The pieces for offshore DC grids are almost in place. There are no technical barriers. Erik Koldby, ABB A/S, Bo Westman and Mats Hyttinen, ABB AB, EWEA

The DC breaker is often seen as a barrier to DC grids But DC switches have been in service since the 1970’s! A typical mechanical DC circuit breaker (MRTB) 4000 A, 125 kV Breaking time 40 ms Transfer breakers developed for bipolar LCC Today the solution is standard SF 6 AC circuit breakers with a passive or an aktive parallel LC resonanse circuit Principle for an active resonance mechanical HVDC circuit breaker But the breaking time make these solutions insufficient for DC grids  U DC  Cigré

© ABB Group Slide 3 PowDoc id  Fast:Breaking times of less than 2ms  PowerfulCurrent breaking capability of 16kA  EfficientTransfer losses are less than 0.01%  Modular Easily adapted to actual voltage & current ratings  ReliableProtective current limitation, functional check while in service  ProvenPower electronic design similar to converter technology  DC breakers are no longer a showstopper for large HVDC grids Hybrid DC breaker is well suited for HVDC grids © ABB Group Slide 3 11MR0339

© ABB Group Slide 4 PowDoc id Hybrid DC Breaker Fast breaking within time delay of selective protection  Normal operation: Current flows in low-loss bypass  Proactive control: Load commutation switch transfer current into Main Breaker switch, the Ultra Fast Disconnector opens with very low voltage stress  Current limitation: Main Breaker switch commutates fault current into parts of the sectionalized arrester bank  Fault clearance: Main Breaker switch commutates fault current into arrester bank © ABB Group Slide 4 11MR0339

© ABB Group Slide 5 PowDoc id A possible development: First Regional HVDC grids Can be built today with proven technology  A typical regional DC grid is defined as a system that constitutes of one protection zone for DC earth faults.  To temporarily and rarely lose the whole HVDC system has a limited impact on the overall power system.  Fast restart of the faultless part of the system  HVDC breakers are not needed  Normally radial or star network configurations  Limited power rating Regional DC grid with optimized voltage level. © ABB Group Slide 5 11MR0339

© ABB Group Slide 6 PowDoc id Line fault handling in regional DC grid DC earth faults vs other faults  DC earth faults  All AC breakers (X) open  DC switches (-) open and isolate the faulty part  Start-up of none-faulty part  Faults in controls, auxiliaries etc cause a trip of the converter:  The rest of the system continues without interruption  In point-to-point transmission, regional HVDC grids and interregional HVDC grids One protection zone for DC earth faults = ~ = ~ = ~ P1P1 P2P2 x x x = ~ = ~ = ~ P1P1 P2P2 x x x Several protection zones for faults in controls, auxiliaries © ABB Group Slide 6 11MR0339

© ABB Group Slide 7 PowDoc id Later step: An interregional DC grid  An interregional HVDC grid is defined as a system that needs several protection zones for DC earth faults.  Developments focus:  HVDC breakers and fast protections  Grid Power flow control/Primary control: automatic control  Master control: start/stop, re- dispatching  On-going Cigré WG B4.52 ”HVDC Grid Feasibility study” © ABB Group Slide 7 11MR0339

Example: 420 kV circuit breakers 1960s: Air blast 1970s and 1980s: Minimum oil 1980s and 1990s: SF 6 You know there will be development – you just do not know how and when – but behave like they did in the 50’es: Act on what is available. Europe needs the new grids! The lack of a DC Circuit breaker is no longer a valid excuse What will the future bring? Inspiration: Look back on AC circuit breakers

© ABB Group Slide 9 PowDoc id © ABB Group Slide 9 11MR0339