Paolo Mazza

This paper investigates the possibility of using simplified test methods for the characterization of IT frequency response in the supraharmonic range. Unlike the traditional reference methods, the simplified method relies on generating low-voltage and low-current signals, which are easier to implement in testing and manufacturers' laboratories. This method has been developed as a valid and rigorous alternative to the reference method, with results obtained from the laboratories of different National Metrology Institutes (NMIs). An inter-laboratory comparison has been conducted on two commercial current and voltage transformers to evaluate the deviation between results obtained by the reference method and those obtained through the simplified method. The experimental results present remarkable consistency between the two methods, with deviations mostly contained within $\pm0.5$~\% and $\pm1$~crad for ratio and phase errors, respectively.

This paper proposes a generation and measurement setup for the characterization of Voltage Transformers (VTs, both inductive as well as Low Power Voltage Transformers, LPVTs) at power frequency and from 9 kHz to 150 kHz with waveforms having amplitudes at MV level. It involves the independent generation of the fundamental tone and of the high-frequency components to produce the desired distorted test waveform. The generation of the fundamental frequency component, in MV amplitude range, is obtained by means of a step-up VT, whereas the high frequency components are generated through a series connected voltage amplifier. As regards the measuring stage, the fundamental tone is measured through a commercial divider. Instead, high frequency tones are measured through a reference device, operating in the range [9, 150] kHz, specifically designed, realized and characterized. As an application, the characterization of two commercial VTs, an inductive VT and a LPVT, are presented.

Stray capacitances (SCs) are a serious issue in high-voltage (HV) applications. Their presence can alter the circuit or the operation of a device, resulting in wrong or even disastrous consequences. To this purpose, in this work, we describe the modeling of SCs in HV capacitive dividers. Such modeling does not rely on finite element analysis or complicated geometries; instead, it starts from an equivalent circuit of a conventional measurement setup described by the standard IEC 61869-11. Once the equivalent model including the SCs is found, closed expressions of the SCs are derived starting from the ratio error definition. Afterwards, they are validated in a simulation environment by implementing various circuit configurations. The results demonstrate the expressions applicability and effectiveness; hence, thanks to their simplicity, they can be implemented by system operators, researchers, and manufacturers avoiding the use of complicated methods and technologies.

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 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, the presence of nonlinear devices cannot be neglected either for their growing numbers or for their effect on the distribution network. In particular, these devices will produce relevant harmonics, which can cause problems such as increase of losses, over-heating in the power transformer malfunction of the circuit switch and, generally, an untimely aging process of connected devices. As a matter of fact, the presence of harmonics is a power quality issue and this distortion phenomena needs to be strictly monitored. The voltage transformer (VT) is the most popular transducer in the power grid due to its robustness and low cost. The frequency response of these transducers is usually characterized by sweep sine test, which can assure their performance under sinusoidal condition, but not under distortion condition. Based on the standard EN 50160, a new characterization method is proposed in this paper, in order to test the VTs under distortion condition. Several VTs with different rated values have been tested. As a direct consequence, it will be shown that, in working conditions similar to the real ones, the use of VT for the measurement of power quality (PQ) index can lead to wrong interpretations and decisions.

The development of an arbitrary MV generator (AMVG) is a quite complex task. The level of voltage amplitude requires a careful analysis of any possible solutions at the level of its design, by finding the best tradeoff between performance and cost. In this perspective, a very attractive solution is represented by the way followed by the authors, in which the cost of the AMVG has been kept low thanks to a simple architecture based on few commercial devices. The performances have been increased by means of suitable pre-processing of the input signal. This approach has been yet applied with the aim to extend the generator bandwidth. In this paper the same approach has been applied also to the compensation of the non-linear effect mainly introduced by the step-up transformer used inside the AMVG. The method requires a two-step procedure: first of all a sinusoidal signal is applied to the input of the AMVG and the corresponding output signal is acquired and its spectrum computed. A new input signal able to produce the partial compensation of the unwanted harmonics introduced by the non-linearity of the system is then synthetized. Despite its simplicity and the adoption of a simple modeling of the system, the experimental results prove that it is possible to reduce about three times the THD of the system.

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 opportunity of verifying and, in perspective, improving the accuracy of voltage, current and energy measuring equipment, both new and already installed in service, is looked at with growing interest, both in the framework of the electricity market and to implement Wide Area Measuring Systems, based on phasor measurements, in order to develop the Smart and Super Grid. The availability of transportable Reference Measuring Systems which can be installed by means of live line working methods is a key factor to achieve such goals in a short time, under reasonable costs and without impact on normal service conditions. New methodologies and equipment developed in Italy are presented, as well as the first results obtained during laboratory and on-site testing, with reference to the improvements implemented on the Simplified Transportable Disconnector conceived in order to connect the equipment to the HV network.