Candidate: Mohsen Khalaf
Title: Cyber Physical Security of Wide-Area Under-Frequency Load Shedding Protection Schemes
Date: June 14, 2019
Time: 2:30 PM
Place: EIT 3145
Supervisor(s): El-Saadany, Ehab - Salama, Magdy A.
Unbalance between generation and load, loss of generators, or disconnection of large interconnection lines are the main reasons for under-frequency in power systems. The purpose of load shedding is to maintain the balance between load and generation in case of emergency. Wide-Area Under Frequency Load Shedding (WAUFLS) is the process of shedding the load in events that cause a significant drop in the frequency of an interconnection or islanded area. Therefore, WAUFLS schemes must be selective, accurate and quick.
The common practice in the past was to use UFLS techniques for local protection. Basically, these techniques shed a certain amount of the load under relief when the system frequency falls below a certain threshold. If the frequency keeps on falling after the first shed, further sheds are performed when lower thresholds are passed. The values of the thresholds and the relative amounts of load to be shed are decided off-line, on the base of experience and simulations. Some of these techniques are based on measuring the Rate of Change of Frequency (ROCOF) when a certain frequency threshold is reached. These developed techniques are called semi-adaptive techniques.
Recently, many adaptive WAUFLS schemes have been proposed. These schemes make use of the frequency derivative and are based on the System Frequency Response (SFR). The initial value of the ROCOF is proportional, through the inertia constant H, to the size of the disturbance. Thus, assuming that the inertia of the system is known, the measure of the initial ROCOF is a backward estimate of the disturbance and consequently an adequate countermeasure in terms of load-shedding can be operated.
Although the frequency is usually considered as a global parameter in the power system, many results in the literature show that the frequency values in neighboring areas within the same grid are slightly different during system transients. This gives a chance to attackers to manipulate each frequency measurement value separately. In some systems, the frequency signals are sent using secure links. However, due to the expensive cost of the secured point to point communication links, not all control systems use it to secure frequency measurements. This work investigates the problem of cyber-physical security on WAUFLS. It is shown that the attacker can affect the operation of these schemes which may result in a system-wide blackout, equipment damage and/or shedding important loads.
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