# Grid Nodes Selection Strategies for Power Quality Monitoring
## 3 Discussion
### Massiveness  
In almost all the reviewed theoretical papers, the validation of proposed algorithms is begun from IEEE schemes. Then, sometimes in addition, the validation is supplemented with other schemes--CIGRE, EPRI, or Brazilian. The largest sizes of test schemes, which were used for validation in the theoretical papers, are summarized in Table 5.  
The IEEE, CIGRE, and Brazilian schemes used in the papers are small--up to 123 busbars. Only in [17] is the EPRI scheme used for a massive system test. Theoretical simulations are limited. Firstly, the usefulness and efficiency of the proposed algorithm remains unclear in the case of a real (large) electric power system. Larger test schemes require more computational resources; however, many authors highlight only fast computational time which was required for their test in small schemes. Secondly, the variety of potential monitoring locations increases with grid size. In the works reviewed it is clearly seen that different results are obtained under various conditions (scenarios) regarding both the quantity of monitors and their locations. In addition, low algorithm adequacy can be expected in the case of small radial grids development, for example, the advance of distributed generators since the most test schemes were created before the worldwide growth of renewable penetration. Many researches use only radial test schemes, i.e., with only one--an infinite--power source. Many test results can be expected to be valid only if system topology remains unchanged. Thirdly, since many algorithms require input data about grid nodes impedance, the final result can be influenced by every grid reconstruction (modification),--for instance, the replacement  
obtain impedances of real (massive) system nodes, and other information which is required for initial data input. On the other hand, there are not many alternatives which could be without any limitations; thus, small-size theoretical tests are suitable at the beginning research stage in anticipation of future worldwide progress. Fifthly, after theoretical validation, there are still many unexpected issues which may be faced in practice during algorithms' application, for example, some locations may be technically unsuitable to install monitors (e.g., absence of appropriate measurement transducer). In addition, many test schemes are not compatible geographically. IEEE schemes have their own unique features representing the North American grid, thus they do not necessary reflect the situation in Europe. For example, IEEE 13 operates at 4.16 kV nominal voltage, IEEE 34 is an actual distribution feeder located in Arizona, USA. Nominal voltage of this feeder is 24.9 kV; the system has very long and considerably loaded lines, loads are unbalanced, and modelled as concentrated and distributed. As a comparison, the Lithuanian power grid is incorporated into post-Soviet BRELL ring where nominal voltage levels are as follows: HV--330 kV and 110 kV, MV--35 kV, 10 kV and 6 kV, LV--0.4 kV. Authors very rarely highlight features of used test schemes and their test results flexibility which can be influenced by various factors: topology (radial, ring), presence of power plants, load percentage, load symmetry, line length, etc. On the contrary, simpler strategies are used during practical research: they are not based on complex mathematics and require less initial data. In eight reviewed papers, including [23] where all voltage levels are investigated, the HV grid is investigated three times, MV--five times, LV--three times. In general, the HV grid has fewer nodes and a better PQ situation. PQ law of the HV grid has its own specifics since European PQ standards are not applicable to HV; thus, each TSO has its own grid code (national regulation) with PQ requirements.