The performance of power systems decreases with the size, the loading and the complexity of the networks. This is related to problems with load flow, power oscillations and voltage quality. Such problems are even deepened by the changing situations resulting from deregulation of the electrical power markets, where contractual power flows do no more follow the initial design criteria of the existing network configuration. Additional problems can arise in case of large system interconnections, especially when the connecting AC links are weak.Thesis By V. Sitnikov W. Breuer, D. Povh, D. Retzmann, E. Teltsch,Siemens Russia Siemens Germany
Dynamic problems can occur which endanger the reliability of the synchronous operation. If power should be transmitted through the interconnected system over longer distances, transmission needs to be supported. This is for example the case in the West-European UCTE system, where the 400 kV voltage level is relatively low for large cross-border and inter-area power exchange. Bottlenecks are already identified, and for an increase of power transfer, advanced solutions need to be applied. Modern Power Electronics provide the necessary features to avoid technical problems in the power systems and they increase the transmission efficiency.
Use of Power Electronics for Transmission
In the second half of the past century, High Voltage DC Transmission (HVDC) has been introduced,offering new dimensions for long distance transmission. This development started with the transmission of power in an order of magnitude of a few hundred MW and was continuously increased to transmission ratings up to 3 - 4 GW over long distances by just one bipolar line. By these developments, HVDC became a mature and reliable technology. Almost 50 GW HVDC transmission capacities have been installed worldwide up to now. Transmission distances over 1,000 to 2,000 km or even more are possible with overhead lines. Transmission power of up to 600 - 800 MW over distances of about 300 km has already been realized using submarine cable, and cable transmission lengths of up to about 1,300 km are in the planning stage.
Source : http://www.ptd.siemens.de/Russia_Power0304.pdf