## 4 Discussion
### PQ Monitor Allocation through the Prism of a Voltage Sags Propagation Mechanism
2 is investigated in Figure 9. However, since the model of generator (swing voltage source) has been selected according to the traditional approach of power-flow studies, it would be desirable but difficult to support this research with practical experiments encompassing various types of generators. Also, it has been found that if a transmission grid fault (Scenarios No. 2 and No. 4) does not propagate upwards, then an occurrence of a voltage transient is highly expected at least in HV lines. The transient can be observed not only in busbars which directly supply the fault node, but also in more distant segments such as the right branch of this paper's test scheme. It would also be beneficial but difficult to support this finding with practical experiments. Moreover, it has been noticed that the same principle is also characteristic to all types of asymmetrical faults. Therefore, considering the above, the following principle, regarding voltage sag propagation upwards, is established for the transmission grid:  
Figure 74: (**a**) Inductive reasoning method; (**b**) Deductive reasoning method. **Principle 5**. : _A sudden occurrence of a high-voltage transient is characteristic of upward propagation of a three-phase fault; and thus, can indicate a nearby fault in either the same or lower level of a high voltage grid._
3. In the case of a downstream propagation from Scenarios Nos. 1-7, the following generalization is established:  
**Lemma 6**. : _If power flows from transformer winding No. 1 to winding No. 2 (and No. 3), and a three-phase voltage sag propagates downstream (from winding No. 1), then the same voltage sag depth is observed at all windings._
4. After the investigation, it has been found that Lemma 6 cannot be applied vice versa (see Scenarios Nos. 5-9). Thus, the additional definition is required:  
**Lemma 7**. : _If power flows from transformer winding No. 1 to winding No. 2 (and No. 3), and a three-phase voltage sag is observed at winding No. 2 (or No. 3), then voltage sag depths at the rest of the windings remain unknown._
5. Firstly, in the case of the only investigated 330/110 kV autotransformer, when a three-phase fault occurs at the tertiary winding (Scenario No. 3), the voltage sag depth at the common winding is 90% (see Table 1). Conversely, when the distance from the tertiary winding (along the same 10 kV line) is increased by at least 0.5 km (modification of Scenario No. 3), no sag is observed at the tertiary windings (see Figure 13). Secondly, after the deeper investigation of the 110/35/10 kV three-winding transformer (M4, M5 and M5c) in the case of Scenarios No. 5 and No. 6, and considering the maximum possible length of MV lines, Lemma 7 is supplemented with the following principle:  
**Principle 8**. : _If power flows from HV/MV/MV transformer winding No. 1 to windings No. 2 and No. 3, a three-phase fault at any point of MV line, directly fed by winding No. 2 (No. 3), propagates upwards to winding No. 3 (No. 2), but does not propagate to winding No. 1._  
Let us continue with asymmetrical faults. More aspects come out in the case of the monitoring and assessment of these faults, due to a shorter propagation path, different but hardly distinguishable patterns (since they depend on many factors) and their rotation (see Tables 4-13). Moreover, PQ observation is limited by technical possibilities, and one of the reasons is that existing voltage transformers were originally designed for energy accounting purposes. For example, most (older) voltage transformers in the Lithuanian MV grid are grounded and measure phase-to-ground voltage, but EN 50160:2010 requires assessment of phase-to-phase voltage of three-wire systems. Next, most older 10 kV voltage transformers do not have a transformer in one of three phases. This phase is grounded in the case of either configuration (star or delta). However, during an asymmetrical fault, Axiom 3 does not apply but the PQ analyzer assumes that the mode is symmetrical, thus the correctness of the voltage estimation procedure cannot be expected.