development of the world's first ultrafast HVDC circuit breaker. Mechanical circuit breakers are too slow for use in HVDC grids, although they have been used for years in other applications. Conversely, semiconductor breakers are fast enough but have a high resistance when conducting, wasting energy and generating heat in normal operation. The ABB breaker combines semiconductor and mechanical breakers to produce a "hybrid breaker" with both a fast break time and a low resistance in normal operation. Costs Generally, providers of HVDC systems, such as GE Vernova, Siemens and ABB, do not specify cost details of particular projects. Such costs are treated as Confidential Business Matters between the supplier and the client. Costs vary widely depending on the specifics of the project (such as power rating, circuit length, overhead vs. cabled route, land costs, site seismology, and AC network improvements required at either terminal). A detailed comparison of DC vs. AC transmission costs may be required in situations where there is no clear technical advantage to DC, and economical reasoning alone drives the selection. However, some practitioners have provided some information: An April 2010 announcement for a 2,000 MW, 64 km (40 mile) line between Spain and France is estimated at €700 million. This includes the cost of a tunnel through the Pyrenees. Conversion process Converter At the heart of an HVDC converter station, the equipment which performs the conversion between AC and DC is referred to as the converter. Almost all HVDC converters are inherently capable of converting from AC to DC (rectification) and from DC to AC (inversion), although in many HVDC systems, the system as a whole is optimized for power flow in only one direction. Irrespective of how the converter itself is designed, the station that is operating (at a given time) with power flow from AC to DC is referred to as the rectifier and the station that is operating with power flow from DC to AC is referred to as the inverter. Early HVDC systems used electromechanical conversion (the Thury system) but all HVDC systems built since the 1940s have used electronic (static) converters. Electronic converters for HVDC are divided into two main categories: Line-commutated converters (LCC) Voltage-sourced converters, or current-source converters. Line-commutated converters Most of the HVDC systems in operation today are based on line-commutated converters. The basic LCC configuration uses a three-phase bridge rectifier or six-pulse bridge, containing six electronic switches, each