## 1 Introduction  
The increased number of power-electronic-based (PE-based) units in electrical power systems causes significant challenges in terms of stability, protection, and security [1; 2; 3; 4]. The most important reasons for that are the change in power system structure, from the conventional model (centralized, top-down-based structure) into a much more distributed, bottom-up-based system [5]. In addition, control systems introduced by PE-based units, such as high voltage direct current (HVDC) transmission lines and wind turbine technologies, may affect the system's stability and controllability [6]. Accordingly, conventional methods of power system stability and security assessments show insufficient performance to deal with the new challenges [7; 8; 9]. To address the security challenges with the PE-based power systems, the first step is to establish a precise definition of security. The security of a power system is defined as its ability to overcome contingencies, such a severe fault, and deliver the energy to customers in abnormal conditions [10]. A general scheme of the security assessment of a PE-based power system is shown in Figure 1. A secure system is also a stable one--both during and after a contingency. Therefore, stability assessment of the system is necessary to ensure its security. However, the definition of power system stability is modified by moving from conventional systems to PE-based ones [11]. In this way, there are some main issues regarding the modern PE-based power systems, which need to be considered:  
* PE-based energy sources are smaller in size in comparison with conventional ones [12; 13; 11]. In addition, some PE-based units, such as wind turbine type IV, are decoupled from the system from a frequency point of view. In general, it can be stated that PE-based energy sources, such as wind turbines and photovoltaics, do not introduce the inertial support as the synchronous generator does. Therefore, they suffer from lower system inertia, which makes the whole system more vulnerable to disturbances [12]. * Based on the grid codes and the relevant standards, grid-connected voltage source converters (VSCs) should support the main grid during a severe fault on the grid side [14; 15]. However, under some specific circumstances, these units should be disconnected from the main system. For instance, if a wind farm is unable to withstand a certain amount of voltage drop for a specific time period, or in case of extreme frequency range, it will be disconnected from the power system [16; 17]. This may cause cascading consequences for the power system. * Uncertainty in the nature of renewable energy sources makes the security assessment of the system more complex for both solar and wind [18; 19]. Grid-connected VSCs' stability challenges are significantly exploited in the literature [20; 21; 22; 23; 24]. Therefore, based on the developed small-signal and large-signal models, one can accurately assess the PE-based unit's stability and security [6; 25]. On the other hand, the power system stability and security are well-matured topics, which are discussed extensively in the literature [10; 26]. In addition, the stability of the PE-based power systems is presented in some recent works [27; 28; 29]. However, only a few approaches regarding the security assessment of the PE-based power systems have been presented so far [18; 30; 31]. The main aim of this paper is to develop a step-by-step guideline for the security assessment of PE-based power systems. The security and stability assessment discussed in this paper is an enlargement of an earlier work by the same authors presented in [32]. However, it should be noticed that in this paper, we try introduce more details regarding the security and stability assessments of large-scale power systems that have high penetration of wind turbines, such as inertial response and synchronization challenges, which are not discussed and assessed in [32].