stresscontrol_hvtz

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Evolution of stress control systems in medium voltage

cable accessories

Dr. Robert Strobl,Tyco Electronics RaychemGmbH / Energy Division

Ottobrunn, Germany

Wolfgang Haverkamp,IEEE/PES MemberTyco Electronics Raychem

GmbH / Energy Division

Ottobrunn, Germany

Dr. Gerold Malin,Tyco Electronics RaychemGmbH / Energy Division

Ottobrunn, Germany

Frank Fitzgerald,PE IEEE/PES MemberTyco Electronics Corporation

Energy Division

Fuquay-Varina, NC, USA

ABSTRACT

Underground cable accessories used in medium voltage

cable systems need a highly reliable stress control

system in order to maintain and control the insulation

level which is designed for estimated life times longer

than 30 years of service. The term electrical stresscontrol refers to the cable termination function of

reducing the electrical stress in the area of insulation

shield cutback to levels that preclude electrical

breakdown in the cable insulation. This paper will

describe the evolution of stress control systems and their

benefits, based on different materials and concepts. The

main focus on this paper will be on the unique Metal-

Oxide-Matrix stress control system, which has never

been attempted before.

Keywords:Stress control technology, Cable accessories

I. INTRODUCTION

In coaxial MV-cable configurations the outer conductive

insulation shield is connected to ground, which contains the

entire radial E-field in the dielectric and determines the

balance between electrical operational and design stress.

This balance is distorted when the outer conductive cable

insulation shield is removed during splicing or terminating

and the shield cutback is left untreated.

Underground accessories used in medium voltage systems

need to provide stress control in order to maintain and

control the electrical stress below the breakdown level ofthe dielectric [1]. The stress control system, like the cable,

should be designed to exceed 30 years operation life.

Stress control is provided in medium voltage cable

terminations for one primary purpose to control the

exceptionally high stresses, which exists at the area where

the shield is terminated. If no stress control were applied,

discharges could occur and the life of the termination would

be limited depending on the stress at the end of the shield

and the discharge resistance of the primary dielectric [4].

Figure 1 shows the stress concentration at the end of the

screen of medium voltage cables when no stress control

system is used.

The field along the dielectric/air interface provides the

highest electrical stress at the edge of the outer conductive

layer. Figure 2 shows electrical discharges (corona) at this

critical area.

This interface has low breakdown strength and the

termination will fail at the shield cut if the field is not

controlled. A stress control is required at the termination ofall shielded power cables which have been developed to

operate at 5kV and higher to eliminate discharge activities

during operation in order to provide more than 30 years life

time.

equipotentia

llines

(%phase/groun

dvoltage

)

ConductorInsulationOuter conductive

layer

equipotentia

llines

(%phase/groun

dvoltage

)


layer

equipotentia

llines

(%phase/groun

dvoltage

)


layer

Figure 1:Uncontrolled cable end potential

distribution

Electrical discharges on theedge of the outer conductive layer

Figure 2:Corona at the outer

conductive layer

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II. GEOMETRIC SYSTEM

The traditional method of reducing the electrical stress and

ensuring long cable services is to install a cone of insulating

material, with an outer conductive electrode, over the cableshield end (see figure 3).

The layer of insulating material between the electrode and

the cable insulation can be seen as an additional

capacitance, resulting in a redistribution of the electrical

potential. Different mathematical algorithms are used to

design the shape of the cone to provide the appropriate

electrical stress distribution. The method is defined as

geometric or capacitive stress control system. This system is

well explained in the literature and widely used. Devices

that utilize this method of stress control are terminations

and splices, where the conical electrode is moulded or taped

from a conductive elastomer with a volume resistivity ofRvol~ 10

2cm. Paper cable accessories consists of a cone

made from metal (Pb or Al), which is then soldered to the

metal cable shield or again taped with paper tapes and

metallic foils.

III. IMPENDANCE SYSTEM

A. Effect of Carbon Black Filler in Polymer Systems

The study of polymer material science has produced a depth

of knowledge that has allowed chemists to modify and tailor

the physical and electrical properties of polymeric materials

for specific applications and requirements. Carbon black

filler has become important compound used to provide

unique electrical properties. With the variation of carbon

black filler content in a high performance dielectric

polymer the volume impedance can be modified to control

the electrical stress in MV cable accessories. However the

volume resistance - component of the entire impedance -

does not vary linearly in relation to the filler content.

This phenomenon is related to the statistical distribution of

the conductive filler in the polymer. A more precise

evaluation of the relation between filler and polymer

confirms that beyond a certain filler concentration sufficient

continuous conductive paths might be available to carry the

electrical current through the polymer system. However the

real measured amount of dispersed conductive particles for

a specific conductivity through the polymer matrix is farless than expected. This effect can be explained in that

particular conductive carbon blacks tend to build so-called

pearl chains, which penetrate the insulated polymer

matrix and form a conductive lattice, which means less

filler will gain the same conductivity as the pure conductive

pigments measured in a test tube. The physical shape of the

carbon black pigments and the polymer material formation

influences the randomly disorganized conductivity matrix

and create different networking pearl chains and therefore

vary the percolation curves. Figure 4 shows the volume

resistivity versus the filler content of different polymers.

Mainly the compounding and manufacturing processes

defined the characteristics of the final product. Producing ameans of stress control for MV and HV applications

requires careful selection of polymer type and carbon black.

This selection of materials and the subsequent processing

method are fundamental in achieving the desired electrical

properties. These properties exist at the steep slope of the

percolation plot. Figure 5 shows the pearl chain model and

the equivalent electrical circuit. Here the pearl chains are

fragmented and unconnected, which leads to the electrical

equivalent of a resistor and capacitor combination. The

equivalent electrical circuit can be designed as a complex

network of resistors and capacitances.

Outer conductive

layer

Conducting cone

Insulation

material

Cable

Insulation

Figure 3:Geometrical stress control cone

0 5 10 15 20 25 30 35

Carbon Black Filler [%]

Spec.

VolumeResistance[c

m]

PolyProp

HDPE

LDPE

1018

102

1010

1014

106

Figure 4: Percolation plot of various polymers

Voltage

R1 C1 R2 C2 R3

C C PEC C C C C C

Model

Equivalent Circuit

Figure 5:Pearl chain model and equivalent

electrical circuit

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The specific volume resistance will exhibit a non-linear

dependency when applying a variable DC E-field across the

polymer matrix. This effect contributes nicely to the stress

control needs for MV terminations and splices and iscaused by potential barriers, which are lowered under

electrical stress. Besides carbon black fillers other pigments

like SiC and ZnO are used for the same stress control

technology, which is described later in this paper as part of

the new ceramic technology for terminations (MetalOxide-

Matrix).

B. Stabilization Effect of cross linking by radiation

The previously described effects are observed for several

thermo-plastic or thermal-elastic compounds. Today,

several technologies are used to cross link polymers and

elastomers. The two major processes are

Chemical Cross- Linking

Radiation Cross- Linking

Chemical cross-linking is the major process used in the

cable industry. The radiation process is more attractive for

advanced material technologies and complex compound

polymers like stress grading as described previously.

For reproducible applications cross-linking by radiation is

preferred. The radiation process leaves the polymer

formulation unaffected and does not initiate chemical by-

products during the chemical cross-linking process, which

might effect the desired behaviour and long term ageingperformance of the material.

The semi-crystal polymer radiated by high-energy beam

dose (several MeV) changes its amorphous part into a three

dimensional crystalline lattice. As a consequence there is a

fundamental change in the physical characteristics of the

doped polymer.

The polymer exhibits elastomeric behaviour beyond the

crystalline melt point and can then be transformed into

different shapes and dimensions and frozen when the

material is again cooled down. Using stress-grading doped

formulations the designed impedance remains stable

through the polymer phase transition and maintains the

electrical stress grading properties within the requiredlimits.

The morphology is temperature stabilized within wide

application ranges of electrical conductive polymers. This

provides improved performance during ageing under

temperature and electrical field operating conditions. The

radiation substantially reduces the amorphous content of

semi-crystalline polymer. The polymer exhibits increased

resistance to chemicals, less MVT (moisture vapour

transmission), improved shape stability (less swelling under

solvent attack), and improved gas sealing characteristics.

C. Stress distribution on Termination and Splices

The impedance polymer stress control layer utilizes the

available cable capacitance to effectively reduce the

electrical stress at the cable shield cutback and along theinsulation interface.

The specific impedance within the range of Zspec ~ 108 -

1010cm [4] provides the required stress control function

depending on cable cross section and voltage class. The

non linear electrical field behaviour dependency of this

stress control material prevents an increase in electrical

stress in cable accessories under transient over voltages and

test conditions. Figure 7 shows the DC current versus the E-

field.

Calculations of the electrical stress distribution along a

termination interface demonstrate that the electrical stress

grows less as the voltage increases. The calculated results

were confirmed by experimental measurement (E-Fieldvector measurements).

Three times higher operation voltage responses only to ~

2.5 stress increase, whereas the geometric stress control

methodology results in equivalent stress increasing in

proportion to the voltage increase.

Furthermore, a combination of various polymer and

elastomer compounds using different types of filler grades

allow cable accessory applications up to 90kV operation

voltages. From a design perspective, the stress control by

material technology allows the designer to create products

for circular cable as well as sector shaped cable [4, 5].

Figure 6:Transmission line circuitCc= Cable Cap. Cs= Stress-control Cap.Ri= Insulation Res. Rs= Stress-control Res.

0 2 4 6 8 10 12 14

E[kV/cm]

DCCurren

Linear Stress Control System

Non Linear Stress Control System

1x10-2

mA

5x10-2

mA

10x10-1

mA

Figure 7:Comparison of carbon black systems

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Figure 10:Ceramic powder and compound pellets

IV. METAL-OXIDE-MATRIX SYSTEM

A. Ceramic technology

The new developed stress control system is based on aspecial ceramic powder and operates differently from the

carbon-black loaded stress control system mentioned earlier

in the paper.

The stress control compound, formulated from polymer and

ceramic powder, provides unique electrical properties.

Figures 8 and 9 show the particles of the ceramic powder

under the electron microscope.

A specifically developed calcination process creates

spherical varistors from each single particle. The centre of

the varistor is electrically conductive, but the marginal

boundary layers where the individual particles build up the

interface are highly insulating. These very thin boundariescontrol the current channel in the ceramic. Each layer

between two particles, which is called boundary grain,

represents a micro-varistor with a defined threshold

voltage. These boundary grains become conductive when

the applied voltage extend beyond across the threshold

voltage. The multiple micro-varistors build a 3-dimensional

electrical network where the electrical properties of the

ceramic powder are mainly influenced by the ZnO-

chemistry and the calcination process, which is very

different from the carbon-black technology [2, 3].

The calcinated ceramic powder (see figure 10) is embedded

in a polymer matrix. This special compound can be

extruded or moulded. The current manufacturing process

provides no limit to the implemented applications.

B. Characteristic of the ceramic technology

Figure 11 shows the characteristic of the ceramic powder

and the relation between the specific impedance in cm

and the electrical field in kV/cm. The material provides an

extreme non-linear characteristic and a threshold voltage

(switching point) is achieved. This characteristic is similar

to that provided by diodes or varistors (usable for both

polarities) and is well known from the semi-conductor

technology (see figure 12).

If the electrical stress (applied voltage) is lower than the

threshold voltage, the material operates as a quasi insulator

in the linear area of the I/U-characteristic. When the

electrical stress increases and reaches the threshold voltage

the ceramic particles (micro-varistors) switches through and

releases free electrodes. The higher electrical stress will be

limited and kept fairly constant along the stress control

system. This technology compensates material overstresses

caused by electrical transients and impulse voltages, which

is very useful for managing service requirements in an

electrical distribution network.

Figure 8:Structure of the calcinated powder

Figure 9:Particle close-up

Figure 11:Characteristic of the ceramic powder

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Figure 12:I/U-characteristic of a varistor

ZnO - Model

Equivalent Circuit

ZnO

Modified PE

ZnO

ZnO

ZnO

ZnO

ZnO

ZnO

ZnOZnO

ZnO

ZnO

ZnOZnO

ZnO

ZnO

ZnO ZnO

ZnO

ZnO

RVAR1 C1 RVAR2 C2 RVARn Cn

Voltage

R1 R3R2

Figure 13:ZnO-Model and equivalentelectrical circuit

The threshold voltage can be adapted as needed to design

requirements for stress control management systems of

cable accessories or other electrical

components/equipments.

C. ZnO-Model and equivalent electrical circuit

A special modified polyethylene is used as a carrier for the

ZnO particles. The boundary layers of the individual ZnO

particles are highly insulated and these very thin boundaries

control the current channel in the ceramic. The equivalent

electrical circuit can be designed as a complex network of

varistors, resistors and capacitances (see figure 13) [2].

D. Electrical performance of ZnO

The typical electrical performance is shown in figure 14 as

an example for a medium voltage termination. The critical

point of a cable is the edge of the outer conductive layer.

The break of the cable shield causes very high electrical

stresses (concentration of the electrical field) and therefore

a stress control system must be used in order to get a

smooth electrical field distribution.

If the electrical stress increases and reaches the switching

point, the individual ceramic particles (micro-varistors)

become conductive according to the current-voltage

characteristic. The electrical stress is always limitedaccording to the switching point design, which avoids

overstresses of the critical areas.

This advanced system with its stress limiting performs very

well at high AC and BIL levels in electrical networks

(transient voltages, overvoltages based on lightning and

switching operations in the electrical distribution network).

The electrical stress is always limited according to the

switching point design. For higher voltage levels a longer

distance for stress controlling is activated and necessary.

The non-linear stress control characteristic provides

excellent electrical performance especially BIL (basic

impulse insulation level). Figure 14 shows the electrical

performance at a 25kV and a 65kV AC withstand voltage

and a 150kV lightning impulse voltage. All electrical data

are based on the 20kV voltage level for medium voltage

polymeric cables.

V. CONCLUSION

The Metal-Oxide-Matrix stress control system is unique

and was never been attempted before. This system providesexcellent electrical stress distribution along the termination

and prevents overstresses of the material specifically along

with high electrical impulses. The system handles

specifically well external overvoltages and transient

voltages in electrical networks. The stress control polymer

matrix loaded with the doped ceramic powder can be

extruded as well as molded. Various applications can be

designed based on this unique technology.

Ele

ctric

al

str

ess

i nkV/m

m

Distance in mm

2

1

100200

25kVAC 65kVAC 150kVBIL

Figure 14:Electrical performance of

ZnO-Micro-varistors

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VI. REFERENCES

[1] Haverkamp W., Lyons P.: World-wide long-term

Experiences with heatshrinkable splice Concept, T&D LosAngeles, IEEE 1996

[2] Strobl R., Haverkamp W., Malin, G.: I(O)XSU-F

Neue Generation waermeschrumpfender Mittelspannungs-

endverschluesse basierend auf ZnO-Technologie,

Elektrizitaetswirtschaft Heft 26/2000 Seite 68 - 73,

Germany

[3] Strobl R., Haverkamp W., Malin, G.: Termination

System for Polymeric Distribution Cables Based on

Ceramic Stress-Grading Technology, erergize, Power

Journal of the South African Institute of Electrical

Engineers, January/February 2000, Page 66 69

[4] Blake A. E., Clarke G., Starr W. T: Improvements in

Stress Control Materials, 7th IEEE/PAS Conference andExposition on Transmission and Distribution, April 1-6,

1979, Atlanta, Georgia

[5] Haverkamp W., Le Baut P.: Heat-shrink Cable

Accessories for plastic cable up to 36kV, March 84

Jicable, France

VII. BIOGRAPHY

Robert Stroblgraduated with a Master of Science Degree

in Electrical Engineering in 1994, and in 1997 he got the

PHD Degree in Electrical Engineering from the Technical

University Graz, Austria. In 1997 he joined Raychem

GmbH, Electrical Products Division in Ottobrunn,

Germany. Previously he worked as a research assistant at

the Institute of High Voltage Engineering, Technical

University of Graz, Austria. His current responsibilities are

development, design and management of cable accessories

projects. His current position is Product Manager for LV

and MV termination cable accessories at Tyco Electronics

Raychem GmbH in Ottobrunn, Germany.

Wolfgang B. Haverkampgraduated from the University of

Essen, Germany with a Master of Science Degree in

Electrical- and Power Engineering in 1966. Hisemployment experiences included the Siemens A.G., Kaiser

Aluminium and Chemical Corporation. In 1980 he joined

Raychem GmbH, Electrical Products Division in Ottobrunn,

Germany. His areas of responsibility have included

managing projects on cable accessory development, their

applications and product management. He is currently

Product Manager for HV Cable Accessories from Tyco

Electronics Raychem GmbH in Ottobrunn, Germany. He is

a Working Group Member of IEEE/ICC.

Gerold Malingraduated with a Master of Science Degree

in Electrical Engineering from the Technical University of

Graz, Austria in 1979 and got the PHD degree in Electrical

Engineering from the Technical University of Graz, Austria

in 1992. His employment experiences include Assistant

Professor and Lecturer at the Institute of High VoltageEngineering, Technical University of Graz as well as

several technical and managing positions at Kabel u.

Drahtwerke AG Vienna. He is a member of national and

international technical committees. In 1991 he joined

Raychem GmbH, Vienna, Austria. His current position is

Business Unit Manager for Cable Network Products at Tyco

Electronics Raychem GmbH in Ottobrunn, Germany

Frank Fitzgerald graduated from the State University of

New York at Plattsburgh in 1974 with a Bachelor of

Science Degree in Physical Chemistry. He attended

Graduate School at Oregon State University for two years

and left to begin working as an electrical engineer at theSatsop Nuclear Power Station. He joined Raychem in 1983

and has several positions including Application

Engineering Management, Area Sales Manager, Technical

Manager for Americas and Product Management. He is

currently responsible for the management of Tyco

Electronics Raychems Nuclear Products world-wide and

for North America cable accessories from Tyco Electronics

Corporation facility in Raleigh, NC.

Tyco Electronics Raychem GmbH

Energy Division

Haidgraben 6

85521 Ottobrunn/Munich

Germany/Europe

Tyco Electronics Corporation

Energy Division

8000 Purfoy Road

Fuquay-Varina

NC 27526-9349, USA.

0-7803-7287-5/01/$17.00 (C) 2001 IEEE

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