Impedance-Based Stability Analysis in Power Systems with Multiple STATCOMs in Proximity
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Multiple STATCOM units in proximity have been adopted in power transmission systems in order to obtain better voltage regulation and share burdens. Throughout stability assessment in this dissertation, it is shown, for the first time, that STATCOMs could interact with each other in a negative way in the small-signal sense due to their control, causing voltage instability, while loads and transmission lines showed small effects. Since this voltage stability problem is induced by STATCOMs, d-q frame impedance-based stability analysis was used, for the first time, to explore the inherent power system instability problem with presence of STATCOMs as it provides an accurate understanding of the root cause of instability within the STATCOM control system. This dissertation first proposes the impedance model in d-q frame for STATCOMs, including dynamics from synchronization, current and voltage loops and reveals the significant features compared to other types of grid-tied converters that 1) impedance matrix strongly coupled in d and q channel due to nearly zero power factor, 2) different behaviors of impedances at low frequency due to inversed direction of reactive power and 3) coupled small-signal propagation paths on the voltage at point of common coupling from synchronization and ac voltage regulation. Using the proposed impedance model, this dissertation identifies the frequency range of interactions in a viewpoint of d-q frame impedances and pinpointed that the ac voltage regulation was the main reason of instability, masking the effects of PLL in power transmission systems. Due to the high impedance of STATCOMs compared to that of transmission lines around the frequency range of interactions, STATCOMs were seen to interact with each other through the transmission lines. A scaled-down 2-STATCOM power grid was built to verify the conclusions experimentally.
- Doctoral Dissertations