# Grid Nodes Selection Strategies for Power Quality Monitoring
## 5 Conclusions  
The main factor promoting PQ monitoring is legislation supplemented by customers' complaints and energy policy. The main PQ standards in the world are: IEEE Std 1159-2019 which currently provides the best guidelines (and includes some recommendations about monitors' displacement), and EN 50160:2010 which does not provide guidelines but sets limit values for a part of the PQ events list. Nowadays, since traditional power systems are facing vast changes and transforming to Smart Grid, PQ legislation does not correspond to modern needs. Currently, worldwide development of PQ monitoring systems is in the initial stages: in order to create conditions for efficient PQ monitors' allocation, besides the additional technical progress needed, solutions and unified approaches to many fundamental issues are essential. A clear and forward-thinking strategy of further research avenues is essential for progress acceleration: according to our findings, primary directions could be a development of organizational monitoring structure (steps, layers), and PQ physics investigation; secondary--PQ target group determination (limitation) for the specific monitor type considering other relevant layers and factors (e.g., grid voltage, propagation mechanism of disturbance, research aim, load type); tertiary--measurement  
Figure 14: Expected annual thunderstorm duration in municipalities of Lithuania [37]. engineering, AI algorithms, ICT, etc. Results, decisions, agreements should reflect in the standards amendments (as unified, universal approach). To conclude, the main highlights of this state-of-the-art review about PQ nodes selection strategies are summarized:  
* Research can be strictly divided into theoretical and practical, and the final allocation result will depend on the objective and PQ target group. According to the proposed criteria classification, the most common criteria selected in theoretical papers is short-circuits and residual voltage evaluation, and in practical papers they were the monitoring of all selected-type substations or busbars, and customers' complaints. In the future, the allocation strategy must have a layered architecture; currently, authors do not pay much attention to the differences between various monitor types and their features: the PQ target group, applicable voltage level, measurement characteristics (e.g., sampling, frequency response), etc. * In theoretical studies, algorithms are usually tested on small size (up to 123 nodes) test systems--IEEE, CIGRE, and Brazilian. In addition, only one example of a large test was found--the EPRI Ckt5 2998-node test. In practical studies, the proposed strategies are used on larger grids since they do not use any algorithm-based approach, and in general, use up to 600 monitors. In the case of the most massive examples (with 3500 and 12,000 monitors), the authors acknowledged that the installation was costly, and more efficient decisions should have been taken. In the future, monitors quantity optimization will be essential not only for CAPEX and OPEX but also for ICT aspects--reduction in bandwidth and simpler data processing. * Currently, PQ physics is not sufficiently studied, and the remaining open questions inhibit the development of a measurement methodology and prerequisite aspects for allocation strategy. For example, special attention should be paid to the following PQ subjects: propagation, indexes (features extraction), assessment of voltage sag burst, impact on equipment (both short-term and long-term), voltage sag mitigation efficiency of power plants, etc. All PQ phenomena are interdependent, but authors often limit their approach to a specific target group (e.g., to balanced voltage sags) and skip the remainder of the list. It is the only way to start (also considering the previously mentioned small test grid size) in anticipation of a unified approach, a universal complex methodology, and AI utilization; thus, the development is now at the beginning stage.