Power stations generate alternating current, AC, and the power delivered to the consumers is in the form of AC. Why then is it sometimes more suitable to use direct current, HVDC, for transmitting electric power?
The vast majority of electric power transmissions use three-phase alternating current. The reasons behind a choice of HVDC instead of AC to transmit power in a specific case are often numerous and complex. Each individual transmission project will display its own set of reasons justifying the choice of HVDC, but the most common arguments favouring HVDC are:
- Lower losses
- Long distance water crossing
- Asynchronous interconnections
- Controllability
- Limit short circuit currents
- Environment
- Lower investment cost
In general terms the different reasons for using HVDC can be divided in two main groups, namely:
| 1. HVDC is necessary or desirable from the technical point of view (i.e. controllability). |
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| 2. HVDC results in a lower total investment (including lower losses) and/or is environmentally superior. |
In many cases, projects are justified on a combination of benefits from the two groups. Today the environmental aspects are also becoming more important. HVDC is in that respect favourable in many cases, as the environmental impact is less than with AC. This is due to the fact that an HVDC transmission line is much smaller and needs less space than AC lines for the same power capacity.
The system characteristics of an HVDC link differ a lot from AC transmissions. One of the most important differences is related to the possibility to accurately control the active power transmitted on a HVDC line. This is in contrast to AC lines, where the power flow can not be controlled in the same direct way. The controllability of the HVDC power is often used to improve the operating conditions of the AC networks where the converter stations are located.
Another important property of an HVDC transmission is that it is asynchronous. This allows the interconnection of non-synchronous networks.