With the high penetration of photovoltaic (PV) systems, fault detection scheme is of great importance for the distribution network operation. The low fault current makes the detection of low-irradiance, low-mismatch, and high-impedance faults a challenging task, especially for PV systems with active maximum power point tracking algorithm and equipped with blocking diodes. Energy loss and potential fire hazard are some consequences of an undetected fault within the PV array. This paper presents a DC-side fault detection scheme for PV systems that consists of two stages. Using the amplitude of the normalized super-imposed component of PV array power, the first stage detects a disturbance while the second stage distinguishes a fault condition from partial shading using a wave-shape based feature. The proposed scheme can properly detect string-to-string, string-to-ground, and open-circuit faults under different fault conditions without the need for the prior information about the PV array and the training data set and is effective for both grid-connected and islanded PV systems. The reliable performance of the proposed fault detection scheme is assessed on the simulation models of two PV arrays.