# Grid Nodes Selection Strategies for Power Quality Monitoring
## 2 Literature Review
### Scientific Papers and Practical Reports
The mentioned selection criteria were: line length and type, number of MV customers, LV customers' density (customers/km\({}^{2}\)), neutral mode. In addition, 200 MV nodes were freely selected by customers (73 installations) and DSOs (127 installations). Measurements were taken according to IEC 61000-4-30. Primary windings of voltage transformers are bonded between phase and ground, thus single phase-to-ground fault or line energization may cause the core saturation of voltage transformers. Moreover, a \"false voltage dip\" term was introduced to describe the core saturation effect on measuring performance, especially on the MV grid operating with isolated neutral. In order to mitigate the effect, measured voltage waveform asymmetry was tracked with a \begin{table}
\begin{tabular}{c c c c} \hline
**Test System** & **Buses** & **Lines** & **Solution** \\ \hline IEEE 14 & 14 & 18 & 4–6 \\ IEEE 30 & 30 & 41 & 12 \\ IEEE 57 & 57 & 80 & 18–19 \\ IEEE 118 & 118 & 186 & 39–45 \\ CEMIG & 48 & 64 & 18–22 \\ \hline \end{tabular}
\end{table} 
Table 1: Results (required number of monitors) when different installation costs are considered. Figure 2: Decision tree with P\({}^{0}\)–P\({}^{6}\) problems: 7-node grid example. digital filter, and a more advanced detection method for the second voltage harmonic was developed. In [9], PQ monitors allocation criteria selection (strategies) before 2013 are briefly summarized. At the EHV and HV level, monitoring of all EHV/HV, EHV/MV, HV/MV substations, connection points of customers, and power stations is considered as a good practice. At the MV level, it is recommended to monitor all EHV/MV and HV/MV, selected MV/LV substations, and connection points of MV customers. At the LV level, random monitoring of LV PCC with fixed and portable monitors (and possibly with smart meters in the future) is considered as good practice. A novel algorithm for optimal PQ monitors allocation in both transmission and distribution systems is presented in [10] (2012). Two parameters are used to obtain an optimal solution--the topological monitor reach area and the coverage control parameter. The monitor reach area evaluation is based on the residual voltage matrix (obtained from the most severe short-circuit simulation) comparison with the coverage control parameter (defined threshold). Both the IEEE 34-node distribution system and IEEE 30-node transmission system are used for validation; however, any examples of application in practice are not presented. The approach is limited: the most severe short-circuit--three-phase short-circuit--occurs rarely (less than 20% of all short-circuit events). Grid operators are also trying to detect single-phase short-circuits (the most frequent event). In addition, the strategy requires a large number of short-circuit simulations (to determine monitor coverage) which would be hardly implemented in the case of any large power system. Many papers validate their approaches (only) on IEEE bus systems. In [11] (2015), the aim is to reach the maximum quantity of both current and voltage measurements with minimum investments. The solution highly depends on system topology. Nodes prioritization strategies are not used. Matrixes, which are filled with Boolean values, are used for the mathematical model which represents grid topology, and both voltage and current observability status in busbars and lines. In addition, economical calculations are added. The created method is tested on IEEE 14, IEEE 30, and IEEE 57 systems; however, the application challenges remain unclear in the case of large power networks. In [12] (2016), monitors allocation criteria are based on the P-median model for non-technical losses assessment. According to the definition provided in the paper, nontechnical losses are losses before billing measurement, and are caused by additional (unexpected, perhaps in the design stage) loads. These losses are the main reason of a voltage drop in lines below nominal value threshold.