# Grid Nodes Selection Strategies for Power Quality Monitoring
## 2 Literature Review
### Scientific Papers and Practical Reports
It was concluded that the proposed approach is a promising tool for the grid operator to assess harmonic levels in MV grid. In [16] (2019) and [17] (2021), the aim is to avoid the multiple estimation problem when impedance-based fault locators estimate fault location distance. If monitors' positions have been determined inappropriately, \"fault can be observed but not necessarily identified\". For example, if impedances are equal between PQ monitor M1 and faults F1 and F2 (see Figure 3), the monitor would not be able to differentiate these faults, because the output would be the same RMS voltage value (i.e., to be exact, would output close RMS values within the tolerance interval of measurement uncertainty). This problem is called the symmetry issue. Therefore, alternative positions should be determined for monitor placement, for example, the possible solution could be M2. The main aim is to achieve maximal fault differentiation with a limited number of monitors. In order to create a database, many faults simulations are necessary. In [16], the proposed algorithm is tested on 13 schemes: IEEE test systems--IEEE14b10 pc, IEEE30b10 pc, IEEE30b5 pc, IEEE30b2 pc, IEEE57b10 pc, IEEE63b10 pc, IEEE118b, IEEE118b25 pc; Brazilian test systems (Brazilian transmission grid configurations in 2016)--SBr9b10 pc, SBr16b10 pc, SBr33b10 pc, SBr65b10 pc, SBr107b10 pc. Different fault types (three-phase, two-phase, two-phase-to-ground, single-phase-to-ground) and fault impedances (1, 5.75, 10.5, 15.25, and 20 \(\Omega\)) are examined. However, in the case of any real power system, it is hard to determine all impedances (including transmission lines) a priori. The authors of the paper notice that the allocation result depends on both variation in fault parameters and fault locations (i.e., either faults occur only in busbars or in both busbars and lines). Moreover, the paper discusses that measurement uncertainty can distort the solution. Computational time in the smallest systems--SBr9b10 pc (9 busbars) and IEEE14b10 pc (14 busbars)--is 2 s and 0.97 s, respectively; in the largest--SBr107b10 pc (107 busbars)and IEEE118b25 pc (118 busbars)--159,957 s (more than 44 h) and 17,492 s (more than 4 h). Despite the very high computational time required for larger grids, it is concluded that the main contribution (novelty) of their paper is the presentation of a new approach, which considers the symmetry issue and proved to be applicable in networks of any size and topology. The authors notice that the result depends on means used for initial data obtaining (faults parameters and locations). If faults are simulated only in buses, the problem is simplified but results' reliability is reduced. In [17], a similar methodology is investigated as in [16], but in a more specific case--an unbalanced distribution system. Firstly, the methodology is tested on a modified CIGRE 15-node test system and IEEE 123-node test system. The paper proposes an extension to the CIGRE model for unbalanced faults simulation, because the model does not consider two following points: (1) a bus may not have three phases; (2) a fault may cover less than three phases. Two different scenarios (topologies) are investigated--when a distributed generation (5 MVA steam turbine synchronous generator) is (1) connected, and (2) disconnected. The IEEE 123 scheme is selected to compare results with [18] (2017), where other authors present their approach for minimizing PQ monitors number required for voltage sags observation in the case of both balanced and unbalanced systems. The main difference between [17] and [18], and the advantage of [17], is the weight of the objective function. Secondly, in addition, the EPRI Ckt5 2998-node test is carried out, and it is concluded that the algorithm in [17] is suitable for practical application in large power systems.