Passive-compensation clamp-on two-stage current transformer for online calibration

Abstract

Digital measurement equipment such as electronic transformers enable real-time functional control and monitoring systems to be built in power grids to accommodate the large-scale access of distributed renewable energy to the grid. However, the current offline calibration of electronic transformers is difficult to implement; therefore, an online calibration method for electronic transformers must be developed to ensure their accuracy. The design of a clamp-on two-stage current transformer (CTS-CT) with a passive compensation loop is presented herein. The compensation loop is utilised to reduce both the amplitude and phase errors, thereby achieving high accuracy. The formulas for calculating the resistance and capacitance of the passive compensation loop are presented. The expanded uncertainties (95% confidence level) of the proposed CTS-CT are estimated to be lower 0.03% for the amplitude error and 1′ for the phase error. Experimental results indicate that the proposed CTS-CT prototype is not affected by the external magnetic field nor the opening and closing times, thereby enabling good-quality online calibration for field applications. Moreover, the proposed CTS-CT adopts neither an active compensatory circuit nor software compensation; therefore, the manufacturing of the CTS-CT is steady and simple, and the calibration procedures are simplified.