# Comparative Technical-Economical Analysis of Transient Stability Improvements in a Power System
## 4 Improved System Description
### Improved IEEE 9 Bus System Modeling with Different SCR Value  
The results of the previous analysis show similar behavior for SVC and STATCOM solutions, although they are devices based on different operating principles. For a more comprehensive analysis, the four solutions are now tested for three different network configurations simulating different network strengths. The purpose of this analysis is to see how these technologies perform under different operating conditions based on different grid strength values. System strength is important for maintaining normal power system operation, for the dynamic response of the power system in the event of a disturbance, and for the return of the power system to stable operating conditions. The concept of system strength refers to the short-circuit power at the nodes. The magnitudes of a strong node are not greatly affected by a disturbance in another node of the network. On the other hand, the magnitudes of a node with low short-circuit power are strongly affected by the disturbance. [16]  
The most basic and easy-to-use metric for determining the relative strength of a power system is the short circuit ratio (SCR). SCR is defined as the ratio between the short-circuit apparent power (SCMVA) of a 3 LG fault at a given location in the power system and the rated power of the resource connected to that location [17]. * To obtain three different network configurations with different system strengths, the analysis is divided into three different cases, applying the same short-circuit fault as in the previous analyses: Case A: The four improvement solutions were tested on the system shown in Figure 2, which was used for the base case. The resource used to determine SCR is the SVC/STATCOM connected to bus 8. Case B: The four enhancement solutions were tested by adding the double line 7-8 to improve the system strength (lower impedance); Case C: The enhancement solutions were tested by opening lines 9-8 to weaken the system strength (more impedance). The value SCR for each of these cases is shown in Table 7, where the power of the resource connected to bus 8 is 50 MVA. To check the effects of the improvement solutions with a different SCR value, the variation of the Gen.2 rotor angle is considered. The results of the analyses for cases A, B, and C are shown in Figures 4-6, respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Base** & **SVC** & **STATCOM** & **EXST4B** & **Line** \\ \hline
**CCT [s]** & 0.405 & 0.403 & 0.407 & 0.404 & 0.473 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Calculation of the CFCT in the four case studies. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Case** & **A** & **B** & **C** \\ \hline SCR Value & 8.72 & 8.96 & 6.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of the SCR values for Case A, B, C.  
From the graph shown in Figure 5, it can be seen that at higher system strength (higher SCR), the rotor angle oscillations are limited to lower maximum values; in contrast, at lower SCR values, see Figure 6, a higher maximum rotor angle value is obtained, corresponding to the lower strength of the system. In all three cases, shown in Figures 4-6, the double line and the fast-acting exciter represent the best improvement solutions due to their effects on the system structure and the direct effect on the generator, respectively. To compare the behavior of the two FACTS devices, the focus is on the different responses of SVC and STATCOM in the three cases, as shown in Figure 7.  
Figure 5: Rotor angle of generator 2, FCT equal to 0.35 s, Case B.  
Figure 6: Rotor angle of generator 2, FCT equal to 0.35 s, Case C.  
These plots show that for strong systems, Case A and B, the solutions SVC and STATCOM have similar effects on the evolution of the rotor angle. On the other hand, in case C, the STATCOM solution gives a better result than SVC in terms of stability of the rotor angle. This is due to the different operating principles of these two devices.