Material considerations for submarine high voltage XLPE cables for dynamic applications. B. Sonerud, F. Eggertsen, S. Nilsson, K. M. Furuheim, G. Evenset.
Material considerations for submarine high voltage XLPE cables for dynamic applications B. Sonerud, F. Eggertsen, S. Nilsson, K. M. Furuheim, G. Evenset Nexans Norway AS Knivsøv. 70 NO-1788 BERG I ØSTFOLD, NORWAY
Abstract- Special consideration is necessary choosing materials and selecting design for dynamic cables due to the mechanical stress, sometimes in combination with a wet design, experienced by the cable. For high voltage cables (>36 kV) this is mainly related to the required (IEC 60840) metallic water barrier and for wet solutions the influence of water treeing and the associated accelerated electrical aging is of concern. Lead is commonly used as a radial water barrier for high voltage cables but the fatigue resistance is inferior to steel and most other metallic materials due to its low melting temperature and poor mechanical properties. For these reasons it is generally not considered suitable as a radial water barrier for dynamic power cables. Alternatives such as longitudinally welded copper sheaths have proved to be viable alternatives to lead and studies on nonmetallic barriers have been performed which show promising results. For wet medium voltage designs or designs which allow some moisture ingress into the insulating material water treeing is often a concern. However, it has been shown that high relative humidity (>70%) is necessary for water tree inception. By delaying the water ingress with sheaths and swelling tapes the time to reach critical humidity levels in the material can be increased from 1 month to several decades. The influence of hydrostatic pressure and compressive stress has been found to diminish water tree growth whereas tensile stress enhances it. For dc voltages there are indications that both humidity and mechanical stress can increase accumulation of charge in the insulating material. It has also been shown that moisture ingress will increase the conductivity which could influence breakdown strength negatively.
pipe can be used as a conductor in a single-phase electric circuit where resistive power losses provide heating. The system is typically supplied by a dynamic riser cable, shown in Figure 1, connected to the platform main power supply at medium voltage with large conductor cross-section to be able to carry the current. Umbilicals carry energy, telecommunication, fluids and chemicals for the control of subsea systems. In power umbilicals medium voltage cables is also used for power supply of subsea pumps and processing equipment. In this paper some aspect related to the choice of materials when designing dynamic power cables and umbilicals are discussed. Focus is the water barrier required for high voltage cables and the influence of mechanical stress and moisture on XLPE insulation for HVAC and HVDC solutions. II. RADIAL WATER BARRIER High voltage submarine power cables must be sheathed [1] to protect the insulation from water ingress; the sheath also needs to be electrically conductive to handle short circuit currents. An example of this can be seen in Figure 2 where a three phase cable with lead sheath as radial water barrier is shown. For static cables this determines the required sheath thickness but for dynamic applications mechanical considerations are also of importance. Lead, which is commonly used as radial
I. INTRODUCTION Applications for dynamic XLPE cables comprise floating wind turbine generators (WTGs), power supply for direct electric heating (DEH) and floating gas and oil platforms as well as power supply for subsea pumps and processing equipment. This includes cables at both medium and high voltage, mainly for ac, but applications for extruded HVDC can also be foreseen. Floating wind turbine generators enable harvesting of wind power in deep water environments and require a dynamic cable connecting the power plant to converter or transformer stations. The voltage generated is typically at medium voltage. Floating oil and gas platforms powered from onshore requires high voltage cables due to the large amount of power to be transmitted and the potentially long transmission distances. DEH cables find their use in heating subsea pipelines in order to avoid hydrate plugs. The
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Figure 1. Crossection of a DEH riser cable
A. Longitudinally welded copper sheath
Figure 2. Crossection of a 3-phase cable with lead sheath water barrier, has a low melting temperature (327°C) and is very soft which makes it easy to apply to the insulated conductor screen by means of continuous extrusion. In addition it has excellent corrosion resistance and high flexibility which is necessary for spooling finished cables on drums or turn tables as well as for installation. The mechanical, fatigue and creep properties for typical sheathing alloys are attained during the sheathing process. At room temperature the material is above the initiation temperature for creep, whereby it slowly deforms permanently at a static load well below the yield point. The fatigue resistance of lead is inferior to steel and most other metallic materials due to its low melting temperature and poor mechanical properties. An example of lead sheath failure can be seen in Figure 3. The mechanical properties are also very dependent on test frequency which makes generalization of test data non trivial [2]. In addition, the heavy weight of lead causes significant strain in the load bearing elements and topside terminations especially for deep water applications. For the reasons listed above, lead is generally not considered suitable as a radial water barrier for dynamic power cables and alternatives are necessary.
Figure 3. Lead sheath failure from fatigue testing.
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Alternatives to lead has been developed, such as a fatigue resistant metallic moisture barrier [3] consisting of a longitudinally welded copper sheath bonded to a semi conductive (SC) polymer by means of a SC glue, shown in Figure 4. Copper has superior fatigue and mechanical properties compared to lead and several times higher conductivity which allows for a reduction in metallic sheath thickness. In addition, the density of copper is ~20% lower than for lead which in combination with a reduced sheath thickness gives considerable weight savings. Copper is also a soft and formable metal but due to its high melting temperature (1396°C) it cannot be extruded onto the cable core without damaging the insulation system; instead a copper strip is folded around the core and sealed with a longitudinal weld seam. To increase the fatigue properties of the sheath and avoid buckling during bending, a SC polymer is bonded to the copper by means of a SC adhesive forming a metal polymer laminate. For a stranded three core cable the SC polymer sheath and adhesive prevent local hotspots if the sheath on one power phase is interrupted e.g. due to mechanical failure [4]. With a SC over sheath, the immediate consequences of a copper sheath break are non-critical, and break detection can be based on sheath current monitoring from the topside end. B. Non-metallic water barrier Another possibility to limit moisture ingress into the cable insulation is to use a non-metallic water barrier. 20 years ago it was found that a minimum of 70% relative humidity (RH) is required for inception of water trees to occur [5]. By delaying the water ingress, and thus the time to reach the critical level of humidity, the service life of the cable can be increased. During the past few years much work has been performed on the design of the outer sheathing layers of the cable and it has been shown that by using a special combination and order of different polymer material based sheathings it is possible to delay the water ingress significantly so that the critical humidity level is reached only after several decades. The basis for this design is extensive knowledge of water permeation properties of materials. Experimentally achieved data of properties of water permeability are use into detailed modeling
enhanced. For electrical trees to be initiated, the local electric field needs to exceed the threshold for partial discharges (PD) as shown in [13].
Figure 4. Power core with copper sheath. of combinations of materials Experimental verification is crucial and parts of this work are presented in [6]. III. INSULATING MATERIAL A. High Voltage AC All polymeric materials are subject to ageing, under the influence of for instance oxygen, mechanical stress and electrical stress. The ageing can cause deteriorated material properties, which in the case of high voltage cables can decrease breakdown voltage and increase dielectric loss. The slow growing water trees and the rapid growth of electrical trees are two severe ageing mechanisms. Considerable research and development work has been performed on the improvement of insulation system, as well as development of cable technology during the past 30 years aiming at minimizing ageing problems [7]. Although the detailed mechanisms for the inception and growth of water trees are not fully known, much data is acquired on the factors affecting water treeing. Apart from humidity levels, the electric stress of the insulation is of significant importance for water treeing [8, 9]. A strong relationship between electric field and number of incepted water trees/mm2 has been shown in [10], where an increase in field from 2.1 kV/mm to 11.2 kV/mm increases the water tree density with a factor > 40. Water trees are often initialized at defects and in voids where the electric field is enhanced. Other essential criteria for water tree inception and water tree growth are the presence of ions, diffusion of water to the inception site and the power frequency. The frequency is possible the main accelerating factor for water tree inception and in particular for water tree growth [11]. Water trees alone are not normally the cause of an electric breakdown. Even cables having water trees bridging the insulation can still retain its insulating properties [12]. However, the risk for electric tree inception is increased at the boundaries of a water tree, where the electric field is strongly
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It has long been indications that mechanical strain affects water tree growth. Recent research confirms that water trees are diminished at compressive zones, whereas water tree growth is enhanced at tensile zones. The effect is most pronounced when the tensile strain is considerable [14]. These results also point toward the conclusion that compressive stress retards water treeing, which is in accordance with preliminary results on the water tree retarding effect of high hydrostatic pressures. The latter results are important regarding DEH cables, as these cables have wet designs and dynamic sections (riser cable). As DEH cables normally have large conductor cross section, the electric field at the inner semiconductor is low, i.e. in the range of 1.5-3 kV/mm. The conductors are also filled with water blocking material, thus preventing water ingress in the conductor and transportation along the conductor strands. Long term testing of DEH cables under harsh conditions, i.e. water immersion, high pressures and high temperatures show that the water tree growth is kept at a low level, and that the electrical breakdown strength is well above the specified limit [15]. In general the problems with water treeing have been significantly decreased due to the high cleanliness and new design of modern semiconducting and insulating materials. Today there are a range of tailor made insulation systems for high voltages and also materials that are specifically designed for water tree resistance [16]. B. High Voltage DC For HVDC cables several essential material properties are affected by moisture ingress, for instance breakdown strength, space charge accumulation and conductivity. These properties are also interconnected and an increase of space charge accumulation and increase of conductivity tends to decrease breakdown strength. The fact that the conductivity of XLPE increases with increasing moisture content is not surprising and has been observed in, for instance, [17, 18]. For high electric field the increase of the conductivity due to moisture ingress in combination with the field and temperature dependence of the XLPE can be the driver for thermal breakdown. Humidity in the material can also affect space charge accumulation which can cause breakdown of the insulation due to field enhancement. One study shows a decrease of space charge density at high field for high moisture content in XLPE whereas the space charge accumulation increased at lower moisture content [18]. The generation of negative heterocharge in XLPE with varying relative humidity has been investigated in [19, 20] where it was found that absorbed moisture and charge density exhibit a linear relationship. There are studies which show that the characteristics of space charge accumulation also changes when XLPE is subjected to
mechanical stress even without any moisture present. In [21] tensile stress was applied to XLPE which increased the amount of heterocharge in the material. For other insulating materials tensile stress has enhanced space charge related breakdown whereas compressive stress suppressed them [22]. For HVDC applications it is obvious that moisture ingress can have a detrimental effect on the insulating system whereas mechanical loading is more ambiguous and analogous to the behavior observed for water tree growth under ac stress. However, it should be noted that the amount of published studies performed on the effect on mechanical loading on dc properties of XLPE is limited.
[8] [9] [10]
[11] [12]
IV. CONCLUSIONS For high voltage ac cables a radial water barrier is necessary due to higher electric stress experienced by the insulating material. Although lead is inappropriate in dynamic applications for many reasons other options are available, such as the longitudinally welded copper sheath or the use of nonmetallic water barriers, possible utilizing a layered sheath system. The options for ac dynamic medium voltage cables comprise wet solutions as well as solutions with a diffusion barrier. As indicated by experience and in literature, the problem of water treeing for medium voltage is mainly a problem of the past as cable technology has evolved as well as the quality of insulating materials. To initiate water treeing a high relative humidity and electric field is necessary, which can be mitigated by either tree retardant materials or proper control of water diffusion. For dynamic applications of extruded HVDC there are not many published studies available, but there are indications that both mechanical stress and humidity will influence space charge accumulation in the material encouraging more studies in this field. REFERENCES [1] [2] [3] [4]
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