# Comparative Technical-Economical Analysis of Transient Stability Improvements in a Power System
## 1 Introduction  
The current energy transition, required by the environmental sustainability problem of recent decades, is changing the paradigm of electricity generation. The current centralized generation model is being replaced by a model of distributed generation and an increasing presence of generation capacity based on renewable energy sources (RESs). The energy transition is changing the characteristics of the electric power system (EPS). One of the most important properties of an EPS is system stability, especially transient stability. The power system is constantly exposed to disturbances of various kinds: faults, power imbalances due to load or generation changes, circuit breaker switching, and so on. These contingencies cause quantities such as frequency, voltages, and rotor angles of generators to deviate from nominal values. Transient stability is the ability of the system to withstand the transients caused by the disturbance event and restore nominal operating conditions. In this context, two concepts of power stability that are generally accepted in definitions of electric power systems are considered, such as [1]:  
1. [label=()]
2. Rotor angle stability: when all synchronous machines in a power system run synchronously with each other even after a disturbance, the power system is said to have rotor angle stability;
3. Frequency stability: is the ability of the system to maintain stable frequencies after disturbances due to discrepancies between generation and load. Transient stability analysis is generally concerned with the analysis of the effects of major disturbances, such as line faults [2]. When a three-phase fault occurs near a generator, the ability of that machine to transfer power changes from possibly 100% to 0%. Therefore, a much wider range of system responses must be considered, and the analysis is essentially nonlinear. Several variables affect transient stability, so improving transient stability can be achieved either by reducing fault severity (e.g., faster fault clearance) or by increasing critical clearing time. Techniques to improve the stability of the system aim to achieve one or more of the following effects [3]:  
1. Reducing the impact of the fault or fault severity: this effect can be achieved by faster fault clearing times, by fast breakers, and thus by reducing the fault severity by decreasing the clearing angle;
2. Increasing synchronization forces to restore stable operation after a disturbance. This effect can be achieved by FACTS by providing the voltage support on a long transmission line;
3. Reducing acceleration or deceleration power by controlling the prime mover to balance mechanical and electrical power;
4. Applying artificial load to a synchronous generator to reduce the acceleration power by increasing the electrical power. Improvement solutions can also be divided into structural and operative solutions. Structural solutions aim to reduce system reactance, which can be achieved by adding parallel transmission lines and/or using a transformer with low leakage reactance. Since these two techniques for reducing system reactance are quite expensive, other techniques such as fixed or variable series compensation based on FACTS can be considered as cost-effective alternatives. Operative solutions can be again divided into preventive and emergency techniques. In preventive control, generator rescheduling and series compensation are commonly used in practice, while in emergency control, most of the techniques aim at supporting the voltage under fault conditions. A significant improvement in transient stability can be achieved with fast-response excitation systems of synchronous generators by increasing the field current to increase the internal machine voltage and thus evacuating more electrical power during the fault, which reduces the acceleration area and leads to an increase in the critical clearing time (CCT). This solution is very effective for faults that occur further away from the generator and where the voltage on the generator bus is greater than zero.