## 5 Stability Enhancement Using FACTS Devices
### Voltage Stability
Regarding optimization techniques, authors [19] have classified them into three groups, i.e., classical analytical-based methods, classical arithmetic-programming-based algorithms, and modern meta-heuristics-based algorithms, as shown in Figure 2. However, in [171] optimization techniques are divided into four different categories, i.e., analytical techniques, conventional techniques, meta-heuristics techniques, and hybrid optimization techniques (a combination of meta-heuristics approaches with analytical approaches or conventional techniques). Irrespective of the advantages of these techniques, the major drawbacks observed in analytical techniques are compromised computational accuracy, lack of solutions about the optimal placement of FACTS devices, and lack of power flow non-linearity consideration [174]. Similarly, conventional optimization techniques are limited due to the difficulty of managing constrained optimization problems. However, most adoptable approaches, to date, are meta-heuristic (stochastic, population-based) optimization techniques. This is because they are vastly efficient in dealing with a highly constrained, multi-modal, multi-objective, and discrete system [175; 176]. Additionally, meta-heuristic optimization techniques can be utilized to determine the optimal position and sizing of various types of FACTS devices simultaneously. In the literature a detailed discussion about meta-heuristic optimization, the various categories, and their significance in terms of the minimization of various engineering problems is presented. But the trend shifted towards hybrid optimization techniques because they reduce the search space of meta-heuristics optimization techniques to obtain a simple structure and require less computational time [171]. Voltage stability for deregulated power networks is an important subject in today's world. Due to several reasons, like limited resources, environmental policies, increased energy demand, etc., policy makers deregulate power networks. This deregulation and advancements in semi-conductor devices open new doors for different methods, algorithms, or techniques. Therefore, several reliable and economical options are available for adopting  
Figure 2: A general list of Optimization techniques used to optimize the FACTS controllers. dereguated networks. In general, FACTS controllers are considered the most reliable, resulti-oriented (because they provide efficient results in real power networks), and secure options available irrespective of old conventional techniques for today's deregulated networks. Useful conclusions about voltage stability using FACTS devices in deregulated power systems are discussed below. 1. FACTS devices are preferable and the best option to eradicate voltage stability problems compared to conventional techniques, i.e., PSS, etc. 2. When comparing series FACTS devices (TCSC and SSSC) and shunt FACT devices (STATCOM), STATCOM performs better for voltage stability support. In general, it is concluded that shunt FACTS devices are more useful than series FACTS devices to provide voltage stability in deregulated power systems. 3. In the series FACTS devices, TCSC and SSSC, the SSSC is superior to the TCSC. 4. In shunt-connected FACTS devices (SVC and STATCOM) and series-shunt FACTS devices (UPFC), the UPFC leads to providing solutions for voltage stability. Consequently, STATCOM and the SVC are preferable. 5. There is one expectational contradiction observed between two different research studies presented in [151; 153]. In [151], STATCOM performs worse than the SVC, TCSC, and UPFC. However, in [153], the SSSC performed worse than the SVC, STATCOM, and UPFC. In both studies, only one series FACTS device was proposed. Generally, series FACTS devices do not perform better than shunt FACTS devices in providing reactive power compensation. However, this is not so in the case of the study presented in [151]. 6. FACTS devices are more beneficial while being used in combination with another FACTS device irrespective of their individual performance. 7.