Application to Stevin Project
In order to limit the visual impact of the new 380 kV line, a new tower top design with insulated cross-arms was chosen for the different sections:
1. Gezelle-Stevin section (8.2 km)
This section will re-use about 60% of an existing 150 kV corridor.
2. Eeklo-Van Maerlant section (17.3 km)
A new OHTL will be constructed in parallel to an existing 150 kV line. Use of pylons with insulated cross-arms will limit visual impact and make the new line look similar to the existing line. On this part, 4-bundle 707AAAC-2Z classic conductors will be installed.
In order to maintain the same tower silhouette on the complete sections and to be able to withstand high compression loads, Elia decided to use pivoting Vee for tangent towers as well as non-pivoting Vee designs for small angles < 10 gon.
According to the IEEE Guide, it is important to verify the stability of a section by optimizing the angles of the insulated cross-arms as well as limiting the number of pivoting Vees that follow. This was assessed by KEMA-DNV GL in different studies, the last being performed for the Stevin Project based on a new wind modelthat also applies to the Belgian coast (see paper B2-110 from 2014 Cigré session).
Building & Accessing Insulated Cross-arms
This issue of construction and ease of access was one of the initial questions from Elia’s maintenance teams. They therefore had a representative in the original working group established in 2008 to address all such questions from the field. During design of the insulated cross-arms with suppliers, some attention was paid to the possibility of blocking the pivoting Vee while pulling the conductors. The original idea was found too weak during mechanical testing and evolved instead to a blocking system that used guy insulators.
Development of an access workbench was part of the project as was adapting some towers at Elia’s training centre in order to carry out pre-testing. The possibility to install prototypes of cross-arms in a safe environment without time pressure and to perform tests as well with a prototype workbench was considered a basic requirement. Development of specific work procedures that take account of all possible maintenance needs on such a tower required a lot of meetings and the trained staff relied on 3D drawings to simulate all the possible working scenarios on these complex structures.
For example, having the possibility to access the end of the cross-arm without heavy trucks or cranes was seen as a basic need since not every tower is easily accessed from a road. The workbench was developed using an external supplier and it took much time to arrive at a proper design that was regarded suitable by field crews. Final tests with the workbench prototype took place a few months before construction and required minor adaptation prior to delivery.
Since composite insulators are relatively new to Belgium, it also became Elia’s responsibility to provide contractors with recommendations regarding their proper handling in order to avoid damage during such operations. For this reason, the insulator specifications required delivery of instructions for handling and assembly as part of the order. Suppliers were also asked to provide training to all contractors. The composite insulator handling Guide issued by Cigré is still a reference but is now being reviewed in the framework of WG B2.57. Elia, along with other TSOs, has also sponsored a document that offers clear guidelines for the correct storage and handling of composite insulators.
Inspection & Maintenance
One of the main challenges for Elia insofar as introducing a new technology to the grid related to proper inspection. In fact, this is regarded as one of the disadvantages of composite insulators since the following questions often arise: How best to inspect them? With which frequency? With what devices? Using internal or only specialized external staff?
As a start, Elia asked its suppliers to provide a 10-year guarantee for their composite insulators. Secondly, suppliers were required to deliver inspection guidelines during the so-called ‘maintenance-free’ period as well as recommendations for maintenance measures after expiry of this period. Since some suppliers are not fully familiar with these types of issues, Elia participated in a joint project with other utilities with the goal of generating inspection guidelines and documentation on all damaged composite insulators. This helped provide a basis for field inspection performed by Elia’s maintenance teams and could be complemented by EPRI’s Field Guide: Visual Inspection of Polymer Insulators.
Development of a new tower design within a TSO took many years since many issues had to be addressed while also taking into account internal rules. Nevertheless, Elia wanted to go for innovation while ensuring the same level of reliability for customers and also guaranteeing the safety of staff and contractors. For these reasons project managers chose not to rely only on internal expertise but to look for guidance through business relationships as well as contracted external experts and test laboratories.
For these types of complex tower structures, simulation and testing are both critical to achieving expected mechanical and electrical performance over the long-term. Elia’s philosophy throughout this project was to become a partner to all stakeholders and to try to find a ‘win-win’ scenario for all, at reasonable cost.