## 4 Discussion  
### PQ Monitor Allocation through the Prism of a Voltage Sags Propagation Mechanism  
As has been mentioned in Section 1, the development of efficient and cost-effective PQ monitoring systems is inhibited by a lack of literature on the topic of the voltage sags propagation mechanism. Currently, only two lemmas regarding voltage and current monitoring are known (see Section 1). Since these lemmas are derived from Ohm's law, their application requires an input of grid impedances, which indicates low practical significance [5]. Therefore, let us summarize the results of Section 3.1, and derive the new rules (principles) which would be easier to apply in practice for PQ monitor displacement in order to optimize monitoring of both symmetrical and asymmetrical faults. This is a method of inductive reasoning--when a general principle is derived by making a generalization based on a specific observation (Figure 74a). Since inductive reasoning is based on empirical research, the correctness of a conclusion is probable but without absolute certainty (and additional evidence may be required). The method depends on the quality of observation. The opposite is the deductive reasoning method (Figure 74b): since it is based on logic, a deduction's conclusions are always bulletproof as long as the premises are true (and assuming that mistakes in the logic chain are avoided). However, deductive reasoning cannot be applied in the case of PQ data, because PQ is a newly emerging (scientific) field [5], hence the theory is not formed. Figure 73: (**a**) Probabilistic surface of power transformers, whose primary winding voltage is either 110 kV or 35 kV, faults through a prism of exploitation age for the first 10 years; (**b**) Probabilistic surface of sleeves faults through the prism of exploitation age. Due to it having the highest level of penetration, the three-phase short-circuit is the simplest case to monitor. Moreover, it is the simplest case to analyze due to its symmetry. \"Which voltage should be measured (assessed)?\"--is a common PQ question arising from both technical and juridical sides. Both voltages are not often monitored simultaneously: for example, see the questions and results of the international survey on PQ monitoring in [68]. Therefore, for the symmetrical case, the following axiom must be established:  
**Axiom 3**. : _In the case of a three-phase fault, both phase-to-phase and phase-to-ground voltage sag depths are equal and independent of neutral mode._  
It is obvious but must be explained that this axiom and all other principles in this subchapter are correct when the essence is not artificially altered, especially with PQ mitigation devices. For example, a single-phase generator is able to distort the original patterns of a three-phase fault. Moreover, it is realistically assumed that the spark of a short-circuit does not damage the transformer and does not physically pass to its other winding(s). Also, the phrase 'at transformer's winding' defines not only the winding but also its near vicinity--let us say up to 100-200 m, but the estimation of a more precise threshold could be a further research avenue. These assumptions (conditions) are fully reasonable and correct, because the main focus is made on voltage sag behavior, but not on the efficiency of PQ mitigation devices. In addition, the condition of an absence of voltage unbalance before fault must be met. When these conditions are fulfilled, other conclusions, derived from Section 3.1.1, are as follows:  
1. Three-phase voltage sag propagation strongly correlates with power-flow direction:  
**Lemma 4**. : _If the directions of power-flows are known (particularly after the fault), then the downstream propagation path of a three-phase voltage sag is known._
2. Upward voltage sag propagation dependence on the 330 kV line length in the case of Scenario No.