Space Charge Accumulation in Polymeric. High Voltage DC Cable Systems

Transcription

1 Space Chage ccumulation in Polymeic High Voltage DC Cable Systems

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3 Space Chage ccumulation in Polymeic High Voltage DC Cable Systems Poefschift te vekijging van de gaad van docto aan de Technische Univesiteit Delft op gezag van de Recto Magnificus pof. d. i. J. T. Fokkema voozitte van het College voo Pomoties, in het openbaa te vededigen, op dinsdag 28 novembe 2006 om 15:00 uu doo Riccado ODEG dottoe in ingegneia elettica, Politecnico di Milano geboen te Lecco (Italië)

4 Dit poefschift is goedgekeud doo de pomoto: Pof. d. J. J. Smit Samenstelling pomotiecommissie: Recto Magnificus, Pof. d. J. J. Smit, Pof. d. eng. J.. Feeia, Pof. i. W. L. Kling, Pof. d. i. E. F. Steennis, Pof. d. J. C. Fothegill, D. i. P. H. F. Moshuis, D. i. M. D. Veweij, Voozitte Technische Univesiteit Delft, pomoto Technische Univesiteit Delft Technische Univesiteit Delft/Eindhoven Technische Univesiteit Eindhoven Univesity of Leiceste, United Kingdom Technische Univesiteit Delft Technische Univesiteit Delft This eseach was funded by the Euopean Commission in the famewok of the Euopean poject enefits of HVDC Links in the Euopean Powe Electical System and Impoved HVDC Technology (contact ENK6-CT ). ISN Copyight 2006 by R. odega Cove: photogaph by uno van den Elshout copyight PhotologiX.nl Pinting: Optima Gafische Communicatie Rottedam, The Nethelands

5 He who loves pactice without theoy is like the sailo who boads ship without a udde and compass and neve knows whee he may cast. Leonado da Vinci i miei genitoi, i miglioi maesti che abbia mai avuto [To my paents, the best teaches I ve eve had] lla mia compagna, Yvonne, pe avemi insegnato a conoscee meglio me stesso [To my gilfiend Yvonne, who taught me moe about who I am]

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7 Peface This Ph.D. thesis was completed within the famewok of the poject HVDC ( enefits of HVDC Links in the Euopean Powe Electical System and Impoved HVDC Technology ) funded by the Euopean Commission. The HVDC poject was launched in Januay 2003 to povide a methodology and associated softwae and hadwae tools to assess the potential technical, economical and envionmental benefits and impacts of high-voltage diect cuent inteconnections embedded in the actual electical powe tansmission and distibution systems of the Euopean netwok. The poject investigated also the potential benefits of using envionmentally moe acceptable high-voltage diect cuent powe cable systems. Delft Univesity of Technology contibuted to the HVDC poject togethe with the following Euopean Patnes: Univesity of Leiceste (UK) Univesity of Suey (UK) Univesity of ologna (IT) Univesity Paul Sabatie, Toulouse III (FR) IECOS (UK) CESI (IT) Pysmian Cavi e Sistemi Enegia S..l. (IT) oealis (SW) TenneT (NL) Tena (IT) RTE (FR) Some of the data pesented in this thesis wee poduced by the poject Patnes in the famewok of the HVDC poject. When so, this is explicitly indicated in the thesis. vii

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9 Table of contents Peface vii Table of contents ix 1. Intoduction Geneal HVDC cable systems HVDC cable insulation Polymeic HVDC cable systems and space chages Histoical development of HVDC cables Space chage and space chage field Reseach and development tend fo space chage phenomena in HVDC polymeic cable insulation Objective of the pesent study Scientific contibution of the thesis to the field of polymeic HVDC cable systems Outline of the thesis Expeimental methods Test specimens Conduction cuent measuements Space chage measuements Geneal Pulsed electoacoustic method (PE) PE method fo multi-dielectic test objects PE method fo cable geomety test objects Space chage paametes ccuacy of the measuements Test conditions Tempeatue conditions Electical conditions Expeimental obsevation of space chage and electic field dynamics Conduction cuent measuements on XLPE and EPR flat specimens Intoduction Results Discussion Summay and conclusions Space chage measuements on XLPE-EPR flat intefaces Intoduction Results Discussion ix

10 Summay and conclusions Space chage measuements on MV-size XLPE cables Intoduction Results Discussion Summay and conclusions Space chage measuements on dual-dielectic mini-cables Intoduction Results Discussion Summay and conclusions Space chage measuements on MV-size models of cable joints Intoduction Results Discussion Summay and conclusions Conclusions Calculation of space chage and electic field in DC cable systems Intoduction Physical model Theoetical backgound Model of the insulation Numeical implementation of the physical model Results of the calculation Calculation vs. measuements Effect of the conductivity function on the calculated pattens Electic field in cable systems fo paticula situations Conclusions Space chage at intefaces in HVDC cable systems Intefaces in HVDC cable systems Space chage accumulation at the semicon-insulation inteface Chage accumulation mechanisms at the semicon-xlpe inteface ccumulation of chage at the semicon-xlpe inteface in the studied specimens Space chage accumulation at the dielectic inteface Mawxell-Wagne theoy fo the intefacial polaization Deviation fom the Maxwell-Wagne theoy: liteatue Deviation fom the Maxwell-Wagne theoy: expeimental esults Thee-laye model fo the dielectic inteface Suggestions on how to impove the macoscopic model fo space chage accumulation x

11 6. Feasibility study fo on-line on-site PE measuements Intoduction Implementation Conclusions Conclusions Space chage at dielectic discontinuities Space chage in cable systems that expeience a tempeatue dop acoss the insulation Recommendations and suggestions fo futhe studies Recommendations Recommendations fo PE testing on HVDC cable system insulation Recommendations fo the design of polymeic HVDC cable systems Recommendations fo the opeation on HVDC cable systems Suggestions fo futhe study ppendix Space chage measuements on multi-dielectics by means of the PE method Geneation of acoustic waves Calculation of electically-induced pessue waves DC voltage applied in absence of space chage and intefacial chage Space chage in the absence of DC voltage and intefacial chage Pesence of intefacial chage in absence of DC voltage and space chage DC voltage applied in pesence of space chage and intefacial chage coustic wave taveling and eflection coustic wave popagation coustic wave eflection Intepetation of detected acoustic signals Test specimens and test pocedues Measuement esults Intepetation of measuement esults Conclusions PE method fo cylindical test objects Diffeent type of PE set-ups Shape of the eath electode pplication of the pulsed voltage pplication of the pulse to the cables Effect of cylindical geomety on the amplitude of acoustic waves xi

12 C. ttenuation and dispesion of acoustic waves in the PE method C.1. Poblem identification C.2. Theoetical backgound C.3. Pocedue fo ecoveing the oiginal acoustic wavefom fom the attenuated and distoted wavefom D. Calibation D.1. Flat homogeneous test object D.2. Flat multi-dielectic test object D.3. Cylindical homogeneous test object D.4. Cylindical multi-dielectic test object E. Equations adopted in the numeical pocedue E.1. Poisson s equation fo the electic field E.2. Calculation of the chage-induced field E.2.1. Field induced by space chage in coaxial intefaces E.2.2. Field induced by intefacial chage in coaxial intefaces E.2.3. Geneal expession of the field E.3. Fouie s heat diffusion equation fo cables and cable joints Refeences List of abbeviations List of symbols Summay Samenvatting Sommaio cknowledgement Cuiculum Vitae xii

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15 Chapte 1 - Intoduction 1. Intoduction 1.1 Geneal HVDC cable systems Since the nineteen fifties, high voltage (HV) diect cuent (DC) cable systems have been used woldwide fo the tanspot of electical enegy. cable system consists of a cable and its accessoies, i.e. joints and teminations. Teminations ae needed at the ends of the cable cicuit, to gade the electic field and to connect the cable to the powe line. Joints ae equied when the cable becomes too long to be poduced in one length. Taditionally, HVDC cable systems have been used only when altenate cuent (C) technology could not be applied. The main eason fo this is that DC links need two convesion stations at the ends of the tansmission link. In the past, the convesion stations often inceased the costs of the poject too much. typical example of HVDC cable application is the sea-cossing enegy tanspot. In this case, C ovehead lines cannot be employed. Moeove, if the length of the sea cossing exceeds km, the use of C cables becomes unfeasible because of the high value of the capacitive eactive powe. Theefoe, HVDC cable systems ae the only technically applicable solution fo long distance undegound o submaine connections. The taditional estictions on the use of HVDC technology ae apidly changing, especially within Euope, giving a potential oppotunity fo application of HVDC cable systems. The actual Euopean tansmission netwok consists in fact of lage HVC synchonous zones, athe scacely inteconnected by HVDC links. New HVDC links ae a means fo einfocing the actual HVC Euopean tansmission netwok. The main dives fo HVC netwok einfocement by means of HVDC links ae [73, 154]: - The libealization of the electical enegy maket and the consequent incease of taded enegy ae leading to lage vaiations of the powe flows. This asks fo additional coss-bode tansmission capacity. - Decentalized powe geneation plants, in paticula wind enegy, ae inceasingly being intoduced in the netwoks. Decentalized geneation plants essentially opeate at vaiable conditions that may have an impact on the powe quality of the netwok, e.g. [136]. - Nowadays, moe difficulties ae encounteed in getting the necessay pemits to build additional ovehead tansmission lines. - The effect of magnetic fields on human health and the visual/noise impact ae ongoing contovesial debates because of which the public efains to accept new ovehead tansmission lines. 1

16 Chapte 1 - Intoduction In this famewok HVDC cable links can povide: - inteconnection between independent netwoks, inceasing the tansmission capacity and inceasing the possibilities fo enegy exchange; - the contol of eactive powe, which suppots the netwok stability and the powe quality (when voltage souce convetes VSC ae used); - elatively shot time fo deliveing a new link. s impotant as the pevious aspects, HVDC cable systems ae consideed envionmental fiendly, because of thei vey low visual impact and noise impact. Futhemoe, when using HVDC cables in a bipola configuation, pactically no magnetic field is poduced HVDC cable insulation The main concen about the employment of new HVDC cable links is the athe high cost of the connection. The cost of the connection could be significantly educed by using polyme-insulated HVDC cable systems (also called extuded HVDC cable systems) instead of mass-impegnated o oil-filled HVDC cable systems (also called lapped o oil-pape HVDC cable systems). The extusion poduction pocess is, in fact, simple and cheape than the pocess fo manufactuing lapped insulation. In addition, extuded cable systems pesent the following advantages if compaed to lapped cable systems: - no use of any impegnants o oil; - moe mechanical igidity, allowing the use of a thinne cable amo; - highe maximal woking tempeatue; - easie cable maintenance and easie component eplacement; - easie pepaation and mounting of cable joints. On the othe hand, HVDC mass-impegnated cables have been poven to be eliable ove many decades, while HVDC polymeic cables have been only ecently employed. The eason fo this will be explained in the next paagaph. 2

17 Chapte 1 - Intoduction 1.2. Polymeic HVDC cable systems and space chages Histoical development of polymeic HVDC cables The fist HVDC cable went into sevice in 1954, fo connecting the Gotland Island in the altic Sea to the Swedish mainland. This was a mass-impegnated cable. Since then, many HVDC cables have been employed all ove the wold. Most of these cables ae mass-impegnated o oil-filled insulated. Though the yeas, the powe tanspoted by the DC links and thei ated voltages have inceased, the cable systems have become longe, the C/DC convesion technology has changed, but massimpegnated and oil-filled insulation have been, in pactice, the only type of insulation to be used up to In that yea, the fist commecial HVDC polymeic cable system was put into sevice. This tend is in contast to what happened in the C cable technology, whee polymeic insulation has been successfully used since many decades. Now it is the dominating C cable technology. The main issue, which needed to be esolved fo the development of the HVDC polymeic cables, is the contol of the space chage phenomena, which affect the eliability of the connection. Nowadays, this concen has been addessed, but only patially solved. s a consequence, mass-impegnated pape is still the dominating technology fo HVDC cable insulation. Recently, some innovative HVDC cable pojects making use of polymeic cable insulation have been completed [52, 130], up to a ated voltage of 150 kv and fo a maximal powe capacity of about 500 MW 1. Howeve, in these pojects, the polaity of the DC voltage must be kept fixed and then the invesion of the powe flow has to be done by inveting the cuent diection. This limits the application of the polymeic cable system to HVDC links using IGT-based convetes (VSC). In the futue, as it has aleady happened fo C cables, polymeic HVDC cables ae expected to eplace the taditional lapped HVDC cables. Howeve, in ode to make this step feasible, moe undestanding about the fundamental space chage pocesses occuing in polymeic DC cable insulation is needed. 1 ccoding to [77], the HVDC polymeic cable technology will be soon upgaded up to 300 kv and 1000 MW. 3

18 Chapte 1 - Intoduction Space chage and space chage field One of the intinsic popeties of the DC cable insulation is the accumulation of chages. Insulating mateials allow a weak electical conduction. This weak flow of chage within the insulation may not be unifom, because of a local non-homogeneity of the mateial. ccoding to the cuent density continuity equation, when an inequality occus between the flow of chages into a egion and the flow of chages out of that egion, chage accumulates in that egion, see equation (1.1): ρ j + = 0 (1.1) t In equation (1.1), j is the cuent density, ρ is the chage pe unit volume (also called space chage density o simply space chage) and t the time. ccoding to Gauss law, a space chage field E ρ is associated to a chage distibution: ρ = E (1.2) ( ) 0 ρ whee 0 is the vacuum pemittivity and the elative pemittivity of the insulation. Theefoe, the electic field E within the insulation in the pesence of space chage is given by the sum of two contibutions: the space chage field and the extenal field E 0 (also called Laplacian field), which is induced by the applied voltage, see equation (1.3). E = E 0 + (1.3) E ρ In the C situation, the flow of chages invets its diection too quickly to allow a significant gowth of space chage at the insulation inhomogeneities, at least fo conventional insulating mateials. This means that the space chage field can be neglected. On the othe hand, unde DC stessing condition, the flow of chage maintains the same diection. This allows a build-up of chage, which, in geneal, significantly affects the electic field distibution inside the insulation. ccumulated chages can be eleased by the insulation when the extenal field is emoved and the insulation shot-cicuited. Howeve, this pocess can last quite a long time, depending on the type of insulation and on the tempeatue. In geneal, chage depletion is much longe fo polymeic insulation than fo lapped insulation. consequence of this phenomenon is that the accumulated chages will be kept within the insulation also when the extenal DC voltage is emoved o when the value of the extenal DC voltage changes. The invesion of the voltage polaity in HVDC cables is a pactical example of such a situation. In this paticula case, the insulation expeiences the sum of the space chage field and the field induced by the DC voltage, which diection has been inveted. This geneally leads to a maximum field nea the inne conducto of the cable. In the wost case, the maximum field can be as high as twice the maximum value of the Laplacian field. 4

19 Chapte 1 - Intoduction Reseach and development tend fo space chage phenomena in HVDC polymeic cable insulation Duing the last thee decades, space chage phenomena in HVDC insulation have been investigated woldwide. Many techniques have been developed fo the expeimental obsevation of space chage. This, combined with a bette theoetical undestanding of the physical pocesses and an impovement of the extusion cable technology, has been instumental to the development of polymeic HVDC cables [30]. Table 1.1 epots an oveview of the most adopted non-destuctive 2 methods fo the measuement of space chage in solid insulation. Nowadays, the scientific community agees that space chage measuing techniques have eached matuity. Howeve, thee ae still two main diections of advancement egading the use of space chage measuements fo the development of polymeic HVDC cable insulation. The fist one consides the impovement of the space chage measuing systems and the enlagement of the applicability field fo space chage measuing techniques, e.g. [54]. Often, combinations of moe insulating mateials o semiconducting and insulating mateials ae used in cable systems. Fo instance, new (nano)composite mateials ae potential candidate fo the cable insulation technology. Consequently, the objects of space chage measuements as well as the measuing conditions ae becoming moe complex. This means that the measuing systems and thei opeational limits have to be upgaded. The second field of advancement fo space chage methods egads the exploitation of the infomation deived fom the measuement esults, e.g. [116]. In addition to the indication of space chage and electic field magnitude and location, futhe quantities can be deived fom the esults of the measuements. If popely analyzed, these quantities can suppot the identification of the chage caies, the natue of the conduction mechanism and even the aging state of the insulation. 2 Destuctive methods fo the measuement of space chage, such as the dust figue method [8] and the pobe technique method [82, 83], wee also used until the beginning of nineteen eighty. The pinciples of those techniques consist of cutting the test object into small slices and detecting the chage pesent at the suface of the cut slice. 5

20 Chapte 1 - Intoduction Table 1.1. Some of the most common non-destuctive methods fo measuing space chage in solid insulations. NON-DESTRUCTIVE METHODS METHOD Themal pulse method [43] Themal step method [36] Lase intensity modulation method [88] Optical methods [142] Pessue wave popagation methods PIPP [51, 143] LIPP [7, 135] Non-stuctued acoustic pulse [106] Pulsed electoacoustic method [6, 13, 60, 61, 94, 99, 100, 143] DESCRIPTION One side of a flat test object is exposed to a tempeatue pulse. s a consequence, a themal wave tavels though the test object. This displaces the space chage, poviding an electic signal at the extenal electodes. y means of deconvolution techniques, the space chage distibution can be obtained fom the electic signal measued at the electodes. This method is simila to the themal pulse method, but instead of a themal pulse a themal step is applied to the test object. sinusoidal-modulated lase-induced heating is poduced at both sides of a flat test object. In this way, tempeatue waves popagate though the mateial, inteacting with the space chage. The esult is a pyoelectic cuent to be measued at the extenal electodes. polaized light passes though the test object, which must be tanspaent. The electic stess distibution and/o the mechanical stess distibution acoss the test object may modify the optical popeties of the mateial. If this happens, the light passing though the test object has a phase delay. y measuing the phase delay of the light, and by knowing how the stess applied to the mateial affect its optical popeties, infomation about the electic/mechanic stess is povided. pessue pulse/step is applied to the test object. pessue wave is geneated and it tavels though the mateial. The acoustic wave displaces the space chage, poviding an electical signal measuable at the extenal electodes. Depending on the way in which the pessue pulse/step is geneated, the method has diffeent names: - PIPP (piezoelectically-induced pessue pulse); the pulse is piezoelectically geneated; - LIPP (lase-induced pessue pulse); the pulse is lase-geneated; - Non-stuctued acoustic pulse; the pulse is geneated by an HV spak between a conducto and a diaphagm. n electic pulse is applied to the test object, esulting in a petubation foce at the space chage location. Consequently, an acoustic wave is geneated. The acoustic wave is detected by a piezoelectic senso afte having taveled though the mateial and though the eath electode. The piezoelectic senso povides a voltage signal which caies the space chage infomation. 6

21 Chapte 1 - Intoduction 1.3. Objective of the pesent study The geneal objective of the pesent study is to obtain a bette undestanding of the majo factos that contol the space chage pocesses in polymeic HVDC cable systems. Electic field pediction methods, which include space chage phenomena, have to be developed to povide tools and pinciples to suppot the design and the opeation of HVDC polymeic cable systems. In ode to achieve this goal, two main factos of influence have been investigated: 1. Cable accessoies ae consideed to be the weakest pat of a cable system, because of the pesence of a dielectic inteface between the cable insulation and that of the accessoy. This thesis aims at a bette knowledge of the polaization phenomena occuing at dielectic intefaces. To that pupose, we developed an accuate methodology fo the expeimental study of the space chage behavio at the dielectic inteface. 2. When opeating, HVDC cable systems expeience a tempeatue dop acoss thei insulation. This thesis aims to povide a bette undestanding about the mechanisms esponsible fo space chage accumulation when a tempeatue dop is pesent acoss the insulation of the cable system. To that pupose, we developed a physical model fo the pediction of space chage dynamics and electic field in loaded HVDC cable systems. The physical model is validated by means of laboatoy investigation Scientific contibution of the thesis to the field of polymeic HVDC cable systems This thesis contibutes to the scientific development in the field of polymeic HVDC cable systems by: - eviewing the pulsed electoacoustic method fo the measuement of space chages at dielectic discontinuities, such as those encounteed in cable accessoies; - developing measuing systems which ae able to detect chages in test objects esembling the eal HVDC cables and cable accessoies; - identifying the main mechanisms esponsible fo the accumulation of chages at dielectic discontinuities and ecognizing the main paametes affecting the accumulation of chage; - expeimentally investigating the effect of a tempeatue gadient on chage accumulation in the insulation of HVDC polymeic cables and cable accessoies; - expeimentally investigating the effect of the voltage polaity evesal on loaded HVDC cables and cable accessoies; - developing a model fo the pediction of chage accumulation in HVDC cable systems and implementing it by means of a numeical pocedue; - developing opeational ecommendations fo the optimization of the electic stess in paticula woking conditions of the cable system. 7

22 Chapte 1 - Intoduction 1.5. Outline of the thesis In Chapte 2, the expeimental methods adopted in this wok ae descibed. They ae: - conduction cuent measuements, - space chage measuements. In paticula, space chage measuements wee pefomed on test specimens of complex geomety. This was done in ode to epoduce the insulation chaacteistics of eal HVDC cables and HVDC cable accessoies as accuately as possible. Special test set-ups had to be designed and constucted, along with the elated pocedues and softwae tools fo the intepetation of the esults and the coection of the output data. This is discussed in detail in ppendixes -D. In Chapte 3, the expeimental esults ae pesented and discussed. Expeimental investigation was caied out on the test specimens unde themal and electical conditions that esemble the woking conditions of HVDC cable systems. In fact, space chage measuements wee pefomed when: - a tempeatue dop is pesent acoss the insulation of the test specimens; - the polaity of the DC voltage, which is applied at the studied test specimen, is inveted. physical model fo the calculation of space chage and electic field in HVDC cable systems is pesented in Chapte 4 as well as its numeical implementation. The esults of conduction cuent measuements wee used as input fo the model, while the esults of space chage measuements wee adopted fo the model validation. The equations adopted fo the calculation of space chage and electic field ae deived in ppendix E. The most citical spots of HVDC cable systems ae the intefaces between diffeent mateials. oth the semicon-dielectic intefaces and the dielectic-dielectic intefaces ae studied in Chapte 5. The expeimental esults of Chapte 3 wee compaed with the esult of the theoetical modeling and the infomation available in the liteatue. Chapte 6 discusses the feasibility of pefoming on-line space chage measuements on actual HVDC cable system on-site. The oveall conclusions of the thesis ae dawn in Chapte 7. Finally, a numbe of ecommendations ae given in Chapte 8, along with some suggestions fo futhe study. 8

23 2. Expeimental methods Chapte 2 Expeimental methods In this chapte, the expeimental investigations pefomed in this wok ae intoduced. In section 2.1, the diffeent test specimens used fo the laboatoy eseaches ae descibed. Section 2.2 deals with the test set-up and the test potocol that wee adopted fo measuing the conduction cuent in insulating mateials. The technique, the test set-ups and the test potocols used fo space chage measuements on diffeent test objects ae discussed in section 2.3. In section 2.4, the expeimental conditions, at which the measuements wee pefomed, ae given. In the thesis, the chaging phenomena occuing in extuded DC cables and accessoies ae investigated. This is the main eason fo choosing the expeimental methods pesented in this chapte. In fact, space chage measuements diectly povide infomation about the chage that may be pesent within the insulation and the consequent field distotion. On the othe hand, the outcomes of conduction cuent measuements ae the basic input fo the model poposed in Chapte 4, which descibes the studied chaging phenomena Test specimens Expeimental investigations wee pefomed on diffeent types of test specimens of inceasing complexity: i.e. flat specimens, dual-dielectic mini-cables, MV-size cables and MV-size models of cable joint. The two main dielectic mateials composing the insulation of the test specimens ae HVDC cable-quality coss-linked polyethylene (XLPE) and ethylene-popylene ubbe (EPR). In addition, semicon electodes wee used. In Table 2.1, some electical popeties of the mateials ae shown. Table 2.1. Electical popeties of the specimen mateials. R [-] σ 20 C [Ω -1 m -1 ] σ 60 C [Ω -1 m -1 ] XLPE EPR semicon - >1 >1 Tape-type semicon - >1 >1 9

24 Chapte 2 Expeimental methods Flat specimens XLPE and EPR flat specimens consist of 30 cm x 30 cm plates in which a semicon electode is coss-linked at one side. Combinations of flat specimens ae obtained by putting two diffeent plates one onto each othe. Unless stated othewise, no lubicants o specific teatments wee applied at the inteface between the two plates. In Figue 2.1.a and in Figue 2.1.b, flat specimens ae epesented. In ode to expel coss-linking by-poducts 1, all flat specimens wee themally teated at 80 C fo 5 days befoe any testing. In Table 2.2, the amount of by-poducts befoe and afte the themal teatment is epoted [122]. This infomation was povided by the manufactue of the studied mateials in the famewok of the HVDC poject. a b Figue 2.1. Flat specimens. a) Dimensions of a flat specimen. b)coss section of a combination of flat specimens. Table 2.2. Coss-linking by-poducts in the studied flat specimens [122]. y-poduct efoe themal teatment fte themal teatment (80 C fo 5 days) Dicumyl peoxide [ppm] < 10 < 10 cetophenon [ppm] 4929 < 10 lpha methyl styene [ppm] < 10 < 10 Cumene [ppm] < 10 - Cumyl alcohol [ppm] 9100 < 10 Dual dielectic mini-cables Dual-dielectic mini-cables, as used in this wok, ae laboatoy models of a coaxial XLPE-EPR inteface. Dual dielectic mini-cables consist of a tiple-extuded constuction made of an innemost laye of semicon, a middle laye of EPR (o XLPE) and an outemost laye of XLPE (o EPR). The total insulation thickness is 2.1 mm (1.5 mm 1 The amount of by-poducts pesent within the insulation affects the electical popeties of the insulation and in paticula its space chage behavio [102, 111, 138]. In time, the insulation eleases the volatile by-poducts. Consequently, the electical popeties of the insulation may change in time, when the amount by-poducts in the insulation diminishes. In ode to neglect this phenomenon and making measuements compaable, the by-poducts wee expelled befoe any testing. 10

25 Chapte 2 Expeimental methods middle insulation mm oute insulation). Taped-type semicon was mechanically pessed against the oute insulation and, ultimately, a conductive eath sceen was applied. In Figue 2.2, the dimensions of a dual-dielectic mini-cable ae shown. efoe any testing, all the dual-dielectic mini-cables wee themally teated at 80 C fo 5 days. In Table 2.3, the amount of by-poducts befoe and afte the themal teatment is epoted. Table 2.3. Coss-linking by-poducts in the studied dual-dielectic cables [122]. y-poduct efoe themal teatment fte themal teatment (80 C fo 5 days) Dicumyl peoxide [ppm] < 100 < 100 cetophenon [ppm] 2927 < 100 lpha methyl styene [ppm] < 100 < 100 Cumene [ppm] < 100 <100 Cumyl alcohol [ppm] Figue 2.3. Dimensions of a MV-size cable Figue 2.2. Dimensions of a dual-dielectic mini-cable. 11

26 Chapte 2 Expeimental methods MV-size cables MV-size cables consist of 4.5-mm thick extuded XLPE cables with a conducto of 50mm 2. In Figue 2.3, the dimensions of a MV-size cable ae shown. efoe any testing, all the MV-size cables wee themally teated at 80 C fo 20 days. In Table 2.4, the amount of by-poducts befoe and afte the themal teatment is epoted. Table 2.4. Coss-linking by-poducts in the studied MV-size cables [122]. y-poduct efoe themal teatment fte themal teatment (80 C fo 20 days) Dicumyl peoxide [ppm] 17 < 10 cetophenon [ppm] 1169 < 10 lpha methyl styene [ppm] 15 < 10 Cumene [ppm] < 10 <10 Cumyl alcohol [ppm] MV-size models of cable joints MV-size models of cable joints ae constucted in the following way. Fistly, the eath sceen and the oute semicon of an XLPE-insulated cable (aea of the inne conducto = 50 mm 2 ; insulation thickness = 4.5 mm) is scaped off fo a length of 80 mm. Then, pat of the exposed insulation is emoved by means of a glass blade and the insulation suface is smoothed by using successive gades of abasive cloth. fte these opeations, the thickness of the emaining XLPE in the cental pat of the opening is 2 mm. t this point, a 100-mm long elastic tube made of EPR (thickness = 2 mm) can be applied on the XLPE by means of a special tool. fte fitting the EPR tube onto a hollow mandel placed ove the exposed XLPE, the tool pulls back the mandel. In this way, the EPR tube stetches ove the XLPE. To make easy this opeation, silicon oil has to be applied at the inne suface of the EPR tube. Finally, an oute semicon is taped on the EPR. In Figue 2.4, a coss section of a joint model is epesented. efoe any testing, all the MV-size models of cable joints wee themally teated at 80 C fo 20 days. Conditioning of used specimens fte a measuement, the specimen used fo the test was conditioned in an oven at 80 C fo at least 24 hous. This was done in ode to eset the space chage histoy of the specimen, allowing to use the same specimen fo diffeent measuements. 12

27 Chapte 2 Expeimental methods a b Figue 2.4. a) Coss-section of a MV-size model of a cable joint. b) Detail of the dielectic inteface Conduction cuent measuements Measuements of DC conduction cuent on flat specimens wee pefomed fo two main puposes. Fistly, fom the measued cuent values, the DC conductivity of the insulation can be infeed. This was done at diffeent electic fields and at diffeent tempeatue conditions. Secondly, esults of cuent measuements can povide infomation on the conduction mechanisms. If the set of data collected fom the measuements at diffeent conditions ae plotted in a voltage-cuent chaacteistic (o in a cuent density-field (J-E) chaacteistic), the electic field above which the conduction mechanism changes can be detected. This paticula value of the electic field is usually called electic theshold. (Seveal woks, e.g. [15, 48, 113, 133, 134], on HVDC polymeic insulation have shown that significant space chage build-up stats when the applied field is above the electic theshold. Recently, this paamete has also been associated with the statup of electical aging [46, 47]. Fo these easons, the electic theshold can be consideed a paamete of utmost inteest not only fo insulation design, but also fo mateial chaacteization and mateial compaison [3, 15, 113, 114]). Test set-up Conduction cuent measuements wee pefomed in a thee-teminal cell by means of a Keithley 617 electomete. In ode to potect the electomete fom ove cuents, the instument was connected to the measuing electode via a seies esisto. The DC voltage was supplied to the test specimen via a Rogowski-pofiled electode made of aluminum. In all tests, a 1-mm semicon laye was applied to the aluminum measuing and guading electodes. In this way, the insulation of the test specimen was always in contact with the semicon. pesonal compute equipped with a GPI inteface was used fo displaying and stoing the acquied data. In Figue 2.5, the test set-up is epesented, wheeas in Table 2.5 some specifications of the set-up ae given. 13

28 Chapte 2 Expeimental methods Figue 2.5. Expeimental set-up used fo measuements of conduction cuent. Table 2.5. Specifications of the set-up used in this wok fo conduction cuent measuements. Conduction cuent set-up Measuing electode: diamete = 28 mm HV electode diamete = 35 mm Guading electode: diamete = 40 mm-350 mm Sensitivity: /m 2 Max tempeatue: 80 C Max voltage: +/- 30 kv Seies esisto: 10 MΩ Test pocedue In Figue 2.6., the typical behavio of the detected cuent density j(t) is epesented. The cuent density is obtained by means of the following equation: j () t () t i = (2.1) m, el whee i(t) is the cuent measued by the electomete and m,el is the aea of the measuing electode. Defining the steady-state value of the cuent density J ss as: J = j t (2.2) ss () t the conductivity σ of the insulation can be infeed fom: 14

29 Chapte 2 Expeimental methods J ss σ = (2.3) E 0 whee E 0 is the extenal field which is applied acoss the insulation unde test. In pactice, in ode to each a quasi steady-state egime, the applied field has to be pesent acoss the insulation fo a sufficiently long polaization time. Fo the test condition adopted in this study, the polaization time vaies fom a few hous, in case of measuements pefomed at a elatively high applied field and tempeatue, to a few days, in case of measuements pefomed at elatively low applied field and tempeatue. In some of the tests, the measued cuent did not stabilize on a constant value even afte a few days. In these cases, a gaphical fitting was used to estimate the steady-state value of the measued cuent. Figue 2.6. Example of detected cuent density. EPR flat specimen at 20 kv/mm and 40 C. 15

30 Chapte 2 Expeimental methods 2.3. Space chage measuements In this section, the following topics ae discussed: some geneal consideations about space chage measuements and an oveview of the most used space chage measuing techniques; the method used in this investigation (pulsed electoacoustic method) and the test set-ups adopted fo measuing space chage; the pulsed electoacoustic method when the test object is a multi-dielectic; the pulsed electoacoustic method when the test object has cylindical geomety; the paametes used fo evaluating the esults of space measuements Geneal Space chage measuements can povide two main infomations: the actual electic field distibution and an indication of the aging state of the insulation. Electic field distibution The pesence of space chage in insulating mateials distots the initial Laplacian electic field distibution. This leads to a local field enhancement within the mateial that may cause insulation degadation and, ultimately, electical beakdown. If the space chage distibution ρ is known, then the electic field E can be calculated by combining equations (2.4) and (2.5) 2 ρ U = (2.4) 0 E = U (2.5) Whee U is the voltage distibution acoss the insulation, 0 the vacuum pemittivity and the elative pemittivity, which is assumed to have no jump vaiations within the dielectic. Figue 2.7 shows the electic field deduced fom the space chage distibution in an XLPE flat specimen. In the figue, it is vey evident that when space chage is pesent, the field is locally enhanced. Space-chage elated aging Up to now, the scientific community does not have a univocal opinion about space chage being a cause of aging of the insulation, a symptom of it o both of them. Howeve, what is unde discussion is whethe thee is a connection o not between tapping of space chage and electical aging has often been mentioned. ccodingly, fom the space chage distibution, an indication of the aging state of the mateial could be infeed. This has been the topic of many publications. In efeence [90], an up-to-date and compehensive discussion about the subject can be found. 16

31 Chapte 2 Expeimental methods a b Figue 2.7. Examples of space chage (figue a) and electic field (figue b) distibutions in a 1.5-mm thick XLPE insulation. pplied voltage= 17 kv. In the pesent thesis, the main goal of space chage measuements is the detemination of the electic field distibution. Fo this pupose, the polaity, the location and the magnitude of the accumulated space chage has to be known. This infomation was obtained by means of the space chage measuing technique descibed in the following section Pulsed electoacoustic method (PE) Pinciple In this wok, the PE method was used fo measuing the dynamic space chage distibution in both flat and cylindical solid insulations. In Figue 2.8, the PE pinciple is schematically epesented. Figue 2.8. Schematic epesentation of the specimen configuation and test set-up fo space chage measuements by means of the PE method. 17

32 Chapte 2 Expeimental methods In the PE method, a pulsed voltage u p (t) is applied acoss a test object (e.g. a flat o cylindical specimen). The pulsed voltage can be supeimposed on a DC voltage U 0. In this case, a decoupling capacito C and a esisto R have to be placed in seies to the pulse souce and DC souce espectively. Depending on the thickness of the specimen, typical values fo the amplitude of the pulse ae kv wheeas the pulse width vaies in the ange ns. Refeence [39] discusses in detail how to choose the pope pulse paametes given a specific size of the test object. If space chage is embedded within the test object, the application of the pulse induces a petubation foce on the space chage distibution. The petubation foce makes the chage to move slightly and, as a esult, stain (acoustic) waves ae initiated at the space chage location. Those waves ae detected by a piezoelectic senso afte having taveled though the test object and though the eath electode. Usually, polyvinylidenefluoide (PVDF) o lithium niobate (LiNbO 3 ) ae used as sensos. In ode to avoid eflections of acoustic waves, a pope acoustic temination is equied at the senso. Fo this pupose, a mateial with the same acoustic popeties of the senso is used in combination with a mateial which is able to absob the acoustic waves. The electic signal povided at the senso is amplified and fed into a scope, whee it is displayed and stoed. Coection of the detected signal Geneally, the electical signal detected at the scope does not diectly epesent the acoustic signal at the senso. This is mainly due to the fact that the senso-amplifie system acts as a high-pass filte [118]. In ode to coect the detected signal, deconvolution techniques [78, 100] have been adopted in this wok. Moeove, acoustic waves ae attenuated and dispesed, while taveling though lossy media. So, the acoustic signal detected at the senso does not diectly coespond to the space chage distibution within the test specimen. In this study, the oiginal acoustic signal is ecoveed fom the attenuated one by means of the pocedue descibed in ppendix C. The pocedue is mainly based on the ideas poposed by Li [97]. Voltage-off and voltage-on measuements Thee ae two main conditions at which measuements can be pefomed. - Voltage-off condition. Space chage is measued while the DC voltage is absent and while the test object is shot-cicuited. The space chage pesent in the bulk of the insulation is detected. In addition, chages, which ae induced by the space chage, ae also detected at both electodes. - Voltage-on condition. Space chage within the test specimen is measued while the DC voltage is applied. The space chage pesent in the bulk of the insulation is detected as well as the chages at the electodes. In this case, electode chages ae induced by both the space chage and by the applied voltage. Calibation of the measuing system must be pefomed in ode to convet the detected signal at the scope in [mv] into a space chage signal in [C/m 3 ]. This is done on the basis of a known chage distibution at the eath electode. In ppendix D, the calibation pocedue is discussed in detail fo diffeent types of test objects. 18

33 Chapte 2 Expeimental methods Test set-ups In table 2.6, the main chaacteistics of the thee diffeent PE set-ups used in this wok ae shown. In all the set-ups, a scope stoed and displayed the output signal (scope type WaveRunne 6050, 500 MHz, 5-Gs/s LeCoy). Table 2.6. Some specifications of the PE set-ups used in this wok. PE set-up fo thin flat specimens (d<1mm) mplifie: gain = 60 d; input impedance = 50 Ω bandwidth = MHz Senso: PVDF, 9 µm Pulse geneato: amplitude = 0-1 kv pulse width = 7 ns pulse ise time = 3 ns Max. tempeatue: 70 C PE set-up fo thick flat specimens (d>0.5mm) mplifie: gain = 50 d; input impedance = 1.5 kω bandwidth = MHz Senso: PVDF, 40 µm Pulse geneato: amplitude = 0-2 kv pulse width = 50 ns pulse ise time = 5 ns Max. tempeatue: 70 C PE set-up fo cable-geomety test objects mplifie: gain = 70 d; input impedance = 1.5 kω bandwidth = MHz Senso: PVDF, 25 µm Pulse geneato: amplitude = 0-4 kv pulse width = 80 ns pulse ise time = 10 ns Max. tempeatue: 70 C 19

34 Chapte 2 Expeimental methods PE method fo multi-dielectic test objects Recently, the PE method has been applied also to lamina test objects composed of two o moe layes of diffeent dielectics (multi-dielectics). Hitheto, seveal kinds of multi-dielectics have been tested by means of the PE method, e.g. [16, 17, 20, 21, 35, 79, 95, 101, 107, 139, 140, 145, 147]. Nevetheless, only little attention has been paid to the fact that, geneally, the PE method does not diectly povide the space chage distibution within the test specimen if the test specimen is composed of diffeent insulation layes [17, 22, 25, 108, 148, 149]. Thee ae two main easons why the output signal of a PE measuement does not diectly coespond to the space chage distibution within a multi-dielectic. 1. In the PE method, a pulsed voltage is applied acoss the insulation to be tested. The pulse induces a tansient mechanical stess within the multi-dielectic which initiates the acoustic signal to be detected. ssuming each laye of a multidielectic is electically homogeneous, the electically-induced tansient mechanical stess is detemined not only by the space chage distibution, but also by the diffeent pemittivities of each laye of the multi-dielectic. The main consequences of this fact ae: - the shape of the space chage distibution within the test specimen is diffeent fom the shape of the mechanical stess distibution induced by the pulsed voltage; - a diffeent calibation facto must be chosen fo each laye of the multidielectic. 2. The PE method is based on geneation and popagation of acoustic waves. In the case of diffeent acoustic popeties of the mateials in contact (acoustical mismatching), waves expeience diffeent geneation, tansmission and eflection coefficients. The main consequences of this fact ae: - the shape of the detected acoustic signal is diffeent fom the shape of the stess distibution within the multi-dielectic; - eflections can occu at the intefaces between two diffeent mateials, leading to possible misjudgment of measuement esults. Theefoe, in ode to coectly evaluate the space chage distibution in a multidielectic by means of PE measuements, the elation between the detected acoustic signal and both location and magnitude of space chage must be known. To contibute to this discussion, the autho studied the electo-acoustic phenomena occuing in a multi-dielectic tested by means of the PE method. This is detailed explained in ppendix, whee the pinciple of the PE technique is eviewed in case the test object is a multi-dielectic. Repesentation of space chage pofiles fo multi-dielectics test objects In this thesis, the pesented space chage pattens fo multi-dielectics do not epesent the actual space chage distibutions within the multi-dielectic. Instead, they epesent the electically-induced mechanical stess distibutions calibated in [C/m 3 ]. If a pope 20