HVDC necessity, configuration and back-to-back

HVDC transmission converts AC to DC at the sending end, sends it on a DC line or cable, and converts it back to AC at the receiving end.

Necessity of HVDC

• Long-distance bulk power: AC transfer is limited by line reactance and stability; DC has no such limit. Beyond about 500–800 km, HVDC is cheaper.
• Long cables: AC cables draw large charging current and become useless beyond 40–60 km; DC cables have no charging current (submarine links, city feeds).
• Asynchronous interconnection of grids with different frequency or not in step.
• Fast and exact control of power flow, useful for power trading and stability.
• Lower losses and cheaper line (two conductors, smaller towers and ROW).
• No increase in fault level when two systems are connected.
• Integration of remote renewables and offshore wind.

General configuration of an HVDC system

Main parts: converter transformers, converter valves (12-pulse thyristor bridges or VSC), smoothing reactors, AC filters and shunt capacitors, DC filters, DC line/cable, earth electrodes and control system.

 AC1 --[Xfmr]--[Rectifier]--Ld---- DC line ----Ld--[Inverter]--[Xfmr]-- AC2
   |                                                                |
 AC filters                                                    AC filters

Types of links:

1. Monopolar: one conductor (usually negative) with earth or sea return. Cheap; used for submarine cables.
2. Bipolar: two conductors, $+V_d$ and $-V_d$, with the neutral earthed at both ends. Most common; if one pole fails the other works as monopolar with earth return (half power).
3. Homopolar: two or more conductors of the same polarity (usually negative, less corona) with earth return.
4. Back-to-back: both converters in one station, no line.
5. Multi-terminal: three or more stations connected in series or parallel.

 Bipolar:
  [R+]======= +Vd line =======[I+]
   |                            |
  earth                        earth
   |                            |
  [R-]======= -Vd line =======[I-]

HVDC back-to-back system

In a back-to-back system the rectifier and inverter are in the same station and connected by a short DC bus with a smoothing reactor; there is no DC line. It is used to connect two asynchronous grids (e.g. 50 Hz and 60 Hz, or two unsynchronised 50 Hz grids) and to control power exchange between them. Since there is no line, a low DC voltage and high current are used, lowering valve cost. It prevents fault and disturbance transfer between the two grids.