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.

Nowadays, voltage transducers in medium-voltage (MV) distribution networks operate in nonsinusoidal conditions while their calibration is still based on simple sine-wave tests. A more significant characterization could be achieved using complex multitone signals. However, this requires an MV generator able to apply arbitrary voltage waveforms to the transducer under test. The authors have recently proposed a simple MV signal generator that can be effectively employed for this purpose. In this paper, the performance of this MV generator has been carefully analyzed. A characterization procedure based on multisine excitation has been proposed and tested. In particular, the main causes of uncertainty have been identified and their impact on the accuracy of the generated voltage evaluated. The analysis considers both linear and nonlinear effects, such as harmonic distortion and hysteresis.

The increasing interest in the use in medium and high voltage networks as well as in the low voltage ones of non-conventional low power instrument measurement transformers entails the availability of dedicated calibration set-up, equipped with flexible and accurate measurement systems. The paper presents a measurement system for the calibration of both inductive and low power voltage and current transformers, which includes a low cost and digital acquisition and measurement equipment. It is based on a portable data acquisition system composed of various current and voltage data acquisition boards. The proposed system can be used for the calibration of transducers with analog output from DC up to tens of kilohertz and allows the comparison of signals of different kind and order of magnitude.

As well known, the availability of an advanced and reliable model describing the real behavior of a physical device may strongly simplify the measurement activities devoted to its characterization. A model driven approach can meaningfully reduce the amount and the complexity of the required tests. This is very important when in field experimental activities are required. A typical example of this situation is the metrological assessment of the instrument transformer (IT) for medium and high voltage applications. The aim of this paper is to present a different approach to the instrument transformer modeling. This proposal leads to an analysis of the instrument transformer that is independent by the waveform shape of the voltage applied to its primary and simply based on the measurement of the input and output signals. In this paper an experimental procedure for the identification of the model parameters of a voltage IT (VT) at the AC-mains frequency is presented and discussed. Finally, the proposed model has been simulated by using the parameters experimentally evaluated. The simulation results have been compared with those obtained through a conventional calibration procedure.

Within the framework of distribution and transmission grids, the knowledge of Instrument Transformers (ITs) behavior in distorted conditions is a topic of great interest. Its relevance stems from the ITs wide use in metering, protection, monitoring and control applications, where their role is to reduce voltage and current to levels compatible with measuring instrument inputs. In force standards require that the performance of measuring instruments is assessed under realistic conditions. On the contrary, performance tests of ITs are generally carried out only at rated conditions, so that their behavior under actual waveforms is not fully known. To cover this gap, a suitable setup for the traceable test of Voltage instrument Transformers (VTs) under a quite large set of static and time-varying test waveforms is developed. The paper, after a short description of the setup, shows the performance of two commercial VTs under some power quality events, that are amplitude and phase modulations and voltage dips.

The importance of power quality monitoring in medium voltage (MV) systems has dramatically increased in the last years. It is well known that a measurement system with wide bandwidth and very low distortion is mandatory for this purpose. Usually, the performance bottlenecks are the transducers which have to be carefully selected according to the required accuracy. Therefore, it is very important to test them in real-world (nonsinusoidal) operating conditions. However, the characterization requires generating arbitrary voltage waveforms with amplitude of several kilovolts. This paper proposes a very simple MV waveform generator employing basic instrumentation which is very common in calibration laboratories. It consists in an arbitrary waveform generator (AWG) connected to a power amplifier which feeds the low voltage winding of a step up transformer. The secondary, high voltage winding is attached to a reference voltage transducer and to the device under test. In general, the shape of the secondary voltage waveform can be very different from the reference voltage of the signal generator. However, assuming that the nonlinearities of the system are negligible, it is possible to estimate the transfer function between the reference voltage of the signal generator and the secondary voltage. Then, it is possible to compute the reference voltage for the AWG in order to obtain the desired secondary voltage waveform. This technique has been implemented and its performance has been tested. The experimental activity has shown that an accuracy of a few percent can be easily achieved.