# Grid Nodes Selection Strategies for Power Quality Monitoring
## 4 Problems and Future Prospects
### Sophisticated Attention to Every PQ Event
Hence, it is impossible to extensively monitor transients; currently, only pilot projects can be executed. Secondly, Hickers also have a complex nature, and a separate EMC standard IEC 61000-4-15 [25] is dedicated to them. In the general case, flicker assessment is not integrated in a casual fixed PQ monitor, thus a separate special device is needed. Even EN 50160 admits that the flicker severity index is not objective: the index tries to analytically evaluate flicker impact on humans with the lamp-eye-brain response model, and the last version of the model dates back to 2010 (or earlier). It is noteworthy that the assessment of harmonics and inter-harmonics also have a separate standard IEC 61000-4-7 [26]; however, many questions remain open, since a complex calculation methodology is used for spectrum assessment (in all scientific fields, even if it is simply called FT--Fourier Transform): windowing function, Heisenberg uncertainty issues, window size, spectrum artefacts, data grouping to determine harmonic value, etc. In addition, most voltage transformers have been initially designed for their primary purpose--energy accounting (i.e., for 50 or 60 Hz component), thus they may not be not suitable for wideband frequency measurements due to distortive magnitude and phase frequency response. This problem is described in IEEE Std 1159 [3] (only for the case of magnitude frequency response) and the Great Britain DSO \"Western Power Distribution\" began to investigate it in [24]. The availability of appropriate measurement transducers highly influences allocation selection criteria. Perhaps, relay protection can be considered for some event monitoring and assessment (e.g., voltage harmonics, unbalance, etc.). Moreover, the nature of each harmonic and its phase are different, and currently not examined in detail (including damage to loads); thus, the need for experiments remains. Harmonics are classified into three groups: positive, negative, and zero sequence. In the general case, harmonics do not propagate through transformers; some harmonics interfere and compensate each other, and some can be resonant. In this paragraph, let us continue the discussion about PQ events, and focus on the most critical problem to industry--voltage sag. To begin with, according to EN 50160 definitions, the standard encompasses only symmetrical phase-to-phase voltage sags, however, it is the rarest fault type. The most common short-circuits are asymmetrical: phase-to-ground short-circuit is the most common fault type, but from the legal point of view, it is not compulsory to research and mitigate them. Different types of voltage sags have dissimilar propagation mechanisms (especially through transformer), which highly relates to monitoring coverage parameter often used in allocation strategies. Authors often limit their strategy for particular type: for example, [10] focus only on three-phase faults. The importance of propagation research to monitors allocation strategy can be reasoned from literature review chapter: for example, [18] proposed a new graphical method for propagation investigation--voltage sag matrix (Figure 4). Besides, the problem when the same voltage sag is observed with several monitors has arisen in [20]. Since the situation is not described in EN 50160, various interpretations are possible. Unified legal definitions would be useful for finding an optimal solution between the data duplication level and monitors' density.