In D-STATCOM (Distribution Static Synchronous Compensator) systems, the harmonic quality of the injected current is strongly affected by several operational and structural parameters, such as the achievable compensation level, modulation strategy, DC-link energy storage, and the characteristics of the AC-side filters. Among these elements, DC-link capacitors represent one of the most failure-prone components in power electronic converters due to aging effects, partial cell degradation, and protection-device malfunctions within capacitor banks. Such degradations modify the effective DC-link capacitance, leading to increased voltage ripple, constrained modulation capability, and a subsequent rise in current harmonic distortion at the point of common coupling (PCC), ultimately deteriorating grid power quality.
To mitigate this issue, this paper introduces a supplementary adaptation scheme integrated as a secondary control layer into the conventional multi-loop control architecture. Instead of modifying the primary current and voltage controllers, the proposed approach dynamically adjusts the reactive power reference, thereby indirectly constraining the current THD within permissible limits. The method operates in steady-state conditions and remains compatible with existing control frameworks. Both simulation results and hardware-in-the-loop validations confirm that the proposed adaptation strategy effectively suppresses harmonic distortion and improves grid power quality, even under significant DC-link capacitance reduction.