Abstract
Most wind turbines are monitored and controlled by supervisory control and data acquisition systems that involve remote communication through networks. Despite the flexibility and efficiency that network-based monitoring and control systems bring, these systems are often threatened by cyberattacks. Among the various kinds of cyberattacks, some exploit the system dynamics so that the attack cannot be detected by monitoring system output, the zero-dynamics attack is one of them. This paper confirms that the zero-dynamics attack is fatal to wind turbines and the attack can cause system breakdown. In order to protect the system, we present two defense strategies using a generalized hold and a generalized sampler. These methods have the advantage that the zeros can be placed so that the zero dynamics of the system become stable; as a consequence, the zero-dynamics attack is neutralized. The effects of the countermeasures are validated through numerical simulations and the comparative discussion between two methods is provided.
Highlights
Wind energy has been recognized as one of the major renewable energy sources for over two decades
These approaches are based on the fact that the zeros of the discrete-time system can be arbitrarily assigned if the zero-order hold (ZOH) is replaced by a generalized hold (GH) or if a generalized sampler (GS) is used instead of simple sampler (SS) [20]
It is seen that the generator angular velocity ωg(t) starts oscillating with increasing magnitude and this is captured by the sampled output ωg(kTs) when GH is used, while the sampled output under ZOH remains almost unchanged
Summary
Wind energy has been recognized as one of the major renewable energy sources for over two decades. If the zero dynamics (which corresponds to the zeros of the transfer function) is unstable, there exists an input signal that drives some internal variable of the system to be unbounded while unnoticed by monitoring the system output Exploiting this fact, one can construct an undetectable cyberattack by copying the zero dynamics of the system. If this attack is applied to the system, by stability, the internal variable will approach the attack signal while the change of the output can hardly be detected [8]. In the first part of this paper, we demonstrate that the sampled-data model of the wind turbine system has unstable zero dynamics and is vulnerable to ZDA; a ZDA is constructed so that the generator angular velocity diverges but its sampled values remain almost constant.
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