Development Process of extruded HVDC cable systems Dominik HÄRING, Gero SCHRÖDER, Andreas WEINLEIN, Axel BOSSMANN Südkabel GmbH, (Germany) dominik.haering@suedkabel.com, gero.schroeder@suedkabel.com, andreas.weinlein@suedkabel.com, axel.bossmann@suedkabel.com ABSTRACT Extruded HVAC cable systems up to 500 kv have been developed successfully in the past decades and several years of operating experience are available. Because of increasing demand in power and the ability to transmit electrical energy over long distances, HVDC cable systems become more important. However, due to DC specific influences on the insulating systems of the cable and accessories, a detailed consideration and evaluation of these effects must be taken into account during the development process of extruded HVDC cables and their accessories. This paper addresses the main influences of DC stress on the components of HVDC cable systems. Fundamental aspects regarding the interface between cable and accessory will be discussed. The paper describes the empiric consideration of an extruded XLPE HVDC cable test system beginning with a model system to an HVDC cable test system with a voltage level of 150 kv from the perspective of a cable system manufacturer. KEYWORDS HVDC, cable system, XLPE, joint, testing INTRODUCTION The increasing power demand worldwide requires the transmission of electrical energy over long distances. Extruded HVAC cable systems have been developed successfully in the past decades and therefore many years of operating experience are available as mentioned in references [1] and [2]. However, based on the capacitive load and the eddy current losses, especially in large conductor sizes, the operation range to longer system lengths of HVAC cable systems is limited. For this reason HVDC cable systems become more important. While oil filled HVDC cables are a well-known technology for more than 50 years, extruded HVDC cable systems have been developed and commercially introduced in the last decade [3, 5]. Actually the work on a new standard for extruded HVDC cable systems is in progress [9]. Based on the low level of experience in this area and in order to deliver a reliable HVDC cable system, detailed development actions are necessary to understand the DC specific effects on extruded cables and their accessories. The main DC specific effects to extruded HVDC cable systems can be addressed as follows: Space charge accumulation in the insulation and interfaces Resistive field distribution Thermal dependence of insulation materials According to this, it can be concluded, that the following aspects must be taken into account during the development process of an HVDC cable system: Space charge behaviour in the insulation materials and interfaces Conductivities of the insulation materials and their temperature and field strength dependence DC and impulse breakdown strength of the cable and accessories With these aspects in mind, basic considerations on an extruded HVDC cable test system, including an extruded cable and a prefabricated joint, has been done and tested for VSC (voltage source converter) applications. This paper provides an overview of the systematic approach of such a HVDC cable test system, while the main aspects will be discussed from the perspective of a cable system manufacturer. GENERAL DEVELOPMENT ASPECTS OF HVDC CABLE SYSTEMS Since the successful development of extruded HVAC cable systems up to 500 kv, a high level of experience regarding operation behaviour of the components is available. In the case of extruded HVDC cable systems, essential differences in the operation behaviour compared to HVAC systems must be considered. The following section addresses those main effects on an HVDC cable system under the applied DC conditions. Electric field distribution under DC conditions The electric field distribution in a conventional AC cable can be easily expressed in the following equation [1]: U E r) = r o r ln ri ( [1] E(r) stands for the electric field strength, U is the voltage, r o the radius of outer insulation screen, and r i the radius of the inner insulation screen. This behaviour is totally different and more complicated under DC conditions. As opposed to the AC field distribution, the field distribution in a DC cable is controlled by the resistivity of the insulation material. Furthermore, the resistivity of an insulation material is strongly dependent on the temperature and the applied electric field. The link between resistivity σ 0 and temperature respective to the electric field has been published in various mathematical expressions. One of the main mathematical expressions from [4] can be stated in equation [2] as: Jicable'15 - Versailles 21-25 June, 2015 1/6
σ ( r) [ αt ( r ) + βe ( r)] = σ 0e [2] where α and β are the coefficients representing the dependence of the temperature and the electric field, σ 0 is the insulation resistivity at a reference temperature, T is the temperature, and E is the electric field. With consideration of equation [2], the electric field distribution in a DC cable is highly dependent on the current load of the cable system. An analytic approximation of the electric field distribution in a DC cable is given according to [5] in equation [3]: with r δ U 0 ro E( r) = r i ro 1 ro o i δ 1 δ a T bu 0 + ln( ro / ri ) ro ri = bu 0 1+ ( r r ) [3] δ [4] where a and b describe the material constants and T the temperature drop in the insulation. Figure 1 shows an example of an electric field distribution of a cable under AC and DC stress with consideration of an operating cable (T conductor > T ambient). The electric field distribution under DC conditions is similar to the AC conditions at the moment the system is energized according to equation [1]. At this moment, no thermal influence is present and only the effect of the electric field has to be taken into account. Thereby, the highest stress occurs on the conductor screen (ISL). However, when the cable is loaded, the current in the conductor leads to a temperature gradient over the cable insulation. As a result, the resistivity of the insulation material changes according to equation [3]. This leads to an electric field inversion in the cable insulation where the electric stress decreases near to the conductor screen and increases near to the outer insulation screen (OSL). The above mentioned electric field distribution under DC conditions leads to a totally different consideration compared to AC systems during the development process of extruded HVDC cable systems. The strong dependence of the insulating materials on the field strength and the temperature, especially, requires detailed knowledge of the insulation materials of the cable system components. More complicated, yet, is the behaviour addressed in the case of HVDC cable accessories. Therefore, several materials with different specific conductivities and their dependencies on temperature and electric field strength must be taken into account. Influence of space charge accumulations One of the main challenges within the development of HVDC materials and components is the understanding of the accumulation of charges in the insulation materials. Even insulating material consists of free charge carriers. Under an applied AC field, the direction of the electric field inverts periodically. In this case the flow of free charge carriers also inverts its direction. Finally free charges remain at their position under AC conditions. Under a DC field the free carriers accumulate especially near to the inner and outer insulation screen of the cable, respective of the accessory or between interfaces of cable and accessory. As a result, the electric field distribution in a cable insulation system under DC conditions will be strongly influenced by the accumulation of space charges. In addition to the above mentioned external field in equation [3], the electrical field distribution in a DC insulation system will be strongly influenced further by accumulations of space charges as stated in equation [5]. E + res = E 0 E s [5] E res shows the resulting field in the insulation, E 0 is the external field as described above and E S is the space charge field. This link leads to the conclusion that strong field enhancements with a critical stress on the insulation system might occur during the the system s operation. Because of these correlations, the materials for DC applications must be chosen with consideration of the effect of space charge accumulations. This leads to the requirement that the investigation of the used DC materials is necessary to get a detailed understanding of the process in the insulation system. For a deeper understanding of the space charge accumulation, figure 2 has been taken from [6] and demonstrates the result of measuring a space charge accumulation by using the PEA method (pulsed electro acoustic). E AC E DC Fig. 1: Schematic distribution of resisitive field (T conductor > T ambient) Fig. 2: Result of a space charge measurement (Courtesy of material manufacturer) Jicable'15 - Versailles 21-25 June, 2015 2/6
Two different materials have been investigated. It can be shown that the standard XLPE obtains a much higher charge density in the area of the electrodes compared to the advanced HVDC XLPE. This result shows the necessity of an optimised material for DC applications. Consideration of space charge effects is difficult because of very long time constants, which depend on the type of insulation, material, field strength, and temperature. However, for reliable operation of the HVDC cable system, detailed considerations of the previously mentioned space charge effects must be taken into account during the development process. BASIC CONSIDERATIONS ON EXTRUDED HVDC CABLE SYSTEMS The basis for a reliable consideration of the HVDC test system, essentially, is defined by its technical requirements. The technical requirements of a HVDC cable system are defined by the electrical, thermal, and mechanical boundary conditions given by the HVDC system configuration (e.g. nominal voltage, load, laying conditions). The thermal boundary conditions are defined by cable construction and system engineering. As already mentioned above, the thermal behaviour of a cable system under DC applications is directly linked to the electrical properties of the insulation system. This means that the requirements on the electrical design of an HVDC cable system are an interaction of thermal conditions and space charge effects as well as nominal and impulse voltages. Transient impulse voltages (e.g. switching and lightning impulse voltages) lead to temporary overstresses on the cable insulation system. Thus, the electrical stress distribution in the insulation system is then the superposition of the nominal applied DC voltage (see equation [5]) and the capacitive field of the impulse (see equation [1]) according to equation [6]: E = E + E sup erimposed res impulse [6] E superimposed shows the resulting field in the insulation, E res is the resulting field of the external DC field and the space charge field, E impluse is the capacitive field of the applied impulse. The level of the impulse voltage E impluse depends on the configuration of the entire HVDC system. As there are no impulse levels given in the standards, the insulation system has to be designed with consideration of the specific project requirements of the HVDC system as mentioned in CD IEC 62895 [9]. Finally, the suitablity of the system components for AC applications in terms of factory acceptance tests and PD measurements must be considered as well. An extruded HVDC test system in a scaled model has been designed, manufactured and tested by addressing the above mentioned influences and requirements. The following section addresses the main aspects of this process of the pre-stage cable system. Consideration of a XLPE-HVDC pre-stage cable system in scaled model The pre-stage cable system in a scaled model consists of an extruded HVDC cable and a prefabricated joint made of silicone rubber. Based on the above mentioned system requirements, basic theoretic considerations in terms of electrical and thermal design of cable system components have been done. The components have been designed in a scaled model for a nominal voltage level of U 0 = ±80 kv. Basic studies on the cable insulation have been done with consideration of the electrical field strength under different thermal and electrical operation conditions. A major focus was on the design of the prefabricated joint. The advantage of this type of joint, compared to other joint types (such as tape molded joints), is the fast and easy installation and the ability to pre-test the insulation body in the factory. However, the challenge of a prefabricated joint design is dealing with different materials and the interface between the cable core and the joint body, especially keeping in mind the DC specific influences mentioned earlier. One of the fundamental details in the design process of the prefabricated joint will be explained in the following section. The calculations of the electric field and potential distributions presented are done using a finite element calculation tool. Figure 3 shows calculations over a section of a conventional HVAC prefabricated joint. The simulation shows the potential distribution of the joint under DC voltage in three different cases. Figure 3a represents the joint under a zero currentload condition. In this case, similar conductivities of the XLPE and the silicone rubber can be assumed and lead to an even potential distribution in the interface between XLPE cable core and joint body made of silicone rubber. According to equation [2], the conductivity of the materials strongly depends on the temperature and the strength of the electric field. Considering the temperature distribution in a joint during the operation, different conductivities must be taken into account. Figure 3b shows the potential distribution for the simple case of σ XLPE < σ SiR. The different conductivities of the materials lead to an uneven potential distribution in the joint. The result is a strong and critical field enhancement in the area of the inner field control element of the conductor connection. As shown in figure 3c, in the case of σ XLPE > σ SiR, the potential distribution leads to a strong electric field enhancement in the area of the geometric field control element. a - σ XLPE = σ SiR b - σ XLPE < σ SiR c - σ XLPE > σ SiR Fig. 3: Equipotential lines of a joint in the case of different conductivities of silicone rubber Jicable'15 - Versailles 21-25 June, 2015 3/6
Figure 4 is a diagram of the potential distribution with the interface between the XLPE of the cable and the joint made of silicone rubber. The graphed lines refer to the results determined as addressed in figure 3. Thereby, the even potential distribution in the case of zero current-load can be shown in with the green line. The blue line represents the case of σ XLPE > σ SiR and the red line represents the case of σ XLPE < σ SiR. Fig. 4: Potential distribution in the interface XLPE and joint body The simple field calculation of the joint shows the highly sensitive behaviour regarding different temperatures resp. conductivities. The main aim of the joint constructions must be a decoupled and less sensitive behaviour between the conductivites of XLPE and the silicone rubber of the prefabricated joint body. Furthermore, the electrical stress on the insulation system in the case of a superimposed voltage must be considered under different conditions of thermal operation. To overcome the stated challenges, different variations of prefabricated HVDC test joints were designed and tested. The respective joints use advanced materials made of silicone rubber. The materials have been chosen with consideration of space charge effects in the joint and its interfaces, dielectric properties and conductivities. The joint body construction includes different types of field control systems (geometric / resistive) and design field strengths. Figure 5 is the graph representation of the potential distribution in the interface between cable core and joint. All parameters are equal to the ones discussed in figure 3 and 4. However, an advanced design of an HVDC joint has been calculated. It can be shown that the HVDC joint realizes an even potential distribution in the interface between cable core and joint body. The simulation shows a less sensitive behaviour of the joint regarding different operation temperatures. This distribution enables a reliable thermal operation range without the occurence of critical local field enhancements. After completing the theoretical consideration of the prestage system components of an HVDC cable in a scaled model with different variations of HVDC joint construction was manufactured and tested, the produced components of the pre-stage cable system were factory tested with an AC high voltage test with a PD measurement in order to check production quality and to avoid false interpretation of the test results. The cable in model scale consists of a 300 mm 2 aluminium conductor. The conductor screen, insulation, and insulation screen are made by a triple extrusion process. The materials used have advanced space charge properties and enable a maximum conductor temperature of 70 C. The insulation system is designed for an operation voltage of U 0 = ±80 kv. The cable screen consists of a 25 mm 2 copper screen, bedding tapes and an outer laminated aluminium foil. An extruded outer PE sheath enables the outer mechanical protection of the cable. The cable was produced with consideration of the degassing behaviour of the crosslinking byproducts. To evaluate the technical performance of the cable and the different joint constructions, detailed tests have been carried out with consideration of the above mentioned DC specific influences on the insulation system. The design of the joints differ in material, field control system and design field strengths. Several tests have been carried out to evaluate an appropriate DC insulation system under all applied thermal and electrical conditions. Figure 6 shows, as an example, the results of a long term DC test carried out with consideration of three different joint constructions. The investigation was conducted using several test objects so that a statistical statement can be reached. Fig. 6: Schematic overview of long term test results on HVDC pre-stage joints Fig. 5: Potential distribution in the interface XLPE and joint body The long term tests have been carried out under increased voltages at ±160 kv and temperature heating cycle conditions at a maximum conductor temperature >75 C. The long term test reveals the suitability of joint design no. 3 in terms of the described long term stress. The results obtained were supported by additional tests e.g. impulse voltage tests and limit field strength tests. Jicable'15 - Versailles 21-25 June, 2015 4/6
After evaluation of the addressed test results, fundamental design rules were defined and transformed in a common design of a HVDC pre-stage joint. The mentioned consideration of the HVDC pre-stage cable system was concluded by an adapted type test according to CIGRÉ recommendation 219 respectively 496 in [7] and [8]. The components have successfully passed the adapted type test. Finally, the result of the described development process of an HVDC pre-stage cable system indicates the suitability of the components for HVDC applications. Testing of a 150 kv extruded HVDC test cable system The results obtained from the process developed for the pre-stage cable system in a scaled model were implemented in general design rules (field strengths, material properties, production parameters). Based on those parameters, the obtained results were implemented and transformed in an HVDC test system for a nominal system voltage of U 0 = ±150 kv. Figure 7 shows a sketch of the HVDC cable. The ±150 kv HVDC cable consists of a 630 mm 2 aluminium conductor (1). The conductor screen, XLPE insulation, and insulation screen are made by a triple extrusion process (2). The materials used have advanced space charge properties and enable a maximum conductor temperature of 70 C. The cable screen consists of a 25 mm 2 copper screen, bedding tapes and outer laminated aluminium foil (3). An extruded outer PE sheath enables external mechanical protection of the cable (4). Fig. 8: Schematic picture of the HVDC test-setup The cable terminations have been realized by special test terminations made of silicone rubber including a geometric field control element. The cable loop has been closed by a copper connection between the terminations. A current transformer induced a heating current in the test loop. The conductor temperature and temperature drop of the test cable have been determined by comparing sheath temperatures with an additional dummy loop. An aluminium compression sleeve establishes the conductor connection in the joints. The main data of the conducted type test are given in Table 1. Table 1: Adapted HVDC type test procedure for VSC applications Description Load Cycle Test Parameter 24 cycles at U T = ±290 kv 8/16 hours heating / cooling T conductor = 75 C -80 C 3 cycles at U T = 290 kv 24/24 hours heating / cooling T conductor = 75 C-80 C U T = ±150 kv DC voltage with imposed impulse voltages Fig. 7: Sketch of the 150 kv extruded HVDC cable The cable joints have been manufactured with consideration of the defined design rules obtained during experimentation of the scaled model pre-stage system. Based on the electrical, thermal and mechanical design of the components, an HDVC cable and joint for a nominal operating voltage of U 0 = ±150 kv has been produced. To evaluate the reliability of the system, a type test adapted to Cigré recommendation 219 respectively 496 in [7] and [8] has been conducted. The test has been performed for VSC applications. For a detailed understanding of the test procedure, figure 8 shows the schematic test-setup of the adapted type test. Superimposed Voltage Test Subsequent DC Test Switching impulse withstand tests Lightning impulse withstand tests T conductor = 75 C-80 C U T = -150 kv 2 hours ambient temperature All tests have been passed successfully. No breakdown or flashover occurred during the tests. Hence, the ±150 kv test cable system shows the suitability for HVDC applications. Additional tests, especially those with long term considerations, are ongoing to support these results. Jicable'15 - Versailles 21-25 June, 2015 5/6
The process described from a pre-stage cable system as a scaled model to the ±150 kv test system shows the successful approach of cable system components under DC conditions. However, based on the mentioned DC specific influences, further investigations and considerations are necessary to obtain a full and reliable understanding of those effects. Because of this, further scientific considerations are planned and ongoing to get a deep understanding of DC fields on extruded cables and accessories made of silicone rubber. A key focus is on the space charge accumulations under increased field strengths in the components and interfaces, consideration of the behaviours of material and the simulation of the insulation systems. Based on those results, general design rules can be implemented in the development of cable system components for higher system voltage levels. CONCLUSION This paper describes the main differences between an AC and DC field in extruded cables and their accessories. Under consideration of the DC specific effects on extruded cable systems, a pre-stage cable system in a scaled model has been designed. Based on the electric, thermal, and mechanical design of the components, the pre-stage system has been manufactured and tested. As a result of the obtained results, an HVDC cable system with a nominal voltage level of U 0 = ±150 kv has been manufactured and tested. The system design was evaluated by a type test adapted to Cigré recommendation 219 respectively 496. The test results show the suitability of the ±150 kv cable system regarding the DC specific influences. Further testing and scientific activities are planned and ongoing for the consideration of higher voltage levels - for the purpose of a reliable and more renewable energy supply in the future. Cable Systems for Power Transmission at a Rated Voltage up to 250 kv, Cigré WG 21-01 [8] 2012, Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500kV, Cigré WG B1.32 [9] 2014, High Voltage Direct Current (HVDC) power transmission cables with extruded insulation and their accessories for rated voltages up to 320 kv for land applications - Test methods and requirements, Comitte Draft, CD IEC 62895 GLOSSARY AC: Alternating Current DC: Direct Current HVAC: High Voltage Alternating Current HVDC: High Voltage Direct Current ISL: Inner Semiconductive Layer OSL: Outer Semiconductive Layer PEA: Pulsed Electro Acoustic Technique PE: Polyethylene SiR: Silicone Rubber VSC: Voltage Source Converter XLPE: Cross-Linked Polyethylene REFERENCES [1] A. Weinlein, U. Peters, U. Laage, H. Memmer, 2015, Worldwide Experiences and Challenges with EHV XLPE Cable Projects 330 kv to 500 kv, 9 th International Conference on Insulated Power Cables - Jicable 2015 [2] J. Kaumanns, A. Weinlein, G. Schröder, V. Stroot, 2011, Development, Qualification and Experience with 500kV XLPE Cable Systems, 8 th International Conference on Insulated Power Cables - Jicable 2011 [3] T. Worzyk, 2009, Submarine Power Cables, Springer Verlag, Heidelberg, Germany, 974 [4] M. Jeronese, 1997, Charges and Discharges in HVDC Cables: in Particular in Mass-Impregnated HVDC Cables, Ph.D. Thesis, Delft University of Technology, Netherlands [5] G. Mazzanati, M. Marzinotto, 2013, Extruded Cables for High-Voltage Direct Current Transmission, IEEE Press, NJ, 59 [6] J. Boström, D. Kung, U. Nilsson, P. Hagstrand, 2011, Material System for Extruded HVDC Cables in Commercial Use Since 1999 and Future Development, ICC Fall 2011 Conference, 10 [7] 2003, Recommendations for Testing DC Extruded Jicable'15 - Versailles 21-25 June, 2015 6/6