# Comparative Technical-Economical Analysis of Transient Stability Improvements in a Power System
## 6 Economic Evaluation  
In the previous sections, the effects of FACTS, SVC, and STATCOM on the transient stability have been analyzed, considering the rotor angle and frequency stability problems. In this section, the comparative analysis between SVC and STATCOM focuses on the economic aspects. This is useful to evaluate whether it is worth the investment. In the following economic analysis, only SVC and STATCOM are considered, while the parallel line and fast-acting exciter are neglected. This assumption is due to the following reasons:  
* The following economic calculation is based only on the savings margins associated with the frequency stability service. This is because there is currently no compensation for stability support. This justifies the exclusion of the transmission line, whose contribution to this service is close to zero;  
The economic evaluation is based on the savings margins for the TSOs due to the installation of the equipment in the transmission system, which is directly operated by them. In contrast, in a deregulated electricity system, the excitation systems of the synchronous generators are owned and operated only by the generation companies;  
For this purpose, a simple payback analysis is performed. This type of analysis is a very simple and widely used method that allows evaluating the period required to  
Figure 11: Frequency trends comparison, STATCOM, and load shedding. Figure 10: Frequency trends comparison, SVC, and load shedding. amortize the initial investment, taking into account the costs and the periodic savings [5]. The unit cost of the STATCOM device, including the installation cost, was derived from the data in the work of [19]. Using the data contained in the work of [4; 5; 6; 9], the unit cost of the device SVC is determined. * SVC: 194.4 \(\epsilon\)KVAr;
* STATCOM 240 \(\epsilon\)KVAr;  
With these values and taking into account the MVAr rating of the two devices, the total cost can be easily determined:  
* SVC: 194.4 \(\epsilon\)/kVAr \(\times\) 50 MVAr = 9.72 M\(\epsilon\);
* STATCOM: 240 \(\epsilon\)/kVAr \(\times\) 50 MVAr = 12 M\(\epsilon\);  
As can be seen in Section 5, Figures 10 and 11, SVC and STATCOM allow load shedding of 100 MW for the analyzed case study. The load shedding is converted using VOLL, which has a mean value of 30 k\(\epsilon\)/MWh in Italy. [20]  
The benefit of SVC and STATCOM is the avoided cost of load shedding. The expression used to determine the total avoided cost is based on the product of the duration of load shedding due to faults (TFT, Total Fault Time) in the transmission system in one year and the VOLL. The TFT can be calculated as the product between the typical annual independent failure rate of the high voltage transmission line AL [21] and the total length of the transmission lines L (878.4 km for the modified system IEEE with nine buses). In addition, a total system recovery time of 10 s, TSRT (0.0027 h/year), is considered. Then, the total avoided cost for SVC and STATCOM (2) can be determined:  
\[\text{VOLL}\times\text{P}_{\text{LS\_SVC}}\times\lambda_{\text{L}}\times \text{L}\times\text{TSRT}=25.64\ \frac{\text{k\  
the best solution is the fast-acting exciter, while the addition of a new parallel line has a limited effect. The use of SVC and STATCOM, even if they improve the rotor angle stability, shows limited effects. On the other hand, SVC and STATCOM are suitable solutions for improving frequency stability. Their use makes it possible to contain frequency fluctuations during the transient phase and avoid load shedding by installing them in the considered network. At SVC and STATCOM, the economic aspect is also taken into account. The result of the simple cost-benefit analysis shows that the investment required for these devices is not profitable, considering only the impact on the stability of the transient phase. The results obtained in this paper can be summarized as follows: Due to their reactive power and voltage support, SVC and STATCOM devices are useful for improving transient stability.