HVDC

The HVDC research group focuses on projects such as the design of on- and offshore converter stations and platforms as well as the simulation of grid connections of renewable energies into HVAC and HVDC grids (conventional and VSC). Another focus is E-Mobility.

 

Group Description
The traditional electric power system based on 3-phase AC transmission technology in general functions well and with good levels of availability and reliability. However, there are challenges mainly arising from the increasing the penetration of renewables, which increase the level of variability and unpredictability in electric power grid operation, the needs for reserve power for power balancing, and will require more flexible power flow control. 
 High Voltage Direct Current (HVDC), especially the Voltage Source Converter (VSC) HVDC, is a feasible and attractive technology to fulfil these challenges. HVDC link is a mean of transmission of electric power based on power electronics, which has distinct advantages in controllability and flexibility.  The benefits from applying VSC-HVDC in the traditional AC grid include independent control of active and reactive power, very fast control response, black start ability and so on.HVDC was first used in power systems to interconnect asynchronous AC systems. The ends of the HVDC link are independently controlled one from each other and this avoids the propagation of perturbations between the two AC grids. Nowadays, the application of HVDC has been largely extended.
 
Long-term Goals
●Connections of large offshore wind farms to the onshore AC grids.It is important to study the control methods for the HVDC convertersso as to achieve both globaland local power system stability, AC grid reinforcement, and so on.
●Hybrid AC-DC network or DC grids. HVDC links are increasingly used inside a same AC network in order to enhance the power transmission capacity and meet the growing demand in terms of power exchange capacity. Design and study of impact of different control methods are critical. In the future, DC micro grids or DC grids are also research interests. 
●Global Energy Interconnection (GEI). According to the concept of the GEI, HVDC plays the key role for building cross-country, and even cross-continent transmission corridors. It is important to make feasible planning of HVDC links with consideration of various factors in a long-term horizon, such as development of technologies, politics, and so on.  
Regarding the connections of large offshore wind farms to the onshore AC grids, we are looking at different technologies of HVDC links, including VSC-based HVDC and diode-based HVDC. Different technologies involve different research interests. For VSC-based HVDC, converter topologies, modeling, control schemes of converters are significant research topics. VSC-based HVDC may also provide grid support to the AC grid, such as active/reactive power support, system stability enhancement. Diode-based HVDC could significantly reduce the cost of offshore wind power, but it brings challenges on the issues of overall system efficiency and local system stability.
In the future power system, the increasing development of HVDC transmission lines inside a same AC network will impose serious challenges on power system. Different methods of controlling HVDC converters may have different impact on stability of the system. HVDC system should be able to provide different services according to the grid environments. For instance, in a weak AC grid, HVDC system should be able to compensate reactive power when voltage deviation appeals. For an HVDC embedded AC power system, it is critical to analysis the whole system in terms of system stability, based on which supplementary control is an important research topic.