1) the main branch consists of a breaker that is able to sustain arcing. this is usually an ac vacuum interrupter (Vi) with an enhanced driving mechanism to reduce opening time. 2) the current injection branches include precharged capacitors. this topology shows two branches. this design enables two operations in a short time, which may be essential considering the time it may take for capacitors to charge. the second branch is not needed if the required breaker duty cycle consists of only a single opening. Switches Vi3a and Vi3b are similar to Vi1. the natural resonant frequency of the lc circuit can be increased to practical values in the range of 2-3 khz, which would reduce the required sizes of L 1 and C 1 . the resistors R 1 are required for capacitor charging. 3) the energy absorber consists of banks of surge arresters. these arresters are based on standard surge arresters widely used in ac systems for overvoltage protection. 4) the residual breaker, Vi2, interrupts only a small current. this device does not need to have high opening speed. 5) the current-limiting inductor L dc reduces the rate of the fault current rise. the mechanical dc cB presented is also called a current injection dc CB, because it employs precharged capacitors C 1a and C 1b, which substantially increase the speed of each operation. Operating Principles Figure 3 illustrates an interruption of a 15-ka peak dc fault current with a mechanical dc cB on a 400-kV dc system, 18 18 Arcing Time 16 16 14 Idc 12 10 Current (kA) Current (kA) 14 IVI1 8 6 Trip Signal 4 2 ILC 10 8 6 2 VI2 Fully Open IS3 VI3 Begins to Close 4 VI1 Begins to Open VI1 Fully Open 0 VI3 Fully Closed 0 0 5 10 15 Time (ms) (a) 20 25 30 0 5 10 15 Time (ms) (b) 20 25 30 25 30 100 600 VdcCB 80 400 Energy (MJ) Vdc Voltage (kV) 12 200 0 60 ESA 40 Fault Current Suppression Time -200 20 -400 0 5 10 15 Time (ms) (c) 20 25 30 0 0 5 10 15 Time (ms) (d) 20 figure 3. The four graphs show the interruption of a 15-kA peak dc fault current with a mechanical dc CB on a 400-kV dc system, assuming a dc fault at t = 0 s. (a) Contacts require about 8 ms to fully separate. (b) The fault current is fully extinguished after an additional 20 ms. (c) The graph indicates the arrester voltage level. (d) Energy dissipation by the arresters (E SA) is about 83 MJ. may/june 2019 ieee power & energy magazine 87