## 1 Introduction  
In the modern power systems, there is an increasing attention on the integration of renewable energy resources (RES), energy storage devices, and high voltage direct-current (HVDC) links. In order to support frequency control in these low-inertia systems, the recent trends of research are oriented in proposing different virtual inertia emulation approaches [1; 2; 3]. To meet the changes, the conventional control scheme such as the load frequency control (LFC) is also adapting to handle the new scenario where controlled areas are interconnected by parallel alternating-current (AC) and HVDC transmission lines and energy storage systems (ESS) are added for emulating virtual inertia. On the other hand, this transformation has introduced a high dependence on data communications, as the control loops involved in the new LFC would use communication networks such as the supervisory control and data acquisition (SCADA) system to transmit measurements and control data. However, the communication channels in the SCADA network for data exchange, especially the ones for wide-area measurements, are usually unprotected, leaving the LFC system more exposed to cyber threats [4; 5; 6]. In fact, it has been reported that the conventional LFC loops of multi-area systems with pure AC interconnections can be vulnerable to a large number of malicious intrusions [7; 8]. Furthermore, a deliberate attack targeted on the LFC system can have a direct effect on the system frequency and further cause severe damages to the stability and economical operation of the grid [9].  
Different from the conventional LFC, in the context of modern power systems with AC/HVDC interconnections and emulated inertia by the ESS, more controllable resources would be added in the new LFC system to enable an improvement of the dynamical response. Furthermore, having more controllable devices also increases the vulnerability to cyber attacks. Though the cyber security concerns of conventional LFC in the normal AC system have been given considerable attention, it remains insufficiently answered how the new LFC in the hybrid AC/DC grid behaves under different cyber attacks. In general, each attack can be viewed in light of corrupting one (or multiple) of the following aspects of data; confidentiality, integrity, and availability Pan2017a. From the perspective of attack impact on the physical power system, the data integrity and availability attacks are more of interest for cyber security analysis. In fact, it has been reported that both data availability and integrity attacks can corrupt the conventional LFC system; see the related work in the following subsection. Thus, in this article, we are motivated to explore the effects of the typical data availability and integrity attacks--Denial of Service (DoS) and false data injection (FDI) attacks--on the new LFC considering AC/HVDC interconnections and the matter of virtual inertia. Here, the DoS and FDI attacks are mainly considered as they are major types of data availability and integrity attacks. For the DoS attack, it is one of the major threats against the availability of data [10]. The FDI attack is also known as a major class of integrity attack. Moreover, recent incidents, like the 2015 Ukraine blackout caused by hackers, have implied the feasibilities of DoS and FDI attacks on the smart grid devices of the real world [11]. Notably in this end, we have known that a DC grid has a low tolerance to a fault; we would also like to know how the new elements of HVDC link and inertia emulation module by ESS would affect the dynamic behavior of LFC under an intentional DoS or FDI attack.