Why HVDC?

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:

1. Lower investment cost
But, there may be larger reasons for selecting HVDC than pure investment costs!
A HVDC transmission line costs less than an AC line for the same transmission capacity. However, the terminal stations are more expensive in the HVDC case due to the fact that they must perform the conversion from AC to DC and vice versa. But above a certain distance, the so called "break-even distance", the HVDC alternative will always give the lowest cost.
Typical investment costs for an overhead line transmission with AC and HVDC.
The break-even-distance is much smaller for submarine cables (typically about 50 km) than for an overhead line transmission. The distance depends on several factors (both for lines and cables) and an analysis must be made for each individual case.
The importance of the break-even-distance concept should not be over-stressed, since several other factors, such as controllability, are important in the selection between AC or HVDC.
2. Long distance water crossing
There are no technical limits for the length of a HVDC cable.
In a long AC cable transmission, the reactive power flow due to the large cable capacitance will limit the maximum possible transmission distance. With HVDC there is no such limitation, why, for long cable links, HVDC is the only viable technical alternative.
The 580 kilometer-long NorNed link will be the longest underwater high-voltage cable in the world in 2007 and thereby surpassing the present longest, the Baltic Cable transmission between Sweden and Germany with its 250 km.
3. Lower losses
HVDC transmission losses come out lower than the AC losses in practically all cases.
An optimized HVDC transmission line has lower losses than AC lines for the same power capacity. The losses in the converter stations have of course to be added, but since they are only about 0.6 % of the transmitted power in each station, the total HVDC transmission losses come out lower than the AC losses in practically all cases. HVDC cables also have lower losses than AC cables. The diagram below shows a comparison of the losses for overhead line transmissions of 1200 MW with AC and HVDC.
4. Asynchronous interconnections
Many HVDC links interconnect incompatible AC systems.
Several HVDC links interconnect AC systems that are not running in synchronism with each other. For example the Nordel power system in Scandinavia is not synchronous with the UCTE grid in western continental Europe even though the nominal frequencies are the same. And the power system of eastern USA is not synchronous with that of western USA. The reason for this is that it is sometimes difficult or impossible to connect two AC networks due to stability reasons. In such cases HVDC is the only way to make an exchange of power between the two 
A HVDC link can be a firewall against cascading disturbances networks possible. There are also HVDC links between networks with different nominal frequencies (50 and 60 Hz) in Japan and South America.
5. Controllability
One of the fundamental advantages with HVDC is that it is very easy to control the active power in the link.
In the majority of HVDC projects, the main control is based on a constant power transfer. This property of HVDC has become more important in recent years as the margins in the networks have become smaller and as a result of deregulation in many countries. An HVDC link can never become overloaded!
In many cases the HVDC link can also be used to improve the AC system performance by means of additional control facilities. Normally these controls are activated automatically when certain criteria are fulfilled. Such automatic control functions could be constant frequency control, redistribution of the power flow in the AC network, damping of power swings in the AC networks etc. In many cases such additional control functions can make it possible to increase the safe power transmission capability of AC transmission lines where stability is a limitation.
Today's advanced semi-conductor technology, utilized in both power thyristors and microprocessors for the control system, has created almost unlimited possibilities for the control of the HVDC transmission system. Different software programs are used for different kind of studies.
Normally a positive sequence program for example ABB’s SIMPOW (now transferred to STRI AB) or PTI’s PSS/E program is used for load-flow and stability studies.
6. Limit short circuit currents
An HVDC transmission does not contribute to the short circuit current of the interconnected AC system.
When a high power AC transmission is constructed from a power plant to a major load center, the short circuit current level will increase in the receiving system. High short circuit currents is becoming an increasingly difficult problem of many large cities. They may result in a need to replace existing circuit breakers and other equipment if their rating is too low.
If, however, new generating plants are connected to the load center via a DC link , the situation will be quite different. The reason is that an HVDC transmission does not contribute to the short circuit current of the interconnected AC system.
7. Environment
There are a number of environmental advantages by transmitting power with HVDC or HVDC Light.
Positive effects on the power systems
Many HVDC transmissions have been built to interconnect different power systems by overhead lines or cables. By means of these links the existing generating plants in the networks more effectively so that the building of new power stations can be deferred. This makes economic sense, but it is also good for the environment. There is an obvious environmental benefit by not having to build a new power station, but there are even greater environmental gains in the operation of the interconnected power system by using the available generating plants more efficiently. The greatest environmental benefit is obtained by linking a system, which has much hydro generation to a system with predominantly thermal generation. This has the benefit of saving thermal generation ( predominately at peak demand ) by hydro generation. Also the thermal generation can be run more efficiently at constant output and does not have to follow the load variations. This can be done easily with the hydro generation.
Reduced ROW (right of way) for a DC line.
One bipolar HVDC overhead line can be compared to a double circuit AC line from reliability point of view. Therefore a single HVDC line with two conductor bundles has less environmental impact than a double circuit AC line with six conductor bundles - it requires less space and has less visual impact.
Minimum environmental impact with HVDC Light.
The HVDC Light technology has made it possible to use extruded polymer cables for DC. This has made the use of land cables an interesting alterative over traditional overhead lines in the 50 - 550 MW range for rather long distances ( Gotland HVDC Light, 70 km; Murraylink , 180 km).
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).
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.