Conventional Voltage Transformers Research Articles

Development of Broadband Resistive–Capacitive Parallel–Connection Voltage Divider for Transient Voltage Monitoring

The on-site measurement of transient voltages is of great significance in analyzing the fault cause of power systems and optimizing the insulation coordination of power equipment. Conventional voltage transformers normally have a narrow bandwidth and are unable to accurately measure various transient voltages in power systems. In this paper, a wideband parallel resistive–capacitive voltage divider is developed, which can be used for online monitoring of transient voltages in a 220 kV power grid. The structures of the high-voltage and low-voltage arms were designed. The internal electric field distribution of the high-voltage arm was analyzed. The influence factors and improvement techniques of the upper frequency limit were studied. The parameters of the elements of the divider were determined. The voltage withstand performances and scale factors under lightning impulses and AC and DC voltages, the temperature stabilities of scale factors and the step response and bandwidth of the developed voltage divider were tested. The results show that the deviations of the scale factors under various voltage waveforms and different temperatures ranging from −20 to 40 °C are within 3%. The withstand voltage meets the relevant requirements specified in IEC60071-1-2011. The step response 10~90% rise time is approximately 29 ns, and the 3 dB bandwidth covers the range of DC to 10 MHz.

An Innovative Approach to Express Uncertainty Introduced by Voltage Transformers

The importance of monitoring voltage harmonics in distribution grids has raised because of the increasing diffusion of power electronics-based nonlinear devices. In this respect, the role of voltage transformers (VTs) is vital since they represent the input stage in power quality monitoring systems, and they are responsible for one of the major uncertainty contributions. Nevertheless, their metrological performances are still quantified in terms of accuracy class, namely, rating maximum ratio error and phase displacement. The aim of this article is moving from the concept of maximum error to that of uncertainty, while proposing a new approach to evaluate and express the accuracy of VTs employed for harmonic measurements. The method is applied to the calibration of a conventional inductive VT by means of a proper setup that allows injecting complex multitone waveforms. Experimental results clearly show the advantages of the proposed approach, which permits a better description of the metrological behavior with respect to the usual ratio error and phase displacement.

Characterization of Voltage Instrument Transformers Under Nonsinusoidal Conditions Based on the Best Linear Approximation

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.

On-line fault detection technique for voltage transformers

Conventional voltage transformers (CVTs) are still widely used as transducers in power networks, thanks to their high reliability, insulation capability, low drift over time and temperature. Their rugged construction is very often used as justification for skipping periodical tests and calibrations, that require putting them off-line, thus avoiding a time-consuming and expensive procedure. For this reason, in the last years, a growing interest has been addressed towards the study of online diagnostic and calibration procedures. The typical approach is based on the frequency response analysis that permits, under sinusoidal excitation, to detect possible deterioration of the behavior of CVT. Anyway, the real interest is to check the CVT fleet already installed and operating on the grid without requiring their disconnection from the grid. As well known, distribution grid voltage features a non negligible harmonic distortion, which may allow the online evaluation of the frequency response of the transformer, by simply connecting a reference transducer. Unfortunately, being the harmonics much lower than the fundamental, this approach cannot be employed in a straightforward way because of the nonlinear behavior of the CVT. This paper proposes an innovative condition monitoring technique of CVTs based on a simplified Volterra model. This opens the way to a new approach to the on-site characterization of CVTs, exploiting the actual voltage of the grid and thus not requiring its disconnection.

Medium Voltage Divider Coupled With an Analog Optical Transmission System

Similar to low-voltage power systems, also in medium voltage networks there is a growing need for power quality (PQ) analyses, required by the increasing diffusion of distributed generation sources equipped with switching converters. This leads to the employment of transducers and measurement techniques able to convert and acquire voltage signals with better accuracy on a wider frequency range than conventional voltage and current instrument transformers. Voltage dividers (VDs) are expected to become the most used transducers in PQ assessments, since, with respect to conventional voltage transformers, they show higher frequency bandwidth and better linearity. The main disadvantage of the VDs is the absence of galvanic insulation between the transducer and the measurement/protection devices. Therefore, in this paper, a prototype of a VD for phase-to-ground connection with an insulating transmission system is presented. It is constituted by a high-impedance resistive-capacitive VD and by an analog electronic insulation interface. Such an electronic interface, inserted between the output of the divider and the input of a measuring instrument, realizes the galvanic insulation by means of an optical transmission system ensuring, simultaneously, wide frequency bandwidth and high accuracy. The design, simulation, and characterization of the realized insulated equipment are presented. Its performance well fit the prescription of the international standards about instrument transformers for PQ measurements.

Experimental Comparison of Conventional and Optical VTs, and Circuit Model for Optical VT

This paper compares the performances of an optical voltage transformer (OVT) and a conventional voltage transformer (VT) in a laboratory for steady state and transients. It also presents a developed equivalent circuit model for OVT. In addition, OVT impulse response has been investigated, and field records have been evaluated to confirm that optical systems can satisfactorily replace magnetic transformers. Both experimental results and field records show that the transient performance of an OVT is better than that of a conventional one, and the performance of OVT is comparable to that of the magnetic VT. Differences are found within experimental errors. A transfer function for OVT is derived by using the obtained frequency response characteristics. A circuit model is developed from the derived transfer function by using a network synthesis method. The model is verified through the frequency response results and tested with PSPICE simulation program. The results show that the developed model can be used as a legitimate OVT model.