High voltage direct current (HVDC) transmission has emerged as a vital technology in the current energy landscape as countries look for solutions to integrate large-scale renewables into the grid. Given the ability of HVDC transmission lines to efficiently transmit bulk power across long distances, the technology’s adoption is gaining significance the world over. A crucial component of HVDC systems is the circuit breaker, which plays a pivotal role in ensuring grid stability, reliability and safety.

Circuit breakers are essentially electrical devices to safeguard the transmission system from fault currents and isolate faulty sections. At its core, it operates by halting the flow of electricity after the identification of a fault by protective relays. Unlike conventional alternating current (AC) circuit breakers, HVDC circuit breakers need to address the unique challenges posed by the continuous and bi-directional flow of direct current. The HVDC circuit breakers (DCCB) must be equipped to deal with high voltages and currents efficiently while minimising energy loss. Simultaneously, DCCBs need to ensure rapid fault clearance to avoid system instability. HVDC requires high-speed cut-off in a few milliseconds and there is a huge demand for downsizing and cost reduction of DCCBs as they are critical for reliable operation of meshed HVDC-grids and multi-terminal HVDC links.

Global Transmission Report presents an overview of DCCB technologies, latest advances and ongoing applications/projects.

Existing technologies

Circuit breakers have a diverse range of designs but they all function based on basic principles. When electrical faults occur, potentially causing hazardous spikes in current, these devices detect any excess current flows by sensing the resultant heating or magnetic effects. Upon detecting a fault, the device initiates a switch to interrupt the circuit, momentarily halting the flow of current and thus mitigating the surge.

Mechanical circuit breakers use transducers to respond to excessive currents, triggering switch mechanisms to physically separate the circuits. This generates an electric arc where the current jumps between contacts, necessitating rapid cooling, containment, and extinguishing. Mechanical circuit breakers offer the advantage of minimal conduction loss but sometimes a delay in the triggering of contact separation mechanisms can compromise safety. Also, the formation of an arc may harm certain electrical equipment limiting its lifespan. Further, extinguishing the arc between separating contacts is more challenging in DC systems due to the absence of periodic zero-current intervals as in AC systems, allowing the arc to persist. Typically, electrical arcs in circuit breakers are managed through the use of a chute, vacuum interrupter, SF6, oil, magnetic coil or puffer.
Mechanical circuit breakers have largely been used in AC and low-voltage DC systems, with challenges hindering their adaptation to medium-voltage DC and HVDC systems. DCCBs often incorporate additional circuitry to introduce artificial zero currents into the line to extinguish the arc making them more complex than their AC counterparts.

Solid-state circuit breakers utilise semiconductor-based transistors to interrupt current flow within the semiconductor material itself. Unlike their mechanical counterparts, these switches do not produce arcs and can swiftly block current during electrical faults. This feature makes them suitable for applications where circuit breakers are positioned close to the protected equipment, such as battery packs in electric vehicles. However, solid-state circuit breakers incur high energy losses, with a significant portion of the current dissipated as heat, necessitating separate cooling systems. Currently, scaling this configuration to medium or high voltage levels while maintaining cost-effectiveness and efficiency presents substantial challenges.

Traditionally, mechanical DCCBs have a long operation time, ranging from 30 to 50 milliseconds (ms). Although this has been reduced significantly in recent years, which is in the range of 8–10 ms, these devices are still not useful for fast isolation of fault lines in DC grids especially those based on voltage source converters (VSC). The solid-state DCCBs have a much shorter operation time than the mechanical DCCBs, and can block fault currents in a very short time, which is typically within 1 ms but their on-state loss is too high, making them infeasible in VSC-based DC grids.

Hybrid circuit breaker which combines mechanical and solid-state components emerged as an alternative for DC grids. By integrating mechanical switches with semiconductor-based interrupters, these hybrid solutions achieve higher breaking speeds and greater flexibility in handling various fault conditions. Hybrid circuit breakers offer lower energy losses and enhanced reliability compared to conventional designs. However, they typically lack the same rapid response times which is constrained by the mechanical switch’s speed. The method of commutation may also compromise the hybrid circuit breaker’s overall efficiency.
The inaugural hybrid DCCB, introduced by ABB in 2012, marked a milestone in HVDC technology. The hybrid DCCB developed by the Global Energy Interconnection Research Institute Company Limited (GEIRI) has demonstrated the capability to interrupt fault currents of up to 26.4 kA within 3 ms and has been successfully deployed in the Zhangbei ±500 kV VSC-based DC grid in China.

However, despite the exemplary performance of the typical two-port hybrid DCCB in terms of on-state loss and fault current interruption, its high cost presents a significant barrier to widespread adoption. Recent studies have proposed novel integrated multiport DCCBs to overcome the issues posed by traditional hybrid DCCBs.

Recent Advances in DCCBs

Given the challenges in the use of conventional circuit breaker technologies in HVDC systems, research and advances in new DCCB designs are underway. Variants of hybrid DCCBs have emerged such as insulated gate bipolar transistor (IGBTs)-based circuit breakers and thyristor-based circuit breakers. These breakers can rapidly interrupt fault currents without relying on external triggering signals. This self-commutation capability enhances system reliability, reduces maintenance requirements, and enables fault isolation within microseconds, thereby minimising disruption to power transmission operations. However, these models are associated with a high amount of energy dissipation requiring large energy absorbers which adds to the size and cost of circuit breakers.

The development of DCCBs based on a series LC circuit (or LC DCCB) is another novel circuit breaker design that is currently in the experimental stage. A study by the College of Engineering, University of Alabama, US, aims to enhance the speed of DC fault isolation by leveraging the voltage withstand capabilities of fast disconnectors during contact movement. The novel approach suggests that a disconnector could integrate a series capacitor to convert DC to AC, allowing a conventional AC circuit breaker to interrupt the AC current. The objective is to design a mechanical DC circuit breaker topology that offers both high performance and cost-effectiveness. The LC DCCB consists of two switches (a fast disconnector and an AC circuit breaker), an inductor and a capacitor. The experimental findings from a 900 V laboratory prototype LC DCCB demonstrate the successful clearing of DC faults, achieving a commutation of 130 A and a peak DC current of 190 A. Furthermore, comparisons between LC DCCB, hybrid DCCB, and mechanical DCCB on a 320 kV system, showed certain performance and simplicity advantages. The mechanical LC DCCB operates swiftly due to early capacitor insertion, resulting in reduced peak current and energy dissipation. Notably, experiments and testing on LC DCCB hardware prototypes have shown positive outcomes under the European Union’s (EU) Progress on Meshed Offshore HVDC Transmission Networks (PROMOTioN) project. The development of DCCBs was a crucial part of the project, which aimed to develop interoperable, reliable and cost-effective technology of protection for meshed HVDC offshore grids in the North Sea. One of the four technological pathways under the project focussed on HVDC switchgear including a first-time performance demonstration of existing DCCB prototypes to showcase the technology readiness of this crucial network component. The project report, released in December 2020, notes that at the beginning of PROMOTioN, practical DCCB installations were non-existent, and the technology had solely been validated through theoretical studies, simulations, and small-scale laboratory setups. PROMOTioN conducted comprehensive literature reviews, modelling, analysis, and demonstrations of various HVDC circuit breaker technologies to enhance comprehension of their application, performance qualification, and technological maturity.

Three technologies of industrial HVDC circuit breakers up to 350 kV system voltage interrupting 20 kA of fault current have been publicly demonstrated in an independent industrial high-power laboratory (KEMA Laboratories). These include:

• ABB – Hybrid HVDC circuit breaker – 350 kV, 20 kA, 3 ms
• Mitsubishi Electric – Mechanical HVDC circuit breaker with active current injection – 200 kV, 20 kA, 7 ms
• SCiBreak – VARC HVDC circuit breaker – 80 kV, 12 kA, 2 ms

The demonstrations were the first time that independent verification of the complete fault current interruption process was carried out in a lab-simulated full-power HVDC grid environment.

In a key development, in 2023, Mitsubishi Electric acquired Scibreak, a Sweden-based DCCB manufacturer, to incorporate Scibreak’s technology and know-how into the Mitsubishi Electric Group, to realise the early commercialisation of DCCBs, and strengthen the HVDC system business globally.

Ongoing applications and projects

Several transmission system operators are running pilot projects to test the application of technology. The Network DC Project of UK’s SSE Networks Transmission (SSEN Transmission), which is the transmission business of Scottish and Southern Energy (SSE), obtained a GBP6 million grant from the UK’s energy regulator, Office of Gas and Electricity Markets (Ofgem), in July 2023. The project aims to implement DCCBs in onshore HVDC hubs. DCCBs could help combine HVDC links that join two points in the network and an export cable from a wind farm in one hub, without needing to build additional stations to change the electricity current from DC to AC and back. This will require less infrastructure to deliver net zero, reduce environmental impact and deliver a more flexible and cost-effective network. The project is being carried out in collaboration with the University of Edinburgh, Carbon Trust, National Grid Electricity System Operator (NGESO), SuperGrid Institute, National HVDC Center, and Mott MacDonald, and is scheduled to run from September 2023 to December 2026.

Italy’s transmission system operator (TSO) Terna’s 2023 Development Plan, unveiled in April 2023 with an investment of over EUR21 billion up to 2032, introduced the concept of the Hypergrid which will comprise five new HVDC backbones and multi-terminal DC network (MTDC) that pass through most of the Italian regions. The main objective of the project is to ensure full interoperability and synergy between the HVAC network and the different HVDC projects, providing for different implementation times with the modular approach. This will be possible by resorting to multi-terminal type configurations with the use of DCCBs in the Sardinian, Ionian-Tyrrhenian and Adriatic backbones to optimise the operations by allowing the meshing of the DC network.

Conclusion

In summary, many advancements are underway for the development of innovative DCCB solutions that offer improved performance, enhanced reliability, and greater flexibility in managing fault conditions. As the demand for long-distance power transmission and renewable energy integration continues to grow, the ongoing evolution of HVDC circuit breaker technology will remain essential for ensuring the stability and sustainability of global power grids.