**6.1. Functional principle of ICAAS**

As part of the project, thoroughly described in (Weindl, Verfahren zur Bestimmung des Alterungsverhaltens und zur Diagnose von Betriebsmitteln der elektrischen Energieversorgung, 2012), the fully automated and Integrated Cable Accelerated Ageing System (ICAAS), incorporating a realistic accelerated ageing model, was developed, Figure 7. It involves voltage and current generation, PD detection and measurement, tan measurement, as well as some other diagnostic parameters (Dr.-Ing. Weindl & Dipl.-Ing. Mladenovic, 2009) (Mladenovic & Weindl, IEEE Electrical insulation Magazine, 2012). Artificial ageing and diagnostic measurements are performed on each cable sample with nominal line-to-line voltage *Un* 20 kV and nominal current of *nI* 239 A . In order to ensure realistic accelerated ageing processes, the ageing and measurements were made under conditions comparable to normal service conditions at 50 or 60 Hz (IEEE Trial-Use Guide for Accelerated Aging Tests for Medium-Voltage Extruded Electric Power Cables Using Water-Filled Tanks, 1998), ("IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems, 2001).

T1air, T6air, T2cable,T4cable and T5cable are temperatures within the thermal tank. Tcable-e, T3cable-t are the temperatures of electrically and thermally aged cables respectively, and Tcable-ir is the temperature of the termination. Tprim, Toil and Tair are respectively the temperatures of the primary windings of the current generating transformer, the oil and the air. Istr, Ires, Iprim and Isec are respectively the controlling currents in the autotransformer, the resonant circuit, and the primary and secondary sides of the current generating transformer. UPD and I<sup>δ</sup> are respectively the diagnostic currents through the insulation. Ua and Ub are (redundant) ageing voltages measured by the voltage divider.

**Figure 7.** Simplified structure of the ICAAS ageing, measurement and control system

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

264 Dielectric Material

**6.1. Functional principle of ICAAS** 

Shielded Power Cable Systems, 2001).

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

As part of the project, thoroughly described in (Weindl, Verfahren zur Bestimmung des Alterungsverhaltens und zur Diagnose von Betriebsmitteln der elektrischen Energieversorgung, 2012), the fully automated and Integrated Cable Accelerated Ageing System (ICAAS), incorporating a realistic accelerated ageing model, was developed, Figure 7. It involves voltage and current generation, PD detection and measurement, tan

measurement, as well as some other diagnostic parameters (Dr.-Ing. Weindl & Dipl.-Ing. Mladenovic, 2009) (Mladenovic & Weindl, IEEE Electrical insulation Magazine, 2012). Artificial ageing and diagnostic measurements are performed on each cable sample with nominal line-to-line voltage *Un* 20 kV and nominal current of *nI* 239 A . In order to ensure realistic accelerated ageing processes, the ageing and measurements were made under conditions comparable to normal service conditions at 50 or 60 Hz (IEEE Trial-Use Guide for Accelerated Aging Tests for Medium-Voltage Extruded Electric Power Cables Using Water-Filled Tanks, 1998), ("IEEE Guide for Field Testing and Evaluation of the Insulation of

T1air, T6air, T2cable,T4cable and T5cable are temperatures within the thermal tank. Tcable-e, T3cable-t are the temperatures of electrically and thermally aged cables respectively, and Tcable-ir is the temperature of the termination. Tprim, Toil and Tair are respectively the temperatures of the primary windings of the current generating transformer, the oil and the air. Istr, Ires, Iprim and Isec are respectively the controlling currents in the autotransformer, the resonant circuit, and the primary and secondary sides of the current generating transformer. UPD and I<sup>δ</sup> are respectively the diagnostic currents through

the insulation. Ua and Ub are (redundant) ageing voltages measured by the voltage divider. **Figure 7.** Simplified structure of the ICAAS ageing, measurement and control system the diagnostic parameters, load conditions or cable temperatures can be evaluated.

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

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 discharge up to approximately 50 kV.

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 267

On the other hand, an inappropriate choice of the ageing conditions could result in too rapid ageing, caused perhaps by physical and chemical processes within the insulation system which do not occur under normal field operation. Thus, too high ageing currents and too high conductor temperatures could cause abnormal degradation of the paper insulation (cellulose) of the cables. Also, too low ageing temperatures would result in a low fault rate within the intended ageing period (two years). The determination of the electrical and environmental ageing conditions during the preliminary work is described in detail in (Mladenovic & Weindl, Determination of the Environmental Conditions for the Accelerated

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

**Figure 11.** Partial overview over central components of the ICAAS system (transformers for voltage

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

and current generation, overvoltage protection, rectifiers, etc.) and cable samples

out several times during complete ageing period.

Ageing of MV-PILC Cables, 2009).

**6.3. Measurement techniques** 

overview over some ICAAS components is shown.

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 input/output values.

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

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

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

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

On the other hand, an inappropriate choice of the ageing conditions could result in too rapid ageing, caused perhaps by physical and chemical processes within the insulation system which do not occur under normal field operation. Thus, too high ageing currents and too high conductor temperatures could cause abnormal degradation of the paper insulation (cellulose) of the cables. Also, too low ageing temperatures would result in a low fault rate within the intended ageing period (two years). The determination of the electrical and environmental ageing conditions during the preliminary work is described in detail in (Mladenovic & Weindl, Determination of the Environmental Conditions for the Accelerated Ageing of MV-PILC Cables, 2009).
