Driven by ease of installation as well as service stability and good behaviour in the event of failure, dry type outdoor cable terminations and dry pluggable solutions for GIS and transformers have been used widely for over 25 years. More recently, driven by growing demand for such cable accessories in onshore and offshore substations, a special design has been realized for a range of applications, including pluggable terminations for GIS and transformers requiring off-shore certification, pluggable solid epoxy-joints and pluggable surge arresters. Moreover, cost optimized dry outdoor terminations now include an option to integrate the surge arrester and also the possibility to install all products onto ultra-bendable HV cables having EPR insulation.
Adapting any high voltage component for special new applications requires verification of design, which is one of the most important steps during the development and qualification process. One of the key challenges for the HV cable accessory is being suitable across different cable constructions that vary depending on manufacturer, customer requirements and operating conditions. Moreover, in order to verify accessory design, internal tests as well as official qualifications witnessed by or in a third party HV laboratory have to be performed.
This article, based on a presentation at the 2015 INMR WORLD CONGRESS by Matthias Freilinger of Pfisterer in Switzerland, discusses these applications and offers examples of how modern day cable accessories are being adapted to meet these needs.
Adapting Dry Outdoor Terminations for Substations
Dry type terminations are known to offer advantages, such as:
- No need for oil or gas insulation;
- Easy and safe installation on the ground;
- Reduced installation costs;
- Explosion proof behaviour.
Such terminations can be divided into two sub-groups: flexible or self-supported for various applications.
Both versions are well known on the market. The flexible type of termination is considered among the most cost effective solutions while the supported termination offers maximum mechanical stability, including a rotational system for lifting onto towers.
The first step in adapting these terminations to meet the requirements of substation applications is to combine both advantages. A flexible termination can be installed onto the cable while on the ground. It is then lifted onto a prepared support with an open baseplate. Due to height restrictions of 6 m and less cable weight, an optimized solution has been developed in regard to mechanical design, ease of installation and cost effectiveness.
Integrated Surge Arrester
In a second step, a combination product has become another option. Replacing the post insulator by integrating a surge arrester in its place offers significant space and cost savings during substation design. This combination product requires only the space of one conventional termination and is fixed onto the same support.
The cable screen is no longer connected to the base plate of such a combination product since the arrester needs a direct earthing point. Bonding of cable screen can instead be applied to the bottom of the termination or via a link box. In order to evaluate the ideal arrangement of such a combination product, an FEM calculation was done using two different programs.
The main findings of these calculations include:
- Distribution of potential along the metal oxide discs of the arrester is more homogenous and voltage along the upper discs is reduced due to the influence of the cable termination. This effect becomes stronger with shorter distances;
- Electric field strength along the arrester’s silicone sheds is normally higher at the upper part near the high voltage potential. Depending on a shorter distance to the termination, this effect can be equalized or even reversed (i.e. a higher electric field on the bottom sheds);
- By reducing distance between arrester and termination, electric field strength at the termination is ‘forced’ from the upper to the lower part and is significantly higher at the area close to the arrester.
These findings lead to the conclusion that there is an optimum distance for the combination product arrangement. Moreover, apart from the influence on electric field distribution of such a product, different customer requirements also have to be considered. Both solid earthed and coil earthed systems are used in energy transmission and this leads to two different surge arrester solutions. In addition to the two earthing systems, there are also different philosophies among network operators. For example, there is a ‘safety first and money is not an issue’ philosophy that focuses only on tube type arresters that offer an explosion resistant design. Additionally, they offer a high cantilever force that supports the cable termination on a high mechanical level. But realizing these benefits requires higher investment cost.
By contrast, a focus on optimizing cost leads to a cage type arrester. Here, should a rare arrester failure occur, in the worst case there can be damage to the functionality of the termination and both have to be replaced. The lower cantilever forces compared to a tube type arrester are still considered high enough to assure sufficient mechanical performance for the combination product.
Additional Tests on Dry Type Outdoor Terminations & Materials
Standard tests of dry type outdoor terminations are normally performed according to IEC 60840 or 62067. Due to increasing demand for this type of accessory – especially for substation projects – additional tests have been performed to satisfy special customer requirements. One is the salt fog withstand test according to UX LK 208 Rev. 01 and based on IEC 60507 (second edition 1991-04). For example, one high voltage cable termination type (EST170-C53L as shown in Fig. 6) has already passed this test, confirming the required specified salinity of 112 kg/m3 at test voltage of 98.1 kV (170 kV / √3).
This same termination type has also been subjected to and passed a wet power frequency withstand voltage test according to UX LK 208 Rev. 01 and consisting of application of 2.5 U0 (218 kV) during 15 minutes while the test object is under artificial rain, as per IEC 60060-1.
Moreover, during the development stage a test to evaluate resistance to tracking and erosion was performed according to IEC 60587 (third edition, 2007-05) and yielded satisfactory results.
The mechanical behaviour of the self-supported termination has also been verified several times through specific tests on the support insulator as well as on the complete termination.
Dry Type Pluggable Solutions for On & Offshore Applications
Pluggable terminations, bushings and other such accessories have become a significant part of the high voltage cable accessory world for many years now. Recently, this product range has been enlarged for additional applications at even higher voltage levels (e.g. designed up to 550 kV). Moreover, to be able to satisfy the differing needs of transformer manufacturers (testing IEC 60137, housing EN 50299), GIS manufacturers (testing IEC 62271‑203, housing IEC 62271‑209) and cable manufacturers (testing IEC 60840 and IEC 62067), it is important to understand the differences and offer a wide product range, e.g.
- Pluggable terminations (separable socket and plug);
- Pluggable surge arresters;
- Pluggable SF6 filled back-to-back joints;
- Pluggable dry cast resin joints;
- Covered discs for pollution protection of sockets;
- Protective caps for pollution protection of plugs;
- Dummy plugs for voltage insulation;
- Earthing and short circuit devices;
- Earthing caps;
- Current testing caps;
- Gas-insulated blind covers.
Dry Type Pluggable Cable Joints
Due to increasing demand for offshore applications of HV cable accessories, a new dry type pluggable (back-to-back) joint has been developed. This accessory combines the advantages of standard dry type pluggable terminations and joints and also allows easy connection between different cables. Main benefits include:
- Dry type design (no oil or gas insulation);
- Pluggable connection;
- Fast and easy installation with connectors;
- Compact design;
- Connection of two different types and sizes of cable;
- Resistant to salt water and UV;
- Epoxy body fully offshore proven.
Fig. 9 illustrates how the advantages of this type of accessory can be exploited by using it for connection between sea and tower cable in offshore wind power generation applications.
To satisfy growing requirements on HV cable systems, the advantages listed above also permit use of dry type back-to-back joints for other applications, e.g.:
- Refurbishing HV cable systems through connection of old (already installed) and new cable;
- Temporary installation for construction sites;
- Backup solutions;
- Connecting test equipment and test objects (e.g. during commissioning).
This construction has been successfully proven and certified for offshore use through several design tests and additional type tests according to IEC 60840 for voltage levels up to Um=100 kV and conductor cross-sections up to 1600mm2. Moreover, there is already development of such a product for Um=170 kV.
Offshore Certification & Installation
In the case of offshore substations, environmental conditions as well as special application have to be considered during project management. Apart from saltwater and high UV, there is limited available space. In addition, the entire substation has to be installed and fully tested onshore and later shifted offshore where the sea cable part is finally connected. A full family of pluggable systems for medium as well as high voltage levels is mandatory. Pluggable cable accessories offer a fully encapsulated solution for offshore substations. The outer part has to be touch proof and on ground potential. All metal and heat shrink parts have to withstand harsh salt fog conditions (DIN 81243) and high UV.
An example of offshore certification is shown in Fig. 12. In addition to design and classification of materials, this includes salt fog tests and ageing tests under UV radiation.
Pluggable cable accessories require less space and additionally offer the possibility of separating different parts of the system, such as the transformers, the gas insulated switchgear or the cable system. Installation and testing of these can be done individually and connection of the sea cable can finally be done with minimum effort to achieve a fully tested substation.
Dry HV Cable Accessories for Ultra-Bendable EPR Cables
Ultra-Bendable HV Cables
Ultra-bendable high voltage cables are especially suited for offshore applications where high flexibility, very small bending radii and mobile or fixed installation are key requirements. Moreover, due to the features mentioned above, this type of cable could also be used for onshore substations and other applications where available space is limited.
Adapting Dry Type Cable Accessories to Ultra-Bendable HV Cables
In order to guarantee proper jointing and termination of ultra-bendable HV cables, it was necessary to implement, as a standard dry-type HV cable accessory, a new mechanical torque connector, especially suited for highly flexible copper conductors. Behaviour and design were successfully verified with a test based on IEC 61238-1 (second edition, 2003-05) for an 800 mm2 conductor cross-section (see Fig. 15).
In addition, performance of HV cable joints, outdoor terminations and dry type pluggable connectors installed on ultra-bendable HV cable has been verified through several design and type tests according to IEC 60840 (for Um=170 kV and conductor cross section up to 800 mm2).
Special Applications of Dry Type Cable Accessories for Ultra-Bendable HV Cables
Ultra-bendable high voltage cables and dry type pluggable terminations combine the advantages of compact and flexible design and are therefore also suited for:
- Temporary installation at construction sites;
- Backup solutions;
- Connection between test equipment and test objects (e.g. during commissioning).
One example of exploiting the above benefits are cable links equipped with dry type pluggable connectors or outdoor terminations that allow transport on a drum mounted onto a trailer (as in Fig. 17).
New system needs represent a challenge for HV cables and accessories. In order to fulfil new requirements in a reasonable time and without significant influence on project duration and cost, know-how, flexibility and product modularity all play a major role. For example, dry type HV cable terminations and dry type pluggable connectors have both shown to be good solutions for substation applications. Relying on engineering know-how, modular products and robust design has also made it possible to implement new features such as surge arresters, connection to ultra-bendable HV cables and dry type pluggable joints – all in relatively short time.