A current transformer (CT) reduces high currents to small, usable values. We utilize CTs for metering and protection purposes. Proper CT selection ensures accuracy, safety, and reliable protection.
Identify the Purpose of CT
Metering CT
A metering CT measures current for billing and monitoring. Therefore, it must work accurately within the tolerable upper limit of the rated current. Because we always predict that the measuring current can increase by 10 to 20% excess during overloading conditions. So, to measure only up to this range, a current transformer must have a high accuracy class (0.2, 0.5). Obviously, it is enough to maintain the high accuracy class up to this range only. Certainly, this is a cost-effective approach for CT design. Additionally, a CT must not saturate under normal load. Because saturation significantly disturbs the accuracy.
Protection CT
A protection CT, on the other hand, has to detect fault current. It may not stay very accurate during short circuits. However, the core of the CT must remain unsaturated during the flow of faulty current. Therefore, it requires high ALF (5P10, 5P20). Also, it needs a high knee point for a PX or PS class CT.
Choose the CT Ratio
For that, we first find out the maximum load current. Then add a 20–30% margin. After that, we select the next higher standard ratio available in the market. For example, suppose the load is 380 A. Therefore,
380 × 1.25 = 475 A → We must choose 500/1 A or 500/5 A current transformer (CT), for both metering and protection.
Select the Core Type
We generally choose the first core as the Metering core. Then, we design the remaining cores for protection purposes in a multicore current transformer. We can create the protection core either for ALF classes or PS/PX classes. The ALF classes (5P10, 5P20, etc.) serve the purpose of general protection. In contrast, PS or PX class cores are used for differential, REF, and busbar schemes. We always need to match the core to the proper application.
Choose the Accuracy Class
Metering Class: We may follow IEC 61869 for the accuracy of current transformers. We use 0.1, 0.2, 0.5, and 1.0 as the classes of the metering CT. These values represent the errors of the current transformers. It is a combination of ratio and phase angle errors. For each metering class, there are corresponding ratio error and phase angle error. Here, the low value implies better accuracy. We use a 0.2 class for energy metering purposes related to billing. For only current monitoring with ammeters, we can use 0.5.
Protection Accuracy: We use 5P10, 5P15, 5P20, and 10P10, etc., for general protection purposes. Here, 5P refers to 5% composite error. Similarly, 10P means 10% composite error. On the other hand, the numbers 10 and 20 after P represent the Accuracy Limit Factors (ALFs). Obviously, the higher ALF implies better fault performance.
PS and PX Class of Current Transformers
PS implies Protection Special. On the other hand, PX means Protection eXact. The CTs with PS or PX classes are special protection-class current transformers. They give very high accuracy during faults and avoid saturation, especially in differential and REF (Restricted Earth Fault) protection. Nowadays, we often use the PS or PX class also for all normal protection purposes. These CTs do not follow normal 5P/10P accuracy. Instead, they follow excitation curve parameters, allowing protection engineers to calculate performance precisely. Engineers use parameters like knee-point voltage (Vk), secondary resistance (Rct), and excitation current (Ie).
Playlist on Current Transformers