# Grid Nodes Selection Strategies for Power Quality Monitoring
## 3 Discussion  
### Criteria  
Grid nodes' selection criteria highly depend on the research aim, resources, and size of the campaign. Research can be classified into three groups: theoretical, practical, and state-of-the-art reviews. Most papers, which have been reviewed in this work, are either directly focused on the topic or at least are slightly related. In total, 19 references were reviewed: 10 theoretical researches, 8 practical campaigns, and 1 literature review. PQ monitoring campaigns of Australia, Belgium, China, Estonia, Great Britain, Italy, and Turkey are presented. Almost all papers notice that PQ monitors are expensive, thus both appropriate allocation strategy and cost-effectiveness analysis are essential in order to maximize project's economic benefit. After the literature review, it can be noticed that nodes selection criteria can be classified into two large groups--abstract and exact. Abstract criteria are (can be) found only in the case of practical research. The criteria are described without any concretization: terms such as critical substation, specific load, and randomly selected nodes are used. Two main aspects can be provided to explain abstract criteria origin:  
1. Not informative definition and description of the strategy; however, indeed some arguments can be found in favor of the selection. If the strategy description was more detailed, a criterion would be classified as exact. Not detailed description complicates the analysis. 2. Even if the grid operator declares not using any strategy and criteria, indeed there are some hidden (invisible) aspects which influenced the choice. The reasons could be: communication access in substation, technical conditions (such as appropriate and safe ambient environment to install monitoring devices), existence of voltage transformers, etc. In general, more convenient places for measuring are selected. Not having a detailed description complicates our discussion, the criteria, and paper classification and distorts results. The strategy remains uncertain in all abstract cases:1. Which substation is considered as a critical? It could probably be a major hub of HV grid--an important substation or switching station. Moreover, an old substation or a substation in poor technical condition can be considered to be critical. 2. Which loads are specific? It could probably be industrial equipment or appliances which are sensitive to PQ issues. For example, voltage fluctuation, harmonics or flickers can distort the output signal of medical or other high-precision equipment; voltage sags can be the reason for false protective relay operation, or can cause electric motor stoppage and consequently interrupt the whole industrial process. Moreover, power electronics can be referred to PQ pollutant equipment. 3. Which algorithm is used for random selection (random number generation)? Are all nodes and all voltage levels equally weighted? Or does it (random selection) mean more convenient places for measuring? In the reviewed papers, two approaches were distinguished which have their own specifics and are (can be) treated as random nodes selection: (1) \"200 freely selected MV nodes\" in [8], and (2) selection of 12,000 sites (HV, MV, LV) in [23]. In the first case, the MV nodes sample is extracted (from whole system) for the further random selection of 200 nodes; however, the algorithm (of random selection) remains unclear; the selection perhaps is intuitive. The second case is a blind selection of majority elements in the system (i.e., statistical population). This strategy led to the further conclusion that such an approach is not cost-effective. There is no paper which uses random number generation for PQ nodes selection. The summary of the papers reviewed is given in Table 2--allocation strategy and its application. A strict division among the papers can be noticed: authors test their methods either with theoretical validation or during practical PQ monitoring campaign.