# Power Grid Reliability Evaluation Considering Wind Farm Cyber Security and Ramping Events
## 1 Introduction  
As the predominant source of renewable energy until 2017, global wind power capacity has expanded 11% to 539 GW [1]. The dynamic performance and reliability of power systems are increasingly relying on the operation of wind farms. However, the cybersecurity of wind farms is challenged by their simple and reliable communication protocols, fixed control flows, long-update cycles of software, and stable topologies [2]. On the one hand, the operation of a wind farm is affected by the intermittence of wind speed and the low inertia of wind turbines (WTs). On the other hand, the cyber threats of a wind farm bring new uncertainties that lead to the abnormal operation and undesired-tripping of WTs. For the secure and reliable operation of a power grid, it is very necessary to develop reliable evaluation methods that incorporate the cyber security of wind farms. To quantify the availability and relationship of the cyber-physical effects [3] of a wind farm, a multi-state Markov model [4; 5] was broadly applied. Furthermore, graphical methods were developed for a better expression of the cyber-physical relationship, such as the stochastic Petri net [6], the Bayesian network [7], the complex network [8], and the attack tree [9]. Though the cyber-physical relationship has been modeled by different approaches, it is challenging to quantify the frequency of attacks and system defense mechanisms. [PERSON] et al. [10] proposed a preliminary study of cyber-attack effects on power grid reliability. The forced outage modes of critical components caused by a cyber-attack were investigated. Data driven approaches have also been popular to estimate the time to compromise or restore the vulnerabilities of an information network [11]. The meantime-to-compromise (MTTC) the vulnerabilities is a widely used unit to quantify the frequency of cyber-attacks [7; 12; 13]. Once the probabilities of a cyber-attack and its cyber-physical relationship are determined, the states of cybersecurity can be sampled for further reliability analysis. Thus far, there has been a lot of investigation and research on the cybersecurity of wind farms. However, most of them have not considered the impact of cyber-attacks on power grid reliability. In [14], the attack scenarios involving WTs control, wind farm disruption and substation disruption were discussed in detail. The dynamic simulation of a WT was tested in [15], and the impact caused by malicious modifications on the WT control parameters was discussed. In [16], power system reliability, considering the sudden tripping of WTs, was evaluated via a reliability analysis. With the specific attack paths, the MTT was applied to estimate the frequency of cyber-attacks. However, the contributions of system defense is rarely considered. Though the tripping attack results in great harm to power systems, it is fortunately and usually one-off and unsustainable. Nowadays, a growing concern is the intention to damage the WTs rather than stop them. If the control system of WTs is intruded, the malicious command can mislead the WTs to run in a dangerous state. The impacts of cyber-attacks against the mechanical components of WTs should be further investigated. Wind power ramping events (WPRs) [17] are typically known as a cause of large power fluctuations. The load shedding resulted from WPRs exceeded 1000 MW/year according to the report of the Electricity Reliability Council of Texas system [18]. Uncertainties associated with wind power integration challenge the operational adequacy of conventional power systems. The modeling of WPRs, especially the feature of ramping rate [19], should be fully considered to evaluate the operational adequacy of power systems. To address those problems, the main contributions of this paper are as follows: (i) A stochastic attack-defense model is proposed to estimate the frequency of cyber-attacks. (ii) Two typical attack scenarios, including an imperfect attack, are modeled to assess the impact of cybersecurity in the wind farm.