The AC current transformers used for the accurate measurement of AC electric currents are designed to transform the AC primary current to a proportional AC voltage signal or a lower proportional AC current signal level that is appropriate for micro-processor based systems. The VA burden that is imposed on the AC current transformer secondary signal can have a significant impact on the accuracy performance of the AC current transformer. To achieve the Accuracy Class desired for specific application, the AC current transformer model and the AC primary to AC secondary ratio must be carefully matched with the VA burden of the monitoring device to provide the optimum accuracy performance.
AC Current Transformer Secondary
The secondary burden or impedance on the AC current transformer has a direct impact on the accuracy performance of the AC current transformer. Overloading the AC current transformer secondary coil can have a significant adverse impact on the AC current transformer accuracy performance. Careful consideration should be given to the interface between the monitoring device and the current transformer to assure optimum accuracy performance. AC Voltage Secondary
For a typical AC current transformer with a AC voltage secondary signal, maintaining a secondary burden of ≥ 10,000 ohms is typically adequate to assure optimum accuracy performance. Minimal power is drawn from the AC current transformer secondary coil. AC Current Secondary
For a current transformer with an AC current secondary signal, the secondary burden is critical to the accuracy performance of the current transformer.
For the purposes of this BLOG post, the CTSB0816 Split-core current transformer with a 5A secondary output signal at Rated Primary Current will be discussed. This example will compare two of the same model CTSB0816 split-core current transformer, each configured for different Rated Primary Currents:
- Model CTSB0816-500A/ 5A with a Rated Primary Current of 500A and secondary signal at Rated Primary Current (500A) of 5A and
- Model CTSB0816-5000A/ 5A with a Rated Primary Current of 5,000A and secondary signal at Rated Primary Current (5,000A) of 5A.
The comparison of the two models shows the significant difference in burden allowed to meet a specific Accuracy Class performance. In this example, to obtain an Accuracy Class 0.5 for the CTSB0816-500A/ 5A the secondary burden must be less than 2.5VA, whereas the CTSB0816-5000A/ 5A the secondary burden must be less than 20.0VA.
Consideration in the calculation of the Current Transformer Burden (VA) should not only consider the measuring device burden, but especially for the lower VA burdens, the length of lead wire from the current transformer to the measuring device. For the CTSB0816-500A/ 5A configuration the current transformer coil impedance (inductance and resistance) and the length of the wire from the current transformer to the measuring device could significantly impact accuracy performance.
Download CTBS0816 product information: CTSB Brochure (pdf 349kb)