## 4 Discussion
### PQ Monitor Allocation through the Prism of a Voltage Sags Propagation Mechanism
Such an example of an asymmetrical fault is given in Figure 75 (from the results of the PQ measurement campaign in the Lithuanian DSO grid). A sudden increase in two measured currents, followed by the disconnection of the line, is seen in Figure 75a. Since the short-circuit is asymmetrical, estimation of phase-to-phase voltages, given in Figure 75b, is incorrect. Let us begin with the two-phase-to-ground short-circuit. The following conclusions are derived from Section 3.1.2:  
1. Principle 5 applies. 2. Similarly (but not analogously) to Principle 5, the interconnection with voltage unbalance has been found in the case of upstream propagation of an MV grid fault (Scenarios Nos. 5-7). The difference in upstream propagation is that the unbalance weakens, while the transient does not. Since voltage unbalance is usually caused by loads, the application of this relationship can be expected only after full interoperability between PQ and others Smart Grid's applications which measure the consumption and the unbalance--in particular, meter data management (or an advanced metering infrastructure) and substation relay protection and automation. In the case of Scenario No. 6, this principle is explained in Figure 32, and is also characteristic of a two-phase fault. In general, this unbalance (to be exact, only the negative sequence) should be higher than 2% (e.g., in Scenario No. 6, the negative sequence unbalance of both phase-to-ground and phase-to-phase voltages of M4 is 2.3%); however, more in-depth research on the topic is required. Currently, this principle is established as follows:  
**Principle 9**. : _If power flows from either an HV/MV or MV/MV transformer winding No. 1 to winding No. 2 (and No. 3), a sudden occurrence of voltage unbalance at winding No. 1 is characteristic of upward propagation of both two-phase-to-ground and two-phase faults; and thus, can indicate the fault at any point of the MV line connected to winding No. 2 (or No. 3)._
3. In the case of Scenario No. 4, when a two-phase-to-ground fault occurs in the HV grid, the voltage unbalance is observed at all windings of the 330/110 kV autotransformer (see Table 6 and Figure 31). The distance between the fault node and the autotransformer is 55 km. Since the HV grid's lines are considerably longer, more in-depth research is needed in order to establish a generalization for upward propagation of two-phase-to-ground faults in an HV grid (similar to the previous case), including separate examination of differences between a transformer and autotransformer. Figure 75: Asymmetrical fault in the Lithuanian 10 kV grid. (**a**) Available measurements of current; (**b**) Estimated phase-to-phase voltage: since only two voltages were measured and Axiom 3 does not apply, the estimation of voltage is incorrect. In the downstream propagation path, rotation is often (but not always) observed in interruption, sag, swell, and RVC events. This rotation is not random (chaotic), which means there is a possibility of feature extraction. The example of voltage pattern dependence on winding configuration is given in Figure 32. However, more in-depth research on the topic is required in anticipation of AI algorithms. 5. In the case of downstream propagation, Lemma 4 applies partially, due to observed conversions from interruption to sag (e.g., the phase-to-ground voltage of both phase A and phase B in Scenarios No. 6 and No. 7). 6. More observations are needed for a generalization of phase-to-ground voltage swell behavior caused by a two-phase-to-ground fault in an HV grid. Meanwhile, in the case of an MV grid (Scenarios Nos. 5-7), it is seen that voltage swells do not propagate through transformers in both directions. Moreover, in the case of an LV grid (Scenarios No. 8 and No. 9), voltage swells do not propagate upwards through the 10/0.4 kV transformer. These principles are also characteristic to a single-phase fault (see Table 12).