Methods for on-site qualification and calibration of inductive instrument voltage transformers for harmonic measurements

The measurement of harmonic and interharmonic voltages is essential for the assessment of voltage quality in electrical networks. As the number of emitting loads in the higher voltage levels increases, the interest of network operators in accurate knowledge about harmonic levels in the network also increases. In many cases, traditional inductive voltage transformers (IVTs), which are only designed for high accuracy at rated frequency, are used to measure voltage harmonics. This challenges the accuracy of harmonic measurements and the reliability of a compliance assessment. Since a direct, invasive measurement of the IVT's frequency response is often not possible, this paper presents an in-situ two-stage approach for dealing with the IVT's frequency response to ensure accurate voltage harmonic measurements. In the first stage, the application bandwidth is estimated without the need to connect additional equipment to the primary voltage. In the second stage, the IVT's frequency response including its temperature dependency can be calibrated using a non-invasive measurement method.