The main components used for integration in the power system of distributed renewable energy sources, basically work in dc, f.i. batteries, photovoltaic systems and fuel cells. In addition, most common loads already use dc power internally, usually provided by rectifiers. Consequently, the usage of generated energy can be made more effective if directly delivered to load through a dc grid or microgrids, with improvements in efficiency, costs and reliability. The HVDCs are already widely used for their convenience over long-distance bulk power transmission. Despite this wide usage of the dc power network, the Power Quality definition and assessment in dc is still in an unstandardised situation. This paper focuses on a specific PQ phenomenon typical of dc networks: voltage ripple. Observations are made on the sources of this phenomenon and a methodology is proposed for measuring the intensity of this disturbance by introducing synthetic indices, also with reference to the most recent technical standards, with the specific scope to the compatibility level assessment. The proposed measurement techniques are applied to experimental data acquired on the power supply of a dc railway system.