## 2 Conventional Power System Security Assessment  
A power system's reliability is defined as its ability to cope with its demand and withstand any outage and contingency [26; 35]. It is generally divided into two categories, including adequacy and security, as shown in Figure 2[19; 35]. Adequacy measure represents the ability of a power system to cope with its demand at all times, considering planned and unplanned outages and uncertainties [36]. The system adequacy is associated with the planning of power systems for long-term facility expansion, short-term marketing, and operational planning, including unit commitment and economic dispatch among available generation systems. Meanwhile, the system is subjected to different kinds of uncertainties within its operations, which may have planned or unplanned sources. It is worth mentioning that the power system security becomes a more challenging issue when information and communication technology (ICT) is counted in the security assessment [37]. The planned outage can be due to the maintenance of sub-systems and components. Moreover, the unplanned uncertainties can be induced by generation-demand unbalance, load forecasting, renewable power uncertainty, failure of any component, short-circuiting, large load changes, and so on. The occurrence of any contingency in the system may cause system variables violation. These violations could be thermal overloading of lines, system bus voltage, generator rotor angels, and system frequency. They are associated with the different phenomena in the power system and may disturb the power delivery. Therefore, the system must be able to withstand and/or respond to any sudden contingency to maintain power delivery. This ability is called power system security [10; 38]. As can be seen from Figure 2, there is a close relationship between the security and stability concepts. However, there is a slight difference between them. A secure system is the one that is stable for all credible contingencies. However, for the stability of the system, only the current system condition needs to be checked (without any contingency assessment) [10; 39]. To guarantee the system security, it is high of importance to examine all likely contingency occurrences through analytical analysis and simulations. Therefore, the system operator should evaluate the different contingency occurrences to understand the system behavior after any outage. The impact of any likely disturbance on the system's stability and the voltage and thermal limits must be analyzed. For the cases which violate the system's performance, appropriate corrective and/or preventive actions should be applied to sustain the system's security. The system's security assessment is classified into two categories, including static and transient/dynamic security [40]. A static security assessment evaluates the system voltage and the equipment's thermal limits violations after any contingency to ensure that they remain in acceptable intervals [32; 41]. Otherwise, the protection system will separate the components, or it may isolate a part of the system [42; 43; 44]. However, before entering into the steady state region, the system stability should be assured after a contingency [45]. Following the causes of instabilities, power system stability issues are conventionally classified as voltage stability, angular stability, or frequency stability issues [10; 26]. Voltage stability is associated with the ability of a power system to maintain the voltage of all buses after any contingency. Voltage instability may occur once the system controllers and/or operators cannot prevent progressive voltage rise or fall under a disturbance [10; 46]. Voltage instability causes loss of load, transmission line outages, loss of generation, and consequently, cascading outages. In the case of widespread voltage instability, voltage collapse may occur, leading to a blackout.