Some additional improvements are still needed to enrich or complete the circuit breaker’s functionalities, extend its performance range and facilitate its interface with grids. Even so, this prototype opens the door to the development of Ultra-Fast Mechatronic Circuit Breakers suitable for any DC transmission voltages.
“We see this innovative breaker as a very cost-effective solution, since it offers many advantages such as the ability to partition a network into sub-networks.” As Grieshaber states: “All these results represent a major step towards realising the Supergrid, which will effectively integrate renewable energy sources and enable power trade and bulk supply across continents.”
Why faults occurring in a DC grid must be cleared as fast as possible.
Meshed DC networks are still in the research and development stage, since only point-to-point or radial multi-terminal DC links are in operation so far. Therefore, several possible design strategies can be studied for HVDC networks, depending on integrating existing DC links with different voltages or not. In the latter option, i.e., when the whole network is operated at a single DC voltage level, one possibility is to design a DC grid in the same way as an AC grid: DC substations would be connected together in a meshed configuration, each DC overhead line or cable being protected by a DC breaker at each end. AC generator units and loads would be seen through AC/DC converters whose technology and design could influence the behaviour of the DC grid.
“The similarities stop there,” explains Lucas Violleau, Circuit Breaker Engineering Specialist at RTE, the French power transmission operator. “AC grids can face faults for hundreds of milliseconds before losing their stability, because a large amount of kinetic energy is stored in generator units with high inertia. In a DC grid the energy reserve is smaller than in AC grids and is only made up from the system capacitance and inductance. When a pole-to-pole fault occurs, the voltage collapses in less than 10 ms. To avoid a complete DC network outage, the fault must be cleared as fast as possible in that period of time. This requires high-performance direct current circuit breakers, especially for meshed DC grids with high short-circuit currents as demonstrated in the TWENTIES project.”
The TWENTIES project: increase the share of renewables
The TWENTIES project was co-funded by the European Commission’s seventh Framework Programme. When launched in 2010, its objective was to leverage real-life, large-scale demonstrations to pinpoint the benefits of and demonstrate a number of critical technologies by 2013. The technologies in question are those aimed at improving the pan-European transmission network (a.k.a. the ‘Supergrid’). At the same time, they will give Europe a capability to include an increasing share of renewables in its energy mix but maintain its level of reliability performance. One of the TWENTIES project demonstrations addressed the technical challenges around the implementation of meshed HVDC networks, which hold the promise of major benefits.
