# Grid Nodes Selection Strategies for Power Quality Monitoring
## 3 Discussion
### Criteria
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Monitors Allocation Criteria** & **Application in Practice** & **Theoretical Validation** \\ \hline
[15] (2019) & Expected unallowable level of harmonics & No & IEEE 34; \\  & & & CIGRE MV grid \\ \hline
[16] (2019) & Multiple estimation problem: to reach maximal & No & 8 IEEE, 5 Brazilian TSO test systems \\  & faults (voltage sags) differentiation & No & CIGRE 15; IEEE 123; EPRI \\ \hline
[17] (2021) & Multiple estimation problem: to reach maximal & No & CIGRE 15; IEEE 123; EPRI \\  & faults (voltage sags) differentiation & & Ckt5 2998 \\ \hline
[18] (2017) & Faults (voltage sags) observability & No & IEEE 13, IEEE 123 \\ \hline
[19] (2020) & Interaction between emission source coverage, sensitive user coverage, damaged equipment & \\  & ratio, short-circuit capacity, customer’s complaints, line losses, outages, component of & Application in Eastern China & No \\  & “all grid coverage” idea, and component of & & \\  & optimal economic benefit & & \\ \hline
[20] (2018) & Only at MV side at all HV/MV substations of & Monitoring campaign in & \\  & the regional network & Italian grid in 2014 & No \\ \hline
[21] (2009) & TSO grid: frequency of faults, power & Monitoring campaign & \\  & consumption density, modern industrial loads & in Turkish TSO grid & No \\  & & in 2006–2007 & \\ \hline
[22] (2018) & Customers’ complaints & Monitoring campaign & \\  & & in Estonian LV grid & No \\  & & in 2004–2017 & \\ \hline
[23] (2017) & Random selection (or no criteria) to cover the & Monitoring campaign in & \\  & majority of grid: measurements were taken in & Australian HV, MV, and LV & No \\  & & 12,000 sites & grid since 2002 & \\ \hline
[24] (2021) & (1) LCT penetration; (2) EV chargers, LCT & Monitoring campaign in & \\  & connection prospects, PQ issues; (3) similar & British DSO grid & No \\  & network selection for comparison; (4) remote & in 2018–2021 & \\  & monitoring feasibility & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._PQ issues, short-circuits, residual voltage. It is the most common criterion which focuses mainly on voltage sag observability. Until nowadays, almost all papers agree that voltage sags have remained the main reason of industrial outages. Short-circuit and residual voltage are closely related terms since voltage sags can be analyzed through a prism of residual voltage and are mainly triggered by short-circuits. Sometimes, the criterion can encompass focus on another specific PQ event, for example, harmonics. * Faults frequency, power supply reliability. This criterion highly correlates with focus on short-circuits and voltage sags but includes only their severe consequences--supply interruptions which cause outages, and are the reason for poor reliability indexes (SAIDI, SAIFI, MAIFI, etc.). * Renewable energy penetration, EV chargers. Currently, this criterion is chosen for investigation only in modern papers with the beginning of green energy trends and growth of renewable capacity. Solar and wind power plants are large PQ emission sources due to usage of power electronics equipment and wind flickers. * Power losses. In general, this encompasses the classical definition of losses caused by conductor heating when current flows (i.e., conductor resistance). This leads to voltage drop in lines which also correlates with poor PQ situation. The effect of voltage drop is clear in long and highly loaded feeders. The effect of voltage rise is clear in long unloaded feeders (capacitive mode). RMS voltage variation beyond allowed limits (in most cases \(\pm 10\%\) nominal voltage) is harmful for equipment. * Customers' density, quantity; line length. The criterion is oriented to grid regions with a higher number of clients. \"Line length\" is interpreted as a sum of lengths in the region or overall line length connected to the busbar, i.e., the priority is not given to a longer or shorter feeder as it was in the previous case (No. 9). * Industrial regions or loads.