Doctoral School of Electrical Engineering 

BME-VIK, Department of Electric Power Engineering

Supervisor: Dr. TAMUS Zoltán Ádám

Development and implementation of a novel electrode system for unshielded low voltage cables used in NPPs and PV systems

Introducing the research area

Low voltage (LV) power cables are of high importance in Nuclear Power Plants (NPPs) and Photovoltaic (PV) systems, as they are not only used to supply power, but the safe and reliable operation of these plants depends on these cables (Figure 1).

 

Figure 1 Low Voltage Cables Layout (a) NPPs [1], (b) PV systems [2].

During the service period of these LV cables, they encounter diverse types of stresses (Figure 2a.), which result in irreversible changes to the polymer materials (insulation and jacket) of the cable, known as aging. But in the case of NPPs and PV systems, in addition to other stresses, environmental stress has also gained due attention, as γ-irradiation and temperature in NPPs and thermal stresses in PV systems are common for several reasons (Figure 2b.). Temperature and radiation stress cause physiochemical changes inside the polymeric materials that affect the integrity of the cable polymer materials. It is therefore particularly important to assess the condition of the cable, known as condition monitoring (CM), either under normal operating conditions or design-based events (DBEs) or even for the case of the life extension of NPPs, as any premature failure of the LV cables can result in equipment unavailability of the equipment or even plant shut down or transient situations.

Figure 2 (a) Aging and Types of Stresses, (b) Cause of Temperature and Radiation Stresses.

Brief introduction of the research place

My research work was conducted at the Department of Electric Power Engineering, Budapest University of Technology and Economics, under the supervision of Dr. Tamus Zoltán Ádám. It is affiliated with the High Voltage Laboratory research group, which – in addition to education, research, and R&D laboratory – is also a commercial testing lab. Looking back on a rich history, the laboratory has been participated in several research topics and produced publications on lightning protection, electrostatics, live line work, and insulation diagnostics, etc. While my research work was executed under the KNN_16 funding scheme from the National Research, Development, and Innovation Fund of Hungary under Project No. 123672.

History and context of the research

Recently, a wide range of condition monitoring (CM) techniques have been developed and adopted (Figure 3) [3-8]. These CM techniques have focused on the measurements of electrical, mechanical properties, thermal analysis, and physicochemical properties, which are divided into destructive and non-destructive techniques.

Figure 3 Condition Monitoring Techniques used in Recent Times [3].

Electrical techniques are classified as non-destructive techniques, and in electrical techniques, researchers focused on LV multi-conductor shielded cables, where the goal was to separately study the effect of aging on the insulation and jacket material, as the integrity of both components is critical to maintaining the qualified conditions of cables. Measurement of electrical properties of insulation can be easily executed by the different connections of the test equipment to the shielding and conductors of the cable under test.

The research goals, open questions

The on-site measurement capability and non-destructive nature of the electrical/dielectric measurement methods necessitate the development of these methods for testing unshielded single conductor (1C) cables, such as power cables. However, in the case of the unshielded 1C cables, measuring the dielectric properties of the dielectric polymeric components, i.e., the jacket and core insulation, is not a simple task as the insulation and jacket together form a composite insulation. Because this cable has only one conductive core, the connection of the electrical test equipment is not obvious. Heeding the problem faced in the determination of aging in LV unshielded 1C cables, the aim of is research work was to study the impact of thermal and radiation stresses on the electrical/dielectric properties of the composite insulation (jacket and insulation) of LV unshielded 1C cables. The study focused on unshielded 1C LV cables used in NPPs and PV systems. The research goals are:

i. Implementation of non-destructive electrical CM methods on unshielded 1C cables.

ii. Identification of electrical aging markers through the estimation of electrical parameters to understand the degradation of the entire polymeric material of the cable, i.e., insulation and jacket.

iii. Establish a correlation between the electrical aging markers and the physical degradation of the various kinds of LV cable materials.

iv. To establish the practicability of non-destructive techniques as viable techniques instead of destructive techniques.

Methods

The main goal of this research is to implement non-destructive electrical techniques as viable CM techniques for LV unshielded 1C cables. One solution is using a surface electrode on the jacket. By connecting the equipment to the surface electrode and the conductive core, the resultant of the dielectric characteristics of the jacket and core insulation can be measured. Moreover, using non-destructive electrical CM techniques by this connection enables the evaluation of the overall state of the cable without separating the insulation and jacket, thus making the task of the overall condition assessment of the unshielded 1C cable feasible. In this research work, the electrical/dielectric parameters of the cable jacket and insulation are evaluated using dielectric spectroscopy over a wide frequency range, Extended Voltage Response (EVR), Polarization-Depolarization Current (PDC) while the mechanical properties are evaluated by Shore D hardness. Thermal and radiation aging result in cross-linking, chain scission, and oxidation, which affect the dielectric polymer properties such as conductivity and polarization. Electrical/dielectric properties such as complex permittivity, polarization, and depolarization current response to these properties, so the adopted techniques would help achieve the objectives of the research work.

Results

Development of Electrode System

By covering the outer surface of the unshielded 1C cable with a conductive layer, the application of non-destructive dielectric measurements can be implemented for the condition assessment of the cable (Figure 4). Using the proposed technique, several measurements were conducted on the thermally and irradiated aged unshielded 1C cable by using dielectric spectroscopy and EVR methods to prove the applicability of the measurement solution to this type of cable. The results of multiple measurements have also shown that the technique is suitable for the detection of global aging of the cable jacket and electrical insulation, however, it cannot detect the localized problems such as hot spots.

 

Figure 4 Cable sample preparation with the developed electrode system.

Dielectric Spectroscopy Measurement of Thermally and Irradiated Stressed NPP Cable

Although dielectric spectroscopy has been used to study the degradation in the LV NPPs cables, most research has focused on the shielded cables, where the focus of research has been solely on insulation. However, to extend the applicability of the technique to the unshielded 1C cable with the developed electrode technique, the impact of thermal stress and radiation stress on the XLPE/CSPE single conductor unshielded 1C power cable was studied (Figure 5).

 

Figure 5 Dielectric spectroscopy measurement (a) OMICRON Dirana Setup: 100 mHz – 1 kHz, (b) Wayne Kerr Precision Component Analyzer: 2 kHz – 500 kHz.

Figure 6 Cross plots of Real and Imaginary and part of permittivity v/s Hardness for (a) Thermally, (b) Irradiated aged LV NPP Cable.

The complete cable samples were exposed to an accelerated temperature of 120 ° C for 10 different periods and a dose rate of 500 Gy / h for 5 different periods. Dielectric spectroscopy was implemented in two frequency ranges: 100 mHz to 1kHz and 2 kHz to 500 kHz as a non-destructive electrical CM technique. Because there is no fix ground reference in the unshielded 1C cable, the application of the technique is not obvious. By connecting the surface electrode and the conductive conductor, the dielectric spectroscopy technique was implemented, as the jacket and insulation functioned as composite insulation.

As a result of the thermal stress, an increase in the real part of permittivity () at 0.1 Hz and the imaginary part of permittivity () at 100 Hz was observed, which is associated with the interfacial polarization. The monotonic increase of the permittivity at 0.1 Hz and 100 Hz for the irradiated samples also revealed the intensity of interfacial polarization. The linear variation of the selected electrical aging markers, i.e.,  at 0.1 Hz and  at 100 Hz, with Shore D hardness, and aging showed the implementation of the CM techniques to the LV unshielded 1C cable in the case of NPP environment (Figure 6).

EVR Measurement of Thermally and Irradiated Stressed NPP Cable

The EVR method is based on the determination of the insulation condition through conductivity and polarization processes, after the charging and discharging phase, respectively. In the past, the technique was applied only to HV and MV equipment and only to multiconductor shielded cables in the LV case. Its application to the NPP scenario has not been explored. To extend the application of the EVR technique to the NPP case, thermally and irradiated stressed XLPE/CSPE unshielded single-conductor cable samples were analyzed (Figure 7 (a)). The conduction-related decay voltage slope (S_d) increased with aging for both stress types, indicating an increase of conductivity with aging. While for thermal aging, the return voltage slope (S_r) initially decreased and then increased in later aging periods. The behavior showed an initial decrease and then an increase in interfacial polarization. The phenomenon of interfacial polarization was increased in the case of radiation stress, where an increasing trend with the absorbed dose was observed. The correlation values for the S_d and S_r with aging and Shore D hardness showed the applicability of the EVR technique to the cable type and the NPP environment (Figure 7 (b) and Figure 8).

Figure 7 (a) EVR measurement setup, (b) Cross plots of Decay voltage slope v/s Hardness for Thermally aged LV NPP Cable.

Dielectric Spectroscopy & EVR Measurement of Thermally Stressed PV Cable

The effect of the thermal stress on LV DC cables deployed in PV systems was investigated using dielectric spectroscopy, EVR, and Shore D hardness as the CM techniques. Thermal stress during the service period of PV cables is an important concern. The XLPO/XLPO PV cable with different chemical composition and size in comparison to the NPP cable was exposed to 120°C temperature for 12 aging periods.

 

Figure 8 Cross plots of (a) Decay voltage slopes, (b) Return voltage slope, v/s Hardness for Irradiated aged LV NPP Cable.

With the developed electrode system, dielectric spectroscopy and EVR methods were implemented on thermally aged XLPO insulated, XLPO jacketed LV unshielded 1C photovoltaic cable samples. It was revealed that:

- The  at 10 mHz and the  at 100 mHz show a good correlation with the Shore D hardness measurement results.

- The S_r decreases in the initial stages of thermal aging then begins to increase, indicating severe damage to the insulation. This trend has been proved by Shore D hardness measurement.

The results demonstrate that dielectric spectroscopy and EVR methods can be used as condition monitoring techniques for XLPO insulated and XLPO jacketed LV unshielded 1C photovoltaic cable (Figure 9). With aging, the cable became harder as shown by the Shore D hardness measurement, which supported the observations of the electrical measurements of EVR, as S_r values increased in the latter part of the aging period while a decreasing trend was observed in the initial stages of the aging periods. The increasing trend of S_r and the hardness values in the latter stages of the aging period show that the cable was damaged (Figure 10).

 

Figure 9 Cross plots of (a) Real part of permittivity, (b) Imaginary and part of permittivity v/s Hardness for Thermally aged LV PV Cable.

Figure 10 (a) Return voltage slope at 1 sec discharging time, (b) Shore D hardness, v/s Aging Time (Hours) for thermally aged LV PV Cable.

 

Expected impact and further research

Understanding the aging of polymeric materials in LV cables has recently become a focus of interest for researchers. This resulted in a search for suitable CM techniques to perform the task. But due to the complex composition of the materials and the complex aging, many new findings have yet to be unearthed. This research work is also a step forward in finding CM techniques where the adopted ones have the advantages of being simple, non-destructive, and compatible especially for unshielded 1C single conductor cables.

In the present research, the results obtained by the electrical and mechanical measurements showed the effectiveness of the techniques to be used as the non-destructive CM techniques for the unshielded 1C LV power cables, with different cables having different construction and composition and the cables being exposed to different stresses. Appropriate electrical aging markers were also selected with the objectives of the research in mind. The strong correlation and good trend with aging confirmed the authenticity of the aging markers. However, it is still believed that there are emerging research issues that can be addressed, including:

i. The results obtained in this work can be validated by chemical tests such as OIT, gel content, and FTIR, which provide a deeper understanding.

ii. The mechanical tests such as EaB of the LV unshielded 1C cables also help validate the findings of the electrical measurements.

iii. The estimation of remaining life was also a topic of interest for researchers. The aging markers selected in this work can be used to estimate the remaining life of the cables.

iv. Aging is a multi-factor phenomenon, where different stresses act simultaneously. Thus, the study of the behavior of the LV unshielded 1C cables in a multi-stress environment using the adapted methods can further help in understanding the degradation and make the methods more dependable.

v. Because the work was performed under laboratory conditions, the application of these methods to field conditions will aid in practical applications.

Publications, references, links

Journal Publications

[J1]   Mustafa, E., Németh, R. M., Afia, R. S. A. and Tamus, Á. Z., “Parameterization of Debye Model for Dielectrics Using Voltage Response Measurements and a Benchmark Problem,” Periodica Polytechnica Electrical Engineering and Computer Science, vol. 65 (2), pp. 138-145, 2021. (Q3, Scopus)

[J2]   Afia, R. S. A., Mustafa, E. and Tamus, Á. Z., “Electrical and Mechanical Condition Assessment of Low Voltage Unshielded Nuclear Power Cables Under Simultaneous Thermal and Mechanical Stresses: Application of Non-Destructive Test Techniques,” IEEE Access, vol. 9, 4531-4541, 2021. (IF = 3.745, Q1, Indexed: WoS, Scopus, SCIE)

[J3]   Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Dielectric Loss and Extended Voltage Response Measurements for Low Voltage Power Cables used in Nuclear Power Plant: Potential Methods for Aging Detection due to Thermal Stress,” Journal of Electrical Engineering. (IF = 1.18, Q2, Indexed: WoS, Scopus, SCIE, SCI)

[J4]   Afia, R. S. A., Mustafa, E. and Tamus, Á. Z., “Dielectric Spectroscopy of Low Voltage Nuclear Power Plant Power Cables Under Simultaneous Thermal and Mechanical Stresses,” Energy Reports, vol.6 (9), 662-667, 2020. (IF = 3.595, Q1, Indexed: WoS, Scopus, SCIE)

[J5]   Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Application of Non-Destructive Condition Monitoring Techniques on Irradiated Low Voltage Unshielded Nuclear Power Cables,”  IEEE Access, vol.8, 166024-166033,2020. (IF = 3.745, Q1, Indexed: WoS, Scopus, SCIE)

[J6]   Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Condition Assessment of Low Voltage Photovoltaic DC Cables under Thermal Stress using Non-Destructive Electrical Techniques,” Transactions on Electrical and Electronic Materials, vol. 21 (5), 503-512, 2020. (Q3, Indexed: WoS, Scopus, ESCI)

[J7]   Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Study of Electrical Integrity of Low Voltage Nuclear Power Cables in Case of Plant Life Extension,” IFIP Advances in Information and Communication Technology, vol. 577, 2020. (WoS, Scopus)

[J8]   Afia, R. S. A., Mustafa, E. and Tamus, Á. Z., “Investigating the Complex Permittivity of Low Voltage Power Cables Under Different Stresses,” IFIP Advances in Information and Communication Technology, vol. 577, 2020. (WoS, Scopus)

[J9]   Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Investigation of Complex Permittivity in XLPO based Photovoltaic DC Cables due to Thermal Aging,” Lecture Notes in Electrical Engineering, vol. 598 (1), 2019.

[J10]  Afia, R. S. A., Mustafa, E. and Tamus, Á. Z., “Thermal Aging of Photovoltaic Cables based Cross-Linked Polyolefin (XLPO) Insulation,” Lecture Notes in Electrical Engineering, vol. 598 (1), 2019.

[J11]  Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Condition Monitoring Uncertainties and Thermal-Radiation Multistress Accelerated Aging Tests for Nuclear Power Plant Cables: A Review,” Periodica Polytechnica Electrical Engineering and Computer Science, vol. 64 (1), 2019, pp. 20-32. (Q3, Scopus)

[J12]  Mustafa, E., Tamus, Á. Z., Afia, R. S. A. and Asipuela, A., “Thermal Degradation and Condition Monitoring of Low Voltage Power Cables in Nuclear Power Industry,” IFIP Advances in Information and Communication Technology, vol. 553, 2019. (WoS, Scopus)

[J13]  Afia, R. S. A., Tamus, Á. Z. and Mustafa, E., “Effect of Combined Stresses on the Electrical Properties of Low Voltage Nuclear Power Plant Cables,” IFIP Advances in Information and Communication Technology, vol. 553, 2019. (WoS, Scopus)

Conference Papers

(Peer-Reviewed)

[C1] Mustafa, E., Afia, R. S. A., and Tamus, Á. Z., “Impact of Thermal and Mechanical Stresses on the Degradation of XLPE/CSPE Nuclear Power Plant Cable: Analysis of the Dielectric Response at Very Low-Frequency Range,” in 8^th International Conference on Power and Energy Systems Engineering, CPESE’21, Fukuoka, Japan, September 10-12, 2021. (Under Review)

[C2] Afia, R. S. A., Mustafa, E., and Tamus, Á. Z., “Ageing Assessment of XLPE/CSPE LV Nuclear Power Cables Under Simultaneous Radiation-Mechanical Stresses,” in 8^th International Conference on Power and Energy Systems Engineering, CPESE’21, Fukuoka, Japan, September 10-12, 2021. (Under Review)

[C3] Mustafa, E., Afia, R. S. A., and Tamus, Á. Z., “Application of Novel Electrical Aging Markers for Irradiated Low Voltage Nuclear Power Plant Power Cables,” in International IEEE Conference AND Workshop in Obuda on Electrical and Power Engineering, CANDO-EPE ’20, Budapest, Hungary, Nov. 18-19, 2020.

[C4] Afia, R. S. A., Mustafa, E., and Tamus, Á. Z., “Extended Voltage Response Measurement of Low Voltage Nuclear Power Cables under Simultaneous Thermal and Mechanical Aging,” in International IEEE Conference AND Workshop in Obuda on Electrical and Power Engineering, CANDO-EPE ’20, Budapest, Hungary, Nov. 18-19, 2020.

[C5] Mustafa, E., Afia, R. S. A., and Tamus, Á. Z., “Investigation of Electrical and Mechanical Properties of Low Voltage Power Cables under Thermal Stress,” in IEEE International Conference on Diagnostics in Electrical Engineering (Diagnostika’20), Pilsen, Czech Republic, Sep. 1-4, 2020.

[C6] Afia, R. S. A., Mustafa, E., and Tamus, Á. Z., “Condition Assessment of XLPO Insulated Photovoltaic Cables Based on Polarisation/Depolarisation Current,” in IEEE International Conference on Diagnostics in Electrical Engineering (Diagnostika’20), Pilsen, Czech Republic, Sep. 1-4, 2020.

[C7] Mustafa, E., Afia, R. S. A., and Tamus, Á. Z., “Investigation of Photovoltaic DC Cable Insulation Integrity under Thermal Stress,” in IEEE 3^rd International Conference on Dielectrics, ICD’20, Valencia, Spain, July 5-9, 2020.

[C8] Afia, R. S. A., Mustafa, E., and Tamus, Á. Z., “Assessment of Nuclear Power Plant Power Cables Under Thermal and Mechanical Stresses,” in IEEE 3^rd International Conference on Dielectrics, ICD’20, Valencia, Spain, July 5-9, 2020.

[C9] Afia, R. S. A., Mustafa, E. and Tamus, Á. Z., “Evaluation of Thermally Aged Nuclear Power Plant Power Cables Based on Electrical Condition Monitoring and Regression Analysis,” in International IEEE Conference AND Workshop in Obuda on Electrical and Power Engineering, CANDO-EPE ’19, Budapest, Hungary, Nov. 20-21, 2019.

[C10]   Mustafa, E., Afia, R. S. A. and Tamus, Á. Z., “Electrical Integrity Tests and Analysis of Low Voltage Photovoltaic Cable Insulation under Thermal Stress,” in IEEE 7^th International Youth Conference on Energy, IYCE’19, Bled, Slovenia, July 3-6, 2019.

[C11]   Afia, R. S. A., Mustafa, E., Asipuela, A. and Tamus, Á. Z., “Non –Destructive Condition Monitoring of Nuclear Plant Power Cables,” in IEEE 7^th International Youth Conference on Energy, IYCE’19, Bled, Slovenia, July 3-6, 2019.

[C12]  Asipuela, A., Mustafa, E., Afia, R.S.A., Tamus, Á. Z., and Khan, M.Y.A., “Electrical Condition Monitoring of Low Voltage Nuclear Power Plant Cables: tanδ and Capacitance,” Proceedings of IEEE 4th International Conference on Power Generation Systems and Renewable Energy Technologies (PGSRET’18), Islamabad, Pakistan, Sep. 10-12, 2018, PAPERID-138, pp. 503-509.

[C13]   Mustafa, E., Afia, R.S.A., and Tamus, Á. Z., “A Review of Methods and Associated Models used in Return Voltage Measurement,” Proceedings of IEEE International Conference on Diagnostics in Electrical Engineering (Diagnostika’18), Pilsen, Czech Republic, Sep. 4-7, 2018, PAPERID-275, pp. 69-72.

[C14] Afia, R.S.A., Mustafa, E., and Tamus, Á. Z., “Mechanical Stresses on Polymer Insulation Materials,” Proceedings of IEEE International Conference on Diagnostics in Electrical Engineering (Diagnostika’18), Pilsen, Czech Republic, Sep. 4-7, 2018, PAPERID-276, pp. 170-173.

List of Publications on MTMT

https://m2.mtmt.hu/gui2/?type=authors&mode=browse&sel=authors10061384

List of references

[1]  Light Water Reactor Sustainability Program-Evaluation of Localized Cable Test Methods for Nuclear Power Plant Cable Aging Management Programs, PNNL-25432, May 2016.

[2]  K. Vyas, “Managing Solar Cables and Connectors for Safety and Longevity of PV System,” New Delhi, India, 2017.

[3] International Atomic Energy Agency (IAEA), “Benchmark Analysis for Condition Monitoring Test Techniques of Aged Low Voltage Cables in Nuclear Power Plants,” Vienna, Austria, 2017.

[4]  International Atomic Energy Agency (IAEA), “Assessing and managing cable aging in nuclear power plants,” Vienna, Austria, 2012.

[5]  M. Subudhi, “Literature review of environmental qualification of safety-related electric cables: Summary of past work. Volume 1,” Upton, NY, Apr. 1996. DOI: 10.2172/226028.

[6]  Electrical Power Research Institute, “Initial Acceptance Criteria Concepts and Data for Assessing Longevity of Low-Voltage Cable Insulations and Jackets,” California, USA, 2005

[7]  R. F. Gazdzinski, W. M. Denny, G. J. Toman, and R. T. Butwin, “Aging Management Guideline for Commercial Nuclear Power Plants- Electrical Cable and Terminations,” California, USA, 1996.

[8]        Brookhaven National Laboratory, “Condition Monitoring of Cables, Task 3 Report: Condition Monitoring Techniques for Electric Cables, BNL-90735-2009-IR,” 2009.