# Power Quality Impact and Its Assessment: A Review and a Survey of Lithuanian Industrial Companies
## 5 Discussion
### Voltage Sag Tables  
Such matrices, as constructed in this paper, and their properties depend not only on the values or weights of the elements, but also on both voltage and time intervals. Currently, EN 50160:2010 and IEEE Std 1564-2014 do not argue the intervals of their tables, but it can be supposed that these intervals should be somehow related with the settings of relay protection and automation, thus the absence of argumentation does not mean the absence of practical value. Indeed, one of the most important factors determining voltage sag parameters is the reaction of relay protection and automation. These devices can be divided into two groups: (1) devices that disconnect the line upon detection of improper conditions (e.g., overcurrent protection (OCP), undervoltage protection (UVP), differential protection, distance protection, zero sequence overcurrent protection, etc. ), and (2) devices that automatically attempt to restore power supply after the disconnection, in particular automatic circuit recloser (ACR) and automatic transfer switch (ATS). Due to their latency and the requirement for selectivity, grid protections do not always work quickly enough as needed to protect the load from harmful effect or to prevent its downtime. Let us examine two realistic examples of the Lithuanian 6 kV grid presented in Figure 12. In order to meet the selectivity requirement, descending from a higher to a lower voltage level, ATS time settings must increase, UVP time settings must increase, and OCP time settings must decrease at any expected current value in the time-current curve. Both UVP and OCP times must be lower than the time of ATS. For example, in Figure 12a, when the depth of voltage sag is equal to 50%, one feeder will be disconnected by the UVP after 1.5 s and the ATS will be triggered after not less and usually more than 2 s, the other--after 2.2 s and ATS will be triggered after not less than 2.6 s, and such intervals can be too lengthy to maintain electric motors and other industrial equipment operating and thus to avoid outage. An effective way to speed up the operation of grid protection is installation of optical-fiber cables for signal transmission, as well as supplementation of ATS with rate of change of frequency (i.e., first time derivative of frequency) relays. \begin{table}
\begin{tabular}{c c c c c c c c c|c} \hline \hline \multicolumn{1}{c}{**Pair**} & \multicolumn{10}{c}{**Similarity**} \\ \hline  & & & & & & & & & & & \\ \cline{5-10}  & 3.3 & 7.3 & 13.9 & 23.8 & 32.3 & 23.4 & 17.6 & 12.3 & 5.5 & \\ \cline{5-10}  & 7.4 & 17.3 & 30.5 & 45.5 & 56.7 & 32.8 & 21.9 & 16.5 & 9.6 & \\ \  
To continue, when analyzing from the PQ perspective, some traditional concepts of relay protection and automation must be expanded. Typically, all protective relays try to disconnect a PQ victim (i.e., line or device) from electricity network as soon as possible; however, some loads can be critical, thus their at least slightly longer operation can be beneficial, especially those that are used for human and environmental security (e.g., fire-fighting pumps). The purpose of relay protection and automation can not only be power system protection but also its monitoring--after transferring the parameter measurements to the operator's data analysis system, they will be extremely useful for determining the type of fault, its location and propagation path, as well as the protection response and logical sequence [1,2]. Data collection and analysis will play a very important role in Smart Grids--absolutely correct decision making can be only achieved with knowing the whole picture, but before that, there is still a lot to be done, including the application of communication technologies and the development of AI algorithms [1,8]. Another example relates to ACR. Some grid faults disappears when the voltage is disconnected, therefore an automatic attempt to restore the power supply is beneficial and reasonable.