# Comparative Technical-Economical Analysis of Transient Stability Improvements in a Power System
## 1 Introduction
Static VAr compensator (SVC) and static synchronous compensators (STATCOM) are able to control the voltage/reactive power at their connection point. They can improve the voltage profile of the system by controlling the voltage on one of the required buses [3; 4; 5; 6; 7]. The diffusion and the increasing interest in the introduction of innovative solutions in the power grid are also justified by the objective of increasing the production of electricity from RES (as indicated in the European Roadmap and by ENTSO-E) [8]. However, achieving this goal brings some challenges related to the stability of the electrical system. In order to overcome these challenges, TSOs are implementing various modernization measures based on power electronic devices and the conversion of phased-out production units (e.g., coal) for the provision of ancillary services (e.g., synchronous condenser), in line with circular economy principles. These modernization actions can also be carried out by revising the current grid structure when the solutions presented so far are not effective, such as the construction of new power lines. The transient stability studies allow verifying if the system can withstand the transients caused by a disturbance event. The stability limit is determined by calculating the maximum clearing angle and the critical clearing time (CCT), which describes the time that it takes for the generator to go from the initial rotor angle to the critical rotor angle. The greater the CCT, the more severe the disturbance the generator or system can withstand. For a complete comparison, the technical analysis alone is not sufficient to fully assess the actual feasibility of a solution under study. It is also important to consider the economic aspect of the solution in question. Cost can be one of the most important decision criteria for both TSOs and utilities. For this reason, there are different types of economic evaluations in the literature, see the work of [6; 9]. In this paper, a comparative analysis of solutions to improve transient stability, both rotor angle stability and frequency stability, is performed. These solutions are SVC, STATCOM, a fast excitation system, and the doubling of a transmission line. The objective is to understand and evaluate what advantages these solutions bring to transient stability. The numerical simulations are performed using PowerWorld Simulator. The IEEE 9-bus system is used as a case study [7]. This is the real contribution of this paper:  
* Sensitivity analyses were performed to determine: the worst-case location of a three-phase line fault for rotor angle stability and the most effective location to install the double line and SVC or STATCOM;
* An analysis of the impact on rotor angle stability is performed for different network strength by varying the SCR (short-circuit ratio) values at the FACTS installation bus;
* An evaluation of the impact on transient frequency stability, specifically, the impact of installing SVC and STATCOM. These solutions provide TSOs with dynamic control during contingencies;
* An economic assessment of the use of FACTS will be conducted. This economic analysis focuses on the economic savings, e.g., avoided load shedding, achieved by TSOs when using these devices. The paper is organized as follows. Section 2 presents the solutions to improve transient stability. The studied system and the steps of the simulation are described in Section 3. In Section 4, the proposed solutions to improve the rotor angle stability are tested. The same devices are then used to improve the frequency stability: This analysis is described in Section 5. Economic considerations of FACTS devices, SVC and STATCOM, are developed in Section 6. Conclusions are presented in Section 7, where the improvement solutions are summarized, and their main features are highlighted.