# Grid Nodes Selection Strategies for Power Quality Monitoring
## 3 Discussion
### Massiveness
In general, dealing with PQ issues in the HV grid is not a priority because: (1) many phenomena do not propagate through the transformer (including from LV to MV and from MV to HV, i.e., upstream--against the power flow); (2) it is almost impossible to fully mitigate propagation of severe effects (short-circuits, transients) in the TSO grid and avoid their propagation to the DSO grid. Along with PQ mitigation devices installed in the HV grid (synchronous generators, lighting arresters, static synchronous compensators, etc. ), preventive measures must be taken considering all possible risks and hazardous effects, for example, wind park flickers, probability of lighting and insulators' ageing. Contrary to the HV grid, LV grids are more polluted, and have a larger number of nodes which implies that permanent monitoring with fully functional monitors is almost impossible. At the moment, PQ-measuring smart meters and protection devices seems to be promising to cope with this problem of massiveness. However, many authors do not include monitor types classification in proposed strategies, rarely analyze their functionality limitations and cost efficiency. To compare with a casual fixed PQ monitor, smart meters are (can be) less accurate, have a lower sampling frequency, and do not offer many processing options of measured data, but these limitations highly correlate with project cost reduction. A fully functional and accurate fixed monitor in the LV grid could be used only in special cases: for example, for sensitive and critical loads in hospitals, industry, or laboratories, where poor PQ can not only cause damage or cause disconnection, but harmonics and flickers can also distort the output signal which is critical for decision making (e.g., medical images). An alternative option to reduce limitations for non-critical applications is the use of portable monitors. However, it often remains undefined how long temporary measurements should be taken for both--better PQ situation analysis and better ratio of duration to measured nodes quantity. According to EN 50160, most PQ parameters can be assessed only from a weekly array. It is noteworthy that, in the case of a problem, user equipment must additionally be verified for EMC. The experiment must be based on IEC EMC compatibility standards, despite any initial tests at design and manufacturing stage (required for CE marking). The tests usually require complex procedures such as equipment disconnection from grid and artificial line impedance stabilization network. Focus on the MV grid is slightly more common in the papers, and a possible reason could be that the MV grid is between HV and LV through a prism of PQ: (1) EN 50160 is applicable to MV; (2) the majority of industrial motors and renewable power plants (including their power transformers) are directly connected to the MV grid; (3) grid size is in-between HV and LV, thus it is possible to reach at least partial coverage; (4) most severe faults propagate to the LV side, but do not propagate to the HV side; (5) PQ situation is in-between HV and LV, since many emissive loads are on the LV side, harmonics do not propagate through transformer to MV winding, and voltage sags propagation is slight. Practical campaign sizes are summarized in Table 6. In the case of using portable monitors, monitors quantity to measured nodes quantity ratio along with measurement duration are given (if known). As has been mentioned before, in [20,23] the authors concluded that massive monitors installation remains costly and more effective allocation strategies should be applied.