Porcelain Insulators & Covered Conductors – A Problem of Dielectric Compatibility

EDUARDO HILSDORF

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Eduardo Riani Hilsdorf Isoladores Santana – Brazil – PPC Insulators  Manuel Luis Barreira Martinez  

High Voltage Lab - Federal University of Itajubá – M.G. – Brazil

Porcelain  Insulators  and  Covered  Conductors    A  Problem  of  Dielectric  Compa8bility  

Objectives

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•  Check the possibility of application of porcelain line posts insulators solid core in compact lines

(Spacer Cables);

•  Comparatively evaluate the performance of polymeric pin type insulators and porcelain line posts

solid core.

•  Evidence that the geometry of the components influences the distribution of the electrical field;

•  Existence of electric utilities in Brazil using in Spacer Cables systems porcelain, line post

insulators;

•  Evidence of problems with polyethylene pin type insulators applied under salt polluted conditions

(coastal areas).

Motivation

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Background & Initial Discussions Spacer Cable Systems

•  First systems operated at 5 kV, with acrylic spacers and porcelain pin type insulators

•  For 15kV voltage this conception was maintained.

•  Field experience shows the components which had worked at 5 kV don’t work well in 15 kV.

•  The increase of the dielectric stresses applied to conductors, resulted in tracking of the conductors

insulation and subsequent erosion of spacers, reducing the system reliability;

•  The majority of failures involved insulators;

•  The trend to failure was accelerated by the replacing of common pin type from radio free pin type

insulators;

•  The development of new insulator conception was necessary to increase reliability;

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Background & Initial Discussions Spacer Cable Systems

•  The use of polyethylene for cables and spacers produced positive results that it rendered obvious

the use of the same material for insulators;

•  Consequently, mainly as for the application of porcelain pin type insulators, radio free, a habit was

consolidated not to use porcelain insulators in compact networks;

•  This practice spread out and utilities adopted, as a standard, polymeric pin insulator in compact

networks.

Compact Lines – Field Operation !  Contamination !  Electrical Field !  Internal defects (bubbles) !  Partial internal discharge !  Dielectric compatibility !  Mechanical, thermal and environmental stresses !  Sun exposition (ultraviolet rays) !  Tracking and erosion

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Figure 1 – Photo of sections in polymeric spacers exhibiting non-uniform material.  

Figure 2 – Electronic radiography of insulator containing internal air bubbles coming from the manufacturing process

Figure 3 - Photos of insulators including internal defects, which degradated in operation a time after their installation in network.

Application of porcelains to electrical insulator

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Figure 4 - Radio - Free Treated

Figure 5 – Illustration of radio-free treatment effects on electrical field lines in ANSI 55-6 Class insulator.

!  Conventional Lines !  Radio free treatment !  Semiconducting glaze on the top and thread !  Reduces the effects of radio-interference !  Reduces punctures problems !  Covered Cables incompatibility

EXPERIMENTAL METHODOLOGY Evaluations by Finite Element- Computational Simulation

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•  Using Comsol Multiphysics® applied to 15 and 25 kV covered conductors, with pin type polymeric

insulators, solid core line post porcelain insulators and solid core line post porcelain insulators

with semi-conductor glaze (radio-free) were simulated;

•  Voltage of 2U0, that is, 16 and 29 kV for systems with 15 kV and 25 kV class voltage respectively.

Figure 6: a)  Ceramic line post insulator P1 , b)  Polymeric pin type insulator.  

Evaluations by Finite Element – 15kV Computational Simulation

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INSULATOR  REFERENCE   BASIC  DIMENSIONS  (  MM)  

P1  H   210  D   125  /  45  

P  STUDY  H   208  D   125  /  45  

P2  H   208  D   125  /  55  

P3  H   205  D   125  /  63  

POLYMERIC  PIN  TYPE  15KV    H   133  D   139  

P1  -­‐  RT  H   200  D   125  /  60  

 Where: H – height of insulator since support up to cable position; D – diameter of shed / body diameter.

E1 – Electrical field gradient in the interface of conductor surface and insulating layer E2 – Electrical field gradient in the interface of conductor and insulator support

Evaluations by Finite Element – 25kV Computational Simulation

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Where: H – height of insulator since support up to cable position; D – diameter of shed / body diameter.

E1 – Electrical field gradient in the interface of conductor surface and insulating layer E2 – Electrical field gradient in the interface of conductor and insulator support

INSULATOR  REFERENCE   BASIC  DIMENSIONS  (MM)  

P4  H   305  D   150  /  80  

P1  H   210  D   125  /  45  

POLYMERIC  PIN  TYPE  -­‐  25  KV  H   152  D   190  

 

Evaluations by Finite Element – RF treatment

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Figure 7 - Study of electrical field around post type ceramic insulator P1 - RT radio-free.

Figure 8 – Study of electrical field on P1 ceramic insulator line post type without radio-treatment

Analysis by Corona Cameras (UV Camera) !  Applied voltage was 38.5 kV, that is about 5U0 for a 13,8kV system class voltage.

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Figure 10 – Image of corona effect on insulators designed for 15kV voltage class a) Ceramic insulator solid core line post type, b) Polymeric insulator pin type, c) Ceramic insulator pin type.

Test of Dielectric Compatibility with Multi-stresses.

Parameters for test are: !  Temperature on external surface of the

cable: 60 ºC; !  Rain cycles of 5 minutes aspersion followed

by 15 minutes without rain; !  Aspersion: 1 mm/minute; !  Water conductivity: 750 µS/cm; !  Applied voltage: 2U0; !  After thirty continuous testing days, the

sets, cable, insulators and spacers shall not exhibit any kind of damage, tracking, puncture or erosion.

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Figure 11 – Basic installation diagram for the test of multi-stress electrical compatibility in conformity to LAT-EFEI.

Test of Dielectrical Compatibility with Multi-stresses.

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 Figure 12 – Images of the starting of the dielectric compatibility test on 3 line post ceramic insulators, 15 kV voltage class, P2 reference, carried out on September 16th 2011.

Two arrangements tested with two different cables.

Gray color at left was manufactured with two layers, one internal semi-conductive next to conductor, and the second, external, in standard XLPE.

Black color at right was manufactured with one single layer in XLPE.

Test of Dielectric Compatibility with Multi-stresses.

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Figure 13 – Image of the dielectric compatibility test on line post type ceramic insulator, 15 kV voltage class, P2 reference, phase "b", on September 16th 2011 + 5 days.

Figure 15 – Image of the dielectric compatibility test on line post type ceramic insulator, 15 kV voltage class, P2 reference, phase "b" on September 16th 2011 + 10 days.

Figure 16 – Image of dielectric compatibility test on line post type ceramic insulator, 15kV voltage class, reference P2, phase "b", on September 16th 2011 + 25 days.

THE INFLUENCE OF CABLE DESIGN AND MANUFACTURING

Since defects have arisen along the cable with semi-conductive layer developing towards insulators and along the entire cable without semi-conductive layer in a random way, the need to investigate this phenomenon was taken into account.

Other types of covered conductors of North American origin exist. In general they are manufactured with three layers, where: •  A semi-conducting layer, independent from the cable class of voltage; •  A layer of low density polyethylene with predominant insulating function; •  An external layer of high density polyethylene HDPE with electro-mechanical function.

In this case, a new test was carried out on 3 sets of 15kV voltage class insulators P2 reference A branch of covered conductor of North-American origin was used, duly followed up

through daily visual inspection During the test and at the test end, no damage, tracking or erosion were observed on

interface region between cable and insulators, as can be verified in the images on Figures 19 and 20. Therefore, the test set up withstood the combined stresses, as claimed by the standards.

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THE INFLUENCE OF CABLE DESIGN

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Figure 19 – Image of start of dielectrical compatibility test with 3 insulators ceramic post type, 15kV class voltage, P2 reference, carried out in October 2012.

Figure 20 - Image of the end of dielectrical compatibility test on 3 ceramic insulators post type, 15kV class voltage, P2 reference carried out in October 2012.

CONCLUSIONS 1/2 !  It is possible to obtain suitable values of electric field gradients for porcelain insulators, solid core

post type, calculating them through finite element method, !  With a corona camera, was not possible observe any significant difference in corona level between

ceramic insulators post type, and polymeric pin type insulators. On the contrary, for ceramic pin type, the electric activity level was extremely high, confirming that its usage is not recommendable with covered cables;

!  Through dielectric compatibility test, it was observed that it is possible to use solid core post type insulators in configurations applying covered conductors. It is, however, recommended to apply dielectric compatibility tests for the validation of each specific project, since the studies of electric field by finite elements indicate that the insulator design has a relevant influence on the result of electrical field distribution on the system parts involved;

!  The behavior of the interface between cables and insulators do not present noteworthy problems, as images emphasizing electric activities in UV field indicate. The same, however, cannot be said as regards to the cables, which are not suitable, even if the insulators – spacers and remaining accessories are developed for this specific application, as indicated by results of tests and suitable studies. Therefore, in this paper it is not considered that the insulators were the inductors of the problems observed on the cables.

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!  It is important to register that, due to cable insulating failures, in regions next to insulators, polymeric pin insulators can exhibit carbonization, transition of phase solid - liquid, or even fire development involving insulator and cable, taking to system interruptions and potential conditions of accidents involving third party: This problem does not occurs in porcelain insulators because, even when the cable loses the insulation, the porcelain insulator is able to maintain system operating, since, on account of the level of dissipated – consumed power during failure process, they do not develop phase transitions and do not come up to flammability condition.

!  Part of the problem verified in this paper, that is, the failure aspect and development, is related to the fact that national normalization for covered conductors does not consider the application of the dielectric compatibility test. Therefore a lag exist between the manufacturing of covered conductors and that of their accessories, as demonstrated in this paper, for both porcelain and polymeric materials, as reported. This procedure creates distinct requirements for parts which must operate together, what is a problem for the purchaser, mainly with the ageing of compact networks, because it will be observed only when it will turn into an insoluble problem.

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CONCLUSIONS 2/2

Thank you

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eduardo.hilsdorf@seves.com.br