The conventional current transformers (CT) with iron core are still the most widely used components for protective relays measurements. Due to the non-linear magnetizing characteristics of the iron core, CTs are prone to saturation under heavy fault and transient conditions. Saturation levels depend on the fault current magnitude, CT burden, primary time constant of a DC offset current, level of CT core remanence, and dimensioning factor of the CT. When CT saturates, the primary currents cannot be reproduced correctly, leading to the misoperation of protection devices connected to the CT. These incidents are on the rise due to increased fault levels of the expanding power system. The saturation free time of the CT, which is the time elapsed for the CT to saturate from fault inception, plays a major role in the correct operation of protection relays. Utilities try to maximize this saturation-free time by specifying strict CT requirements when needed. However, with improved saturation detection algorithms in modern relays that require shorter saturation-free times, such requirements can be relaxed. These developments can enhance cost efficiencies through reduced CT dimensioning and less costly secondary circuit design.
Testing a wide variety of cases in a simulation platform similar to actual system conditions is the key for utilities to optimize the CT requirements. Real-time digital simulation (RTDS) testing provides the greatest tool to verify CT design requirements. Using the RTDS with the hardware in loop (HIL) testing replicates the actual system conditions to simulate protective devices' performance. This method offers several advantages compared to the conventional processes and tools utilized by most utilities to evaluate the relay behavior under CT saturation circumstances. Such processes and tools are developed based on simplified models and assumptions, which do not represent the true behavior of the system under the test. HIL testing using RTDS would allow engineers to understand the relay features thoroughly. However, the cost and time required to prepare the test plan and test setup to execute the simulations could hinder the engineers from moving forward with this testing approach.
We present methodologies that would help reduce the cost of RTDS testing of bus differential protection elements in the presence of CT saturation. We discuss how a 1-Amp relay input can be used instead of a 5-Amp input for testing to reduce the number of amplifiers required for testing. In addition, it is shown how to vary the CT saturation free time by changing the relay burden instead of requiring relatively high magnitude secondary test currents that exceed regular test set current limits. Automated test scripts have been used to run several test cases to reduce the test execution times and thoroughly test the relay behavior. Various test cases have been simulated, and the efficacy of the adopted methods is demonstrated
M. Khanbeigi, A. Hangilipola, M. Leung, J. Holbach, H. Khani, A. Momeni, A. Davari, Presented at PAC World Global Conference 2021, Virtual Event, August 2021