## 5 Conclusions  
DC power quality phenomena are becoming more critical due to the higher presence of DC grids in distribution networks. Nevertheless, detailed knowledge of these DCPQ phenomena is still in its infancy. Actual waveform data measured at operating DC grids are needed to define the PQ phenomena and develop appropriate assessment methods. This paper presents a flexible digitiser system with specific trigger mechanisms based on the DC signals and the AC PQ parameters of the AC network to which the studied DC grid is connected. The digitiser is built with a commercial National Instruments chassis with two high-performance NI PXI 6124 data acquisition boards (16 bits, 4 MS/s) controlled by a computer with an Intel i5-11400 with six cores and 128 Gb RAM. The measurement software was developed in Python and ran in a Linux distribution (Ubuntu 20.04.4 LTS) as the operating system. Three DC signal-based triggers have been developed and tested with synthetical waveforms, namely _DC Reference trigger_, _DC Delta trigger_ and _DC Slope trigger_. The first two are appropriate to capture slow voltage and current variations with different shapes, while the last one has demonstrated excellent capabilities for capturing transients. The developed system can also use triggers based on the AC PQ parameters of the network to which the DC grid is connected. These triggers are fired when the AC PQ parameters fall outside the ranges provided in EN 50160 [21] for PQ in distribution grids. The digitiser has been extensively tested in the laboratory to guarantee synchronisation between the two acquisition boards, accuracy, performance of the triggers and long-term stability. These tests allow it to run for long periods in unattended mode. Finally, the equipment has been installed in a real operating DC grid, and the first waveform captures using the developed triggers were shown. Some of these captures could be traced to the root cause using the data coming from the SCADA of the grid. In any case, the implemented system presents some limitations that are important to take into account when configuring the sampling frequency, measurement windows or number of analysed data, since an extremely demanding performance might lead into an abnormal operation and deficient storage of all detected PQ phenomena. In addition, the selection of very restrictive values when setting the thresholds of different available triggers may cause an indiscriminate capture of ordinary events, which is far from the purpose of this system, focused on gathering valuable information regarding relevant PQ distortion in the monitored grid. For future developments, a significant database of PQ events is required and expected to be obtained at this end of this project. Once a sufficient number of phenomena are collected, a detailed analysis of them will be possible to be carried out. Then, considering the results of this analysis, an eventual proper definition of parameters, and also corresponding limits, might be achieved as a final outcome. Finally, the selection of these parameters and limits are expected to contribute to covering the current lack of reference normative documents in this matter. Figure 10: Triggered slope event in the real micro-grid: complete waveform (**a**) and zoomed (**b**). **Author Contributions:** Conceptualization, M.A.O. and J.J.P.-A. ; methodology, M.A.O., J.J.J.P.-A. and J.J.M. ; software, M.A.O., validation, J.J.P.-A. ; writing--original draft preparation, M.A.O. and J.J.P.-A. ; writing--review and editing, J.J.M. ; supervision, J.J.M. All authors have read and agreed to the published version of the manuscript. **Funding:** This work has received funding from the project 20 NRM03 DC grids (Standardisation of measurements for DC electricity grids) of the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.