## 1 Introduction
### Related Work  
The LFC is known as a typical automatic closed-loop system that maintains the grid frequency and scheduled tie-line power between controlled areas by tuning the setpoints of generators for active power output, based on the wide-area transmitted measurements [12]. Research activities have been carried out to look into the attack impact on conventional LFC systems with pure AC transmission lines. In the early work of the authors of [13], effects of data integrity attacks on the operating frequency stability of LFC are introduced. Then, it is demonstrated in [7] how different FDI attacks on the LFC loop can affect the system frequency and electricity market operation. The work in [14] introduced a systematic method, based on reachability, for evaluating the impact that an FDI attack can have on the LFC system. Experimental tests of various cyber attacks on LFC using Cyber-Physical Security Testbeds can be found in [8]. In [15], the modeling language Modelica is introduced to support impact assessment of FDI attacks on the LFC. There are also some studies that have started to explore the effects of data availability attacks on LFC loops. The work in [16] illustrates that DoS attacks are able to make the dynamics of a LFC system unstable. The work in [17] focuses on the impact of time-delay attacks on the dynamic behavior of a multi-area LFC, indicating that such an availability attack can be more harmful in the area where there are load changes. Other related research is about communication delay or packet loss in the LFC system. For instance, a linear discrete-time model that includes the effects of different communication delays in the LFC model is proposed in [18] to explore the stability issues. Similar studies on communication delay/packet loss in conventional LFC models of normal AC systems can be found in [19; 20; 21].  
The conventional LFC models have been modified to adapt to the reformulation of traditional power systems [22]. One of such aspects is about the deployment of HVDC transmission lines between controlled areas [23; 24]. Besides, new functionalities are added in the frequency control to consider the matter of virtual inertia emulation [25; 26]. In the pioneering work in [27], a method for evaluating the effects of virtual inertia on the dynamic behavior of a two-area LFC system is developed. As mentioned earlier, widespread application of communication networks in the LFC-related loop could make it vulnerable to various cyber attacks. However, as far as we know, there is still a lack of studies on the cyber security research for the new LFC system in the hybrid grid with AC/HVDC interconnections and emulated inertia by storage devices. The following references have started to evaluate the attack impact on the part of HVDC system control or the inertia emulation process; however, none of them have focused on the overall LFC loop considering new elements of HVDC links and emulated inertia by ESS. The work in [28] has studied the effects of cyber attacks on the dynamic voltage stability of a HVDC system. The authors of [29] try to evaluate the impact of cyber attacks on the HVDC transmission oscillation damping control. The work in [30] demonstrates the risk/impact of a cyber-physical attack in which loads providing emulated inertia control services are attacked. The work in [31] has interpreted the effects of FDI attacks on the LFC of a low-inertia power system. Our recent work in [32] aims to propose a comprehensive framework for vulnerability and impact analysis of stationary FDI attacks on the hybrid AC/DC grid. To conclude, research efforts are still needed to evaluate the effects of different types of data integrity and availability attacks on the LFC system with new elements of HVDC link and also ESS for virtual inertia.