CVT transients revisited — Distance, directional overcurrent, and communications-assisted tripping concerns

Several classic papers explain the fundamentals of capacitive voltage transformer (CVT) design, operation, and transient response. Distance elements can overreach, particularly in high source-to-line impedance ratio (SIR) applications, which can result in undesired Zone 1 operations. Because this continues to be a problem in real applications, this paper revisits documented field cases using event data in hopes of shedding new light on this known problem. Solutions to distance element overreach are shared, from modified reach and time delays to modern solutions such as CVT transient detection logic. How does the protection engineer know what type of CVT is used? How can the protection engineer calculate the SIR from real-world event data? This paper gives practical guidance for the user to answer these fundamental questions. New data and research included in this paper update the topic. We investigate the CVT transient effect on directional element stability, directional overcurrent applications, and various communications-assisted protection schemes. We also share field cases of directional element and directional comparison blocking scheme misoperations and solutions and practical recommendations for mitigating the problems in all cases.