# Grid Nodes Selection Strategies for Power Quality Monitoring
## 2 Literature Review
### Scientific Papers and Practical Reports
In addition, an Integer Linear Programming model is used to estimate monitors observability in regions with the most important loads. An algorithm validation is performed on IEEE 14, IEEE 30, and IEEE 118 systems, and with a case study in the Brazilian DSO grid (40-busbar radial circuit belongs to metropolitan region of Recife, main city--Pernambuco). This network fragment is chosen as a representative example of other similar parts with high nontechnical losses. In the chosen grid, the total losses (consisting of nontechnical and technical) are equal to approximately 32% of the total load, nontechnical losses--24%. The algorithm solved a problem with 11 PQ monitors (i.e., the smallest possible amount). In addition, the authors highlighted some implementation challenges and drawbacks: no flexibility in monitor type selection and the model may provide a different solution when both DSO and TSO systems are considered (i.e., when the TSO grid is not simplified to an infinite power source). In [13] (2016), similar to [11], the aim is to reach maximal grid coverage. The main motivation of [13] is Brazilian grid codes (for both transmission and distribution systems). According to the authors, one of the tasks of Brazil PQ regulation is an introduction of penalties in order to increase the correctness of grid players' activity. To begin with, identical to [11], two lemmas derived from [PERSON]'s law were used:  
**Lemma 1**. : _If voltage \(V_{1}\) of bus No. 1 is observable and current \(I_{12}\) of line between buses No. 1 and No. 2 is observable, then voltage \(V_{2}\) of bus No. 2 is observable._  
**Lemma 2**. : _If voltage \(V_{12}\) through line between buses No. 1 and No. 2 is observable, then current \(I_{12}\) of the line is observable._  
In anticipation of further discussion, it can be remarked that the application of these lemmas requires input data of impedances of all branches or at least values of loads. This requirement for the initial data is very complex in the case of a real power grid. In [13], similar to many theoretical PQ monitors allocation researches, the optimization problem is mathematically described as a minimization of the multiplication product of cost matrix and binary existence matrix (see Equation (1)). If a monitor is installed on a busbar, the existence value is 1, else--0. The algorithm is tested on the CEMIG (Brazilian) transmission system with 65 buses and 98 lines (500, 345, 230, and 138 kV). Four different scenarios are examined, taking into consideration both the allocation availability and costs (which can be either unitary or proportional to a number of installations since it correlates with a required quantity of measurement transformers). Calculation time (for 65 nodes) is relatively long--34-124 s. Similar to [7], the DRF of each scenario is calculated, and the solution with greatest DRF value is chosen: global DRF value interval (of all four scenarios) is 1.7607-2.7975, local--1.7607-3.4722. In [14] (2018), the aim is to increase voltage sag observability and minimize cost with a Multi-objective Evolutionary Algorithm. The algorithm is tested only on small test systems: IEEE 13, IEEE 34, and IEEE 37. Two scenarios for IEEE 13 test are created: (1) when faults impedance is always equal to 0 \(\Omega\), and (2) when values from 0 to 5 \(\Omega\) are possible. The authors concluded that the cost-benefit ratio is acceptable in both cases. IEEE 34 and IEEE 37 are examined only under conditions of the first scenario. If the algorithm had been successfully tested in large-scale systems, it would be a useful tool for voltage sag monitoring campaigns planned in practice. In [15] (2019), the main allocation criterion is harmonic (up to 6.4 kHz) resonance conditions in MV network. The methodology is tested on 15-node and 24-node distribution systems which are based on the CIGRE MV test system, and the IEEE 34 system. In the case of both the 15-node and 24-node systems, the optimal solution is 4 PQ monitors (same quantity but different locations).