HVDC > HVDC Classic > Why HVDC? > Asynchronous connection
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.
For smaller asynchronous interconnections
HVDC Light is the proper choice.