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电平VSC多端高压直流输电模型和适当的创新控制概念，以对交直流电网的稳定性做出重大贡献。在德国Energiewende的背景下，互联系统正在向混合和多模式能源系统转变。可再生能源的高普及率、常规发电厂和核电站的关闭、由此导致的惯性不足、新颖的混合和多模式结构以及能量在所有网络级别上的双向流动，都要求整个能源系统的控制策略发生变化。为了确保未来电网的稳定，新型多电平电压源-变流器可以做出有价值的贡献，因为它的控制存在多个自由度。因此，需要对同时对交流电网施加频率下降、对直流电网施加电压下降的加权下垂常数的方法进行研究，以便为下垂常数和加权因子的选择提供适当的方法。因此，将使用粒子群优化算法和细菌觅食算法等优化方法，因为它们有望在大规模系统中获得非常好的结果。在优先计划的测试和集成环境中实施模型的目的是为了与其他贡献者比较和验证结果。由于多电平变流器的拓扑结构允许交直流侧的解耦，因此存储在变流器单元中的能量起着非常重要的作用。交流侧和直流侧之间的功率偏差不会直接影响直流电压，但会影响变流器的能量。因此，将电压下降法改进为能量下降法。为了保证该方法对系统稳定性的适当贡献，必须将优化方法应用于能量下垂方法，一旦深入研究了不同的下垂方法并获得了深入的见解，第二个资助期应将重点放在非线性控制方法上。下垂方法总是分别提供有功功率偏差与频率、电压或能量支持之间的线性特性。但是，随着能源系统转换的不断推进，线性控制方法可能不再适合新的电网拓扑结构。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.