An Efficient Digitizer for Calibration of Instrument Transformers

A new, low cost and efficient digitizer for the calibration of the instrument transformers (IT) is presented in this paper. The proposed digitizer provides digital output directly proportional to ratio and phase errors of the instrument transformer under test, without the need of a separate analog-to-digital converter and signal processor. The existing methods for calibration of IT use either complex AC bridge based balancing techniques or systems based on expensive Data Acquisition Systems (DAS). The proposed digitizer is based on the high precision, dual-slope analog-to-digital conversion technique. The proposed digitizer is suitable for the comparison method of calibration of the Current Transformer (CT) or the Potential Transformer (PT) in which the test IT is compared with a standard IT of same ratio but having much higher accuracy. The proposed digitizer is designed in such a way that it accepts the signals from both the standard IT and a difference signal which is a function of the error, and the conversion process ensures that it delivers digital outputs of the ratio and phase errors of the test IT directly. A suitable prototype of the proposed digitizer has been developed in the laboratory and tested in different stages to verify its performance. The results obtained from the experimental studies showed the practicality of the proposed digitizer.