# Power Quality Impact and Its Assessment: A Review and a Survey of Lithuanian Industrial Companies
## 3 Materials and Methods
### Impact Assessment  
The following points system is introduced: (1) 0 pts is given when the event is not dangerous, (2) 0.5 pts is given when the event is partially dangerous, and (3) 1 pt is given when the event is dangerous. The impact is assessed by using two approaches: (1) when the weights are not assigned, and (2) when the weights are introduced according to IEEE Std 1564-2014. The standard proposes the following formula for voltage sag severity estimation:  
\[S=\frac{1-U}{1-U_{\text{curve}}(t)}\, \tag{12}\]  
where \(U\)--voltage sag magnitude; \(t\)--event duration; \(U_{\text{curve}}(t)\)--magnitude value of the reference curve for the same duration. The values are given in Table 6: it can be noticed that (1) when there is no event, the severity is equal to 0, (2) when the parameters of the event are above the SEMI F47 curve, the severity is between 0 and 1, (3) when the parameters are on the curve, the severity is equal to 1, and (4) when the parameters are below the curve, the severity is higher than 1. Despite the fact that such an approach is more suitable for outage risk assessment and it can be argued whether it is suitable for the assessment of damage to equipment (or, in other words, for predictive maintenance), no differentiation was made in this regard; therefore, the companies assessed the impact as a whole. On the other hand, for the respondents, such differentiation would have made this survey more difficult rather than clearer. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Voltage, p.u.**} & \multicolumn{5}{c}{**Duration, s**} \\  & **t \(\leq\) 0.02** & **0.02 \textless{} t \(\leq\) 0.2** & **0.2 s \textless{} t \(\leq\) 0.5** & **0.5 \textless{} t \(\leq\) 10** & **t \textgreater{} 10** \\ \hline
**U = 0.8** & 0.20 1 & 0.40 1 & 0.67 1 & 1.00 2 & 2.00 3 \\
**U = 0.6** & 0.40 1 & 0.80 1 & 1.32 3 & 2.00 3 & 4.00 3 \\
**U = 0.4** & 0.60 1 & 1.20 3 & 1.98 3 & 3.00 3 & 6.00 3 \\
**U = 0.2** & 0.80 1 & 1.60 3 & 2.64 3 & 4.00 3 & 8.00 3 \\
**U = 0.0** & 1.00 1 & 2.00 3 & 3.33 3 & 5.00 3 & 10.00 3 \\ \hline \hline \end{tabular}  
* The value above the SEMI F47 curve: equipment must be able to continuously operate without interruption under the conditions identified in this area. 2 The value on the SEMI F47 curve. 3 The value below the SEMI F47 curve.  
\end{table} 
Table 6: Voltage sag severity according to IEEE Std 1564-2014. In addition to Table 6, the standard gives the following algorithm voltage sag severity estimation:  
\[S=\left\{ \begin{array}{ll}1-U,&t\leq 0.02\;\text{s},\\ 2(1-U),&0.02\;\text{s}<t\leq 0.2\;\text{s},\\ 3.3(1-U),&0.2\;\text{s}<t\leq 0.5\;\text{s},\\ 5(1-U),&0.5\;\text{s}<t\leq 10\;\text{s},\\ 10(1-U),&t>10\;\text{s},\end{array} \right. \tag{13}\]  
where \(U\)--voltage sag magnitude; \(t\)--event duration. Since the table of EN 50160:2010 does not coincide with the table of IEEE Std 1564-2014, some adjustments must be made. Two methods are used for the recalculation of the weights--the worst-case method (Table 7) and the interpolation method (Table 8). Using the first method, the highest severity value is taken: in the case of interval overlap, the highest weight \(S_{i}(t_{i})\) is always in the right cell, thus its index is always the highest. The second method is based on interpolation according to the following formula:  
\[S=\sum_{i}S_{i}(t_{i}).\frac{\tau_{i}[\%]}{100}, \tag{14}\]  
where \(S_{i}(t_{i})\)--the value of voltage sag severity given in the table cell with index \(i\); \(\tau_{i}\)--overlapping duration (at most three cells are covered per row). It can be seen from the tables that the values in the first and fifth columns are slightly lower in the second case. The values of each table filled in the questionnaire are added up, and the sum is expressed as a percentage, i.e., the closer the value is to 100%, the more sensitive the equipment is to interference. Three sums are calculated for each case: method No. 1--the weights are not used; method No. 2a--the weights are taken from Table 7; method No.