Impact of Degraded Photovoltaic Modules on Maximum Power Extraction in Series-Parallel Photovoltaic Arrays

Photovoltaic (PV) arrays operating under maximum power point tracking (MPPT) are commonly analyzed under the assumption that total extractable power can be estimated by summing the individual module maximum power points. In practice, however, electrical mismatch caused by module degradation, aging, or non-uniform operating conditions can invalidate this assumption, even when the inverter correctly tracks the global maximum power point. This paper investigates intrinsic power loss mechanisms arising from the presence of a severely degraded PV module embedded within otherwise healthy series-parallel PV arrays. Using circuitlevel SPICE modeling with explicit representation of cell substrings, bypass diodes, and an MPPT-controlled inverter input, the study systematically analyzes single-string and multi-string configurations operating under inverter-side MPPT control. Correct global MPP tracking is assumed by construction, thereby isolating loss mechanisms that are independent of MPPT failure or local-maximum trapping. Simulation results demonstrate that the actual array-level power loss consistently exceeds intuitive MPP-summation estimates. An analytical bound is further derived, showing that the MPP-summation loss constitutes a strict lower bound on the achievable array-level loss for any heterogeneous PV array operating under MPPT control. In single-string and two-parallelstring configurations, bypass diode activation in the degraded module constitutes the dominant loss mechanism at the global MPP. In contrast, for arrays with three or more parallel strings, bypass diodes remain inactive at the global MPP, yet substantial loss persists due to enforced voltage equilibrium and current redistribution among parallel strings. In all cases examined, intrinsic electrical coupling shifts the global operating point away from the arithmetic sum of individual module MPPs. In addition to steady-state efficiency loss, the study identifies reverse current absorption in weakened strings under low-load and inverter-off conditions as a distinct and operationally relevant phenomenon. This Quadrant IV operation causes degraded strings to absorb power from healthy strings and dissipate it internally, raising important reliability concerns. The results highlight fundamental limitations of MPP-summation-based loss assessment and underscore the necessity of array-aware electrical modeling when evaluating degradation impact in practical PV systems.