Back-to-back DC and its advantages

Back-to-back DC

A back-to-back (B2B) HVDC link is an HVDC system in which the rectifier and the inverter are in the same station, with no DC transmission line between them. The DC link is only a few metres of bus with a smoothing reactor. It is used to connect two AC systems, not to carry power over distance.

 AC grid A          B2B station          AC grid B
 (50 Hz)   +---------------------------+  (50/60 Hz)
 ===|======|[Rectifier]==Ld==[Inverter]|=====|=====
           +---------------------------+
                 DC link (no line)

Because there is no line, the DC voltage is chosen low (tens of kV) and the current high, which makes the valves cheaper.

Advantages

• Asynchronous interconnection: links two grids of different frequency (50 Hz and 60 Hz, e.g. Japan) or same frequency but not in synchronism (e.g. India's regional grids earlier).
• Controlled power exchange: power flow is set exactly by the converter control, so the link can be used for power trading.
• No transfer of faults or disturbances: a fault in one grid does not raise the fault level of the other, and oscillations are not passed across.
• Stability support: fast power modulation can damp low-frequency oscillations.
• Low cost and losses: no DC line, low DC voltage and simple insulation.
• Voltage support: a VSC-type B2B can also supply reactive power to both sides.