Engineers designing power monitoring systems should carefully select the required current transformers based on very specific characteristics :
1. Accuracy. In most applications, measurement accuracy has a direct impact on the efficiency of the entire system. The exact degree of power calculation obviously depends on the accuracy of the current transformer. Class 1 power meters may require current transformers with accuracy greater than 1%, and this accuracy is generally associated with expensive materials and manufacturing processes. An alternative solution is to calibrate the power meter for each transformer used. Taking into account the specific characteristics of each transformer, it is permissible to set the power meter in its most precise mode of operation and transfer the variables from one transformer to another. As we saw later in this article, understanding the linearity, drift, and repeatability and compensating for their overall inaccurate readings opens the door to various new technologies.
2. Drift. The drift of the transformer is related to the duration of the reading timeout independent of the initial system calibration. Some of the changes in its characteristics may be due to the surrounding humidity and temperature or aging of components. Low drift potential - meaning that the transformer is highly resistant to these limiting factors - is a very important feature for building high performance, stable, reliable power meters.
3. Linearity. The linearity of the transformer refers to the stability of its characteristics over the entire operating mode range.
The high linearity of analog sensing components is essential for accurate measurement of a wide range of primary currents, especially at low current values. Several techniques provide good performance only for a limited range of measurements, so the current in the application area is limited to very high or very low.
4. Phase shift. The accuracy of the actual effective power or energy calculation is not only related to the accuracy of the AC current and voltage transformers and the magnitude of the linearity, but also to the possible phase shift between the two correlation measurement. Of course, the phase shift should be as low as possible.
5. Integration. Due to the self-powered supply, the current transformer does not require any additional wiring in addition to the two output lines of the main power monitoring device. Many of these transformers provide calibrated standard outputs for integration in power monitoring systems. Typical 1A and 5A or 333mV outputs match most standard power meters on the market. High-accuracy power meters require specific calibrations for each transformer that can no longer be interchanged. These transformers can then produce low current output, and contacting these low currents during system operation is safer than traditional 1A/5A signals. In addition, the current output is also virtually undisturbed, so it is preferable to use a current signal output instead of a voltage signal output when connecting long distance cables to the power meter.
6. Price. The price of the transformer is of course important, especially when the three-phase power measurement requires three precision current transformers. However, the price of current transformers should not be considered separately, but also the installation and maintenance costs. Although solid-core transformers are more costly, the overall performance is more reliable and easier to install and replace the clamp-type transformers does reduce the system cost.
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