Current Transformers (CTs) play a crucial role in power systems. They perform both metering and protection functions. However, the same CT can not perform both metering and protection functions. This is because each of the protection and metering applications has different accuracy, burden-handling, and dynamic performance requirements. Hence, CTs are designed as either Metering CTs or Protection CTs. Although in most cases, both CTs are combined in a multi-core current transformer.
Metering CT
A metering CT accurately measures current under normal load conditions. Therefore a metering CT maintains its accuracy only up to 1.2 × rated current. Because this is the most economical approach. This also reduces the size of the core.
Protection CT
A protection CT reproduces a faithful current during faults. The fault current often rises to 20–30 times the rated current. A protection CT ensures the correct operation of the relays during short circuits. The knee voltage of a protection CT core is significantly higher than that of a metering CT core. That means the core must remain unsaturated for a high current value, even if the accuracy may not remain effective.
Constructional Differences
We use high-permeability CRGO steel for metering CTs. Nowadays, many manufacturers use a nanocrystalline core for the purpose. A nanocrystalline core offers a much smaller magnetizing current. Hence, it helps to measure currents with great accuracy during normal operating conditions. The metering cores are smaller compared to the protection cores. Because they work with a low burden, meaning they supply only a small burden (load) of either a wattmeter, or an energy meter, or an ammeter. Due to this design, the flux density in a metering CT stays close to its rated value and may saturate early if the current becomes too high.
In contrast, protection CTs have a larger cross-sectional core. A larger cross-section ensures that it can handle a high amount of flux without saturation. In other words, we can say that it can deal with fault currents. A fault current is much higher than the rated current. The cross-sectional size is optimized to maintain a linear magnetizing curve during faults for a specific number of secondary turns. Obviously, it ensures that protection relays receive the correct current signal.
| Feature | Metering CT | Protection CT |
|---|---|---|
| Core Material | High-permeability CRGO steel for accuracy. Also, nanocrystalline core materials for more precise accuracy. | Large cross-sectional CRGO steel core to avoid saturation under faults. Some manufacturers prefer not to use nanocrystalline material in protection cores due to cost optimization. |
| Core Cross-Section | Smaller | Larger |
| Flux Density | Close to the rated point | Much below the saturation point, even at fault currents |
Accuracy Requirements
Metering Accuracy
We specify a metering CT with typical Accuracy classes such as 0.1, 0.2, 0.2S, 0.5, 0.5S, and 1.0 classes, etc. Each class allows a specified ratio and phase displacement error. The table below shows the ranges of errors according to the standard classes.
Ratio Error Limits at Different Percentages of Rated Current
| Accuracy Class | ± Ratio Error (%) at 1% | 5% | 20% | 100% | 120% |
|---|---|---|---|---|---|
| 0.1 | — | 0.4 | 0.2 | 0.1 | 0.1 |
| 0.2 | — | 0.75 | 0.35 | 0.2 | 0.2 |
| 0.5 | — | 1.5 | 0.75 | 0.5 | 0.5 |
| 1.0 | — | 3.0 | 1.5 | 1.0 | 1.0 |
| 0.2S | 0.75 | 0.35 | 0.2 | 0.2 | 0.2 |
| 0.5S | 1.5 | 0.75 | 0.5 | 0.5 | 0.5 |
Phase Angle Error Limits (in Minutes) at Different Percentages of Rated Current
| Accuracy Class | ± Phase Angle (min) at 1% | 5% | 20% | 100% | 120% |
|---|---|---|---|---|---|
| 0.1 | — | 15 | 8 | 5 | 5 |
| 0.2 | — | 30 | 15 | 10 | 10 |
| 0.5 | — | 90 | 45 | 30 | 30 |
| 1.0 | — | 180 | 90 | 60 | 60 |
| 0.2S | 30 | 15 | 10 | 10 | 10 |
| 0.5S | 90 | 45 | 30 | 30 | 30 |
Here, we observe that the standard accuracy range is maintained only from 100% to 120% of the rated current. Although 0.2S and 0.5S classes maintain it from 20% to 120%. We refer to these 0.2S and 0.5S classes as special classes. These special classes include error limits at 1% of rated current, as shown in the table. Obviously, it implies the special classes have more stringent low-current accuracy.
Here, saturation occurs intentionally to protect measuring instruments from the fault current.
Protection Accuracy
We specify the protection accuracy classes as 5P10, 5P20, 10P10, 10P20, etc. Here, the first digit specifies the allowable composite error. For example, 5P20 class has 5% composite error up to 20 times the rated current. This 20 is ALF of the CT. This parameter implies the CT remains unsaturated during the fault to ensure correct relay operation. Here, P stands for protection.
PS Class
A PS Class provides high accuracy under fault conditions. The PS classes are characterized by Knee Point Voltage (Vk), magnetising current (Im), and secondary resistance (Rs). We use PS classes for unit protections like transformer, generator, busbar, and line differential protections, and restricted earth fault (REF) protection.
Why PS Class is Important
Protection schemes like differential and REF work on small current differences. Any CT saturation can cause maloperation (false trip), or failure to operate (security risk). PS class CTs prevent this by matching the magnetizing curve of all the CTs used for a unit protection scheme. We match the linear portion of the magnetizing curve by matching the Knee Point Voltage (Vk), magnetizing current (Im), and secondary resistance (Rs).