Impacts of Superconducting Cables on the Dynamic Response of Current Transformers and Protective Relaying Devices

Replacing conventional ac transmission lines with superconducting cables provides performance advantages since superconducting cables have lower series inductance and very low ac resistance. However, the very low resistance will also have an impact on the dynamic response of current transformers (CTs) and on the response of protective relays that process their current measurements. Whenever a short circuit occurs on an ac power system the resulting current will have both fundamental frequency ac response and a decaying dc offset. The amplitude of the dc offset is dependent on the point on the ac voltage wave where the fault occurs, which is random. The rate of this decay depends on the R-L time constant of the ac circuit involved in the current path. A larger time constant increases the odds that a CT will enter saturation. Superconducting cables will increase the R-L time constant, and potentially increase the severity of CT saturation, creating error in the secondary current used by the protective relays. For many protection functions this error will delay the protective action, but in a few cases it may cause circuit breakers to trip in error. This paper will model a sample system with differing levels of penetration of superconducting cables and look at the impact of the superconducting cables on the dynamic CT response and on different protective relay functions and discuss potential solutions.