which are measured, controlled and saved in pre-defined time intervals are shown in yellow, while the digital signals are shown in grey. In order to record all the necessary data and to control the ageing conditions, a Supervisory Control and Data Acquisition (SCADA) system was designed and developed (Freitag C., Entwicklung und Implementierung eines Steuerungs-, Regelungs- und Messsystems zur Realisierung einer automatisierten Versuchsanlage für die beschleunigte Alterung von Mittelspannungskabeln, 2008). It handles more than 100 analog/digital

After the realization of ICAAS the first 24 installed cable samples were used within an pretest lasting for 6-9 months, in order to check and adjust the operation of the ageing and

The test field was made up of nearly 100 PILC cable samples, with a nominal voltage *<sup>n</sup> U* of 20 kV and a length of 13,5m, which were arranged in selective and representative agegroups. Most of the cables that were investigated had been in service for times between 20 and 60 years. Others were brand new or had been stored for 10 years. In Figure 10 cables stored in the ICAAS thermal tank, installed and prepared for the artificial ageing are shown.

measurement systems, and to point out the most appropriate ageing conditions.

Up to 64 samples can be artificially aged and monitored simultaneously.

**Figure 10.** Cable samples in the thermal tank of the ICCAS system

Of the greatest importance for the ageing experiments are, beside the selection of ageing parameters, the measurements of the ageing voltage, the cable conductor temperature, the leakage currents (mainly capacitive) through the cable insulation, the environmental temperature and measurements of the diagnostic parameters. In Figure 11 a partial overview over some ICAAS components is shown.

**Figure 11.** Partial overview over central components of the ICAAS system (transformers for voltage and current generation, overvoltage protection, rectifiers, etc.) and cable samples

Within the project period, several diagnostic measurement techniques have been developed, optimized and performed on each cable sample. Measurements of the dissipation factor on 50Hz, (Freitag C., to be published), with an accuracy of better then 10-5, as well as partial discharges are performed regularly in pre-defined time intervals (at least daily). Measurements of return voltage and polarization /depolarization currents have been carried out several times during complete ageing period.

The initial condition of all cable samples was documented using PD analysis, tan() measurements and return voltage measurements (RVM), (Mladenovic & Weindl, IEEE Electrical insulation Magazine, 2012). The resulting data are to be used as reference indicators for the later measurements. Anyway, the measurement of the PD is unique due to its local character, but should be correlated with measurements indicating the general cable condition like tan . However, a direct comparison of the measurement values is not possible due to their complex and not negligible dependency on environmental and test parameters, (Weindl, Mladenovic, Scharrer, & Patsch, 2010) (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, Comparison of the parametric Partial Discharges and Dissipation factor Characteristics of MV PILC Cables, 2012). For this reason series of parametric studies under controlled test conditions were made, with the aim of documenting the dependence of tan , PD and return voltage on the cable condition, test voltage and cable temperature. In the following, some characteristic profiles of the dissipation factor and PD for different ageing groups will be presented and discussed.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 269

(a)

(b)

(c)

**Figure 12.** Selected initial profiles of tan

and temperature dependency for the different cables

showing a resonant temperature region (a) and the voltage
