Evolution of stress control systems in medium voltage cable accessories

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1 Evolution of stress control systems in medium voltage cable accessories Dr. Robert Strobl, Tyco Electronics Raychem GmbH / Energy Division Ottobrunn, Germany Wolfgang Haverkamp, IEEE/PES Member Tyco Electronics Raychem GmbH / Energy Division Ottobrunn, Germany Dr. Gerold Malin, Tyco Electronics Raychem GmbH / Energy Division Ottobrunn, Germany Frank Fitzgerald, PE IEEE/PES Member Tyco 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 stress control 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 of the 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. Outer conductive layer Insulation equipotential lines (% phase/ground voltage) Conductor Figure 1: Uncontrolled cable end potential distribution 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 of all shielded power cables which have been developed to Electrical discharges on the edge of the outer conductive layer Figure 2: Corona at the outer conductive layer operate at 5kV and higher to eliminate discharge activities during operation in order to provide more than 30 years life time. 1

2 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 cable shield end (see figure 3). Outer conductive layer Conducting cone Insulation material Cable Insulation Figure 3: Geometrical stress control cone 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 of R vol ~ 10 2 Ωcm. 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. 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 far less 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. Spec. Volume Resistance [ Ω cm] Carbon Black Filler [%] PolyProp Figure 4: Percolation plot of various polymers Mainly the compounding and manufacturing processes defined the characteristics of the final product. Producing a means 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 Model HDPE LDPE III. IMPENDANCE SYSTEM C C C C PE C C C C 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 Equivalent Circuit R1 C1 R2 C2 R3 Voltage Figure 5: Pearl chain model and equivalent electrical circuit 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. 2

3 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 is caused by potential barriers, which are lowered under electrical stress. Besides carbon black fillers other pigments like SiC and are used for the same stress control technology, which is described later in this paper as part of the new ceramic technology for terminations (Metal Oxide- 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 byproducts during the chemical cross-linking process, which might effect the desired behaviour and long term ageing performance 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 required limits. 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 Figure 6: Transmission line circuit C c = Cable Cap. R i = Insulation Res. C s = Stress-control Cap. R s = Stress-control Res. electrical stress at the cable shield cutback and along the insulation interface. The specific impedance within the range of Z spec ~ Ωcm [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- DC Current 10x10-1 ma 5x10-2 ma Non Linear Stress Control System Linear Stress Control System 1x10-2 ma E[kV/cm] Figure 7: Comparison of carbon black systems 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-Field vector 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]. 3

properties. Figures 8 and 9 show the particles of the ceramic powder under the electron microscope. The calcinated ceramic powder (see figure 10) is embedded in a polymer matrix.

4 IV. METAL-OXIDE-MATRIX SYSTEM A. Ceramic technology The new developed stress control system is based on a special ceramic powder and operates differently from the carbon-black loaded stress control system mentioned earlier in the paper. Figure 10: Ceramic powder and compound pellets Figure 8: Structure of the calcinated powder 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. 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 Ωcm 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). Figure 9: Particle close-up 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 boundaries control 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 chemistry and the calcination process, which is very different from the carbon-black technology [2, 3]. Figure 11: Characteristic of the ceramic powder 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. 4

5 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 limited according to the switching point design, which avoids overstresses of the critical areas. Figure 12: I/U-characteristic of a varistor The threshold voltage can be adapted as needed to design requirements for stress control management systems of cable accessories or other electrical components/equipments. Electrical stress in kv/mm kV AC 65kV AC 150kV BIL C. -Model and equivalent electrical circuit A special modified polyethylene is used as a carrier for the particles. The boundary layers of the individual 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]. - Model Modified PE Equivalent Circuit R 1 R 2 R 3 Distance in mm Figure 14: Electrical performance of -Micro-varistors 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 R VAR1 C 1 R VAR2 C 2 R VARn C n Voltage Figure 13: -Model and equivalent electrical circuit D. Electrical performance of 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. V. CONCLUSION The Metal-Oxide-Matrix stress control system is unique and was never been attempted before. This system provides excellent 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. 5

6 VI. REFERENCES [1] Haverkamp W., Lyons P.: World-wide long-term Experiences with heatshrinkable splice Concept, T&D Los Angeles, IEEE 1996 [2] Strobl R., Haverkamp W., Malin, G.: I(O)XSU-F Neue Generation waermeschrumpfender Mittelspannungsendverschluesse basierend auf -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 [4] Blake A. E., Clarke G., Starr W. T: Improvements in Stress Control Materials, 7th IEEE/PAS Conference and Exposition 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 Strobl graduated 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. Haverkamp graduated from the University of Essen, Germany with a Master of Science Degree in Electrical- and Power Engineering in His employment 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. Graz, Austria in 1979 and got the PHD degree in Electrical Engineering from the Technical University of Graz, Austria in His employment experiences include Assistant Professor and Lecturer at the Institute of High Voltage Engineering, 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 the Satsop 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 Raychem s 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 Ottobrunn/Munich Germany/Europe Tyco Electronics Corporation Energy Division 8000 Purfoy Road Fuquay-Varina NC , USA. Gerold Malin graduated with a Master of Science Degree in Electrical Engineering from the Technical University of 6

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