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 paper, 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 SCRs 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 q-axis should have a fast bandwidth; with a fast PLL, the outer loop q-axis control bandwidth should be reduced. In all cases, the outer loop d-axis should be slowed down to reach the power transfer limit.