PD Measurements Related to 525 kV HVDC Corridor Projects

Utility Practice & Experience

Background

The German energy transition to renewables provides for the phase-out of nuclear energy by the end of 2022 and the phase-out of coal before 2038. Onshore and offshore wind farms are located mainly in the north while solar energy is mainly in the south and the increasing share of these renewable but volatile energy supplies will require reinforcing the countries existing 380 kV AC grid.

Main load centers are in the southwest and the sites of the de-commissioned nuclear power plants provide powerful access points to the 380 kV AC grid. The three HVDC corridor projects under construction will therefore improve the balancing of electricity from renewables while ensuring the supply of electricity after the last three nuclear power plants are shut down at the end of 2022.

Originally, it was planned to build the corridor projects as overhead lines and, while there was widespread public approval for the energy transition, protests arose. Therefore, in 2015 the federal government prioritized cables over overhead lines, accepting the higher costs and a complete re-plan of the corridor projects. At the time, only 320 kV HVDC extruded cables were available and with limited service experience, mainly from using them as export cables for offshore wind farms and for a few land projects. The first edition of IEC 62895 High Voltage Direct Current (HVDC) power transmission cables with extruded insulation and their accessories for rated voltages up to 320 kV for land applications – Test methods and requirements was published and arrived just in time for the recently scheduled projects.

Fig. 1: German 525 kV HVDC cable corridor projects (Ultranet: overland line 400 kV HVDC).

In order to reduce the number of cables and especially joints needed for the planned transport capacity by almost half, a nominal voltage of 525 kV would be preferable. But extruded HVDC cables at 525 kV were not yet qualified, and the corresponding standard was limited to 320 kV. Therefore, to get started, the tests of 525 kV cable systems were initially performed based on the CIGRE TB 496 recommendation. In 2018 and 2019, several pre-qualification tests (PQ tests) on HVDC 525 kV extruded cable systems (i.e. cable, outdoor terminations and joints) were successfully passed. These PQ tests did not yet include gas insulated switchgear interfaces since at the time no type test certificates were available for this type of accessory. After intensive discussion, DIN IEC 62895 (including the German Annex) was published in 2019, particularly to extend the scope from 320 kV to 525 kV. Based on these passed PQ tests, at the end of 2019 German TSOs decided for HVDC 525 kV extruded cables for the corridor projects. Awards were largely made in 2020 and current estimates of the total cost of these HVDC cable corridor projects falls between 50 and 70 billion €. In August 2020, the world’s first type test approval for HVDC 525 kV gas insulated switchgear interface was successfully passed.

The commissioning of HVDC cable corridor projects is scheduled for 2025 earliest. SuedLink, the longest cable system will be about 700 km long. Commissioning of SuedLink could be delayed until 2028.

525 kV HVDC Cable System

The German corridor projects cable systems will consist of HVDC extruded cables, joints and outdoor terminations. The cable design is similar to common HVAC cable designs with the key difference being the conductor, which, even for highest cross section, does not require AC loss optimization (see Fig. 2).

Fig. 2: 525 kV HVDC extruded cable design.

Three different polymeric insulating materials will be used in the German corridor projects for the main insulation of the 525 kV HVDC cables:

• Cross-linked Polyethylene, XLPE

• Polypropylene, PP

• XLPE with inorganic filler

Only the filled XLPE is suitable for operation in “classical” HVDC systems with LCC converters, where the load flow direction is changed by the polarity of the DC voltage. With SVC converters, polarity reversal of the voltage is avoided. Unlike long AC cable systems, HVDC cable systems have no need for cross-bonding to minimize screen losses. Accordingly, only straight through joints would be needed (see Fig. 3)

Fig. 3: Types of joints (simplified, not to scale).

Nevertheless, screen interruption joints with external and accessible screen connections will be installed every 5 km to 12 km. The design of (DC) screen interruption joints is similar or identical to AC cross-bonding joints. This allows grounding of the cable screen and, if necessary, access for sheath testing and fault location. It also allows for distributed PD detection.

Inductive PD sensors at the earthing connection of straight through joints with additional earthing provide only very low PD sensitivity.

Due to the exceptional length of the cable systems in the German corridor projects, each of them will consist of sections from different cable manufacturers. For after-installation testing constraints, the length of each cable section will be about 100 km long. The obvious choice to connect two cables is a joint. However, special transition joints would be required to connect HVDC cables from two different manufacturers. Depending on the number of manufacturers, transition joints would have to be developed for all combinations. In addition to the development effort required, this would be a nightmare for spare parts logistics.

For the above reasons, each cable section will be equipped with outdoor cable terminations that can be easily connected to the cable termination of the next cable section, which will also allow separate testing of each cable section. This is necessary not only for technical but also for commercial reasons. The technical facility for connecting two cable sections is called a “cable transition station”. A 525 kV HVDC outdoor cable transition station will be about the size of a small switchyard.

Fig. 4: Outdoor cable transition station.

To minimize visual impact, cable transition stations could alterna­tively be installed underground. However, this would require correspondingly large and elaborate buildings. The most compact solution for connecting HVDC cable systems from different manufacturers would be to use gas insulated switchgear interfaces in back-to-back configuration. With two integrated disconnect switches and a suitable outdoor test bushing installed only during voltage testing, separate testing of each cable section after installation would be feasible and relatively simple. But due to late type approval and a still missing pre-qualification test certificate, this solution is not expected to play a role in the first corridor project.

Plan to attend the 2022 INMR WORLD CONGRESS in Berlin, where cable expert, Professor Ronald Plath of the Technical University of Berlin will make a presentation on the German HVDC corridor projects currently under construction. The focus will be on after-installation testing according to the relevant standards since the German TSOs plan to perform not only DC but also AC voltage testing in combination with on-site partial discharge measurements.