Polypropylene Cable Insulation E-Book

Boxue Du

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 564

Veröffentlichungsjahr: 2024

Beschreibung

An introduction to a cutting-edge, environmentally friendly insulation material

The installation and maintenance of high-voltage cables is an infrastructure problem with potentially major environmental impacts. In recent years, polypropylene has emerged as an environmentally friendly material for insulating high-voltage cables, particularly HVDC power cables and HVAC power cables. Polypropylene Cable Insulation begins with an introduction to high-voltage cables and the development of polypropylene insulation before describing the dielectric properties and applications of this insulation in both HVDC and HVAC contexts. The result is a thorough, accessible guide to an essential part of any environmentally friendly power grid.

Readers will also find:

• Detailed explorations of the relationship between space charge behaviors and trap characteristics
• Discussion of topics including polarization and dielectric relaxation, electrical treeing degradation, partial discharge, and more
• Graphs and tables illustrating experimental results

Polypropylene Cable Insulation is ideal for electrical power engineers, power transmission system operators, and any engineers or researchers working in power transmission and/or distribution cables.

Leseprobe

Table of Contents

Cover

Table of Contents

Series Page

Title Page

Copyright Page

About the Author

Preface

Acknowledgements

1 Introduction

1.1 Background

1.2 State of the Art of PP Modification Method

1.3 Effect of Microstructures on Dielectric Properties

1.4 Effect of Operating Conditions on Dielectric Properties

1.5 Content of This Book

References

Part I: Polypropylene Insulation for HVDC Cables

2 Space Charge and Dielectric Breakdown

2.1 Introduction

2.2 Effect of Elastomer on Space Charge and Breakdown Characteristics

2.3 Effect of Inorganic Nanofiller on Space Charge and Dielectric Breakdown

2.4 Effect of Organic Compounds on Space Charge and Dielectric Breakdown

2.5 Conclusion and Outlook

References

3 Electrical Treeing Phenomenon

3.1 Introduction

3.2 Electrical Treeing Under Impulse Superimposed on DC Voltage

3.3 Effect of Ambient Temperature on Electrical Treeing

3.4 Effect of Bending Deformation on Electrical Treeing

3.5 Methods for Suppressing Electrical Treeing

3.6 Conclusion and Outlook

References

4 Insulation Thickness Optimization for HVDC Cables

4.1 Introduction

4.2 Electric Field Distribution Calculation Model for HVDC Cables

4.3 Space Charge and Electric Field Under DC Voltage

4.4 Space Charge and Electric Field Under Polarity Reversal Voltage

4.5 Insulation Thickness Optimization for HVDC Cables

4.6 Conclusions

References

Part II: Polypropylene Insulation for HVAC Cables

5 Polarization and Dielectric Relaxation

5.1 Introduction

5.2 Effect of Blending Modification

5.3 Effect of Monomer Grafting

5.4 Effect of Thermal Ageing

5.5 Conclusion and Outlook

References

6 AC Electrical Treeing and Dielectric Breakdown

6.1 Introduction

6.2 Electrical Treeing Dependent on Crystalline Morphology

6.3 An Insight into Electrical Tree Growth Within Heterogeneous Crystalline Structure

6.4 Methods for Suppressing Electrical Treeing

6.5 Conclusions

References

7 Electrothermal Aging and Lifetime Modeling

7.1 Introduction

7.2 Aging Mechanism and Lifetime Models

7.3 Thermal Aging

7.4 Electrical–Thermal Aging

7.5 Conclusions

References

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Advantages and disadvantages of OF‐, MI‐, and XLPE‐insulated cable...

Table 1.2 Crystallinity of iPP/sPP/aPP blends.

Chapter 2

Table 2.1 Index for polymer samples investigated in this subsection.

Table 2.2 Density at electron trap centers.

Table 2.3 Index for polymer samples investigated in this subsection.

Table 2.4 Effect of different grafted molecules on the dielectric properties...

Table 2.5 Index for polymer samples investigated in this subsection.

Chapter 3

Table 3.1 Absorption peaks of common group.

Table 3.2 Electrical tree morphologies of PPH.

Table 3.3 Mechanical properties of PP samples.

Table 3.4 Index for PP samples investigated in this work.

Chapter 4

Table 4.1 Electric field distortion rate at different temperature coefficien...

Table 4.2 Electric field distortion rate at different electric field coeffic...

Table 4.3 Electric field distortion rate at different insulation temperature...

Chapter 5

Table 5.1 Index for PP blend samples investigated in this subsection.

Chapter 6

Table 6.1 Index for PP samples investigated in this subsection.

Table 6.2 Electrical tree morphology of iPP/sPP blends at 50, 70, and 90 °C ...

Table 6.3 Electrical tree morphology of iPP/sPP blends at 70 °C under AC vol...

Table 6.4 Index for PP samples with different crystallization process.

Table 6.5 Index for PP samples with different nucleating agent.

Table 6.6 Weibull distribution parameters of samples.

Table 6.7 Index for PP samples investigated in this subsection.

Table 6.8 Weibull distribution parameters of samples.

Table 6.9 Composition of the samples.

Table 6.10 Weibull distribution parameters of samples.

Table 6.11 Lattice parameter of crystal.

Chapter 7

Table 7.1 Derivation of PQT voltage for EHVAC‐extruded cable, HVDC‐extruded ...

Table 7.2 Fitting equation between characteristics parameters and insulation...

Table 7.3 The fitting result of

S

1

.

Table 7.4 VEC of specimens at different temperatures.

List of Illustrations

Chapter 1

Figure 1.1 Schematic diagram of the grid connection and transmission of larg...

Figure 1.2 Cable typologies that can be conveniently used in relation to the...

Figure 1.3 Molecular configurations of PP. (a) iPP, (b) sPP, (c) aPP.

Figure 1.4 Schematic diagram of multiscale aggregation structure of PP insul...

Chapter 2

Figure 2.1 SEM photos for the PP/POE and the PP/PBE blends with various elas...

Figure 2.2 (a) Scheme of charge trapping and de‐trapping process in energy b...

Figure 2.3 Deep trap distribution for electron for the two types of blends. ...

Figure 2.4 Shallow trap distribution for electron for the two types of blend...

Figure 2.5 Dependence of the trap level distribution and DC breakdown streng...

Figure 2.6 Relationship between the elongation at break, tensile strength, a...

Figure 2.7 Schematic diagram of microscopic morphology at the interface betw...

Figure 2.8 Schematic illustration of the experimental setup for PEA.

Figure 2.9 Effect of the nanofiller content on space charge distribution at ...

Figure 2.10 Relationship between the total charge and the polarization time....

Figure 2.11 Effect of the nanofiller content on the electric field at 70 kV/...

Figure 2.12 DC breakdown strength of different specimens.

Figure 2.13 (a) The charging current and (b) volume resistivity of different...

Figure 2.14 (a) Illustration for the preparation process of TiO

2

@BNNS nanofi...

Figure 2.15 EDAX results of TiO

2

@BNNS nanofillers calculated at 900 °C. (a) ...

Figure 2.16 XRD patterns of BNNS and TiO

2

at room temperature, and calcinati...

Figure 2.17 Space charge distributions of (a) PP at 50 °C, (b) PP at 90 °C, ...

Figure 2.18 (a) Average charge density and (b) electric field distortion rat...

Figure 2.19 DC breakdown strength of PP composites at (a) 30 °C, (b) 50 °C, ...

Figure 2.20 (a) Surface potential decay and (b) trap level distribution of P...

Figure 2.21 Three‐dimensional electrical potential distributions of (a) PP/B...

Figure 2.22 Chemical formula of the voltage stabilizers. (a) Tinuvin 622. (b...

Figure 2.23 FT‐IR spectra: (a) PU, PU622, PU770, and PU944 composites; (b) a...

Figure 2.24 Space charge distribution in PU and its voltage stabilizer compo...

Figure 2.25 Space charge depolarization in PU and PP/ULDPE/voltage stabilize...

Figure 2.26 Relationship between mean volume charge density of different sam...

Figure 2.27 Weibull distribution of DC breakdown strength at different tempe...

Figure 2.28 3D electrostatic potential distribution in (a) T622, (b) T770, a...

Figure 2.29 Electronic energy band in PP, ULDPE, and three different voltage...

Figure 2.30 Relationship between the trap level and the trap level density....

Figure 2.31 Scheme of melt‐free radical grafting procedure.

Figure 2.32 (a) FT‐IR spectra of the samples. (b) OIT of the samples by DSC....

Figure 2.33 Space charge distributions and electric field distortion rate of...

Figure 2.34 Effect of antioxidant grafting on DC breakdown strength.

Figure 2.35 Relation between DC‐prestressed breakdown strength and temperatu...

Figure 2.36 DC‐prestressed breakdown strength after the hetero‐polarity pres...

Figure 2.37 Surface electrostatic potential distribution of the samples. (a)...

Figure 2.38 DOS and HOMO–LUMO orbitals of the samples.

Figure 2.39 Energy level and corresponding molecular orbitals. (a) Before co...

Chapter 3

Figure 3.1 Diagram of the impulse superimposed on DC voltage. (a) Positive D...

Figure 3.2 Typical electrical tree morphologies of different DC amplitudes c...

Figure 3.3 Relationship between the tree length and the treeing time of diff...

Figure 3.4 Charge transportation process under different DC amplitudes. (a) ...

Figure 3.5 Relationship between the accumulated damage and the treeing time ...

Figure 3.6 Breakdown time of different DC amplitudes and +25 kV impulse‐comb...

Figure 3.7 Typical electrical tree morphologies of (a) +5 and (b) −15 kV DC ...

Figure 3.8 Relationship between the tree length and the treeing time of (a) ...

Figure 3.9 Relationship between the accumulated damage and the treeing time ...

Figure 3.10 Breakdown time of −5 and −15 kV DC and +25 kV impulse‐combined v...

Figure 3.11 Diagram of the impulse superimposed on DC voltage in this subsec...

Figure 3.12 Typical electrical tree morphologies of different impulse amplit...

Figure 3.13 Relationship between the tree length and the treeing time of dif...

Figure 3.14 Relationship between the accumulated damage and the treeing time...

Figure 3.15 Schematic of the amount of injected space charges with the chang...

Figure 3.16 Breakdown time of different impulse amplitudes and −25 kV DC‐com...

Figure 3.17 Schematic of the measuring system for electrical tree.

Figure 3.18 Typical profiles of electrical trees in the XLPE and the PP unde...

Figure 3.19 Typical propagation characteristics of electrical trees in the P...

Figure 3.20 Electrical tree inception probability for 10 minutes in the PP a...

Figure 3.21 Typical profiles of electrical trees with different impulse freq...

Figure 3.22 Typical propagation characteristics of electrical trees in the P...

Figure 3.23 Typical morphologies of electrical trees under different conditi...

Figure 3.24 Relation between the inception probability and the temperature w...

Figure 3.25 Relation between the tree length and the treeing time under diff...

Figure 3.26 DSC curves of PP cable insulation.

Figure 3.27 Relation between the fractal dimension and the TTB of breakdown ...

Figure 3.28 Relation between the accumulated damage and the TTB of breakdown...

Figure 3.29 Typical breakdown morphologies of electrical trees under differe...

Figure 3.30 SEM images and EDAX analysis of the original sample and breakdow...

Figure 3.31 ATR‐IR spectra of the (a) original sample and (b) breakdown chan...

Figure 3.32 (a) Experimental setup of PP samples under bending deformation c...

Figure 3.33 (a) Inception time of electrical trees. Tree length variation at...

Figure 3.34 Morphological characterization of typical electrical trees corre...

Figure 3.35 Gaussian fit for tree length classified by typical morphologies ...

Figure 3.36 Morphological characterization of electrical trees and standard ...

Figure 3.37 Standard deviation (

σ

) of Gaussian fit for the electrical t...

Figure 3.38 Simulation results in different curvature radius for PPH. (a1) (...

Figure 3.39 Effect of deformation on crystalline morphology. (a1) (a2) PPH. ...

Figure 3.40 Variation of electrical tree morphologies with temperature and c...

Figure 3.41 Chemical structural formula of the chosen polycyclic compounds. ...

Figure 3.42 Tree morphologies in PP and PP‐BN0.1 under different temperature...

Figure 3.43 Relationship between the tree length and treeing time with diffe...

Figure 3.44 Relationship between the accumulated damage and treeing time wit...

Figure 3.45 Tree morphologies with different contents of polycyclic compound...

Figure 3.46 Relationship between the tree length and treeing time with diffe...

Figure 3.47 Relationship between the accumulated damage and treeing time wit...

Chapter 4

Figure 4.1 Experimental and fitted conduction current versus electric field ...

Figure 4.2 Electric field distribution in

G

and

B

materials considering the ...

Figure 4.3 Variation of electric field distribution with temperature gradien...

Figure 4.4 Effect of temperature gradient on (a) space charge and (b) electr...

Figure 4.5 Electrical field distributions varying with temperature gradients...

Figure 4.6 Relationship between the

E

max

and the temperature gradient.

Figure 4.7 Model of space charge behavior under the Δ

T

within the HVDC‐extru...

Figure 4.8 Schematic diagram of polarity reversal voltage (a) without relaxa...

Figure 4.9 Effect of Δ

T

on (a) space charge and (b) electrical field evoluti...

Figure 4.10

E

max

before and during polarity reversal under different Δ

T

.

Figure 4.11 Space charge behaviors in the vicinity of the inner and outer se...

Figure 4.12 Charge variation rate on the inner side of the insulation under ...

Figure 4.13 Effect of PRP on (a) space charge and (b) electric field evoluti...

Figure 4.14

E

max

under different PRP as the Δ

T

is 30 °C.

Figure 4.15 Space charge behaviors in the vicinity of the inner and outer se...

Figure 4.16 Charge variation rate on the inner side of the insulation under ...

Figure 4.17 Relationship between

n

and thickness of the insulation.

Figure 4.18 Relationship between

k

4

and thickness of the insulation.

Figure 4.19 Relationship between

K

and thickness of the insulation.

Figure 4.20 Relationship between temperature coefficient and electric field ...

Figure 4.21 Relationship between electric field coefficient and electric fie...

Figure 4.22 Relationship between insulation temperature difference and elect...

Figure 4.23 Effect of insulation thickness on (a) temperature gradient, (b) ...

Figure 4.24 Electrical field distributions varying with insulation thickness...

Figure 4.25

C

E

and

E

max

varying with insulation thickness.

Figure 4.26 Effect of insulation thickness on (a) space charge and (b) elect...

Figure 4.27 Electric field distribution at 0, 60, and 120 seconds.

Figure 4.28

E

max

and

C

E

under different thicknesses.

Chapter 5

Figure 5.1 The variation of different types of polarization with frequency u...

Figure 5.2 Example for FDS fitting of grafted‐PP samples based on the H–N mo...

Figure 5.3 Real part of permittivity ln(

ε

′) as a function of frequency ...

Figure 5.4 Imaginary part of permittivity ln(

ε

″) as a function of frequ...

Figure 5.5

α

Relaxation strength (Δ

ε

α

) and

δ

relaxation ...

Figure 5.6 Relationship between the relaxation time constant (ln(

τ

δ

...

Figure 5.7 Trap center densities (a) and energy levels (b) of deep traps and...

Figure 5.8 Activation energy of the

δ

relaxation as a function of the s...

Figure 5.9 Molecular structures of grafting monomers: (a) dibenzylidene acet...

Figure 5.10 Schematic of free radical initiated grafting of PP: (a) the deco...

Figure 5.11 (a) FT‐IR spectra of grafted‐PP samples. (b) Oxidation induction...

Figure 5.12 DSC curves of grafted‐PP samples: (a) DSC melting curves and (b)...

Figure 5.13 Real part and imaginary part of permittivity of grafted‐PP sampl...

Figure 5.14 α Relaxation of grafted‐PP samples varying with temperature: (a)...

Figure 5.15 Relationship between Δε

α

and temperature of grafted‐PP samp...

Figure 5.16 Schematic of the dipole moment of grafted‐PP samples: (a) PP, (b...

Figure 5.17

δ

Relaxation of grafted‐PP samples varying with temperature...

Figure 5.18 Relationship between

τ

δ

and temperature of grafted‐PP ...

Figure 5.19 Relationship between

τ

δ

and temperature of grafted‐PP ...

Figure 5.20 Real part and imaginary part of permittivity of thermal‐aged PP ...

Figure 5.21

α

Relaxation of thermal‐aged PP samples varying with temper...

Figure 5.22 Relationship between Δ

ε

α

and temperature of aged‐PP sa...

Figure 5.23 Relationship between Δ

ε

α

and temperature of aged‐PP sa...

Figure 5.24

δ

Relaxation of aged‐PP samples varying with temperature: (...

Figure 5.25 Relationship between

τ

δ

and temperature of aged‐PP sam...

Figure 5.26 Schematic of the double potential well model of

δ

relaxatio...

Chapter 6

Figure 6.1 Electrical tree deterioration in polypropylene cable insulation. ...

Figure 6.2 Breakdown strength of inner, middle, and outer layers in polyprop...

Figure 6.3 Crystal morphology of iPP/sPP blends: (a) sPP0, (b) sPP5, (c) sPP...

Figure 6.4 Differential scanning calorimetry (DSC) curves of iPP/sPP blends....

Figure 6.5 Avrami index of iPP/sPP blends.

Figure 6.6 Inception time of electrical tree for isotactic polypropylene (iP...

Figure 6.7 Variation of electrical tree length of different sPP contents at ...

Figure 6.8 Expansion coefficient and fractal dimension of electrical tree in...

Figure 6.9 Time‐varying fractal dimension of sPP0, sPP5, and sPP15 at 70 °C ...

Figure 6.10 Weibull distribution of AC breakdown strength in iPP/sPP blends:...

Figure 6.11 Trap level characteristics of iPP/sPP blends.

Figure 6.12 Schematic diagram of charge accumulation under AC voltage: (a) s...

Figure 6.13 Setup of the mesoscopic electrical tree.

Figure 6.14 Crystalline morphologies of iPP with different cooling methods: ...

Figure 6.15 Melting curves of iPP samples with different cooling methods....

Figure 6.16 POM images and binarized images of mesoscopic electrical tree mo...

Figure 6.17 (a) Length and (b) accumulated damage area of mesoscopic electri...

Figure 6.18 Pearson correlation coefficient between the mesoscopic and macro...

Figure 6.19 Heterogeneous structure of samples (a) before crystallization, (...

Figure 6.20 X‐ray diffraction pattern of samples.

Figure 6.21 DSC curves of (a) heating melting, (b) isothermal crystallizatio...

Figure 6.22 (a) PRPD spectra during electrical treeing. (b) POM images and t...

Figure 6.23 Partial discharge (a) counts and amplitude varying with α‐nuclea...

Figure 6.24 Effect of heterogeneous mesoscopic structure on breakdown streng...

Figure 6.25 Heterogeneous mesoscopic structure of (a) crystal region and (b)...

Figure 6.26 (a) Time‐dependent curve of the DACF and (b) relative permittivi...

Figure 6.27 Voronoi Network‐based PP heterogeneous mesoscopic structural mod...

Figure 6.28 Electric field distribution in mesoscopic structure of (a) PP‐p ...

Figure 6.29 Crystalline morphologies of PP/elastomer blends with different n...

Figure 6.30 Schematic diagram of experiment system.

Figure 6.31 Schematic diagram of electrical tree morphology characterization...

Figure 6.32 Electrical tree morphologies of unmodified PP/elastomer samples:...

Figure 6.33 Degradation area and PD characteristics of electrical treeing: (...

Figure 6.34 Electrical tree morphologies of PP/elastomer blends with differe...

Figure 6.35 PD characteristics of PP/elastomer blends with different nucleat...

Figure 6.36 Degradation area of PP/elastomer blends with different nucleatin...

Figure 6.37 Breakdown strength of PP/elastomer blends with different nucleat...

Figure 6.38 Cross‐sectional images of 35 kV PP‐insulated cable (a) before an...

Figure 6.39 Mesoscopic crystal morphology of the samples during the isotherm...

Figure 6.40 (a) X‐ray diffraction pattern, (b) DSC curves within the heating...

Figure 6.41 (a) Phase‐resolved partial discharge spectrums and (b) mesoscopi...

Figure 6.42 (a) Partial discharge counts and amplitude, (b) correspondence b...

Figure 6.43 Weibull distribution of dielectric breakdown strength.

Figure 6.44 Heterogeneous mesoscopic structure: α‐crystal, β‐crystal, and am...

Figure 6.45 Mesoscopic heterogeneous structure of PP with various crystal ph...

Figure 6.46 Effect of crystal phase structures on

E

max

and breakdown strengt...

Chapter 7

Figure 7.1 The difference between electrical branch growth and breakdown....

Figure 7.2 Free energy diagram: the solid line is in no electric field and t...

Figure 7.3 DMM model and HVDC lifetime test results.

Figure 7.4 Results of electrothermal ALT at 80 °C DC voltage.

Figure 7.5 Different stress application methods.

Figure 7.6

E

–

t

characteristics of specimens.

Figure 7.7 Electrical stress lifetime curve at various temperatures with con...

Figure 7.8 Thermal aging life curve by the Arrhenius model.

Figure 7.9 TI and HIC under the Eyring and Arrhenius models.

Figure 7.10 FT‐IR spectra of PP samples with different aging times.

Figure 7.11 XRD spectra of PP samples with different aging times.

Figure 7.12 DSC curves of aged‐PP samples.

Figure 7.13 SEM images of PP insulation after aging for 0, 7, 14, 30, and 60...

Figure 7.14 Tensile strength and elongation at break of aged‐PP samples.

Figure 7.15 Conductivity of aged‐PP samples at 30, 50, 70, and 90 °C.

Figure 7.16 Effect of thermal aging on AC breakdown strength of PP blend ins...

Figure 7.17 Relationship between aging time and FDS characteristics paramete...

Figure 7.18 Fitting curves between FDS characteristics parameters (Δ

ε

δ

...

Figure 7.19 The flow chart of condition assessment of PP insulation based on...

Figure 7.20 Material “time–pressure” testing device circuit.

Figure 7.21 Fitting functions of “

E

–

t

” characteristics for samples at differ...

Figure 7.22

E

–

t

characteristics at different temperatures for specimens. (a)...

Figure 7.23 (a1)–(a3) Remaining lifetime fitting results for the RAMU, Fallo...

Figure 7.24 Fitting results of the three‐dimensional function graph of sampl...

Guide

Cover Page

Table of Contents

Series Page

Title Page

Copyright Page

About the Author

Preface

Acknowledgements

Begin Reading

Index

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IEEE Press Editorial BoardSarah Spurgeon, Editor‐in‐Chief

Moeness Amin

Ekram Hossain

Desineni Subbaram Naidu

Jón Atli Benediktsson

Brian Johnson

Tony Q. S. Quek

Adam Drobot

Hai Li

Behzad Razavi

James Duncan

James Lyke

Thomas Robertazzi

Joydeep Mitra

Diomidis Spinellis

Polypropylene Cable Insulation

Boxue Du

Tianjin UniversityChina

Zhonglei Li

Tianjin UniversityChina

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About the Author

Boxue Du

He received the ME degree in Electrical Engineering from Ibaraki University, Hitachi, Japan, in 1993, and his PhD degree from Tokyo University of Agriculture and Technology, Tokyo, Japan, in 1996.

From 1996 to 2002, he was an Associate Professor at the Niigata Institute of Science and Technology, Niigata, Japan. Concurrently, from 2000 to 2002, he served as a Visiting Scientist at Niigata University, Niigata, Japan. Since 2002, he has been a Professor and the Director‐Founder of the Institute of High Voltage, within the School of Electrical and Information Engineering at Tianjin University, Tianjin, China. He is currently a Chair Professor and has been ranked the #1 Surface Charge by ScholarGPS.

His research interests primarily encompass advanced insulation materials for electrical equipment, polymer dielectrics for energy storage, and high‐voltage insulation technology. Prof. Du is deeply engaged in both teaching and research in high‐voltage technology. He has spearheaded Key Projects, Joint Funds, and Funds for International Cooperation by the National Natural Science Foundation of China.

Prof. Du has published 8 monographs and over 900 academic papers, including more than 275 articles in IEEE Transactions, accumulating over 8800 citations. He holds over 30 authorized invention patents and has been recognized as one of the highly cited scholars by Elsevier and one of the top 100,000 scientists globally. His editorial contributions include roles as Contributing Editor‐in‐Chief and Associate Editor for IEEE and IET journals, as well as membership on the editorial boards of journals such as High Voltage, CJEE, Scientific Reports, and IET Nanodielectrics. He has been honored by the IEEE International Standards Committee with the Special Contribution Award and the Special Advancement Award for his exceptional contributions. Prof. Du is an active member of IEEE DEIS TC and the CIGRE D1.64 Committee.

Zhonglei Li

He received the PhD degree in electrical engineering from Tianjin University, Tianjin, China, in 2016. He is currently an Associate Professor with the Key Laboratory of Smart Grid of Education Ministry, School of Electrical and Information Engineering, Tianjin University. His research focuses on addressing the strategic demands of HVDC power cable transmission for large‐scale offshore wind power in China. Specifically, he investigates key issues related to improving the electrical properties of DC cable insulation and regulating electric fields in cable accessories.

He has led several research projects funded by the National Natural Science Foundation of China, including a Young Scientists Fund, a General Program, a subproject of a Key Program, a subproject of the Smart Grid Joint Funds of the NSFC, and a subproject of the China National Key R&D Program. He has authored over 80 SCI‐indexed papers, including publications in IEEE Transactions on Dielectrics and Electrical Insulation (IEEE TDEI), and has an H‐index of 26. Additionally, he has published two monographs: “DC Cable Accessory Insulation” by Science Press.

He has been recognized by several prestigious programs, including the Young Elite Scientists Sponsorship Program by the China Association for Science and Technology (CAST), and the World’s Top 2% Scientists list published by Elsevier and Stanford University. He has also been selected for the Technology Leading Talent Plan of Tianjin University. His future research aims to further explore methods for suppressing space charges and enhancing dielectric properties.

Preface

Power cables are essential components for electrical energy transmission within power systems. With the advancement of new energy sources, such as offshore wind power, submarine cables have become critical for energy transmission. Over the past few decades, extruded cables with polymer insulation have emerged as the predominant choice for both submarine and terrestrial applications. The continuous pursuit of stronger insulation materials to achieve higher voltage levels and increased transmission capacity has been relentless. This pursuit serves as the impetus for the publication of this book.

Polypropylene, renowned for its excellent electrical performance, is considered an environmentally friendly cable insulation material for the next generation of power systems. The use of polypropylene insulation, which does not require a cross‐linking process during production, represents a significant advancement in the global drive for green energy development and the pursuit of carbon neutrality.

This book focuses on polypropylene cable insulation, offering an in‐depth examination of polypropylene insulation for high‐voltage DC cables and high‐voltage AC cables in Part I and Part II, respectively. It discusses many thought‐provoking and critical topics, including space charge, electrical tree, lifetime modeling, and insulation breakdown. The methodologies and innovations discussed within these pages will play a pivotal role in shaping the future of energy transmission, facilitating the transition to renewable energy sources and enhancing the reliability of power systems globally. It is hoped that this book will provide valuable guidance for the development of future high‐voltage cables for power systems.

This specialized book in the field of electrical engineering is particularly geared towards professionals involved in high‐voltage power transmission. It is also suitable for researchers, graduate students, and advanced undergraduate students working in the area of high‐voltage cable transmission.

In conclusion, this book not only addresses the technical aspects of polypropylene cable insulation but also underscores its critical role in achieving sustainable and efficient power systems. It bridges the gap between theoretical research and practical application, offering valuable guidance for the next generation of electrical engineers. As we move towards a future increasingly reliant on renewable energy, the knowledge contained within these pages will prove invaluable. It is with great anticipation that I invite you to delve into this work, confident that it will inspire innovation and progress in the field of high‐voltage power transmission.

Boxue Du

Zhonglei Li

Tianjin University

Acknowledgements

The successful publication of this book has been made possible through the invaluable assistance and support of numerous individuals and organizations. The academic scholars, including Prof. Weijiang Chen, Prof. Yoshimichi Ohki, Prof. Jinliang He, Prof. Shengtao Li, Prof. Hong Zhao, and Prof. Pingkai Jiang, contributed significantly to the project and contents of this book.

This book has also received significant supports from the industry. We extend our profound gratitude to the State Grid Electric Power Research Institute, State Grid Smart Grid Research Institute, China Electric Power Research Institute, State Grid Tianjin Electric Power Research Institute, State Grid Shandong Electric Power Research Institute, State Grid Liaoning Electric Power Research Institute, and State Grid Jiangsu Electric Power Research Institute for their essential contributions to collaborative researches and applications of the polypropylene cable insulation.

Our heartfelt thanks also go to Jiangsu Shangshang Cable Group, Kunming Cable Group, Ningbo Orient Wires & Cables Group, Zhongtian Technology Group, and Far East Cable Co., LTD., whose unwavering support enriched the technical content of this book.

Finally, we express our deep appreciation to Wiley Press for their crucial support in bringing this publication to fruition.

1Introduction

1.1 Background

Power cables are key equipment for achieving high‐capacity power transmission. The earliest cables can be traced back to the 19th century. In its development history of over 150 years, the voltage level and transmission capacity of cables have continuously increased, making important contributions to the development of society and economy. Especially after entering the 21st century, with the advancement of internationalization and the development of new energy sources such as offshore wind power, submarine cables are considered the most important means of transmission technology. Figure 1.1 is the schematic diagram of the grid connection and transmission of large‐scale offshore wind power based on the high‐voltage and large‐capacity power cable systems.

Power cable insulation is a protective medium that isolates high potential and is crucial for ensuring the safe and reliable operation of cable transmission systems. In other words, cable insulation needs to have high breakdown field strength, low dielectric loss, resistance to electrical tree discharge, and long service life, as well as good heat resistance, flexibility, and mechanical strength. Cable insulation materials are constantly advancing and developing in pursuit of better performance. The initial cable insulation used oil‐filled (OF)‐insulated cables and later developed into mass‐impregnated (MI) traditional or polypropylene‐laminated paper (PPLP)‐insulated cables. OF‐insulated cables have the advantages of safety, stability, reliability, and long service life. However, the length of OF‐insulated cables is greatly limited due to the need for oil supply equipment, and there are also environmental pollution hazards. Compared with OF‐insulated cables, MI‐insulated cables do not require oil supply equipment, break the length limit of OF‐insulated cables, and have no environmental hazards. They have higher insulation performance and heat resistance and are recognized as the most reliable type of high‐voltage cable insulation at that time. Until the 1950s, with the emergence of plastic extruded cable insulation represented by cross‐linked polyethylene (XLPE), cable insulation had a new development direction. Compared to the first two, extruded insulation has advantages such as high‐temperature resistance, high transmission power density, high strength, lightweight, environmental protection, and easy installation. Therefore, it has been widely applied and developed after its release. In 1957, General Electric (GE) Company produced the earliest XLPE AC cable by using the advanced production processes in chemistry. In 1999, the world's first commercialized XLPE‐extruded DC cable was applied in the Gotland Island II Project in Sweden. The voltage level of this DC cable was ± 80 kV, the transmission power was 50 MW, and the cable length was 140 km. The advantages and disadvantages of the three types of cable insulation are compared in Table 1.1, and Figure 1.2 indicates the cable typologies that can be conveniently used in relation to the length, power, and voltage of the link and the type of transmission system.

Figure 1.1 Schematic diagram of the grid connection and transmission of large‐scale offshore wind power based on the high‐voltage and large‐capacity power cable systems.

At present, the most commonly used extruded insulation material for power cables is cross‐linked polyethylene insulation. Its main producers include Dow Corporation in the United States and Nordic Chemical Company, which basically occupy the world market for DC cable insulation materials. The “superclean” XLPE insulation material launched by Nordic Chemical can meet the insulation material performance requirements of 500 kV high‐voltage AC and ±640 kV high‐voltage DC cables [1].

Table 1.1 Advantages and disadvantages of OF‐, MI‐, and XLPE‐insulated cables.

Cable insulation

Advantage

Disadvantage

OF

① Safe and reliable operation ② Long service life ③ High conveying power

① Complex installation ② Oil supply equipment needs to be installed ③ Potential environmental pollution

MI

① Safe and reliable operation ② No oil supply equipment required ③ Cable length unlimited ④ No potential environmental pollution

① Complex installation ② Restricted working temperature

Extruded XLPE

① High operating temperature ② High‐rated capacity ③ Lightweight and low loss ④ Easy installation

① Space charge injection and accumulation ② Thermosetting materials cannot be melted and reused

Figure 1.2 Cable typologies that can be conveniently used in relation to the length, power, and voltage of the link and the type of transmission system.

However, with the widespread application of XLPE, its shortcomings are constantly exposed. (i) The production process of XLPE high‐voltage DC cables requires controlling the environment and temperature of the cross‐linking process, resulting in complex production processes, high‐energy consumption, and low‐production efficiency. (ii) After the XLPE cable is extruded, a long‐term degassing treatment is required to reduce the impact of crosslinking by‐products on DC conductivity and space charge accumulation. (iii) The cross‐linking by‐products, mainly including acetophenone, cumyl alcohol, and α‐methylstyrene, accelerates the aging of cable insulation, which makes it difficult to achieve a lifetime of 40 years [2]. (iv) XLPE is a thermosetting polymer, and after cable retirement, the insulation cannot be melted again for processing and utilization [3], making it difficult to recover and treat, which has a significant adverse impact on the environment. Given the many issues mentioned above, the application of XLPE insulation material in high‐voltage DC cables has significant limitations, and the development of thermoplastic insulation materials is a new research direction.

Thermoplastic‐insulated cables have many advantages, such as no cross‐linking process is required in the production process, simple process, and low‐energy consumption, no need for degassing, shortened production cycle, no cross‐linking by‐products (which does not affect the electrical performance of insulation), and recyclable by melting. Since the 1970s, thermoplastic insulation materials such as low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (HDPE) have been used in high‐voltage cable insulation, but their heat resistance level or mechanical properties do not fully meet the requirements of high‐voltage DC cables. Polypropylene (PP) material has excellent insulation, heat resistance, mechanical properties, and recyclability, making it suitable for the manufacturing of high‐voltage DC cables. The melting point of PP is about 40–50% higher than that of polyethylene, and the long‐term working temperature is higher than that of XLPE [4]. Good heat resistance is of great significance for improving the working temperature and voltage of cables. PP is a nonpolar material with high breakdown field strength (≥100 kV/mm under AC voltage and ≥300 kV/mm under DC voltage) and high‐volume resistivity (~1016 Ω m), which does not change significantly with temperature. Under the same insulation thickness, the PP‐insulated cable system has higher operating voltage, higher long‐term operating temperature, and larger transmission capacity of the line, compared with that with XLPE insulation. In another word, PP cable insulation can reduce the thickness of the insulation layer and increase the current carrying capacity of the cable system. PP has a high charge injection threshold and less space charge accumulation [5]. PP‐insulated cables can reduce the energy consumption of the cable cross‐linking process by 1000 kWh/km and reduce carbon emissions by 80% throughout their entire life cycle, according to data from Prysmian. In summary, studying PP as an insulation material for high‐voltage DC cables has broad application prospects.

As early as the 1990s, scholars began to study the electrical properties of PP as an insulation material for high‐voltage DC cables. Scholars from Osaka University collaborated with Mitsubishi Cable to prepare 600 V and 22 kV cables using PP as insulation. The study found that the AC breakdown field strength and dielectric loss of PP cables at different temperatures met the requirements of the cables. Further research on 22 kV cables found that the lifespan of cables with PP as the main insulation is higher than that of XLPE, and the suppression effect on water trees is better than that of XLPE [6]. In 2010, Prysmian released a high‐performance thermoplastic elastomer (HPTE) insulation material developed based on PP materials. P‐Laser cables prepared using HPTE as the matrix have better electrical and mechanical properties than traditional XLPE cables [7]. In 2015, Prysmian developed a 320 kV DC high‐voltage cable system, and, in 2016, it announced the successful development of 525 kV/2.6 GW and 600 kV P‐laser‐insulated cables, but none of them have been put into engineering application. In 2015, A. Vaughan et al. from the University of Southampton [8] prepared a model cable by blending PP with propylene–ethylene copolymer. The study found that the mechanical and electrical properties of the cable were better than those of XLPE cable. In 2019, P. K. Jiang from Shanghai Jiao Tong University and Shanghai Huapu Cable Company jointly developed 35 kV and below PP power cables and announced their successful development [9]. In 2023, the authors of this book collaborated with Nanrui Group Corporation to develop functionalized‐graft‐modified PP cable insulation. In 2023, Prysmian will supply the cable system for the BalWin offshore wind farms in the North Sea of Germany, where P‐laser cable based on PP‐based insulation will be used for land cables.

PP is polymerized from propylene monomers, and there are a large number of asymmetric carbon atoms on the main chain of PP. According to the different arrangement of methyl groups on the main chain in space, PP can be divided into three molecular configurations, as shown in Figures 1.3. Isotactic polypropylene (iPP) has all methyl groups located on one side of the molecular chain, while syndiotactic polypropylene (sPP) has alternating methyl groups located on both sides of the molecular chain. The irregularly located methyl groups on both sides of the molecular chain are called atactic polypropylene (aPP). Among them, iPP and sPP are crystalline materials, while aPP is an amorphous material. The density of iPP is 0.92–0.94 g/cm3, and the melting point is approximately 165 °C. The density of sPP is 0.89–0.91 g/cm3, and the melting point is approximately 135 °C. The density of aPP is 0.90–0.91 g/cm3, and the softening temperature is 90–130 °C. Among the three molecular configurations of PP, aPP is an amorphous polymer and is not suitable for use in insulation materials. The crystallinity of sPP is relatively low, and, in the presence of nucleating agents, the crystallinity is only 30–42%. Moreover, the crystallization rate is very slow, and it depends heavily on the regularity of the opposite structure, resulting in poor processability [10]. At the same time, the thermal deformation temperature of sPP is about 30 °C lower than that of iPP. The most common type of iPP, with high melting point, high thermal deformation temperature, excellent processability, and a wide range of adjustable spherulite size, is suitable as an insulation material for high‐voltage DC cables.

Figure 1.3 Molecular configurations of PP. (a) iPP, (b) sPP, (c) aPP.

However, iPP also has drawbacks that limit its ability to become cable insulation, including strong rigidity, high brittleness, insufficient flexibility at room temperature, poor low‐temperature impact performance, and the tendency of iPP to form large spherulites, leading to a significant reduction in the strength of the breakdown field. These drawbacks result in the iPP substrate cable being prone to cracking under stress, making it difficult to directly produce and process, and making it difficult to bend after cable formation, which is not conducive to cable laying and use. So it is necessary to modify iPP to improve its mechanical toughness, low‐temperature impact, and breakdown field strength before it can be applied in power cable insulation. However, most toughening and modification methods will inevitably lead to varying degrees of degradation in electrical and heat resistance properties. In recent years, scholars from various countries have conducted research on the modification of polypropylene insulation, with the goal of synergistically regulating the electrical, mechanical, and heat resistance properties of polypropylene insulation. This book will review the state‐of‐the‐art of PP‐based cable insulation and discuss its dielectric properties and their modification methods for AC and DC cables, respectively.

1.2 State of the Art of PP Modification Method

1.2.1 Nanocomposites

Extensive research efforts have been dedicated to the enhancement of the dielectric properties of polypropylene (PP) insulations. With the laboratory research and industrial application of nanomaterials, nano addition has become a commonly used method of PP modification [11, 12]. Nanoparticles refer to groups of atoms or molecules with a size of nanometers, which have small particle size and large specific surface area, and show significant nano‐effects, such as quantum size effect and volume effect. In addition to their own special properties, nanoparticles can also endow materials with different functions, such as optoelectronic, magnetic, and dielectric and anti‐aging properties [13]. Nonetheless, it is imperative to acknowledge that nanoparticles are burdened by exceedingly high surface energy, rendering them prone to substantial agglomeration tendencies.

The addition of nano MgO particles could significantly inhibit the injection and accumulation of space charges in PP material. The composite material with 3 phr MgO had improved DC breakdown strength, with an increase of 29.3% compared to pure PP [14]. The study found that the addition of nano MgO reduced the space charge density of PP/POE blends and increased the strength of the DC lightning breakdown voltage [15]. In the European project GRIDABLE, experimental data showed that compared with pure PP (6.9 × 10−16 S/m), 4.5 wt% PP/SiO2 composite insulation had lower conductivity (3.8 × 10−16 S/m). This could be attributed to the ability of nano SiO2 particles to inhibit charge accumulation and improve trap site distribution [16]. There were attempts on small‐scale production of inorganic nano/PP composite insulations. The doping of SiO2 particles could only improve the performance of low‐probability breakdown areas at room temperature, making the breakdown data more concentrated [17]. There were also some studies focusing on the effect of nano particles on the polymer microstructures. Inorganic nanoparticles would affect the crystalline morphology, crystallization rate, and crystallinity of the PP, resulting in changes in the properties of PP composites. The changes of the crystallization behavior of PP/ZnO nanocomposites were studied in [18]. The results showed that nano‐ZnO could promote the nucleation of PP, and the crystallization temperature increased by 7 °C with the content of 0.06 wt%. The crystal morphology results proved that nano‐ZnO played a positive role in refining spherulites. Moreover, it was found that nano‐CaCO3 can help to form a uniform spherulite structure and eliminate the boundary between spherulites, confirmed by a scanning electron microscope. At the same time, the crystallization time of the composite material was significantly shortened, indicating that the crystallization rate was improved [19].

However, due to the large difference in polarity and high surface energy between nanoparticles and PP, nanoparticles may exhibit a strong tendency to agglomerate in the PP matrix, resulting in an increase in interfacial defects and ultimately the deterioration of the composite properties [20]. In [21], the PP composite insulation with 2 wt% nano‐clay had a shorter service life, mainly due to the different thermal properties of the nanoparticles and the polymer matrix, and the weak coupling effect between the nanofillers and the matrix. Under long‐term electric field and higher temperature, the dissociation of the nanoparticles may be caused. Inorganic nanoparticles are prone to cause agglomeration and separation phenomenon due to dispersibility problems, which may form micropores inside the insulation. Such electrical weaknesses can lead to a decrease in breakdown field strength.

To improve the dispersion of nanoparticles in PP matrix, some surface treatment or modification methods were used. The nano SiO2 was modified with trimethylsilane, which could fill the gaps of spherulites inside PP and block the charge transport. The AC and DC breakdown strengths were, respectively, increased by 19.9% and 52.3%, respectively, compared to those of the pure PP [22]. In [23], two‐dimensional hexagonal boron nitride (h‐BN) nanosheets were coated by non‐covalent polymerization of dopamine hydrochloride (PDA) to obtain BNNSs@PDA. The results showed that the expansion of electrical trees was effectively hindered under the electric field. The dielectric loss was only 0.006, and the theoretical energy storage density reached as high as 7.42 J/cm3. In addition, organic–inorganic hybrid nanofillers have attracted the attention of many researchers because of its better compatibility compared with inorganic particles. Polyhedral oligomeric silsesquioxanes (POSSs) had a cage structure consisting of a skeleton alternately connected by Si–O. The groups connected to Si atoms on the top corners could be reactive or inert groups, which mainly determined the chemical or physical interaction between POSS and the polymer matrix. Compared with traditional fillers such as SiO2 and Al2O3, nano POSS was a type of organic–inorganic hybrid particles with a strong comprehensive performance due to the remarkable small‐size effect, close bonding interface, and good compatibility, which had capacity in the regulation of polymer properties [24, 25]. Some studies showed that POSS helped to improve the corona resistance and suppress partial discharges (PDs) of the insulation material [26]. After adding 3 wt% of octamethyl‐POSS, the average AC breakdown strength of PP increased by 22% [27].

However, in the field of electrical properties improvement of polymers, the application of other kinds of organic–inorganic hybrid nano particles has not been found yet. The feasibility of doping such nanofillers to thin insulations has not been verified. The effects of nanofiller type on the crystallization, melting, and other behaviors of PP insulations are not clear enough. The relationship among the molecular structures of organic–inorganic hybrid nanofillers, the micro morphologies, and dielectric properties of PP insulations needs further research. At the same time, the complex voltage stresses under operating conditions have not been considered when evaluating the breakdown performances of modified PP insulations.

1.2.2 Polymer Blending

The properties of different polymers can be synthesized through proper physical blending, which is a widely used material modification method. After PP and ethylene–propylene copolymer were blended, when the mass fraction of the ethylene–propylene copolymer is 50% and the ethylene content is 9 mol%, the breakdown field strength of the blend increased [28]. Studies showed that the trap density of PP/ethylene–octene (EO) blends was twice that of PP/ethylene–propylene (EP) blends, which effectively reduced electric field distortion inside the blends [29]. When PP/ethylene–octene copolymer (POE)/styrene–ethylene–butylene–styrene block copolymer (SEBS) was (100/10/10), the low‐temperature impact performance of the blend increased by 1.82 times and the elongation at break raised to ~614.1% [30]. In [31], PP/ULDPE blends with 15 wt% ULDPE had excellent mechanical toughness. In order to further improve the electrical characteristics, the doping of 0.01 wt% nano‐graphene introduced a large number of deep traps in the composite. The space charge accumulation of the PP/ULDPE/graphene composite material was reduced, and the breakdown strength increased by ~14.3% compared with that of the pure PP at 30 °C. Polyvinylidene fluoride (PVDF) with low molecular weight had good compatibility with PP. Although the dielectric constant was elevated to 3.6, but the breakdown strength of the composite insulation was only about 439 kV/mm. Furthermore, boron nitride nanosheets (BNNSs) helped to improve the compatibility, resulting in higher Young's modulus and tensile strength [32]. It was worth noting that the dielectric loss of PP/PFDF composite insulation reached 0.54%, which was usually 1–2 orders of magnitude higher than that of the pure PP, too high to be suitable for applications in power systems [33, 34]. At the same time, the two‐phase interface produced by different polymers would increase the dielectric loss and affect the space charge accumulation. Comparing the effects of EPDM, POE, and EVA on the electrical and mechanical properties of PP composites, it was found that the parameters such as melting temperature and crystal form of the blends did not change much, and their toughness was significantly improved. However, there were a large number of interfaces between different materials, which led to serious accumulation of space charges in the blends and a decrease of 5–15% in breakdown performance [35]. What’s more, the crystallization behavior of PP was significantly affected in the blend, which may reduce the crystallinity of the polymer insulation.

It can be seen that in order to improve polymer properties through blending, not only it is necessary to further explore the optimal mixing ratio, but also the coordination relationship between various properties is also crucial. How to realize the regulation of the long‐term electrical resistance and loss characteristics of composite materials still needs further research.

1.2.3 Chemical Copolymerization and Grafting

Copolymerization refers to the polymerization of two or more compounds into one substance under certain conditions. Based on the study of the thermal and dielectric properties of ethylene–propylene block copolymer, ethylene–propylene random copolymer, and ethylene–propylene–butene random copolymer, it was found that random copolymer had a higher melting point. And, the limited space charge injection resulted in relatively high breakdown field strength [36]. Some scholars compared the dielectric properties of polypropylene and ethylene–propylene block copolymer polypropylene (EPC). Results showed that the DC breakdown field strength of EPC at high temperatures was more stable than that of the PP, and the amount of space charge accumulation was less [37]. The melting and crystallization peak temperatures of copolymerized PP were lower than those of homopolymerized PP. This was because the chemical structures of the molecular chain of the former had changed greatly compared with the latter, which directly affected the arrangement of the molecular chain and the crystallization behavior, further affecting its macroscopic thermal parameters [38]. However, the copolymerization process is complicated and the by‐products are difficult to control. Besides, PP molecules are easily entangled with other polymer molecules, which may form smashed crystals. Or, the copolymerization chemistry will reduce the molecular chain regularity, adversely affecting the crystallinity of the material and even destroying the mechanical or electrical properties.

Scholars at home and abroad used chemical methods to graft maleic anhydride onto PP molecules, aiming to introduce deep traps to inhibit space charge injection and accumulation. It was proved that maleic anhydride‐grafted polypropylene (PP‐g‐MAH) could significantly promote the dispersibility of ZrO2 nanoparticles. The DC breakdown strength of the ternary system composed of PP, PP‐g‐MAH, and ZrO2 was elevated by 43.3% compared with that of unmodified PP [39]. After the antioxidant‐hindered phenol group (AO) was grafted to the main chain of the PP molecules using esterification reaction, the thermal oxidation stability of the material was significantly enhanced [40]. Studies showed that by grafting styrene copolymer to PP, the chain segment relaxation of the dielectric could be improved by heterogeneous nucleation. The reduction of dielectric loss reached approximately 37.6% [41]. The benzoyl peroxide (BPO) was used to initiate the graft reaction of PP molecular chain and styrene monomer. The graft modification had no obvious effect on the thermal parameters of the material, and the breakdown strength was ~ 498 kV/mm at 30 °C. The benzene ring group grafted on the PP molecular chain had a large volume, and the rigid group could not be rotated. The steric hindrance effect hindered the movement and orderly folding of the PP molecular chains, and, finally, reduced the crystallinity [38].

There are also some studies on the chemical graft modification of the surface layer of polymers. After fluorine was grafted onto the surface, the trap distribution of the PP insulation could be controlled. Charges were mainly bound in the fluorinated layer, which was difficult to inject into the insulation. At the same time, the electric field formed by the charge accumulation inside the fluorinated layer would weaken the external electric field and further reduce the charge injection [42]. Under the catalysis of ultraviolet light, the C–H of the surface layer in the PP insulation was replaced by C–OH [43]. The grafted PP–OH insulation always had better insulation resistance than that of the unmodified insulation. The chemical deep trap introduced by the surface hydroxyl (–OH) layer helped to capture the injected charge, reduce the number of carriers, and prevent the internal degradation of the insulation by preventing the carriers from hitting the molecular chain [44].

It can be seen that chemical treatment methods have the potential to improve the dielectric properties of insulations, but such research is still in the laboratory‐level preparation stage. At present, due to the low success rate and by‐products of chemical graft technology, how to achieve precise and efficient control is the main problem to be solved.

1.2.4 Crystallization Regulation

Research has shown that there is a strong relationship between the microscopic characteristics and the macroscopic properties of the PP insulation. In addition to the influence of molecular weight and molecular chain regularity, the crystallization behavior is also controlled by heterogeneous nucleus and crystallization conditions. The control of the crystallization process was evaluated and showed the potential for the application to the structure optimization of PP insulations [45]. The low crystallinity of PP induced an increase in electron transport in the amorphous region, which resulted in an increase in impact ionization. Furthermore, the low density of the spherulite polymer caused a decrease in the dielectric breakdown strength of the PP material [46, 47].

Under different crystallization conditions, PP molecular chains showed different spatial arrangements in the crystal lattice and could form five crystalline forms including α, β, γ, δ, and pseudo‐hexagonal state [48–50]. Among them, the α‐crystal form belonged to the monoclinic crystal system, which is the most thermodynamically stable crystal form. PP generally tended to crystallize to form the α‐crystal form with normal preparation conditions. α‐Nucleating agents could induce α‐crystals in PP, which were divided into three categories: inorganic, organic, and polymer types. The inorganic nucleating agents mainly included mica, titanium dioxide, and calcium carbonate, which had cheap price but poor compatibility with polymers, so could not be used in mass production [51]. Organic nucleating agents, such as dibenzylidene sorbitol, aromatic carboxylates, and substituted aryl heterocyclic phosphates, had good dispersibility, showed high nucleation efficiency with low contents and developed most rapidly among all α‐nucleating agents. The main representatives of polymer nucleating agents were polyvinyl naphthenic compounds, but the related technology was not yet mature, and there was almost no report on the advent of industrialized products.

The results showed an increase in the crystallization peak temperature of the composites and a preferred orientation of the polymer crystallites as the concentration of the nucleating agent content increased [52]