HVDC > HVDC Classic > Why HVDC? > 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.
For more detailed investigations of the performance of the inner control loops of the converter and its interaction with nearby network is simulated in a full three-phase representation program such as PSCAD/EMTDC.
PSCAD/EMTDC is used for detailed investigations of the performance of a HVDC link.