Frequency calibration of voltage transformers by digital capacitance bridge

Abstract

In modern power systems, several conducted disturbances and power quality phenomena are commonly experienced also at medium voltage level. Therefore, voltage and current transducers, which scale voltage/current waveforms down to levels compatible with the measuring instruments, are expected to accurately work over a proper range of amplitudes and frequencies, keeping the signal integrity needed to properly perform their measuring tasks such as energy billing, protection coordination and power quality monitoring. Thus, there is an increasing need for characterized and traceable methods to assess the performances of transducers in a wide frequency range. This paper focuses on a method for the calibration of medium voltage measurement transformers from power frequency to 2.5 kHz, which involves a two step procedure based on the use of a digital capacitance bridge. The system is designed for the characterization of voltage transformers at rated primary voltage up to 20/√3 kV, applying a harmonic frequency sweep, whose magnitude can be selected from 2% to 10% of the primary voltage.