In view of the global rise of Inverter-Based Renewables (IBRs), both subsynchronous and supersynchronous oscillations are emerging as critical threats to grid stability and secure operation. Due to the finite bandwidth of their control loops, IBRs can be represented by an equivalent impedance seen by the network, whose real part, that is, the resistance, can assume positive, zero, or even negative values depending on the frequency range considered. This characteristic can consequently result in the appearance of poorly damped, critically damped or even unstable resonances, with exponential amplification of currents and voltages.

Among the various methodologies available in the literature for assessing the harmonic stability of IBR-rich systems, the impedance-based approach stands out. Once the impedance of the power electronics-based generator and the impedance of the network to which it is connected are known, the Nyquist stability criterion can be applied. When obtaining the IBR impedance model, the main emphasis is placed on the dynamics of the grid-side inverter. However, in two-stage photovoltaic generators, it is also important to account for the impact of the DC/DC converter and its controller. This work, therefore, aims to evaluate the impact of the second conversion stage on the equivalent solar generator impedance.

The developed model consists of an LCL filter, a three-phase inverter, a DC/DC converter and a photovoltaic array, along with their typical control loops. To this end, the Component Connection Method (CCM) is used, which is a systematic methodology for the modular construction of complex systems in state-space representation. Through algebraic manipulations on the obtained state space equations, it is possible to derive the equivalent impedance of the solar generator.

The results include a sensitivity analysis of the impedance profile in relation to the DC side parameters, such as the gains of the DC/DC converter current controller and the photovoltaic array characteristics. Furthermore, this analysis is extended to different values of the capacitor placed between the DC/AC inverter and the DC/DC converter. The obtained results contribute to the ongoing discussions on the harmonic stability issues in modern power systems with high insertion of IBRs. They highlight the frequency ranges most affected by the DC/DC converter parameters and the role of the DC-link capacitor in decoupling the dynamics between the two stages of photovoltaic systems.