The calibration of measurement transformers represents a classical task in the practice of electrical measurements. The great part of the commercial instruments expressly designed for this purpose founds their working principle on a scheme based on an idea of Kusters and Moore. Though they can assure a very high accuracy, the need to employ a high-performance electromagnetic circuit makes them very expensive and usually not suitable for measurements at frequencies higher than 50 or 60 Hz.\ud For this reason, this kind of instruments cannot be employed for calibration of the new generation of current and voltage transducers, like electronic measurement transformers, which employment is growing in all the applications where wide bandwidth is required. In this paper a new method for the calibration both of electromagnetic voltage and current measurement transformers (VT and CT) and electronic voltage and current measurement transformers (EVT and ECT) is discussed and a deep metrological characterization is carried out. The novelty of the proposed method is represented by a completely different approach to the measurement of the ratio and phase errors of the measurement transformers. The method is based on proper digital signal processing of the signals collected at the secondaries of the transformer under test and of a reference transformer, when the same signal is applied to their primary. Since no auxiliary electromagnetic circuits are required, this solution can be easily implemented, in a simple and cost-effective way. In spite of its simplicity, the tests developed on a prototype clearly point out that the proposed system is suitable for the calibration of measurement transformers with precision class up to 0.1 in the frequency range from 50 Hz to 1 kHz

The most recent IEC standards about voltage transformers warn about nonlinearity, which may have significantly impact on the harmonic measurement performance. However, there is a lack of scientific literature about this topic: usually their characterization consists of frequency response measurements, which are clearly not able to capture the nonlinear behavior.In this paper, an innovative approach based on simplified frequency domain polynomial models developed by the authors is proposed. The method is applied to two different medium voltage inductive transformers. Models are identified and validated with a large set of realistic primary waveforms injected by a proper setup. Experimental results confirm the remarkable accuracy of the proposed models especially for low-order harmonics, which are the most affected by nonlinearity.

Monitoring voltage harmonics represents one of the most important tasks in power quality assessment. In particular, the employed instrument transformer plays a key role in the achieved accuracy. Its harmonic measurement performance is typically evaluated by measuring its frequency response function. However, nonlinearities may have a non-negligible impact on measurement uncertainty: for example, this occurs as far as inductive voltage transformers are considered. This paper proposes a simple technique allowing the compensation of the most significant nonlinear effect, which is the harmonic distortion produced by the large fundamental primary voltage. The method is firstly derived and introduced by means of numerical simulations, and then implemented through a proper experimental setup. Results highlight remarkable accuracy improvements when realistic voltage waveforms are measured.

Voltage instrument transformers are usually tested at the rated frequency. In order to assess their performance in measuring harmonic components, typically, the frequency response function (FRF) is evaluated. Therefore, this conventional characterization does not consider nonlinear effects that may have a nonnegligible impact on the accuracy, especially when the transducer under test is represented by an inductive voltage transformer (VT). In this paper, a simple procedure for the characterization of voltage instrument transformers is presented. The method is based on the concept of best linear approximation of a nonlinear system. It requires applying a class of excitation signals that resembles the typical voltage waveforms found in power systems. Results consist of the FRF that permits the best linear compensation of the transducer response, and sample variances that allow quantifying the impact of noise and nonlinearities on the accuracy. The method is presented and explained by means of numerical simulations. After that, it has been applied to the characterization of a conventional inductive VT. Experimental results show how the accuracy of the transducer under test is heavily degraded by nonlinear phenomena when low-order voltage harmonics are considered.

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