A search for new tower designs was launched in Belgium in early 2008 in conjunction with two new 400 kV projects: Stevin (extension of the 400 kV grid to the coast) and Brabo (reinforcement of the 400 kV grid in the Port of Antwerp). Requirements were:
• minimal visual effects on landscape
• low EMF and noise levels under line
• transmission capacity of 3 GVA/circuit
A Working Group was established to conduct research and make proposals to management. The trigger was the trend within Europe toward designs that improve public acceptance of new lines by all stakeholders. This led to an international survey of compact designs since 1950, with the pros and cons of each. The conclusions of this study were that there is diversity in the circa 10 basic designs in regard to silhouette; some innovative designs are applied locally (e.g. near a town), others for a complete line; and use of steel poles is superior aesthetically to lattice towers but carries extra costs.
Elia, the Belgian TSO, compared 5-6 new tower designs against its standard 380 kV lattice tower. Comparison was as objective as possible using scores and weighted factors covering aspects such as: permits & authorizations; integration into landscape & EMF levels; financial costs; safety and ease of maintenance. After this work, Elia retained the 2 options with the best scores, namely the steel pole and a lattice tower with insulated cross-arms (versus the classic lattice tower).
An analysis was done of Total Cost of Ownership of the complete standard line using different tower design options. Results were quite similar for the different designs. Therefore, the final choice was left to the project managers and their recommendation was use of lattice towers with insulated cross-arms and having an average span of 350 m. However, a lot of additional work remained in regard to applying this technology, including:
• Specifying the design & delivery of insulated cross-arms;
• Qualifying suppliers;
• Working out procedures & tools;
• Developing inspection & maintenance guidelines.
This INMR article from 2015, contributed by Jean-François Goffinet of Elia, reviewed what was done in these different areas.
Development of New Tower Design
As a TSO is typically conservative and wants to maintain good performance regarding average interruption time (AIT) and energy not supplied (ENS), a need was felt to come up with proven designs using suppliers having good reputations and long experience.
Designing Insulated Cross-arms
Composite insulator technology came to maturity in the 1990s and led to publication of numerous IEC standards since 2000. In parallel, Cigré published a Guide in Electra (2002) and IEEE issued a Guide for Braced Insulator Assemblies for OHTL 60 kV and Greater (2008) – even though this technology had been used for years in the U.S. It is important for any TSO to be able to rely on international standard even if these are only a starting point and provide minimum requirements. Becoming familiar with these standards requires time and experience due to the great amount of tests to be performed on composite insulators. At first, it can give newcomers to this technology a feeling of uncertainty; in fact it’s more a question of trying to eliminate bad designs.
Starting for the first time with composite insulator technology made Elia choose not to rely only on its limited internal expertise with such insulators and insulated cross-arms. In fact, as part of the Elia Group, Elia Belgium could also rely on the experience of its sister company 50Hz in Germany that already had some experience with composite insulators and their specification. Moreover, due to our close contact with neighbouring TSOs (e.g. TenneT and their development of Wintrack towers with insulated cross-arms, it was obvious for Elia to rely on them for information. Specifications were issued in 2011 taking into account experience by both 50Hz and TenneT. In parallel, an insulation co-ordination study for Elia’s 380 kV compact transmission line was made to assess electrical performance of its design. This provided additional input for specification on maximum electric field levels on the insulated cross-arms.
Finding Suppliers at Right Price
Having good connections with other TSOs and being a member of Cigré helped Elia engineers to share information about potential suppliers. A first contact with different suppliers took place at the Cigré session in Paris (2008) as well as during the INMR WORLD CONGRESS in Crete (2009). Thereafter, an Assessment Matrix (list of questions) was sent to all potential suppliers of insulated cross-arms in the frame of a request for information. The goal was to gather data about their products and ability to meet Elia requirements. Due to the importance of the insulated cross-arms in the construction of new 400 kV lines as well as in uprating existing 150 kV lines, Elia decided to qualify 2 suppliers in order to:
• be sure to have a qualified supplier on time;
• have redundancy in 2 products;
• share all risks (e.g. financial, technical) with 2 suppliers.
The strategy was also to build long-term partnerships with both suppliers based on a 6+2 year frame agreement, signed in 2012. Elia’s goal was to go through a learning curve when it came to this new technology and selection of the 2 suppliers was based on European Tender practices while also following internal Elia rules and other criteria.
Design Review & Testing
Based on lack of internal expertise and in order to best assess the designs offered by the 2 suppliers, Elia contracted an external testing organization in Sweden for different tasks, including:
1. evaluating electrical design of the insulated cross-arms by simulations based on the drawings and characteristics of the materials used;
2. reviewing available design and type test reports;
3. making recommendations for documentation and testing.
The goal of point 1 was to assess the design by electrical field calculations in relation to Elia requirements. This study led to modification of certain pieces before going to the test laboratory, thereby saving time and money. Due to the seeming complexity for newcomers of IEC standards for composite insulators, it was felt that point 2 could be more efficiently performed by experts in the field. When testing such complex systems, a first step is to assess mechanical capacity of all pieces. Therefore, application of load scenarios on the complete system is mandatory but requires correct set-up as well as proper measurement devices.
These tests proved useful since they revealed weaknesses in the original design of certain pieces (e.g. hinges) as well as in various connection parts. After these two assessments, all electrical tests were passed. This process might appear easy at first glance but in fact completing all such testing takes from 1 to 2 years, depending on required adaptations in design and availability of laboratory test space.