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

建议的工作计划包括开发新型多级VSC多端HVDC模型和适当的创新控制概念，以便为AC和DC电网的稳定性做出重大贡献。在德国“能源转型”的背景下，互联系统正在向混合和多模式能源系统转变。可再生能源的高度渗透，传统和核电站的关闭，因此导致缺乏惯性，因此，新型的混合和多模式结构以及跨所有网络级别的能量双向流动必须改变整个能量系统的控制策略。为了确保未来电网的稳定性，新型的多级电压源-变频器由于其控制中存在多个自由度，可以做出有价值的贡献。因此，需要研究将频率下垂和电压下垂同时应用于交流电网和直流电网的加权下垂常数的方法，以便为下垂常数和加权因子的选择提供适当的方法。因此，将使用粒子群优化和细菌觅食算法等优化方法，因为它们对大规模系统有很好的效果。在优先计划的测试和集成环境中实施模型的目的是与其他贡献者比较和验证结果。由于多电平变换器的拓扑结构允许AC侧和DC侧的解耦，因此存储在变换器的单元中的能量起着非常重要的作用。AC侧和DC侧之间的功率偏差不直接影响DC电压，但影响转换器能量。因此，执行电压下降到能量下降方法的改进。为了保证该方法对系统稳定性的适当贡献，优化方法也必须应用于能量下垂方法。一旦对不同的下垂方法进行深入研究并获得深刻见解，第二个资助期应专注于非线性控制方法。下垂法总是提供有功功率偏差分别与频率、电压或能量支持之间的线性特性。但是，随着能源系统的转型不断推进，线性控制方法可能不再足以适应电网的新拓扑结构。因此，非线性控制方法可以获得更好的稳定效果和更复杂的系统服务。

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.