Wide-Area Communication and Control: A Cyber-Physical Perspective

For several decades the traditional mindset for controlling large-scale power systems has been limited to local output feedback control, which means that controllers installed within the operating region of any utility company typically use measurements available only from inside that region for feedback, and, in fact, more commonly only from the vicinity of the controller location. Examples of such controllers include Automatic Voltage Regulators (AVR), Power System Stabilizers (PSS), Automatic Generation Control (AGC), FACTS control, HVDC, and so on. However, the US Northeast blackout of 2003, followed by the timely emergence of sophisticated GPS-synchronized digital instrumentation technologies such as Wide-Area Measurement Systems (WAMS) led utility owners to understand how the interconnected nature of the grid topology essentially couples their controller performance with that of others, and thereby forced them to look beyond this myopic approach of local feedback and instead use wide-area measurement feedback [1]. Over the past few years several researchers have started investigating such data-driven wide-area control designs using H∞ control [2, 3, 4], LMIs and conic programming [5], wide-area protection [6], model reduction and control inversion [7, 8], adaptive control [9], LQR-based optimal control [10, 11, 12, 13], etc., complimented with insightful case studies of controller implementation for various real power systems such as the US west coast grid [14, 15], Hydro Quebec [16], Nordic system [17, 18], and power systems in China [19], Australia [20], and Mexico [21]. A tutorial on the ongoing practices for wide-area control has recently been presented in [22], while cyber-physical implementation architectures for realizing these controls have been proposed in [23, 24]. One of the biggest roadblocks for implementing wide-area control in a practical grid, however, is that the current power grid IT infrastructure is rigid and low capacity as it is mostly based on a closed-mission specific architecture. The current push to adopt the existing TCP/IP based open Internet and high-performance computing technologies such as the NASPInet [25] would not be enough to meet the requirement of collecting and processing very large volumes of real-time data produced by such thousands of PMUs. Secondly, the impact of the unreliable and insecure communication and computation infrastructure, especially long delays and packet loss uncertainties over wide-area networks, on the development of new WAMS applications is not well understood. For example, as shown in [26]-[28] uncontrolled delays in a network can easily destabilize distributed estimation algorithms for wide-area oscillation monitoring using PMU data from geographically dispersed locations. Finally, and most importantly, very little studies have been conducted to leverage the emerging IT technologies, such as cloud computing, software defined networking (SDN), and network function virtualization (NFV), to accelerate the development of WAMS [29]. Another major challenge is privacy of PMU data as utility companies are often shy in sharing data from a large number of observable points within their operating regions with other companies. Equally important is cyber-security of the data as even the slightest tampering of Synchrophasors, whether through denial-of-service attacks or data manipulation attacks, can cause catastrophical instabilities in the grid. What we need is a cyber-physical architecture that explicitly brings out potential solutions to all of these concerns, that is how data from multitudes of geographically dispersed PMUs can be shared across a large grid via a secure communicate medium for successful execution of critical transmission system operations, how the various binding factors in this distributed communication system can pose bottlenecks, and, how these bottlenecks can be mitigated to guarantee the stability and performance of the grid. Motivated by these challenges, in this tutorial we review the current state-of-the-art practice for wide-area communication and control from a cyber-physical system (CPS) viewpoint, the ways