In line with two goals of the United Nations, i.e., providing affordable and clean energy as well as combating climate change, various converter-interfaced renewable energy sources (RESs) are being integrated into the power systems. The transfer of renewable power generated by the RESs such as offshore wind farms to remote load centers may require the use of direct current (DC) lines, which are connected to the alternating current (AC) grid via AC-DC converters. In addition to facilitating the reliable connection of RESs to the power grid, high-voltage DC (HVDC) lines may be used for the transcontinental exchange of power to transfer power over long distances.
One of the major challenges in the evolution of AC systems to hybrid AC-DC systems is the control of converters. Each converter station owns various control loops that require proper tuning in their stand-alone mode of operation. Furthermore, control loops of adjacent converters may also impact one another, and as a result, there must be coordination among the control design of converters to guarantee stability and appropriate dynamic response of the entire grid. The control loop interactions among the converters worsen with increasing the size of the system and the number of converters, especially when one converter station is already in operation and re-tuning the converter's controllers is not an option. Another important aspect of future AC-DC power grids is the employment of converters built by multiple vendors, who will take part in the development of converter controllers with unique designs and know-how. These independently designed controllers will form a part of the grid control system. In this scenario, the stability of the entire system is of great importance and needs to be verified due to control loop interactions.
This thesis studies both internal and external control loop interactions in voltage-sourced converters (VSCs) embedded in AC-HVDC systems. This thesis, first, studies the internal control loop interactions, where the control loops within one single converter interact with one another, and develops a method to design the individual control loops within a VSC such that the converter stability is ensured. A metric is proposed to measure interaction levels, and the impact of interactions on set-point tracking capability is also investigated.
This thesis, next, considers the connections among various converters either from the AC side or the DC side and studies the external control loop interactions among the adjacent converters. Regarding the external control loop interactions caused by DC side connections, suitable system models are introduced to enable individual control design for the converters in a multi-terminal DC (MTDC)-HVDC grid. As for the AC side external control loop interactions, two scenarios are considered: 1) the converters are in the grid-following (GFL) mode of operation, and 2) the converters are in the grid-forming (GFM) mode of operation. Regarding the GFL mode of operation, the impact of control modes on the interactions is studied, and the control modes causing the highest interaction levels are identified. A novel control design framework is designed to relate the control design of each converter to the interconnected system stability. The multi-vendor issue then is considered, and the interactions are mitigated by designing individual robust controllers or by employing interaction filters. The interaction analyses are then extended to the parallel connection of GFM converters and hybrid connections of GFL and GFM converters. Stability and coupling analyses are performed among GFL and GFR converters. small-signal stability of parallel GFM converters is proved, and real-time simulations and hardware-in-the-loop-test are performed for validating the studies
