## 3 Power-Electronic-Based Power System Security  
### PE-Based Power System Security Challenges  
In this section, the main challenges of power system security caused by the increase of PE-based unit penetration are discussed.  
One of the main differences between the conventional energy sources, such as thermal and hydro-power plants, with renewable energy sources (RES), such as wind turbines and PV power plants, is the lack of inertia in the latter one. PE-based energy sources have no rotational part that acts as the inertia source to support the system's frequency response [59; 60]. Therefore, a disturbance that leads to frequency fluctuations in a system with high penetration of PE-based energy sources, may lead to lower frequency nadir response and a higher ROCOF, which consequently may lead to unsynchronization of the generators. This type of instability is related to the frequency stability and also rotor angle stability, in which systems with less inertia are more vulnerable to the large disturbances, such as loss of generation. It is worth mentioning that low-inertia PE-based power systems show higher ROCOF in comparison with the conventional power systems. This is extensively seen in small power systems, such as microgrids [12; 61].  
On the other hand, the control system of PE-based units is more complex in comparison with the conventional one. Generally, a control system is used to synchronize the PE-based unit with the power system. A standard control system used in the synchronization unit is the phase-locked loop (PLL). The PLL is a nonlinear feedback control loop; its stability is related to its parameters and also to the system configuration. Increasing the number of PE-based units makes the power system weaker, which means that the system states are less controllable. In this case, the PE-based unit connected to the weak grid using the PLL for the synchronization may become unstable. The PLL is sensitive to system fluctuations. Although the voltage phase angle and the frequency are decoupled variables in the system, they are coupled in a standard PLL. Therefore, a phase jump in the system may converge to a wrong estimation of the frequency by the PLL. If a protection system is based on the PLL output signals, then a phase jump may converge to an undesired trip of a PE-based unit [23; 58; 62].  
Besides the voltage, angular, and frequency stabilities, the modern power electronics-based power systems are exposed to electro-mechanical-magnetic (EMM) instabilities due to the interactions of the control and hardware components of converters with electrical, magnetic, or mechanical components of the power systems [11; 63; 64]. The time of interest for this kind of stability is from microseconds related to the switching frequency of converters up to several milliseconds related to the power sharing control loops. So far, EMM stability has not been considered in the security assessments of power systems; the proliferation of converters will highlight the impact of EMM stability on the overall system performance. More security challenges of the PE-based power systems are listed in Table 2.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & & & Lack of inertia/high ROCOF [65] \\ \cline{3-4}  & Contingencies derived by PE-based units’ design & System frequency support & Fast frequency response limitations [66] \\ \cline{3-4}
**PE-based Power System  
+++ ==WARNING: Truncated because of repetitions==