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      Global Energy Interconnection

      Volume 1, Issue 4, Oct 2018, Pages 520-526
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      Recyclable insulation material for HVDC cables in Global Energy Interconnection

      Yao Zhou1 ,Chao Yuan1 ,Qi Li1 ,Qing Wang2 ,Jinliang He1
      ( 1.State Key Laboratory of Power Systems, Department of Electrical Engineering, Tsinghua University,Beijing, 100084, P.R.China , 2.Department of Materials Science and Engineering, The Pennsylvania State University, University Park,PA 16802, USA )

      Abstract

      With the development of modern power systems, especially that of the global energy internet, high-voltage, direct current (HVDC) cable power transmission will play an important role in the future.The key problem of HVDC cable power transmission is the need for novel cable insulation materials that have high performance, recyclability, and higher working temperature to replace traditional crosslinked polyethylene.This paper investigates the thermal and electrical properties of polypropylene (PP)/Al2O3 nanocomposites as a potential recyclable HVDC cable insulation material.The developed nanocomposites exhibit excellent thermal and electrical properties with the introduction of Al2O3 nanoparticles.Particularly,the space charge accumulation is greatly suppressed.

      1 Introduction

      The rapid growth of power load and its asymmetric distribution call for large-capacity, long-distance, lowloss power transmission.Therefore, it is meaningful to construct the global energy internet to solve the problem of asymmetric power generation and consumption.Such a network interconnects the power grids in different countries and even different continents to ensure safe and reliable supply of global energy.Furthermore, it enables the efficient utilization of clean energy on a global scale, for example,wind power in the North Pole and solar power along the equator.In order to support the interconnection of power grids in different continents, future power transmission should have a larger capacity and higher voltage level.High-voltage, direct current (HVDC) power transmission technology is an ideal solution to increase the capacity.For example, the ±1100 kV DC allows a transmission distance of 5000 km and a capacity of 12 GW, which well covers the demand of intercontinental power grid interconnection [1-3].

      The DC transmission system includes a transmission channel and a converter station at each end, and the most commonly used transmission channel is overhead transmission lines.However, the overhead transmission channel is severely restricted by the available land resources.First of all, it is difficult to find routes of overhead transmission lines for hydropower station output and the power access of big cities.By 2020, China will build more than 800,000 km of ultra-high-voltage power transmission lines.Such concentrated power transmission lines would greatly impact the natural landscape and life of residents along the lines.On the other hand, offshore wind power has become an important strategic emerging industry.The 13th Five-Year Plan for Power Development in China indicates that the capacity of offshore wind power will be about 5 million kW, accounting for nearly one quarter of the national wind power.This offshore wind power also cannot be sent out by overhead transmission lines.

      DC cable power transmission is a practical way to transmit ultra-high-voltage power and offshore wind power.Compared with overhead transmission lines, cable transmission has the advantages of low failure rate and reduced environmental damage and impact.At the same time,the use of DC cables as transmission channels is in line with the proposal of CIGRE and the Chinese National Mediumand Long-Term Science and Technology Development Plan(2006–2020), which contributes to resolve the conflict among power, urbanization, and natural environment.

      DESERTEC is a global renewable energy solution based on harnessing sustainable power from sites with the most abundant renewable energy sources.These sites can be connected by low-loss HVDC transmission [4].The original and first region for the assessment and application of this concept is the EU-MENA region, including the European Union, Middle East, and Northern Africa [5].The DESERTEC organizations promote the generation of electricity in these areas using renewable sources such as solar power plants and wind parks, and develop a Euro-Mediterranean electricity network primarily made up of HVDC transmission cable [6].

      Up to now, most polymeric HVDC cables employ crosslinked polyethylene (XLPE) as the insulation material[7-10].However, XLPE cannot fully meet the requirement of modern power systems.As we know, the crosslinking process makes XLPE a thermoset material that is difficult to recycle, and the crosslinking and degassing processes in cable manufacturing also cause much energy consumption and pollution [11-13].Therefore, the development of recyclable polymeric insulation materials has attracted much attention.

      Polypropylene (PP) has shown great potential in cable insulation development, because it is easy to recycle, can be manufactured without the crosslinking process and byproducts, and most importantly, is cheap [14].PP is polymerized from the propylene monomer, and there are many asymmetric carbon atoms in its main molecular chain.According to the different arrangements of methyl groups on the main molecular chain, there are three different stereostructures of PP.The methyl groups can be located on the same side of the chain (isotactic polypropylene, iPP),alternately located on both sides (syndiotactic polypropylene,sPP), and irregularly located on both sides (atactic polypropylene, aPP).iPP and sPP are semi-crystalline materials with melting temperatures of 165 and 135 °C, respectively, while aPP is amorphous [15].Due to the high melting temperature of PP compared with XLPE (about 110 °C), PP-based insulation materials have improved working temperature,so that the capacity of PP-based HVDC cables can be much higher than that of XLPE-insulated cables.

      However, space charge accumulation is still a challenge for the development of eco-friendly HVDC cable insulation materials because space charge accumulation would cause the electric field distortion, and then initial breakdown of the insulation material, which decreases the service life of the power apparatus [16-18].In this study, Al2O3 nanoparticles are introduced into PP to improve the electrical properties of PP, especially the space charge behavior under a DC electric field.The results show that the PP/Al2O3 nanocomposites could maintain the excellent thermal properties of PP,while greatly improving upon the electrical properties of PP, including the DC breakdown strength, DC volume resistivity, and space charge accumulation.The optimal content of Al2O3 for improving the electrical properties of PP is about 3 per hundred ratio (phr).

      2 Experiments

      2.1 Materials

      The PP pellets (Q300f, Basell) and Al2O3 nanoparticles with 50 nm in diameter (Aladdin) were firstly dried in a vacuum oven at 60 °C for 12 h and then melt-blended in a mixer (Haake Poly Lab QC) at 200 °C for 12 min with the rotor speed of 60 rpm.Pristine PP without any nanoparticles was also melt-blended in the mixer for comparison.The Al2O3 nanoparticle contents in the PP matrix were 0.5, 1, 3,and 5 phr.For characterization, film samples with different thicknesses were prepared by compression molding at 200 °C under a pressure of about 15 MPa for 8 min, and then cooled to room temperature under the same pressure.All obtained samples were stored in the vacuum oven prior to measurements to exclude the influence of moisture.Samples for electrical characterization were sputter-coated with gold on both sides to serve as electrodes.

      2.2 Characterization

      Thermal properties of the PP/Al2O3 nanocomposites were verified by differential scanning calorimetry (DSC,TA Q2000).The tests were performed under nitrogen at a heating rate of 10 °C/min between 20 and 200 °C.

      The conduction current was measured with a digital high-resolution electrometer (Keithley 2635B) equipped with a standard three-electrode system.The measurements were performed under 20 kV/mm DC electric field.Before the electric field was applied, the samples were electrically short-circuited for 5 min, and then the leakage current was recorded immediately after the electric field was applied.The DC volume resistivity was calculated according to the leakage current at 10 min after the DC electric field application.

      The dielectric permittivity and loss tangent of the PP/Al2O3 nanocomposites were measured with an Alpha-A high-performance frequency analyzer (Novolcontrol GmbH Concept 40, Germany).The tests were carried out in the frequency range of 10-1–106 Hz at room temperature.

      DC electric breakdown tests were performed using film samples with thickness around 100 μm at room temperature.The samples were sandwiched between two 10-mm ball electrodes and immersed in silicon oil to prevent surface flashover.A DC voltage increasing at the rate of 1 kV/s was applied until the sample breakdown.For each sample, 20 locations were tested, and the test data were processed using two-parameter Weibull statistics.

      Space charge distribution within the samples under DC electric field was measured using the pulsed electro-acoustic (PEA)method at room temperature under 40 kV/mm for 20 min.

      3 Results and discussion

      3.1 Thermal properties

      Fig.1 provides the melting curves of the PP/Al2O3 nanocomposites, showing no obvious difference among the curves.Typically, there are four peaks on each melting curve.The peak located at about 163 °C is the melting peak of isotactic PP (iPP), and the others are the melting peaks of the elastomer phases in the pristine PP.The crystallinity of the nanocomposites is shown in Table 1, which indicates that the introduction of Al2O3 does not change the crystallinity of PP.

      Table 1 Crystallinity of PP/Al2O3 nanocomposites

      Sample Crystallinity (%)0 phr 18.2 0.5 phr 18.2 1 phr 18.5 3 phr 18.0 5 phr 18.2

      3.2 DC conductivity

      Fig.1 DSC melting curves of PP/Al2O3 nanocomposites

      Fig.2 DC leakage current of PP/Al2O3 nanocomposites under 20 kV/mm DC electric field

      Fig.3 DC volume resistivity of PP/Al2O3 nanocomposites under 20 kV/mm DC electric field

      DC conductivity, which determines the DC loss, is a key parameter for dielectric materials operating under a DC electric field.Fig.2 gives the leakage current of the PP/Al2O3 nanocomposites under 20 kV/mm, which is the operating electric field of the insulation material in HVDC cables.The volume resistivity derived from the leakage current is shown in Fig.3.With the introduction of Al2O3 nanoparticles, the volume resistivity of the nanocomposites increases firstly and then decreases.The nanocomposites with 3 phr Al2O3 nanoparticles show the highest volume resistivity, which is 6 times higher than that of the pristine PP.The increased volume resistivity is attributed to the deep traps introduced at the interfaces of PP matrix and Al2O3 nanoparticles, as was shown in previous studies [19-22].The deep traps would capture the charge carriers to reduce their mobility, and thus increase the volume resistivity.

      3.3 Dielectric properties

      Fig.4 (a) Dielectric constant and (b) dielectric loss of PP/Al2O3 nanocomposites

      As shown in Fig.4a, the dielectric constant of PP/Al2O3 nanocomposites shows a weak frequency dependence.Meanwhile, the nanocomposites have slightly higher dielectric constants than pristine PP, due to the higher dielectric constant of Al2O3 nanoparticles and interfacial polarization at the interfaces between the Al2O3 nanoparticles and PP matrix [23-25].Compared with pristine PP, the PP/Al2O3 nanocomposites show similar or even lower dielectric loss, which is beneficial for insulation applications.

      3.4 Dielectric breakdown strength

      Fig.5 shows the Weibull plots of the DC breakdown strength of the PP/Al2O3 nanocomposites with different Al2O3 nanoparticle contents.The DC breakdown strength increases at low loading of Al2O3 nanoparticles and reaches the maximum value at 3 phr Al2O3.However, when the nanoparticle content increases to 5 phr, the DC breakdown strength decreases.The maximum breakdown strength is 387 kV/mm for the nanocomposites with 3 phr Al2O3 nanoparticles, being 19% higher than that of the pristine PP.This increased DC breakdown strength can be attributed to the suppressed space charge accumulation and the corresponding electric field distortion [26-29].

      Fig.5 Weibull plots of DC breakdown strength of PP/Al2O3 nanocomposites

      Table 2 DC breakdown parameters of PP/Al2O3 nanocomposites

      Sample Breakdown strength(kV/mm)Shape parameter 0 phr 326 17.2 0.5 phr 344 15.8 1 phr 359 16.1 3 phr 387 20.8 5 phr 332 14.6

      3.5 Space charge behavior

      Fig.6 Space charge distribution in PP/Al2O3 nanocomposites under 40 kV/mm at room temperature,(a) pristine PP and nanocomposites with (b) 0.5 phr, (c) 1phr, (d) 3 phr and (e) 5 phe Al2O3 nanoparticles

      For insulation materials operating under a DC electric field, the space charge accumulation is a key factor that determines their performance and service life.Fig.6 shows the space charge distribution in PP/Al2O3 nanocomposites under 40 kV/mm at room temperature.In pristine PP,there is obviously serious homocharge injection at both electrodes.With the inclusion of Al2O3 nanoparticles, the space charge injection is gradually suppressed and it almost disappears with 3 phr Al2O3 nanoparticles introduced,which is consistent with the results of the DC breakdown measurement.Again, the suppressed space charge accumulation may come from the deep traps introduced at the interfaces of nanoparticles and polymer matrix, as explained in the literature [30-32].

      4 Conclusions

      For the development of global energy internet with long-distance, large-capacity, and high-reliability power transmission, the DC cable transmission technology will be widely used.New eco-friendly DC cable insulation materials will further improve the environmental friendliness of power systems and promote the construction of global energy internet.

      In this study, thermoplastic polypropylene, a candidate for eco-friendly HVDC cable insulation material, is used as the base material, and Al2O3 nanoparticles are introduced into it to suppress the space charge accumulation and improve the electrical properties.After introducing the Al2O3 nanoparticles, the excellent thermal properties of PP are maintained in the PP/Al2O3 nanocomposites to allow a high working temperature.Meanwhile, the electrical properties including the DC volume resistivity, DC breakdown strength, and space charge behavior of PP are greatly improved with the inclusion of Al2O3 nanoparticles.The optimal Al2O3 nanoparticle content for the electrical properties is about 3 phr.

      Acknowledgements

      This work was supported by National Basic Research Program of China (973 Program) under Grant 2014CB239500.

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      Fund Information

      supported by National Basic Research Program of China(973 Program)under Grant2014CB239500;

      supported by National Basic Research Program of China(973 Program)under Grant2014CB239500;

      Author

      • Yao Zhou

        Yao Zhou received his bachelor degree in Electrical Engineering from Tsinghua University,Beijing, China in 2014.Currently, he is a Ph.D.candidate in the Department of Electrical Engineering, Tsinghua University.His research interests include electrical insulation materials,energy storage materials, and high-voltage engineering.

      • Chao Yuan

        Chao Yuan received his bachelor degree in Electrical Engineering from Hunan University,Changsha, China and his Ph.D.degree in Electrical Engineering from South China University of Technology, Guangzhou, China in 2010 and 2017, respectively.He worked as a visiting Ph.D.student in Chalmers University of Technology, Gothenburg, Sweden in 2014.He is currently a postdoctoral researcher at Tsinghua University,Beijing, China.His research interests include dielectric response,electrical discharges, and charge accumulation on polymeric materials in high-voltage insulation systems, especially for HVDC applications.

      • Qi Li

        Qi Li received his bachelor degree in Polymer Materials and Engineering in 2007, master and Ph.D.degrees in Materials Science in 2010 and 2013, respectively, all from Wuhan University of Technology, Wuhan, China.Afterwards, he joined the Department of Materials Science and Engineering of the Pennsylvania State University in USA as a postdoctoral fellow.He was appointed Associate Professor in the Department of Electrical Engineering, Tsinghua University, in 2016.He was awarded the MRS Postdoctoral Award in 2016.His current research focuses on dielectric polymers and polymer nanocomposites for energy storage and conversion.

      • Qing Wang

        Qing Wang received his Ph.D.degree in Chemistry from the University of Chicago,USA, in 2000.He is a Professor of Materials Science and Engineering at the Pennsylvania State University (Penn State), University Park,USA.Prior to joining the faculty at Penn State in 2002, he was a postdoctoral researcher at Cornell University.His research interests are centered on the development of novel functional materials including ferroelectric polymers, electroactive polymers, ion-containing polymers and polymer–ceramic nanocomposites for applications in electrical and electronic science and engineering, energy storage and energy harvesting.

      • Jinliang He

        Jinliang He received his bachelor degree in Electrical Engineering from Wuhan University of Hydraulic and Electrical Engineering,Wuhan, China in 1988; his master degree in Electrical Engineering from Chongqing University, Chongqing, China in 1991, and his Ph.D.degree in Electrical Engineering from Tsinghua University, Beijing, China in 1994.He became a Lecturer in 1994 and an Associate Professor in 1996 in the Department of Electrical Engineering, Tsinghua University.From 1997 to 1998, he was a Visiting Scientist at the Korea Electrotechnology Research Institute, Changwon, Korea, involved in research on metal-oxide varistors and high-voltage polymeric metal-oxide surge arresters.In 2001, he was promoted to Professor at Tsinghua University.Currently, he is the Chair of the High Voltage Research Institute at Tsinghua University.His research interests include dielectric materials, overvoltage analysis, electromagnetic compatibility, lightning protection and grounding technology, smart sensors and big data application.

      Publish Info

      Received:2018-08-09

      Accepted:2018-08-20

      Pubulished:2018-10-25

      Reference: Yao Zhou,Chao Yuan,Qi Li,et al.(2018) Recyclable insulation material for HVDC cables in Global Energy Interconnection.Global Energy Interconnection,1(4):520-526.

      (Editor Ya Gao)
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