## 1 Introduction  
The growing of distributed energy sources in the power system and the development of power electronics devices have converted DC grids and DC distribution in a competitive alternative to classical AC power systems. DC grids are being developed at different voltage ranges depending on their application. Efficient energy transport over long distances is performed at high voltage DC (HVDC), modern decentralised renewable generation plants can integrate into the electricity grid at DC medium voltage, and low voltage micro-grids allow a more efficient use of appliances and end-user equipment, which typically operates directly in DC [1]. The use of DC grids presents some advantages over traditional AC grids. Among them, the non-existence of reactive power (and the need to control it), the lack of synchronisation needs between connected systems and the absence of skin effect, which translates into fewer losses, etc., can be enumerated. However, some practical obstacles still need attention from the research community, as power quality (PQ) phenomena [2]. Considering low voltage systems and micro-grids, hybrid AC/DC systems combine advantages of both worlds, facilitating the integration of DC technologies in existing AC systems [3]. Nevertheless, they can also share the problems, mainly related with power quality and those produced by the converters. PQ indices are well documented in international standards and scientific literature for AC grids. However, this is different for DC grids, where only a short collection of papers can be found in the scientific literature. The first papers referring to PQ in DC systems were related to harmonic distortion and ripple evaluation [4; 5]. Another early attempt to characterise the PQ phenomena in a DC grid was made by defining several statistic indicators calculated from waveforms measured in a small DC grid [6]. Other works studied the phenomena found in specific applications such as railway traction [7; 8] and small DC micro-grids [9; 10]. The lack of knowledge in DC PQ guided researchers to follow the steps given by the experience in conventional AC grids. Then, attempts to translate AC PQ indices to DC have been carried out, including voltage fluctuations, voltage dips and interruptions, rapid voltage changes and ripple [11; 12]. The parameters were developed theoretically and their performance was evaluated in a limited laboratory experimental setup. Following the same focus, a review of the applicability of AC PQ standards to DC phenomena was published [13], concluding that most of the existing definitions of AC PQ parameters are sufficiently general to be applied to DC micro-grids. Nevertheless, some parameters in the definitions, thresholds, time windows and limits still need to be clarified. A recent review focused on the problem of defining useful PQ metrics for DC grids [14]. The study covered specific applications such as electric transports (including ships, aircrafts and terrestrial vehicles), distributed generation and micro-grids, modern data centers, etc., discussing the PQ indices from the point of view of their first principles origin and also from existing standards. As a conclusion from the previous revision of the state of the art, it is clear that the definition of PQ indices in DC grids requires real-life data which will allow for determining reference levels, thresholds, sampling rates, integration (time) windows, etc. [14]. These data consist of waveforms obtained in measurement campaigns which should be performed in real working micro-grids. Moreover, the effect of AC PQ on the DC side of the microgrids must also be taken into account [2]. In this paper, we present a system which will be used in a measurement campaign performed in a real hybrid DC/AC grid [15]. This is part of the European metrology project \"20 NRM03 DC grids\" which will contribute to the development of new or improved standards regarding definitions, measurement methods, and compatibility levels of PQ in DC grids.