## 4 Discussion
### PQ Monitor Allocation through the Prism of a Voltage Sags Propagation Mechanism
: _If a single-phase fault occurs in either an MV or LV grid, which, respectively, operate in isolated (compensated) and grounded neutral modes, and not at either the secondary or tertiary winding of a three-phase transformer, then phase-to-phase voltage is not distorted._  
5. Upward single-phase voltage sag propagation dependence on the 330 kV line length in the case of Scenario No. 2 is investigated in Figure 10 (see Section 3.1.1). Analogously to Figure 9, it would be beneficial but difficult to support this research with practical experiment encompassing various types of generators. 6. From the juridical point of view, as has been mentioned previously, it is obvious that if Axiom 14 applies (for both MV and LV grids), the phase-to-phase voltage assessment method allows grid operators to avoid legal liability. Conversely, in the case of an HV grid, it is not clear (agreed) which assessment method of fault node is lighter/heavier (see Table 13), but in our opinion, it seems to be obvious that the phase-to-phase assessment method is lighter. The full development of voltage sags propagation theory will advance monitoring solutions and allocation strategies to the next level. The choice of PQ monitor allocation strategy depends on the desired aim [5]. Based on Axioms 12 and 13, it can be concluded that the goal to monitor single-phase faults will require the most resources: since voltage swells are harmful for insulation, this event cannot be ignored in anticipation of the application of predictive maintenance. One of the cost-saving options is usage of limited functionality monitors, for example, PQ-measuring smart meters (see [5] for application details in Lithuania) and protective relays (e.g., negative sequence relays based on Principle 9). Despite this, the study answering many questions and laying a theoretical foundation for the voltage sags propagation mechanism, many aspects still remain unanalyzed. For example, the propagation mechanism in ring topology, and pattern dependence on winding configuration have been only slightly analyzed in this paper. Moreover, along with [5], the idea about interconnection among PQ events has been raised for the first time, slightly investigating the cases of voltage sag relationships with interruption, RVC, swell, transient (also see the first part of Section 4.3), and unbalance. It is noteworthy that there is other obvious relationship between voltage sag (as well as the caused transient), harmonics and inter-harmonics: even if the sag is avoided due to perfect operation of the grid's automation, commutation processes will cause short-time harmonics and inter-harmonics. Anotherfurther research avenue can be related to voltage sags caused by an electric motor starting, including focus on self-starting and motor groups--some details on the topic are given in the end of Section 3.1.5. The third research avenue is the incorporation of PQ mitigation devices (see Section 4.2.2), also specific grids--for example, a traction power network (for railways, trams, and trolleybuses), and HVDC lines. Moreover, it would be beneficial to support the results with practical measurements. However, any artificial creation of voltage sags in real systems is impossible; for that reason, statistical and probabilistic analyses (such as in Section 3.2) gain in importance.