## 1 Introduction
On the contrary, in [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33], investigations of methods for voltage sag source location are based on simulations in the chosen test schemes (IEEE, Brazilian, others), and sometimes the results are validated with additional tests: laboratory testing in [27; 28; 29; 32], practical measurements with one PQ monitor in [29; 30], practical measurements with two monitors (installed on both windings of a 110/20 kV transformer) in [31], practical measurements of currents with protective relays in the Slovenian 20 kV grid in [32], practical measurements with six monitors in the East China 220 kV and 10 kV grids (54 sags were recorded from January 2019 to August 2020) in [33]. The method can be based on either (1) single monitor or (2) multiple monitors [15; 16]. The output of all works [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33] is binary, because the main purpose of source location is to determine \"on which side of a monitoring device the sag originates [27]\", i.e., upstream or downstream, while the main purpose of PQ monitoring is a full record of the event. Despite [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33] not being focused on PQ monitor allocation strategy and the information provided clearly not being sufficient for this task, these references should be cited because they at least slightly investigate a small part of the propagation path or voltage sag features. In general, the related literature can be classified regarding its scope into: (1) identification of the source location, (2) development of an optimal allocation strategy, and (3) the combination of both--the allocation strategy with a focus on voltage sags (see the Venn diagram of Figure 1). State-of-the-art reviews of grid node selection for PQ monitoring are given in [34] (2013) and [5] (2023). In general, the selection depends on the objective, thus the criteria can be very different (or not applied at all, for example, in the case of a pilot project or random/blind displacement). The works can be strictly classified into theoretical and practical [5]. This paper belongs to the group of practical works; and for its scope, the most important papers to cite are [15; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45], i.e., those which are in the overlapping area of the Venn diagram (Figure 1). The theoretical papers [35; 36; 37; 38; 39; 40; 41; 42] validate their approaches on test systems (IEEE schemes are the most commonly used; the Brazilian, CIGRE, and EPRI schemes are used occasionally), meanwhile in the practical studies [14; 15; 43; 44; 45], PQ monitoring campaigns are immediately begun without posterior self-reflection and criticism. The approaches of practical papers are significantly simpler than those of the theoretical papers; on the other hand, theoretical simulations are very limited and are not well prepared for implementation in practice. On the other hand, alternative approaches can hardly be imagined, thus applied methods remain the only option for the beginning. The limitations of the theoretical validation are described in detail in [5]: insufficient size (and therefore unclear adequacy in the case of real power systems), computational resources, required input (in particular, all node impedances, which can be very difficult to implement in practice), only the symmetrical faults are covered, topological flexibility, geographical compatibility (e.g., the difference between North American and European power systems), etc. Since PQ is a relatively recent research field [5; 13], a deficit of exploitation experience has been noticed, along with the rules for allocation. At the moment, only two lemmas of PQ monitor observability have been used in [36; 46]; however, since they are derived from [PERSON]'s law, the practical significance is diminished by the prerequisite for an entire grid model with impedance values. These lemmas are formulated as follows:  
**Lemma 1. **_If voltage \(V_{1}\) of bus No.