## 4 Discussion
### PQ Monitor Allocation through the Prism of a Voltage Sags Propagation Mechanism
Therefore, based on our experience gained during the PQ monitoring campaign in the Lithuanian DSO grid, and considering the above, the following axioms are established:  
**Axiom 10**. : _Phase-to-ground voltage swell, caused either by a two-phase-to-ground or single-phase fault in an MV grid, which operates in isolated (compensated) neutral mode, does not propagate through a transformer in both directions--upstream and downstream._  
**Axiom 11**. : _Phase-to-ground voltage swell, caused either by a two-phase-to-ground or single-phase fault in an LV grid, which operates in grounded neutral mode, does not propagate upstream through an MV/LV transformer._
7. Currently, from the juridical point of view, it is not clear (agreed) which assessment method of fault node incurs lighter/heavier legal liability, i.e., which voltage sag depth array is better: (1) one phase-to-phase voltage interruption with two 10-50% depth sags, or (2) two phase-to-ground voltage interruptions with one swell (see Table 7). In our opinion, the phase-to-phase assessment method is lighter. Let us continue with the two-phase short-circuit; the following statements are derived from Section 3.1.3:  
1. Principle 5 applies. 2. Principle 9 applies. 3. In the case of downstream propagation, Lemma 4 applies partially, due to observed conversions from interruption to sag (similar to the previous case). 4. Voltage swell is not characteristic; thus, analysis of the downstream propagation path should be simpler than in the case of two-phase-to-ground fault. Additionally, according to Table 10, fault node patterns almost do not depend on fault location; however, more observations would be beneficial before generalization. 5. Currently, from the juridical point of view, it is not clear (agreed) which assessment method of fault node incurs lighter/heavier legal liability, i.e., which voltage sag depth array is better: (1) one phase-to-phase voltage interruption with two 10-20% depth sags, or (2) one healthy phase-to-ground voltage with two 40-50% depth sags (see Table 10). Lastly, in the case of single-phase short-circuit, the following statements are derived from Section 3.1.4:  
1. Principle 5 applies. 2. Principle 9 does not apply. 3. Lemma 4 does not apply. Axioms 10 and 11 apply (for MV and LV grids). More observations of single-phase fault behavior are needed for generalization in both cases: (1) when the fault occurs in an HV grid (Scenarios No. 1, No. 2, and No. 4), and (2) when the fault occurs at the three-winding transformer's secondary (or tertiary)winding (Scenario No. 5). If the single-phase fault occurs at the autotransformer's tertiary winding (Scenario No. 3), it does not propagate to the HV grid. Considering the above, the following axioms are established for both MV and LV grids (Scenarios Nos. 6-9):  
**Axiom 12**. : _If a single-phase fault occurs in an MV grid, which operates in isolated (compensated) neutral mode, and not at either the secondary or tertiary winding of a three-phase transformer, then it does not propagate through this transformer in both directions--upstream and downstream._  
**Axiom 13**. : _If a single-phase fault occurs in an LV grid, which operates in grounded neutral mode, and not at either the secondary or tertiary winding of a three-phase transformer, then it does not propagate upstream through the MV/LV transformer._  
4. Single-phase faults do not affect phase-to-phase voltage of both MV and LV grids, except a slight impact on M5 and M5c in Scenario No. 5, when the fault occurs at the tertiary winding of the three-winding transformer. Therefore, when the situation in a three-wire system is assessed according to EN 50160:2010, grid operators will avoid responsibility. Despite this, it will be important to monitor phase-to-ground voltage, for example, in order to evaluate the negative impact on insulation caused by voltage swells. Conversely, if a single-phase fault occurs in an HV grid, the impact on phase-to-phase voltage is clearly seen. Considering the above, the following axiom is established:  
**Axiom 14**.