Engineers who design the power meter monitoring device should carefully select the required current transformers according to their specific characteristics:
(1) Accuracy: In most applications, measurement accuracy has a direct impact on the efficiency of the whole system. The accuracy of power measurement mainly depends on the accuracy of current transformer. A primary power meter may require a current transformer with an accuracy of more than 1%, which is generally related to expensive materials and manufacturing processes. The alternative is to calibrate the power meter for each transformer used. Considering the specific characteristics of each transformer, it is allowed to set the power meter in its most precise mode of operation and transfer variables from one transformer to another. As mentioned in this paper, the compensation for linearity, drift and repeatability, as well as the overall inaccurate readings, opens the door to the study of various new technologies.
(2) Drift: Transformer drift is related to reading timeout persistence independent of initial system calibration. Some changes in its characteristics may be caused by ambient humidity and temperature or component aging. Low drift potential means that the transformer has a high resistance to these limitations. It is a very important characteristic to build a high-performance, stable and reliable power meter.
(3) Linearity: The linearity of transformer is the stability of its characteristics in the whole operating mode range. The high linearity of analog induction components is indispensable for accurate measurement of large-scale primary current, especially in the case of low current value.
(4) Phase shift: The accuracy of actual active power or energy measurement is related not only to the accuracy and linearity of AC current and voltage transformers, but also to the possible phase shift between the two correlated values. Of course, the phase shift should be as low as possible.
(5) Integration: Because of the self-supply, the current transformer does not need any other wiring except two output lines connected to the main power monitoring device. Many of these transformers provide calibrated standard output for integration in power monitoring systems. Typical 1A and 5A or 333mV outputs match most standard power meters on the market. High precision power meters need to be calibrated according to each transformer. Then these transformers may produce low current output, and it is safer to contact these low currents during the operation of the system than the traditional 1A/5A signal. In addition, the current output is hardly affected by interference, so when long-distance wire is needed to connect the transformer to the power meter, the current signal output should be preferred rather than the voltage signal output.
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