Analysis of Controller Bandwidth Interactions for Vector-Controlled VSC Connected to Very Weak AC Grids

Stability assessment of conventional vector current control (VCC) of voltage-source converters (VSCs) in weak grids has not been standardized. In this article, a small-signal model is derived to quantify the maximum active power transfer in a very weak grid across a much wider range of controller bandwidths than has previously been investigated. A novel investigation of the VCC-VSC controller bandwidth interactions between inner and outer control loops, including the phase-locked loop (PLL) dynamics, is demonstrated, and a stability bubble of safe operating points is established. Robustness of the stability bubble under different short circuit ratios is investigated, and dynamic performance considerations are introduced to form a reduced operating region with good transient performance. The controller gains within this region allow rated power transfer in inverting mode and good dynamic performance with no modifications to the conventional VCC structure. For very weak grids, it is recommended that PLL bandwidths between 5 and 30 Hz are avoided. If a slow PLL bandwidth is chosen, the outer loop <tex-math notation="LaTeX">q </tex-math> -axis should have a fast bandwidth; with a fast PLL, the outer loop <tex-math notation="LaTeX">q </tex-math> -axis control bandwidth should be reduced. In all cases, the outer loop <tex-math notation="LaTeX">d </tex-math> -axis should be slowed down to reach the power transfer limit.