# Grid Nodes Selection Strategies for Power Quality Monitoring
## 4 Problems and Future Prospects
### Sophisticated Attention to Every PQ Event
To conclude, in this paragraph two issues directly influencing monitors allocation have been mentioned; however, there are more indirectly related problems related to efficient monitoring (neither mentioned nor solved in EN 50160) which requires unification of approaches and legal definition: time synchronization errors, voltage sag indexes (IEEE Std 1564 research field), damage to equipment assessment, voltage sag hysteresis (according to the definition in IEC 61000-4-30; do not confuse with the ferromagnetic hysteresis loop of a transformer core), assessment of voltage sag burst (voltage sags row with short pause which threshold time must be defined and can be caused by either external triggers (e.g., tree branches) or various commutations, a typical example being the automatic circuit recloser operation). In Figure 5, two simulation examples of faults in the Lithuanian DSO grid are given: in Figure 5a--phase-to-ground voltage of single phase-to-ground fault in 10 kV grid (the most frequent type of fault), in Figure 5b--phase-to-ground voltage of two-phase fault in 35 kV. The nature of the process is influenced by many factors, one of them (important)--neutral mode: the Lithuanian MV grid operates in either isolated or compensated neutral mode. The grid scheme and a more detailed description of scenarios are given in Appendix A. Despite being the most frequent, neither these faults nor phase-to-ground voltage are included in EN 50160, since they almost have no influence on phase-to-phase voltage. The standard encompasses only symmetrical phase-to-phase voltage sag which is the rarest fault. Phase-to-phase supply assessment is important for three-phase loads; however, single-phase loads are influenced by phase-to-ground voltage. Moreover, since phase-to-ground fault in MV grid does not propagate through transformer (including to the LV grid), the proposed amendments would not be disastrous for the grid operator, and will contribute to a great leap forward in the development of PQ monitors allocation strategies. Noteworthy, in the given examples a significant phase-shift (and phase unbalance) is seen at the beginning moment of short-circuit. This could become a reason for the loss of synchronization between monitor and grid, which obviously means PQ data loss (including fault event). Many synchronization loss flags were found during the Lithuanian PQ campaign. Remote monitoring and higher DRF could help to solve this problem, and the detailed description and focus on communication issues will be provided in the next subchapter. In the future, complex approaches must be applied for PQ situation assessment--it is important to monitor the full list of events because there are strong interconnections among them. It can be clearly seen from the given simulation examples. In Figure 5a, one phase-to-ground power supply interruption triggers voltage swells, zero sequence unbalance, and phase unbalance. It is noteworthy that, despite zero sequence component and phase unbalance being measured and controlled by protective relays, in EN 50160, these PQ events are not seriously regarded as serious. Let us examine the relation between voltage sags (Figure 5) and transients (Figure 6). In Figure 6a, a process of commutation overvoltage in 10 kV line is shown, a higher short-circuit frequency leads to a higher number of voltage sags, and consequently to higher number of commutations. In Figure 6b, two-phase-to-ground short-circuits occur in one end of 330 kV 100 km line, and cause the oscillations in the opposite end of the line (see Appendix A). Figure 5: (**a**) Phase-to-ground voltage in the beginning of 10 kV line where single-phase short-circuit occurs; (**b**) phase-to-ground voltage in the beginning of 35 kV line where two-phase short-circuit occurs (blue and yellow curves of short-circuited phases coincides after the fault). Significant phase-shift and unbalance are seen at the beginning moment of both faults (after 0.05 s). The simulation has been performed with model of chosen region of Lithuanian power system.