The Grid is BLIND without CTs

Description

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.

This is an engineering-level introduction rather than a formal training course, but hopefully it gives you a useful look inside one of the most important pieces of instrumentation on the electricity grid.

⚠️ **Safety:** HV equipment and CT secondary circuits can be extremely hazardous. The testing shown or discussed here should only be undertaken by appropriately trained and authorised personnel using the correct procedures and equipment.

Check out the Refoss energy monitoring equipment mentioned below. This isn't a sponsored video but I may receive a commission on purchases.
Refoss Open API Smart Energy Monitor, EM06P: https://refoss.net/collections/refoss-open-platform/products/smart-energy-monitor-em06p?ref=jordan

Exclusive Discount: Your audience can use the code JorT10 for an extra 10% off.

If you'd like to see dedicated videos on **CT testing, differential protection, DC current transformers or protection commissioning**, let me know in the comments.

I'm Jordan, a transmission engineer from the UK. Grab a brew and let's dig into the technology that keeps the grid running.