HVDC is the method of choice for subsea electrical transmission and the interconnection of asynchronous AC grids as it provides efficient, stable transmission and control capability. The extruded HVDC cable system technology is appropriate when power needs to be efficiently delivered through populated or environmentally sensitive areas, or in coastal and submarine applications. HVDC cable links are essential components of sustainable energy systems to connect energy markets and transmit vast amounts of electricity over very long distances, across or between continents.
ABB recently launched the world´s most powerful underground and submarine power transmission cable system. The 525 kV voltage set the world record for extruded HVDC cable technology, doubling possible power flow up to 2600 MW and extending distance while keeping low losses up to 1500 kilometers to enable greater integration of distant renewables and interconnections. The innovative cable system consists of 525 kV extruded HVDC cables as well as the 525 kV DC prefabricated cable joints and terminations.
You can read more about the HVDC cables here.
This article is focused on the technology challenges- including material and production challenges- that ABB overcame in the development of the joint and termination while establishing new standards in robustness and quality control.
Development of the 525 kV DC prefabricated cable joint:
The new 525 kV prefabricated joint is made of rubber and in principle consists of the same number of layers as the cable and one extra layer to stabilize the insulation system. This layer is based on the non-linear resistive field grading technology patented by ABB. The electrical connections are done utilizing ABB screw technology.
Technology challenge #1: Higher voltages demand a higher quality molding process. The process of molding is a challenge since it requires a number of quality control measures. This challenge is especially applicable for a high voltage joint like the 525 kV joint for controlling the conductivity of the field grading material as well as cleanliness of the insulation material. Stringent quality checks of raw materials and rigorous control of the production process were applied to address this challenge.
Technology challenge #2: Ability to withstand higher voltages to enable transmission of large amount of energy over longer distances with lower losses. The DC properties of different insulation materials can cause instable field distribution in the jointing area when increased DC voltage is applied. To prevent this, ABB has applied its patented non-linear resistive field grading technology in combination with geometrical field grading for the new 525 kV prefabricated HVDC joint. This makes the joint extremely robust under DC voltage load changes.
Technology challenge #3: Speed in mounting, while ensuring reliability. The 525 kV prefabricated joint can be mounted within a few days. Moreover the jointing steps are such that two joints can be mounted at the same time. The cable joints are transferred to the site from the factory in sealed condition. The jointing process on site takes place in specifically built containers which enables process regulation and a high level of quality control.
Development of the 525 kV DC termination:
The 525 kV cable terminations connect to overhead bus bars and can be installed indoors and outdoors.
Technology challenge #1: Safety and robustness of the termination. The technical challenge was to ensure robustness and safety of the termination under strongly varying environmental conditions as well as high DC voltage. The cable termination consists of a polymeric composite insulator which offers maximum safety without any risk of explosions. The termination design has a proven robustness for DC applications. The cable termination is filled with non-flammable dielectric gas. Similar to the prefabricated joint, ABB patented non-linear resistive field grading technology in combination with a geometrical field grading provides the required stability at such high DC voltages.
Qualification by international standards:
The 525 kV prefabricated joint and termination are in line with the qualification process specified by international standards. It also complies with the CIGRE Technical Brochure (TB) no 496, which is the latest document governing the qualification of extruded HVDC cables.
ABB has more than a hundred years of experience in high voltage cable accessories and is a pioneer in the field of HVDC accessories. The 525 kV prefabricated cable joint and cable termination are another example of innovation and of ABB's philosophy of creating ground breaking solutions and offerings for its customers.
