Photo Credit: HSP
A. DOUTRELEPOINT
HSP Hochspannungsgeräte GmbH
Germany
High voltage condenser bushings are a key component in AC and DC transmission grids while interconnecting power transformers or shunt reactors to busbars and overhead lines. Insulating high voltage against ground and transmitting the respective currents, bushings must be designed in accordance to the applied electrical field, physical stresses, and impacts such as pollutions, flashovers, seismic events, and others. In the beginning oil-impregnated paper (OIP) bushings were the preferred technology of choice. Nowadays, especially due to increased requirements on EHS (Environmental, Health and Safety), our HSP Hochspannungsgeräte GmbH dry-type epoxy resin-impregnated synthetic (RIS) bushings are the latest development to fulfil these demands.
Characteristics of those dry-type condenser bushings are:
• Explosion-resistance due to silicone rubber shed instead of porcelain housing and the absence of flammable oil
• No utilization of hazardous and environmental polluting materials and components, such as mineral oil or SF6
• Higher currents and overload capabilities
• Suitable for a wide ambient temperature range from -60°C to +55°C
• High seismic resistance
These progressive benefits in the technical design characterize RIS bushings as a key asset of our eco-friendly REGENERA™ portfolio, created by Trench and HSP. Based on the joint commitment to eliminating harmful environmental impacts, we are driving the sustainable change by delivering high-performance, eco-friendly solutions with disruptive benefits for both our customers and the environment.
Different design and manufacturing methods for condenser graded bushings have been applied and available in the electrical power industry for many years. While oil-impregnated paper (OIP) bushings have been well-known and used for decades in transmission and distribution grids, dry-type epoxy resin-impregnated synthetic (RIS) bushings became a valuable alternative for system operators and utilities. OIP bushings are a mature technology which fulfils the technical standards and requirements. However, over their lifetime, OIP bushings may be exposed to several risks, e.g., the potential loss of insulation media, or accelerated ageing of the paper insulation due to overheating. In these and other cases RIS bushings can be adopted to minimize such risks and increase the safety of the respective transformers and reactors.
Nowadays, especially due to increased requirements on EHS (Environmental, Health and Safety), our HSP Hochspannungsgeräte GmbH dry-type epoxy resin impregnated synthetic (RIS) bushings are the latest development to fulfil these requirements.
Principle structure and design of RIS condenser bushings
The main component of a high voltage condenser bushing is the so-called active part, or the condenser core, which, in the case of RIS, consists of a synthetic nonwoven fabric as carrier material impregnated with epoxy resin and concentrically inlayed with aluminium layers forming individual capacitors (see Figure 2).
This condenser core ensures the capacitance grading of the high voltage condenser bushing.
The core is housed in a composite insulator covered with silicone rubber sheds as shown in Figure 1. The gap between the composite insulator and the condenser core is filled with a dry-type nitrogen-based foam. As a result, the bushing is completely dry and free of any hazardous fluids and gases, such as mineral oil, SF6, CO2, etc.
Life cycle Assessments (LCAs) have demonstrated a better environmental performance of RIS condenser bushings in comparison with other established technologies, like OIP or resin impregnated paper (RIP) bushings.
Figure 1: Bushing main components.
Figure 2: Cross section of RIS condenser core
Electrical functionality and material properties of the RIS condenser core
In operation, the capacitance graded condenser bushing is capable to distribute the electrical field consistent as illustrated in figure 3. Therefore, the local electrical field stress is reduced and stretched over the entire length of the bushing.
Figure 3: Illustration of equipotential lines
One of the main material advantages of the RIS condenser cores manufactured at HSP is the non-hydroscopic performance. This ensures that no moisture or humidity will enter the condenser core and harm the electrical performance under high voltage, as opposed to paper (used for RIP bushings), which can accumulate humidity easily while exposed to the ambient environment. This aspect is very sensitive in the manufacturing process, but also during storage, installation, and commissioning of the respective bushings. Figure 4 shows the humidity absorption after storage in water, the results after storage at 23°C und 50% rel. humidity are for paper: 6,24% and for synthetic materials: 0,23 - 0,32%.
Figure 4: Humidity absorption behaviour after storage in water
Our RIS solution for HVDC grids
HVDC grids and inter-connections need specific equipment, such as shunt reactors and power converter transformers. Condenser core bushings are used for a safe and reliable connection of this equipment to the grid.
Especially in this kind of applications, the DC electrical stress leads to a resistive grading, in addition to the capacitive grading of AC electrical stress.Therefore, design and testing of HVDC condenser bushings is elaborated and defined in specific international standards.
As an example, to demonstrate the capabilities of our sophisticated RIS technology for HVDC applications, we developed and extensively tested a 545kV HVDC converter transformer bushing. To verify the design and testing, the individual test sequences have been performed in parallel with both technologies, RIS and RIP. All applicable type and design tests were finished successfully in 2022. Figure 5 shows the test set-up of the even wetting DC voltage test.
Figure 5: Type test of the 500kV RIS-HVDC bushing – Even wetting DC voltage test
Lifetime cycle assessment
The impact of products and services on the environment is becoming more important for TSOs and public utilities, and therefore the use of eco-friendly components and equipment for a more sustainable power transmission and distribution also gains importance. A well-established method to calculate the environmental impact of individual products is the so-called life cycle assessment (LCA). This assessment is used to calculate, among else, the carbon footprint of a respective product from the initial sourcing of raw materials to manufacturing, delivery, installation, commissioning, operation, deinstallation and disposal (Gradle to Grave approach). Figure 6 showcases the comparison of carbon footprint between RIP and RIS condenser bushings under the following base line scenario: Total lifetime of 40 years and a realistic anticipated temperature based on average load of 70% as in-service operation conditions. The result is that RIS condenser bushings have a 45,8% lower carbon footprint than RIP condenser bushings. Also, the operational losses are significant lower for RIS condenser bushings compared to RIP condenser bushings.
