Power distribution networks are changing rapidly as integration of renewables as well as creeping urbanization require updated concepts in power infrastructure. Compactness is no longer only desirable but now even a crucial performance requirement. Similarly, downtime has to be minimized while safety aspects are more complex. This edited recent contribution by Ruben Grund and Michael Zerrer of Pfisterer, discusses these challenges in the context of development of ‘next generation’ cable accessories.
Increasing Transmission Capacity
Urbanization requires higher transmission capacity that could either be triggered by growth in number of customers or due to fluctuating power generation and consumption due to integration of renewables. Moreover, energy has to be transported over long distance since points of generation and demand in most cases do not overlap. This requires detailed consideration of power losses and one approach to reduce these has been increased system voltage, as has occurred over past decades. While the first three-phase AC line in 1891 had a system voltage of 25 kV and carried power over 176 km, these days system voltage of 550 kV is used for extra high voltage cable lines, allowing power transmission up to 1.5 GVA. The main reason for higher system voltages is power loss reduction. For example, increasing system voltage by 10 leads to a reduction in power losses by a factor of 100, using same diameter cable. If cable diameter is reduced by 10, power losses are still reduced by a factor of 10.
Cable Accessories up to UM=550 kV
Accessories are NOW increasingly available for high voltage cables up to 550 kV. This includes dry, pluggable cable connections for transformers and gas insulated substations (GIS), cable terminations, cable joints and pluggable cable joints as well as blind dummy plugs. These components are type tested and can be used for all types of XLPE cable, independent of core diameter, thickness of insulation or manufacturer. All field control units and electrically stressed materials are pre-fabricated and routine-tested, which ensures highest long-term operational safety.
One type of dry pluggable system can handle cable diameters of 3000 mm² CU or AL conductor, carrying up to 4000A nominal current. Maximum diameter over insulation is 144 mm and the principal set up is shown in Fig. 1. An additional benefit is the housing, which is touch-proof, waterproof and salt-water resistant, meaning it can be used in coastal areas as well as in offshore surroundings.
The extensively tested latest technology of cable joints ensures simple installation of the waterproof external housing as well as maximum operational safety. Using a bolted connector allows the conductor to be connected with optimal contact force, without special tools. Treatment of the cable shielding can be adapted individually for cable type and customer needs (see Fig. 2 as example).
The qualification requirement is performing a type and a pre-qualification test, according to IEC. A sample set up as per IEC 62067 is shown in Fig. 3.
A major hurdle is the so-called Annex G test at which the joint is submersed in water at 1 bar pressure and cycled, with number of cycles depending typically on regional specifications. Chart 1 shows heat cycle measurements during such a test, while temperature at a dummy is used for calibration.
Outdoor Cable Terminations
Another key component in modern cable networks is the outdoor termination, now usually offering a water-repellant external housing made of silicone. Minimum creepage distance is 16,600 mm which covers a high pollution class of > 65mm/kV. The stress cone of the termination is also pre-molded and routine tested, which allows fast mounting time and ensures high reliability of these parts. A special centering device, unique to some types of terminations, ensures the right position of the stress cone, both in radial position and with the axial centricity of the important electrical part.
A major hurdle for outdoor terminations is overall size, resulting in a large free length of cable inside the termination. Due to load change, conductor length also changes leading to elongation and therefore a ‘buckling’ effect at high temperatures. The smaller the conductor, the greater this effect. Tests were therefore conducted at a cable with 300 mm² copper conductor and a corresponding over insulation diameter of 62 mm (refer to Fig. 4). Maximum buckling of this arrangement proved to be outside limiting values. Therefore, to prevent buckling of the cable inside the termination’s insulator housing, a special spring device was integrated, as shown in Fig. 5. This allows for length compensation of the conductor inside the termination and at the same time prevents the conductor from moving off-center, thereby also ensuring a constant field distribution independent of temperature change.
As in other terminations, the base plate is insulated from the cable screen, which allows a flexible earthing system according to customer requirements. To allow quick and safe cable connection, the top bolt of the termination as well as the connection in the joint is equipped with a bolted connection that does not require any special tools. This bolted connection offers life-long contact force and assures the connection to the cable connector.
A new cable clamping system has been developed for both the cable connector and termination (see Fig. 6), consisting of mainly two parts – an extension cage and a cable clamp. If the system is to be used on small outer cable diameters, an additional adapter is available. The cable clamp is adjusted on the cable diameter to allow maximum retention force, ensuring minimal pressure on the cable. As such, even huge cable weights can be handled. At the same time, this system fulfills a centering function to ensure proper positioning of the cable within the connector-respective termination.
Decrease in Skin Factor
De-centralized energy production, driven mostly by increasing generation of renewables, requires additional network capacity for energy transfer and thereof distribution. Growing urbanization further drives this by requiring additional distribution capacity. This trend is confirmed by evaluation of cable dimensions in Chart 2, which shows worldwide usage of cable connectors for UM=245 kV. Since 2006, cables diameters have increased by circa 10%.
Increasing conductor cross-section is one option. Nevertheless, due to the ‘skin effect’ the investment is not equivalent to the benefit. Chart 3 compares use of different conductor types and shows the resistance increase with reference to the skin effect. The ratio of AC resistance to DC resistance is shown to be close to equivalent utilizing enameled conductors, which are stranded and have every single wire insulated. Therefore, no current flows between the single strands, resulting in a great reduction of skin effect.
This is a major advantage for cables but it challenges design and therefore the function of cable accessories. Contact surface requires homogeneous current distribution. Former solutions required removal of insulation layers by abrasives or heat, processes which are time consuming and depend highly on workmanship. Furthermore additional cable length is required due to bending of the strands. This therefore contradicts the goal of compact solutions. Latest developments see using the front surface of the cable conductor as the connecting surface. Contact surface is offered with an incompressible yet flexible material. This solution is being used for joints, terminations and pluggable systems. Pfisterer, for example, started developing such a solution in 2011. A patent is pending and components for trial projects have already been delivered.
While increase in transmission capacity is a vital enabler for tomorrow’s grid, network flexibility is another key aspect. Pluggable systems are becoming more and more popular but any enclosed system, such as a cable system, needs additional thought in reference to protection.
Surge arresters are usually either SF6 or air-insulated and both have limitations. Use of SF6 is becoming more restricted and testing requires handling SF6 gas. Similarly, air insulation requires creepage and space and positioning is not optimal. Substation grid layout when using cable usually does not allow an air-insulated arrester to be positioned as close to the transformer as possible.
A pluggable, solid insulated arrester has been developed and tested up to Uc=144 kV (Ur=180 kV) with reference to the latest standards. The arrester’s main insulation is solid and there is no insulation liquid or gas such as SF6 included. Fig. 7 shows a cutaway view of such an arrester. The main part in regard to arrester functionality is the metal oxide discs. These are used as a non-linear component with very low leakage current during operation. The discs are connected to the male part of the plug-in system and insulated by a silicone body, which includes field control elements. The head armature (flange) includes a bursting disk for pressure relief and a ‘turnable’ head for re-directing the gas in the event of failure, according to IEC 60099-4. The housing is made of glass-fiber reinforced resin and allows great mechanical strength as well as protection of the silicone body against the environment. The silicone body itself is protected and touch-proof. A special arrangement of the earthing path allows for connecting monitoring devices or discharge counters, if desired.
Conventional bushing are mainly used on transformers and these days RIP (resin impregnated paper) types are being more widely used. During assembly of such a conventional bushing, the transformer tank has to be opened and therefore its main insulation, in most cases mineral oil, is exposed to the environment. This process could cause difficulties in view of pollution, humidity and assembly time. With reference to a pluggable system, the transformer is enclosed by a socket, which then offers an interface. This interface allows all options without oil work and no need to open the transformer tank. The system can be tested using testing equipment – voltage proof enclosed and connected to a cable system using cable connectors or connected to an overhead line utilizing pluggable bushings. These pluggable bushings are available up to UM 245 kV and solutions for higher voltages are under development. While pluggable bushings offer main advantage in regard to transformers, other applications are made possible with this approach. One example is a temporary installation for short or mid-term applications where downtime is crucial and full flexibility is required. All components are pre-tested and shipped to site and final assembly is but a plugging process. Fig. 8 shows such a temporary arrangement at UM 245 kV.
Evolution of Cable Terminations
The major proportion of all cable terminations today are oil-insulated and consist of a GFR housing and a silicone stress cone for field control. This arrangement is established and still widely in use. However, if referring to assembly requirements, long-term stability and mode of failure, this system does not comply with most customer expectations. Nowadays, cable terminations up to UM 170 kV are dry and have solid insulation. The assembly process is easier, there is no liquid insulation and therefore failure mode is optimized. By way of disadvantage, they are not self-supporting and therefore the next evolutionary step is dry type terminations that are self-supporting, enabling integration of additional functions. These evolutionary steps are shown in Fig. 9. Self-supporting systems and pluggable solutions are already available and tested up to UM 170 kV. Higher voltages will follow.