# Grid Nodes Selection Strategies for Power Quality Monitoring
## 4 Problems and Future Prospects  
### Sophisticated Attention to Every PQ Event  
Dealing with each PQ issue must be start from fundamentals. The allocation problem is not an exception. An initial step should be PQ events targeting. Different target groups of PQ events will lead to a different monitors' allocation strategy. PQ events assessment problems which will be described below must be solved in the future, these solutions are vitally important in order to create an efficient allocation strategy. However, it is impossible to present all experiences and describe everything in one paper, thus only a part of the problems, which became obvious to the authors during PQ monitoring campaign (in Lithuanian DSO grid) and review of the literature, standards, and other legal documents, will be presented. In spite of the fact that European legislation requires the monitoring of the whole list of PQ events for situation assessment, currently it is a difficult task to deal with, firstly, due to many universally unsolved issues (i.e., lack of a universally unified approach) regarding both: uncertainties in PQ events assessment methodologies, and technical possibilities (progress) of monitors which highly correlate with cost. Hence, many papers limit their approach to a selected target group and investigate the most beneficial method for individual (most problematic) events monitoring. Nowadays, authors agree that voltage sag has remained the most problematic PQ event to industry for more than the past two decades. Other main issues are harmonics (which cause power losses, machine vibrations), and RMS voltage drop below a lower limit of 0.85-0.90 p.u. Some authors affirm that their approach is suitable for all of the PQ events list, however, that is only a half-truth. Each phenomenon has a different origin and propagation mechanism, which will determine different measurement requirements and cost-benefit monitors displacement strategy. Reading between the lines, suitability for \"all PQ events\" at least means exclusion of a \"special\" group--flickers and transients. Let us explain why we can call it \"special\". Firstly, in IEC 61000-4-30, no measurement and aggregation requirements are given for both types of transients--impulsive (fast) and oscillatory. These phenomena (including overvoltage caused by lightning) require very sophisticated monitors with a  
\begin{table} \begin{tabular}{c c c c c c c c} \hline \hline
**Voltage 1** & [6] & [8] & [19] & [20] & [21] & [22] & [23] & [24] \\ \hline HV grid & & & 15–37 2 & & 205/20 3 & & 12,000 4 & \\ MV grid & 427 5 & 600 5 & & 3500 5 & & & (jointly) & 46 5 \\ LV grid & 1500+/64 6 & & & & 73/7 7 7 & 7 & (jointly) & \\ \hline \hline \end{tabular}   
* In case of various countries, boundary between HV and MV can be different. In this paper, voltage levels are classified according to thresholds of European standard EN 50160:2010. 2 Fixed monitors in case of different scenarios: grid market scenario requires the largest number. Total number of buses of the test grid is 205. 3 Weekly measurements with 20 portable monitors. 4 Example of massive campaign with various monitors including limited PQ functionality devices (smart meters, protection relays). Detailed information about device types is unknown. 5 Fixed monitors (quantity is equal to number of busbars). 6 With 64 portable monitors. 7 With portable monitors (exact amount is unknown), overall measuring time is 160 weeks. 7 With 64 portable controls. 7 With 6 high sampling frequency and broad dynamic range. Moreover, transducers with adequate magnitude and phase frequency response are required in order to avoid results distortion. It is a very complex task. In [24], the Great Britain DSO faced many problems even with harmonics which are assessed up to 40 th (i.e., spectrum is limited to 2 kHz). According to IEEE Std 1159, typical spectrum content of oscillatory transient can be up to 5 MHz and a typical duration of impulsive transient ranges from less than 50 ns to more than 1 ms [3].