Procedures & Challenges Dimensioning DC Insulators

Insulators

Germany is pursuing the goal of increasing the percentage of total electricity consumption contributed by renewable energy such that by 2050 at least 80% will come from sources such as solar and wind. Moreover, while the proportion of renewable energies will increase, nuclear power plants will be shut down. This ‘energy revolution’ will have great impact on the future infrastructure of high and extra high voltage grids throughout the country. Major solar renewable energy sources are in the south while wind is mainly in the north and both are far from load centers. This will require transmission of energy over great distances. At the same time, energy consumption in Germany is increasing steadily and expansion of the power grid will be indispensable. However, German Transmission System Operators continue to face the challenge of public opposition when it comes to building new lines. Moreover, related licensing processes have become complex, meaning that satisfying all related political requirements is difficult.

As one solution, use of DC links has been proposed to increase transmission capacity as well as to improve dynamic performance of the system. As a first step, existing overhead lines should become hybrid, i.e. equipped with one DC circuit and one AC circuit on the same tower. For the DC circuits, TSO Amprion GmbH has considered the need for new conductors and insulators and also reviewed different considerations to gain knowledge about AC/DC hybrid lines and proper corresponding design of their components.

Insulators in particular require a much closer look since they show significantly different behavior under DC stress than under AC stress. While in the case of AC insulators the main dimensioning parameter is switching impulse under rainy conditions, for DC insulators pollution severity of ambient air defines required creepage and, with that, required insulation distance. In this regard, a combined approach of deterministic and statistical methods was used for insulation coordination and determining minimum required creepage distances.

Fig. 1: Different types of overvoltage.

Data from field experience and laboratory tests as well as artificial and on-site DC pollution measurements were carried out for application of the statistical method using adequate software (e.g. the Insulation Selection Tool).

The following steps were taken as part of this approach:

1. Determining the type of insulator to be considered. Since DC insulators require much higher creepage distances than AC insulators, conventional porcelain insulators were not deemed suitable;

2. Determining required phase-to-ground and phase-to-phase clearances for different kinds of overvoltage in order to prevent flashovers;

3. Determining insulator requirements, including optimum housing profile, insulation length and required unified specific creepage distance (USCD) depending on site pollution severity;

4. Creating insulator drawings based on the above and from simulations with grading rings and different insulator constructions;

5. Producing prototypes according to these drawings;

6. Performing verification type and design tests, according to IEC 61109, for DC insulators and comparing accelerated ageing tests for AC and DC.

Attend the 2022 INMR WORLD CONGRESS in Berlin this October to meet Kübranur Varli who will make a detailed presentation on the methodology and key findings of this program to develop new hybrid overhead lines. She will also review the types of tower and insulator configurations that were studied and which were deemed to best satisfy all requirements.