The photovoltaic (PV) DC busbar, as a high-voltage transmission link between PV arrays and inverters, is prone to faults. To address open-circuit (OC) faults, this paper proposes a diagnosis method based on traveling wave reflection analysis using spread spectrum time domain reflectometry (STDR) and sequence time domain reflectometry (SSTDR). The signal-impedance mapping mechanism is first established by analyzing wave transmission and reflection. An equivalent characteristic impedance model for string-type PV modules is then developed, and impedance variations under healthy and faulty conditions are analyzed to enable fault detection and localization via signal eigenvalue shifts (ΔRPτ). Experiments with varying fault distances compare the performance and applicability of STDR and SSTDR signals. Results show that the method effectively diagnoses and locates OC faults. The reflection peak (RPτ) decreases with increasing fault distance (l), and SSTDR signals demonstrate superior robustness and accuracy compared to STDR, making them more suitable for long-distance PV DC busbar fault diagnosis.