The Importance of Annealing CT Cores

Why do you need to anneal the electrical steel core in a current transformer?

By: Jessica Seelman

The annealing process of a current transformer (CT) core is a critical operation within the manufacturing process that must be performed properly or the CT will not function correctly. Typically, an electrical steel core in a CT will be a toroidal core. Toroidal cores are shaped like a donut and are very efficient because the secondary winding can be evenly distributed which improves the electrical field. Toroidal cores need to be annealed after they are wound into the desired shape. The is because, as the core is being formed, stresses are placed on the steel the grains are separated within the steel. This reduces the electrical performance. To relieve the steel of the stress and realign the grains, you must anneal the toroidal core.

The annealing process can be performed in a continuous flow annealing furnace or in a batch furnace. A continuous flow furnace is most efficient when one will consistently have a large volume of steel to anneal. Continuous flow furnaces are very efficient if operated properly because they have the ability to hold temperature once ramped up to the desired temperature. On the other hand, a batch furnace works well if your demand of steel to anneal is low or inconsistent. In either type of annealing furnaces, the heating profile, atmosphere, and cool down speed is critical. Your steel manufacture should be able to recommend the best practices to follow when annealing in either type of furnace.

So now we know why a core in a CT has to be annealed but how do you determine if the annealing process worked? Once a CT is completely assembled it should go through final tests to ensure it meets the customer’s requirements. One test that is required by the IEEE C57.13.6-2016 standard is an excitation test. The excitation test applies voltage to the CT and measures the exciting current. The minimum exciting current is established based on the copper winding. If the exciting current is lower than the established limit the core was not annealed properly and it will not meet the relaying or metering requirements. The only way to correct this issue is to disassemble the CT and reanneal the core then start the assembly process over again. As you can imagine this creates a great deal of non-value added time and additional resources. This is why the annealing process is so critical. If one continuously has annealing problems it can create non-value added processing time, waste, and potentially prevent you from meeting your customer’s due date.