Sampling current ratio bridge for calibration of current transducers up to 10 kA with 5 ppm uncertainty

A sampling current ratio bridge has been developed, tested and validated for the accurate complex ratio measurement of AC current transducers for primary currents up to 10 kA with expanded uncertainties of 5 · 10^-6 in magnitude and 5 μrad in phase (k = 2). A unique property of the system is the use of digitizers for sampling of the secondary current signals and step-down transformers with a large number of current ratios. This allows for the accurate calibration of a large variety of different current transformers and other types of current transducers.

I. Introduction

In the last few years, a new sampling current ratio bridge has been developed at VSL for the accurate measurement of the complex ratio of AC current transformers (CTs) for primary currents up to 10 kA [1]. In this bridge, the CT under test is compared to a reference CT developed and calibrated by NRC [2]. The sampling current ratio bridge consists of two measurement branches containing a step-down transformer, current buffer and high-accuracy digital voltmeter sampling the secondary current signals. This approach is fundamentally different from the more commonly used analogue ratio bridges. The main advantage is the flexibility in the current ratios that can be tested; even other current transducers such as Rogowski coils and other current to voltage converters can be directly compared to the reference CT by adapting one of the two measurement branches. Even with this flexibility, a ratio uncertaintY of 5. 10⁻⁶ in magnitude and 5 μrad in phase can be obtained for CT calibrations between 45 Hz and 65 Hz.