Controlled-Impedance Source Control for Grid-Connected Converters in Series-Compensated Photovoltaic Systems

Photovoltaic systems integrated with battery energy storage may exchange power with the utility grid through series-compensated networks. Their grid interfaces must support both rectifying operation for grid-to-storage charging and inverting operation for power export from photovoltaic generation or storage. However, interactions between conventional phase-locked-loop (PLL)-based control and series-compensated networks can cause subsynchronous resonance (SSR), limiting stable operation. This paper proposes a PLL-free controlled-impedance-source strategy that shapes the converter terminal characteristic to support bidirectional power exchange. The frequency-dependent impedance is analyzed to establish parameter-selection guidelines, and the steady-state operating points are derived to support voltage-loop design. Root-locus analysis incorporating current-measurement filtering, sampling, and delay effects evaluates the closed-loop stability under rectifying and inverting conditions. Controller-hardware-in-the-loop experiments compare the proposed strategy with conventional PLL-based control at different short-circuit ratios (SCRs) and series-compensation levels. The proposed controller maintains stable operation under tested conditions in which the PLL-based controller exhibits subsynchronous oscillations. Additional inverting-mode experiments show stable operation at SCR = 2.5 and a loss of stability following a transition to SCR = 2.0, consistent with the predicted stability limitation. These results demonstrate the applicability of the proposed strategy to bidirectional grid interfaces in series-compensated photovoltaic–storage systems within the investigated operating range.