EXECUTIVE SUMMARY
Vacuum switchgear has been in extensive use in distribution systems for 30 years for the making and breaking of fault current and the switching of loads of all possible nature. The reliability and performance records of the vacuum switching technology are outstanding in the medium voltage range (up to 52 kV), having led to the domination of vacuum switching technology in distribution systems.
Already in the 1960s, efforts have been made to extend the application of vacuum switching technology to the transmission voltage level. Around 1980, high-voltage vacuum circuit-breakers (HV VCB, high voltage here defined as above 52 kV) were put in service in Japan and by 2010, around 10 000 HV VCB were installed, mainly in industrial applications, but also in utility applications. The reason for preferring vacuum technology in favour of SF6 was mainly the capability to switch very frequently and/or the lower maintenance costs compared to SF6. The voltage level of installed switchgear is (with few exceptions) at present limited to 72 / 84 kV and the technology is almost exclusively metal-enclosed. Reliability studies (on a limited population of circuit breakers) show similar reliability for HV VCB and HV SF6 switchgear of the same rating.In the US, vacuum capacitor bank switches have been used for a few decades up to 242 kV.
Around 2008, intense R&D programs started in China and Europe in order to develop HV VCB. In this case, the main driver is the absence of SF6 gas, recognized as a very strong greenhouse gas. This led to a number of products and applications with voltages up to 145 kV. In China a rapid growth of application in commercial operation is foreseen with at present (2013) some hundreds of HV VCB in service up to a voltage level of 126 kV. In Europe, field tests are being carried out on type-tested devices before the new products come to the market.
All HV VCB products are based on MV VCB interrupter technology. No essentially new technical features were necessary. The main extrapolation is in the geometry of the interrupter that has to be designed in order to deal with the higher voltage rating e.g. by increasing the diameter and the contact gap length. In some cases for voltages above 126 kV, two vacuum gaps in series are applied.Both vacuum and SF6 technology are equally well suited to handle the standardized duties related to fault- and load current switching. Nevertheless, because of the fundamentally different principles of current interruption there exist certain differences which are relevant to application in HV systems. Apart from the medium itself, the main differences identified by the Working Group are:Operational features:
With regard to normal currents up to 2 500 A, there are no significant differences, but above 2 500 A it is challenging to realize in HV vacuum switchgear such normal current ratings. This is due to a number of reasons, because of the heat generation by VCB contact structure and the limited heat transfer capability of the interrupter.
It is easier to check the quality of the interruption medium in the case of SF6 circuit breakers. It is not practical to
monitor the required degree of vacuum in service.
The number of possible switching operations is higher with vacuum than with SF6 due to the higher endurance of the VCB contact system to arcing. This makes vacuum attractive in applications requiring (very) frequent switching operations, such as daily operations.
At a typical 72,5 kV rating, the drive energy of the vacuum circuit breaker may be as low as 20% of that of the equivalent SF6 circuit breaker. The sizes of the vacuum and SF6 devices are comparable.
HV VCB may need more than one interrupter in series above 145 kV. In SF6 technology, single break circuit breakers up to 550 kV have been put in service since 1994 and widely installed in many countries.
There is a large difference in physics between the technologies. For example the arc voltage of VCB is much lower than SF6 CB, several tens of volts against several hundreds of volts. Also, the duration of the arc in fault switching is shorter in vacuum switchgear as the minimum arcing time in HV VCB is typically 5 - 7 ms against 10 - 15 ms for SF6 CB. The consequence of this is that the number of possible switching operations of VCB is generally significantly higher than for SF6 CB.
X-ray emission from HV vacuum circuit breakers up to and including a rated voltage of 145 kV are within the standardized limits of 5 µSv/h under normal operating conditions.
SF6 circuit breakers do not emit X-rays.
The Impact of the Application of Vacuum Switchgear at Transmission Voltages
Introduction
Vacuum switchgear has traditionally been used for medium-voltage applications, typically up to 38 kV. However, advancements in vacuum insulation and interruption technology have enabled its application at transmission voltages (above 72.5 kV). This shift has the potential to revolutionize high-voltage power systems by offering an environmentally friendly, efficient, and reliable alternative to conventional SF₆-based switchgear.
Advantages of Vacuum Switchgear for Transmission Systems
1. Environmental Benefits
- Elimination of SF₆ Gas: Vacuum circuit breakers (VCBs) do not use sulfur hexafluoride (SF₆), a potent greenhouse gas. This reduces the environmental impact and aligns with global efforts to phase out SF₆-based equipment.
- Reduced Carbon Footprint: Manufacturing and maintenance of vacuum switchgear have a lower carbon footprint due to the absence of gas leakage risks and the need for SF₆ recycling.
2. Enhanced Reliability and Performance
- Superior Arc Quenching: Vacuum interrupters achieve rapid arc extinction by using a high dielectric strength vacuum, minimizing energy losses and contact wear.
- Longer Equipment Lifespan: With minimal degradation of vacuum interrupters, these switchgear units require fewer replacements over time, reducing operational costs.
- Higher Operating Cycles: Compared to SF₆ and air-insulated circuit breakers, vacuum circuit breakers support a greater number of switching operations with lower failure rates.
3. Improved Safety and Maintenance Efficiency
- Reduced Fire and Explosion Risks: Unlike oil or SF₆-based breakers, vacuum interrupters do not produce combustible gases, significantly enhancing safety.
- Lower Maintenance Requirements: The sealed-for-life vacuum interrupters eliminate the need for gas refilling, leak detection, and gas purity monitoring, resulting in lower maintenance costs and fewer shutdowns.
4. Compact Design and Space Efficiency
- Smaller Footprint: Vacuum switchgear has a more compact design, reducing space requirements in substations. This makes it ideal for urban and constrained environments.
- Ease of Installation: Modular designs allow for faster installation, integration with smart grid technologies, and adaptability to existing infrastructure.
Challenges of Implementing Vacuum Switchgear at Transmission Voltages
1. Technological Barriers
- High Dielectric Strength Requirements: At transmission voltages, maintaining high dielectric strength in a vacuum requires advanced insulation techniques, including solid insulation enhancements.
- Contact Material Development: The need for durable, high-performance contact materials that can handle large short-circuit currents without excessive wear is critical.
2. Cost Considerations
- Initial Investment Costs: Vacuum switchgear for high-voltage applications requires precision manufacturing, increasing initial capital expenditure.
- Limited Large-Scale Deployment: Since vacuum technology is relatively new for transmission voltages, economies of scale have yet to be fully realized, keeping costs higher than traditional SF₆-based alternatives.
3. Standardization and Regulatory Compliance
- Need for Industry Standards: Global standards for vacuum switchgear at transmission levels are still evolving, requiring rigorous testing and regulatory approvals.
- Grid Compatibility Issues: Integration with existing grid infrastructure, particularly in older substations, may require modifications and upgrades.
Future Prospects and Industry Adoption
The push for SF₆-free technologies, along with increasing regulatory pressures, is accelerating research and deployment of vacuum switchgear at transmission voltages. Some notable developments include:
- Hybrid Systems: Combining vacuum and gas-insulated technologies to leverage the strengths of both.
- Solid-State Vacuum Breakers: Exploring advanced materials and digital control mechanisms to improve switching capabilities.
- Smart Grid Integration: Using digital monitoring and IoT-based diagnostics to enhance performance and reliability.
Conclusion
The application of vacuum switchgear at transmission voltages presents a transformative opportunity for the power industry. While challenges remain, the benefits—ranging from environmental sustainability to enhanced reliability and safety—make it a promising alternative to traditional SF₆-based systems. With continued innovation and regulatory support, vacuum technology is poised to become a cornerstone of future high-voltage power networks.