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 nonnegligible impact on measurement uncertainty; for example, this occurs as far as the inductive voltage transformers are considered. This paper proposes a simple technique allowing the compensation of the most significant nonlinear effect that is the harmonic distortion produced by the large fundamental primary voltage. The method is first 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.

The number of nonlinear devices in distribution grids has rapidly grown in the last years, and this trend is likely to increase in a near future. In this scenario, power quality monitoring and identification of disturbance sources are critical aspects. Inductive transformers are often employed for this purpose, although their metrological performances are guaranteed only near rated frequency. Secondary voltage harmonics are affected by the filtering behavior of the transformer, but also by nonlinearities due to the iron core that may have remarkable impact. In this work a novel post-processing technique allowing to dramatically reduce the detrimental effects of nonlinearities is proposed and discussed. After explaining the methodology, it has been applied to enhance the harmonic measurement accuracy that can be obtained by using an inductive medium voltage instrument transformer. Experimental tests carried out in laboratory show the significant performance improvement achieved thanks to the proposed approach.

The most recent IEC standards about voltage transformers warn about nonlinearity, which may significantly impact the harmonic measurement performance. However, there is a lack of scientific literature about this topic: usually their characterization consists of frequency response measurements that are clearly not able to capture the nonlinear behavior. In this paper, an innovative approach based on recently developed simplified frequency-domain polynomial models 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.

Current transformers for ac power systems applications are usually calibrated by applying sinusoidal currents at different frequencies. The main limitation of this approach is that it does not take into account nonlinearities that may have a significant impact on harmonic current measurement. A more accurate characterization should be based on test currents that are similar to those found in normal working conditions. However, this demands a proper current generator able to generate multitone waveforms with proper accuracy, which is seldom available in laboratories. This paper proposes a simple approach to implement a current generator, based on a waveform generator, a power amplifier, and a transformer which allows boosting the output current capability. Its uneven input-output response can be compensated through linear prefiltering of the desired current waveform. When the generator is employed to reproduce a specific class of current waveforms, such as those typically found in ac distribution grids, an optimal compensation filter can be easily designed. This approach allows canceling systematic nonlinearities introduced by the transformer and the power amplifier, thus significantly increasing accuracy with respect to a conventional frequency response compensation, as confirmed by experimental results.

The widespread diffusion of nonlinear power electronics-based devices in distribution grids has raised the interests towards power quality monitoring. In this respect, the estimation of synchronized harmonic phasors would enable the employment of algorithms that allow identifying the sources of harmonic disturbances, which, until now, have been only tested through numerical simulations or implemented in small-scale. However, the impact of the uncertainty due to the employed voltage transformers and to the other devices that compose the measurement chain has to be analyzed. In this framework, the present paper proposes a class of harmonic synchrophasor estimation algorithms that allows compensating the frequency response function of the instrument transformer. The technique has been tested and validated under realistic conditions considering an inductive voltage transformer. Experimental results highlight its effectiveness, especially when taking into account its ease of implementation. The black-box approach allows abstracting from the operating principle of the voltage transducer.