## 1 Introduction
1 is observable and current \(I_{12}\) of the line between buses No. 1 and No. 2 is observable, then voltage \(V_{2}\) of bus No. 2 is observable._  
**Lemma 2. **_If voltage \(V_{12}\) through the line between buses No. 1 and No. 2 is observable, then current \(I_{12}\) of the line is observable._  
Therefore, the need for simpler practical rules (principles, lemmas) regarding PQ monitor displacement along with voltage sag observability is obvious. In order to achieve this goal, the physical behavior of voltage sags must be investigated not only in a rational (analytical) way but also in an intuitive way. Occasionally, several investigation techniques can be found in the literature--for example, the voltage sag matrix (only for symmetrical faults) in [15; 37; 38; 40; 47] or the voltage-duration plane in [43; 45]. In this paper, a full propagation path (including both TSO and DSO grids) will be monitored in the test grid--the chosen fragment of the Lithuanian power system which is a part of the post-Soviet BRELL ring. It can be regarded as a novel test system, because the analogous validation (i.e., BRELL-based) has not been found in the literature. This paper will cover all types of short-circuits and analyze them through the prism of both phase-to-phase and phase-to-ground voltages. However, it is known a priori that the implementation of the above-mentioned task will not solve the fundamental economic problem to an adequate degree, i.e., will not sufficiently diminish a quantity of the devices. Therefore, additional criteria must be  
Figure 1: Proposed classification model of the objectives of the related literature. introduced: regardless of many available alternatives, the traditional subject--power supply reliability--is chosen to be the second criterion for the PQ monitor allocation strategy. Much political attention is devoted to reliability, for example, in [48; 49; 50]--the 5 th, 6 th and 7 th CEER Benchmarking Reports--power supply interruptions are grouped into planned (outside the scope of this paper), unplanned, and unplanned excluding exceptional events (such as beforementioned Fukushima earthquake and tsunami). Despite being important, long-established and included into the main PQ standards (EN 50160:2010, IEEE Std 1159-2019, and others), the criterion almost cannot be met in the PQ monitor allocation literature, and is only briefly mentioned in [44; 51]. The interconnection (possibly strong statistical correlation) between various reliability indexes and voltage sags should be expected (hypothesis)--in a similar manner to underground cables: according to the 5 th Benchmarking Report [48] (pp. 39-40), which is also cited by the 7 th [50] (p. 112), the correlation between total SAIDI (planned plus unplanned including exceptional events) and the percentage of underground cables in MV grids is 0.6 (and higher than 0.8 without Austria, Estonia, Finland, Poland, and Spain, i.e., with the data of 13 countries). The SAIDI calculation method along with other common indexes (SAIFI, MAIFI) are defined by IEEE Std 1366-2022 [52], and the duration (and, respectively consequences) after the sag/interruption depends on the settings and latency of protective relays and automation, in particular the undervoltage relay, automatic circuit recloser (ACR) and automatic transfer switch. According to the modern literature [53; 54], power system reliability is classified into two interdependent groups: (1) resource adequacy (a focus on power balance (e.g., peak demand, generation and transmission capacities, and operating reserves), which is outside the scope of this paper), and (2) operational reliability (a focus on the ability to withstand sudden disturbances).