## 3 Power-Electronic-Based Power System Security
### A New Guideline for the PE-Based Power System Security Assessment  
In this section, a new guideline to assess the PE-based power system security is proposed. The guideline was developed based on the conventional security assessment with some modifications.  
The conventional security assessment is discussed in [32]. The first step in the security assessment of the power system is related to the static security of the system, in which the load flow feasibility and different system constraints are checked for a system with different N-1 (or even N-2) contingencies. At this stage, the system static security will be checked for all contingencies. Then, if the system static security is guaranteed for all contingencies, the dynamic security will be checked for the whole system. The dynamic security of the system is related to the stability of the system, when the system is subjected to a contingency. In this way, regarding the dynamic security of the system, the small-signal model of it concerning different contingencies is assessed. If the system's small-signal model is stable for different contingencies, then the dynamic security is guaranteed. The last step is to check whether the system's transient security is guaranteed. To do so, the stability of the system subjected to all credible large disturbances is checked. In a conventional security assessment model, the impact of the PE-based unit on the system security is not included.  
Regarding the PE-based power system security assessment, the PE-based units' security is included in all steps of the conventional model. This is shown in Figure 3. In this way, for the static security of the system, no further efforts are needed in comparison with the conventional method. However, for the dynamic and transient security, the PE-based units' model should be included in the assessment. This can be done by including the PE-based security in the assessment loop, as shown in Figure 3. In order to execute the security assessment of PE-based power systems based on Figure 3, the PE-based units' synchronization and the system frequency changes should be checked. For the synchronization of PE-based units, the main outline is to check the performance of the synchronization unit of the PE-based energy source. For instance, if a PLL is used to synchronize the unit with the rest of the system, then it should be able to estimate the PCC phase angle and introduce the system frequency with an appropriate dynamic response [58]. In this way, first, the small-signal model of the grid should be checked, so if all eigenvalues of the small-signal model are in the left half of the imaginary plane, it can be concluded that all the oscillatory modes of the system can be damped. Then, for the transient response, it should be checked whether the synchronization unit can estimate the system frequency with an appropriate dynamic response or not [62]. On the other hand, the frequency nadir and the ROCOF can be checked for the whole system. To do so, an average of the synchronous generators' rotor speed (in pu) is used to define the system frequency, and based on its response to disturbances, ROCOF and system frequency nadir can be defined [65].  
Regarding the dynamic security of the PE-based power systems, the small-signal model of the system should be used for the small-signal stability assessment. In this way, for every N-1 contingency, the system small-signal model is checked as to whether it is stable for all small disturbances. If the system is stable for all small-signal case studies (N-1 contingencies), then the system is dynamically stable.  
The same process should be done for the transient stability assessment. If the system is stable for all large disturbances, then its transient security is guaranteed. However, as it is mentioned in the latter section, PE-based units' control system may be affected by large-disturbances. Therefore, their stability for large disturbances should be checked.