What is the standard current transformer ratio?

The transformation ratio of a current transformer is the ratio of the primary current to the secondary current, such as 500/5, 250/5, 100/5, 75/5, 50/5, 25/5, etc. For example, 500/5 means that when the primary current reaches 500A, the current induced on the secondary side is 5A. The ratio of these two is the transformation ratio.

The secondary side of a current transformer is always 5A. The transformation ratio of a current transformer is the primary current divided by the secondary current. Because of current transformers, we can now make ammeters with a 5A rating. Current relays can also be made with a 5A rating.

To determine the transformation ratio of a current transformer, you first need to understand its operating principles and application scenarios.

Current transformers are primarily used to measure large currents. They do this by converting the high current on the primary side into a smaller current on the secondary side, allowing measurement using a standard-range ammeter. The primary winding of a current transformer is typically connected in series with the main circuit, while the secondary winding is connected to the measuring device or protective device.

The transformation ratio is the ratio of primary to secondary current, typically expressed as I1/I2 or N2/N1 (N1 is the number of primary winding turns, N2 is the number of secondary winding turns), where I1 is the primary current and I2 is the secondary current. For standardization, the secondary current is typically fixed at 5A or 1A. When choosing an appropriate transformation ratio, consider the following factors:

• Maximum operating current of the load: Ensure that the primary current does not exceed the maximum rating of the transformer.
• Measurement accuracy requirements: Different current conversion ratios can affect measurement accuracy.
•  Cost-effectiveness: Select the most cost-effective solution.

A current transformer (CT) is an instrument that uses the principle of electromagnetic induction to convert a large primary current into a smaller secondary current for measurement. A CT consists of a closed iron core and windings. Its primary winding has a small number of turns and is connected in series with the current being measured.

The transformation ratio is designed for different load conditions. High primary currents result in a larger transformation ratio, while low primary currents result in a smaller ratio. However, it must be appropriate. For example, if the maximum primary current exceeds 300A for a prolonged period, a 250/5 current transformer will burn out due to the overload. However, if the primary current is lower, using a larger current transformer will result in inaccurate measurement.

For 10kV high-voltage metering, the voltage transformer ratio is 10kV/0.1kV. The current transformer ratio is determined based on the downstream load. For example, if the downstream transformer is a 500kVA transformer, a 50/5 ratio would be appropriate; if it is a 630kVA transformer, a 75/5 ratio would be appropriate.

 

 

In short, the rated current is calculated based on the downstream load. Choosing a ratio that is too high or too low will result in inaccurate measurement. Current transformer convert large currents into smaller ones, ensuring uniform specifications for secondary components such as metering, protection, signaling, and control.