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Sep 12, 2026 - Detecting Faults on the National Grid How does a 5000 amp transmission circuit get measured safely by a protection relay? Or a 63,000 amp fault? In this video, we take a deep dive into "HV AC current transformers (CTs)": what they are, how they work, why they`re critical to grid protection, what the ratings on their nameplates actually mean, the different types you`ll find on the electricity network, and how we test them during commissioning. We`ll cover: • How CTs transform thousands of amps into a small, usable secondary current • CT ratios, polarity and why getting them backwards can cause protection to trip incorrectly • Metering vs protection CTs • Accuracy classes including 0.5, 5P and PX • Accuracy Limit Factor (ALF), composite error and saturation • CT burden and why cable runs matter • Knee point / excitation characteristics • Toroidal, bar, bushing and split-core CTs • Core balance and summation CTs • Overdimensioned, anti-remanence and linear CTs • Oil and SF6 insulated CTs • CT analysers and what they actually test • Secondary and primary injection testing • Stability and operate shots for differential protection • The potentially serious consequences of getting CT testing wrong Current transformers are effectively the "eyes of the protection system". Without accurate CT measurements, protection relays can`t reliably determine what`s happening on the network or decide when a circuit breaker needs to trip. Similar news:
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