# Power Quality Impact and Its Assessment: A Review and a Survey of Lithuanian Industrial Companies
## 5 Discussion
### Voltage Sag Tables
The causes of such faults are electrical breakdown of air insulation by lightning-induced voltage transients, conductor gallop (dancing), foreign objects thrown on the lines [120], as well as the contact with flora or fauna. In addition to overhead power lines, ACR can also be and is used for the protection of busbars, power transformers, and cable lines, the failures of which often occur in air-insulated parts [120]. According to the traditional concept, from a technical point of view, ACR is considered successful when it successfully reloces the line; however, though a prism of PQ, such a successful ACR is actually unsuccessful if the electromagnetic disturbance causes consumer downtime and, respectively, worsens grid reliability indexes [2]. The success is achieved only when the consumer does not experience downtime after successful ACR, and unsuccessful ACR will in general always be a failure. When analyzing PQ measurements, it is not uncommon to find a successive voltage sag, which mostly occurs due to automation response, particularly due to reclosing events. An example of the phase-to-ground voltage measurement, which was taken in the 0.4 kV feeder of a metalworking company, which specializes in the production of forged and machined parts for the automotive industry, is given Figure 13: the time interval between the start thresholds of two observed voltage sags is 4.4 s, and a total of 3 such pairs were recorded during the month between November and December (which suggests that windy weather had an effect), which is slightly worse than the average situation in the country. However, both primary and secondary causes of such sequences  
Figure 12: Realistic examples of the settings of relay protection and automation, which have a direct influence on the parameters of short-duration RMS variations, their impact on the load and consequences. (**a**) A combination of UVP and ATS; (**b**) A combination of OCP and ATS. (in other words, voltage sag groups or voltage sag bursts) are investigated too little--they can occur not only due to reclosing events but also due to adverse weather or incorrect registration [2; 8; 121]. It is noteworthy that double ACR is also sometimes used in Lithuania: in this case, according to [120], the probability of successful reclosing increases slightly (approximately 0.05-0.15), but unsuccessful reclosing damages the device. Next, other voltage sag tables could also be used for the survey: for example, IEEE Std 1564-2014 additionally mentions the UNIPEDE table, IEC 61000-4-11 table, and IEC TR 61000-2-8 table. Their purposes are different: one tables are used for voltage sag classification, and the others for results registration during EMC tests. Therefore, one question that arises is what table should be used, and another question is time boundary values. In most tables, the shortest duration of RMS variation is equal to a half cycle, including the table of EN 50160:2010 (see Table 5). Respectively, IEC 61000-4-30:2015 requires the size of the window for voltage sag monitoring to be equal to one cycle refreshed each half cycle, which is denoted by \(U_{\text{RMS}(1/2)}\). On the other hand, IEEE Std 1159-2019 states that \"sags have been defined from 2 ms (about one-eight of a cycle [at 60 Hz]) to several minutes in various publications\". The upper time limit is also not entirely clear: the time threshold of short-duration RMS variations is usually considered equal to 1 min, but the boundaries between transient, short and long interruptions have not yet been set unanimously. Regarding the voltage thresholds, the situation is much clearer: according to IEC 61000-4-30:2015, the start threshold of voltage sag is 90% of nominal grid voltage, and the end threshold is same 90% plus 2% hysteresis. One more question, which is relevant for PQ impact assessment studies, relates to end-use equipment grouping, and perhaps a good practice of EMC standards could be useful for such a task.