The energy transition in Germany has been aiming at increased integration of renewable energy into the electrical system. This has lead to new requirements to expand and adapt the country’s transmission system. At the same time, there is limited public acceptance of new power infrastructure and this has resulted in transmission system operators (TSOs) needing long approval processes for any new overhead line projects. While conventional line designs used for these types of projects have proven highly effective and reliable, they have also had significant visual impact on landscapes due to typical corridor widths and tower dimensions. As a result, corresponding with a worldwide trend, several local initiatives were started to develop and implement more compact overhead line designs. Practical experience up to now has shown that, apart from the technological aspects of design, a proper communications plan, validated by an independent acceptance study, is needed to best meet all the challenges related to public acceptance. 50Hertz Transmission GmbH (50Hertz), one of the four German transmission system operators has initiated such an integrated development approach for a compact overhead line design called compactLine. This edited contribution to INMR by Wilhelm Kiewitt and Marian Wuntke discusses the newly developed design and its characteristics. With this, it has now become possible to accelerate public acceptance of necessary grid expansions and reduce the time needed in the past to obtain such approvals.
Technical Requirements by TSO
In order to be efficiently integrated into the existing transmission system, 50Hertz defined a set of technical requirements that had to be fulfilled by a new compact overhead line design. One of the many pre-conditions in this regard was a 400 kV two-circuit layout consisting of suspension and tension pylons that could replace existing overhead line designs. For example, conductors had to be a quad bundle, i.e. 4 x ACSR 434-AL1/56-ST1A, with overall ampacity of 3600A and minimum required ground clearance of 12.5 m. With regard to overall dimensions, the resulting corridor could not exceed a maximum of 60 m width and 40 m average tower height at span length of up to 420 m. Some of these criteria are met by innovative tower designs already available on the market. However no existing design could satisfactorily cover the combination of all these requirements.
CompactLine Research Project
50Hertz initiated a research project in 2013 whose aim was to develop an innovative overhead line with significantly reduced height and width, leading to reduction in visibility as well as right of way (ROW). All acceptance criteria had to be met with a technically feasible solution that fulfilled these requirements. Such a complex project required diverse expertise and experience and the group of researchers, coordinated by 50Hertz, included SAG (an installation services and research center) as well as RIBE, a component manufacturer. Moreover, research institute FGH e.V. and RWTH Aachen University were included in the consortium, also supported by a German insulator manufacturer and the German Federal Institute for Material Research (BAM). The research project was co-funded by the German Federal Ministry for Economic Affairs and Energy.
The research activities of the consortium were defined in three major working packages: ‘a research phase’ (2014-2016), ‘a construction phase’ (2017-2018) and ‘a monitoring phase (2018-2019). Along with these packages, a sequential development process was set up and serves to guiding project researchers.
As for any development of new technical solutions, definition of objectives and technical requirements were crucial to successful and efficient engineering. System development and component prototyping as well as mechanical and electrical testing made up for the core of these various engineering activities. The steps were then monitored by an independent acceptance study to investigate performance of the newly developed design with regard to requirements for public acceptance. Construction of a pilot installation of the entire overhead line as well as in-depth monitoring during its operation represented the final steps of the development process.
To meet design requirements, the whole overhead line concept had to be re-assessed, such that many new components had to be developed and simulated. Various mechanical tests on these prototypes were successfully performed and passed, followed by electrical tests. Apart from tests of components, full-scale tests were conducted on a 1.2 km outside test area to confirm the mechanical functionality of the conductor bundle system and related insulators. The novelty of the compactLine concept meant that there was no practical experience or simulation software to otherwise confirm these parameters. During mechanical full-scale tests, for example, the focus was on dynamic behaviour of the system as well as on stationary mechanical strength of components. Other important aspects, apart from mechanical and electrical tests, were electromagnetic fields as well as audible noise from operation of the line. Extensive simulations were conducted to investigate these aspects of compactLine to optimize conductor phase geometry so as to ensure the new design met all regulatory requirements.
For the compactLine overhead line concept to be technically feasible, all newly-developed components had to pass related testing and simulations to verify its operability. By early 2017, this goal had been achieved and the components for the new design were finalized.
1. Steel Support Rope & Phase Conductor Bundles
One of the main technical characteristics of the new concept is based on significant reduction of conductor sag, without compromising required minimum ground clearance of 12.5 m, i.e. reduction of sag in spans was key to reduced tower heights. This was achieved through use of two additional steel support ropes in every conductor bundle. While new to overhead lines, this technology is common for cable car installations and bridges, where it has been applied for years. The steel rope used for compactLine is a 26 mm diameter rope consisting of stranded, hot-dip galvanized steel wires and having overall rated tensile strength of 791 kN. These ropes are installed with high tension to attach the phase conductors, like garlands, to it. As a result, sag in each span (i.e. the catenary) is no longer defined by the conductors but rather by the steel ropes with conductors attached.
2. Bundle Spacer
Completely new bundle spacers were developed and installed at approximately every 20 m of the conductor bundle and serve to attach the four conductors to the two steel support ropes. This meant that, while tension in the steel ropes is very high, for the conductors themselves it can be rather low yet still with low sag between spacers. However this conductor configuration came with a series of technical challenges. The compactLine bundle spacer not only needs to ensure the distance between the single conductors but also must carry their weight while allowing as much movement as possible. To realize such maximum movement, different designs were considered. The final version proved a 400 mm x 500 mm squared bundle spacer of cast aluminium.
3. Insulator String Sets
Apart from conductor configuration, insulator string sets and fittings for the tension and the suspension towers also had to be developed. The suspension string set is a rather wide ‘V’ configuration of composite insulators with a 88 mm core diameter and the tension string set is a long and narrow ‘V’ configuration of composite insulators with 63 mm core diameter. One of the benefits of the steel support rope concept is reduced lateral movement of conductors due to wind. To take full advantage of this, a rigid ‘V’ string arrangement for suspension insulators was selected to withstand resulting increased compression and tensile forces. Tension insulator sets have to take the high tensile forces from the steel support ropes and, given relevant load factors, these insulators were designed for up to 1320 kN.
4. Suspension & Tension Towers
Due to the highly-tensioned steel ropes and increased weight, resulting mechanical forces on tension and suspension towers are significantly higher than on conventional structures. To comply with these added forces, a complete re-design of the towers proved necessary. Moreover, an essential additional goal for the new tower design was reduction of dimensions at the bottom. The suspension tower was eventually therefore designed as a single tubular, conical steel pole with ring flange connections. The dead-end tower, by contrast, was designed as portal type, consisting of two tubular, conical steel poles supporting the cross-arm. Both tower designs were based on concepts verified from wind turbine towers. Cross-arms consist of steel profiles connected to the tower top by steel tie rods. Due to incorporation of manholes at bottoms as well as below tower tops, access to cross-arms is ensured without need to bring in cranes or lifts. The resulting standard suspension tower has overall width of 38 m and height of 30 m. The standard tension tower has overall width of 36 m and height of 36 m.
Technical Achievements of New Design
A great deal of engineering and testing was necessary to fulfil requirements for compaction and also ensuring that all components were developed according to current standards. Due to significant reduction of conductor sag, in combination with a reduced lateral movement of conductor bundle as well as single level phase configuration, it was possible to greatly condense tower dimensions. In fact, compactLine tower designs are radically smaller in width and height compared to conventional Danube towers. As a result, ROW could also be reduced.
With reduction in suspension tower height (58m à 30m) and average ROW (77m à 55m), the main objectives of the project were achieved. With the innovative compactLine design, it is now possible to replace existing 220 kV double circuit overhead lines by a 400 kV double circuit line corridor without increasing ROW. Such a compact overhead line design has also reduced environmental and visual impact on landscapes. Correspondingly, it is likely that such reduced overhead line geometry will also positively impact the speed of public acceptance.
Communication Concept & Acceptance Study
In order to understand which factors are most relevant for public acceptance of overhead lines and whether compactLine could offer a positive contribution in this regard, a scientific investigation was necessary. An independent university-based research institute conducted a one-year field study on acceptance of overhead lines in general and on comparison of compactLine to conventional designs. To identify most relevant acceptance factors, researchers started to collect background information by interviewing stakeholders such as technicians, environmental experts and local administrations. The result was a catalogue of factors and parameters that influence acceptance of overhead lines in general. Based on these, workshops were conducted with politicians, media experts and other stakeholders to categorize and specify these factors. A comprehensive questionnaire was developed and used in a field survey with nearly 1000 randomly selected participants in different parts of Germany. It contained different questions and scenarios. One goal was to identify the overhead line parameters with highest relevance for acceptance in general. Another was to specifically compare acceptance of compactLine versus the conventional Danube tower design. Analysis of survey data showed that 40% of all those interviewed had a clear preference for compactLine compared to only 7% preferring the conventional tower design. The rest were indifferent.
Besides this clear preference, another main conclusion from analysing findings was that tower height (i.e. visibility of the overhead line) is the decisive factor in perceived adverse environmental impact and strongly influences public acceptance. Altogether, the study showed that these technical line design developments, with reduced height and right-of-way, represent a favourable design alternative that can contribute to greater public acceptance.
After finalization of technical developments and given results of the acceptance study, the innovative compactLine design was judged to be beneficial and a decision made to verify the concept in practice by means of a pilot installation. The pilot installation is approximately 1.8 km long, comprising of five towers (three suspension and two tension). After completion, it will be a fully operational connection of an existing substation with an existing nearby overhead line located south of Berlin. Construction started in late 2017 and commissioning will be approximately one year later. After commissioning and during operation, an extensive monitoring program will be set up to analyse line behaviour to gain experience for further operation and maintenance.
Analysis of Influence on Environment & Health
Along with construction of the pilot installation and the monitoring program, a further study on influence of overhead lines on the environment will be carried out by another independent consulting firm. To that end, the pilot installation will be compared to two existing conventional overhead line connections (400 kV and 220 kV connections) in terms of impact on public and human health as well as on fauna and flora biodiversity, etc. The goal will be to gain more information on these interdependent issues and results of this study are expected in conjunction with the installation phase.