Development of Novel Models and Control Methods for Multilevel-VSC Multiterminal HVDC-Systems for Improving the Stability of Interconnected AC- and DC-Grids

The suggested work program comprises the development of models of novel multilevel-VSC multiterminal HVDC and the appropriate innovative control concepts in order to give a significant contribution to both the stability of the AC- and DC-grid. Against the background of the German ‘Energiewende’ the interconnected system is transforming to a hybrid and multimodal energy system. The high penetration of renewable energies, the shut-down of conventional and nuclear power plants, the consequently resulting lack of inertia, the novel hybrid and multimodal structure and the bidirectional flow of energy across all network levels must therefore require a change of the control strategy of the entire energy systems.In order to ensure the stability of the future grid the novel multilevel Voltage-Source-Converter can make a valuable contribution due to the existence of many degrees of freedom in its control. Therefore, the approach of the weighted droop-constants, which applies a frequency droop to the AC-grid and a voltage droop to the DC grid at the same time needs investigation in order to provide the proper method for the selection of the droop constants and weighting factors. Hence, optimization approaches as the Particle Swarm Optimization and the Bacterial Foraging Algorithm will be used since they promise very good results for large scale systems. An implementation of the models in the test and integration environment of the Priority Program is intended in order to compare and validate the results with the other contributors. Since the topology of the multilevel converter allows a decoupling of the AC- and DC-side, the energy stored in the cells of the converter plays a very important role. A power deviation between the AC- and DC-side does not directly influence the DC-voltage, but affects the converter energy. Therefore, the advancement of the voltage droop- to an energy droop-method is carried out. In order to guarantee a proper contribution of the method to the system stability, the optimization approaches have to be applied to the energy droop-method as well.Once the different droop-methods are intensively investigated and deep insights could be gained, the second funding period should focus on nonlinear control approaches. Droop-methods always provide a linear characteristic between active power deviations and frequency, voltage or energy support respectively. But, as the transformation of the energy system is continuously advancing, linear control approaches might not sufficiently fit to the novel topology of the grid anymore. Nonlinear control approaches could therefore gain better stability effects and more sophisticated system service.

Principal Investigator:

Prof. Dr. Matthias Luther