Low-power Instrument Transformers Research Articles

Trends in Measuring Instrument Transformers for Gas-Insulated Switchgears: A Review

Voltage and current measurements in high-voltage substations are fundamental for stable operation. Conventional instrument transformers (ITs) face challenges in gas-insulated switchgears (GISs), such as size, weight, accuracy limitations, and behavioral instability at abnormal voltages and currents. Non-conventional instrument transformers (NCITs) have emerged to address these issues, complying with International Electrotechnical Commission (IEC) standards and providing millivolt-level signals, enabling downsizing of GIS bays. The transition to digital substations, as mandated by IEC 61850-9-2, requires a shift from the conventional 110 V/5 A outputs to levels ranging from millivolts to volts. Electronic instrument transformers (EITs), compliant with the IEC 60044-7 and 8 standards, offer alternatives to conventional ITs with smaller sizes and wider frequency ranges. However, issues remain with EITs, including limited adoption, the necessity of separate power sources, and susceptibility to electromagnetic interference. Recent standards, transitioning to IEC 61869, focus on low-power instrument transformers (LPITs). Low-power voltage transformers (LPVTs) and low-power current transformers (LPCTs), designed with passive components, present potential solutions by directly connecting to merging units (MUs) for digital signal transmission. This review outlines the current status of various IT standards, covering conventional ITs, EITs based on IEC 60044-7 and 8, and LPITs based on IEC 61869-10 and 11. Advancements in sensor technology relevant to these standards are also explored. The paper provides insights into the evolving landscape of instrument transformers, addressing challenges and offering potential pathways for future developments in digital substations.

Characterization of Instrument Transformers under Realistic Conditions: Impact of Single and Combined Influence Quantities on Their Wideband Behavior.

Instrument transformers (ITs) play a key role in electrical power systems, facilitating the accurate monitoring and measurement of electrical quantities. They are essential for measurement, protection, and metering in transmission and distribution grids and accurately reducing the grid voltage and current for low-voltage input instrumentation. With the increase in renewable energy sources, electronic converters, and electric vehicles connected to power grids, ITs now face challenging distorted conditions that differ from the nominal ones. The study presented in this paper is a collaborative work between national metrology institutes and universities that analyzes IT performance in measuring distorted voltages and currents in medium-voltage grids under realistic conditions. Both current and voltage measuring transformers are examined, considering influence quantities like the temperature, mechanical vibration, burden, adjacent phases, and proximity effects. The study provides detailed insights into measurement setups and procedures, and it quantifies potential errors arising from IT behavior in measuring distorted signals in the presence of the various considered influence quantities and their combinations. The main findings reveal that the temperature has the most evident impact on the inductive voltage transformer performance, as well as the burden, causing significant changes in ratio error and phase displacement at the lower temperatures. As for low-power ITs, establishing a priori the effects of adjacent phases and proximity on the frequency responses of low-power ITs is a complex matter, because of their different characteristics and construction solutions.

Low-Power Instrument Transformers and Energy Meters: Opportunities and Obstacles

Low-Power Instrument Transformers and Energy Meters: Opportunities and Obstacles

Accuracy Type Test for Rogowski Coils Subjected to Distorted Signals, Temperature, Humidity, and Position Variations.

Low-Power Instrument Transformers (LPITs) are becoming the first choice for distributed measurement systems for medium voltage networks. However, there are still a lot of challenges related to their operation. Such challenges include their accuracy variation when several influence quantities are acting on them. Among the most significant influence quantities are temperature, electromagnetic field, humidity, etc. Another aspect that increases the importance of studying the LPITs’ accuracy behavior is that, once installed, they cannot be calibrated for several years; hence, one cannot compensate for in-field conditions. Hence, this work aims at introducing a simple type test for a specific LPIT, the Rogowski coil. First, an experimental setup to assess the effect of temperature, humidity, and positioning on the power quality accuracy performance of the Rogowski coil is described. Second, from the results and the experience of the authors it has been possible to design a specific type test. The test has the aim of finding the limits of the accuracy variations of a single Rogowski coil. Afterwards, such limits can be used to compensate for the in-field measurements, obtaining an overall higher accuracy. The results of this work may contribute to the always-evolving standardization work on LPITs.

Why Should We Test the Wideband Transformation Accuracy of Medium Voltage Inductive Voltage Transformers?

In this paper the results of the tests of the wideband transformation accuracy of medium voltage (MV) inductive voltage transformers (VTs) in the frequencies range from 50 Hz up to 5 kHz are presented. The values of voltage error and phase displacement for transformation of the harmonics of distorted primary voltages are determined. In the case of a typical 50 Hz-type inductive VT with a rated primary voltage equal to (15/√3) kV and (20/√3) kV manufactured by an international company the limiting values of the accuracy classes extension for quality metering required by the standard IEC 61869-6 for the Low Power Instrument Transformers (LPIT) were not exceeded. While, in the same test other MV inductive VTs show poor accuracy and even resonance at multiple frequencies. Unfortunately, this problem also arises from nonlinearity of the magnetization characteristic of their magnetic core. Therefore, for transformation of the sinusoidal voltage in the secondary voltage significant but not easily detectable values of the low order higher harmonics are present. Moreover, for transformation of harmonics of distorted primary voltage the influence of connected capacitance on the obtained values of voltage error and phase displacement was tested.

Power-over-Fiber LPIT for Voltage and Current Measurements in the Medium Voltage Distribution Networks.

In this work, we present the design, laboratory tests, and the field trial results of a power-over-fiber (PoF) low power instrument transformer (LPIT) for voltage and current measurements in the medium voltage distribution networks. The new proposed design of this power-over-fiber LPIT aims to overcome the drawbacks presented by the previous technologies, such as the continuous operation (measuring and data transmission) for a wide current range conducted in the medium voltage transmission lines, damage due to lightning strikes, accuracy dependency on vibration, position and temperatures. The LPIT attends the accuracy criteria of IEC 61869-10 and IEC 61869-11 in terms of current and voltage accuracy and it attends the practical criteria adopted by Utilities companies including voltage measurements without removing the coating of the covered conductors. The PoF based LPIT was developed to be applied at 11.9 kV, 13.8 kV, and 23.0 kV phase-to-phase nominal voltages, and in two current ranges 1.25–30 A and 37.5–900 A. The digital data transmission of current, voltage, and temperature from the sensing unit to the processing unit uses a special synchronism technique and it is performed by two 62.5 µm multimode fibers in 850 nm. The optical powering in 976 nm is also performed by one 62.5 µm multimode fiber from the processing unit to the sensor unit. We presented all details of the sensor design and its laboratory characterization in terms of accuracy and temperature correction. We also presented the results of field tests of the sensor made in two different conditions: in a standard distribution network and an experimental hybrid fiber/power distribution network. We believe that these studies aim to incorporate optical fiber and devices, digital technologies, communications systems in electrical systems driving their evolution.

Effect of the Conductor Positioning on Low-Power Current Transformers: Inputs for the Next IEC 61869-10

Low-power instrument transformers (LPITs) are spreading among the distribution network thanks to their features (e.g., compactness, lightness, enhanced bandwidth, etc.). It is then a fundamental to guide users and manufacturers to a correct usage and manufacturing of the LPITs. Technical committees, which are in charge of writing dedicated standards, often tackle such a task. Focusing on the current type of LPITs, the low-power current transformers (LPCTs), the associated standard is the IEC 61869-10, which is going to be improved in 2021. To this purpose, the work aims at providing new inputs for the future version of such a standard. In particular, the focus is pointed towards the effects of the conductor positioning on the window-type LPCT accuracy. Literature and gained experience in the field are the two pillars that have been used to refine the outcomes of the work, which are provided in terms of suggestions for each technical aspect, as discussed in the standard, related to LPCTs.

Low-Power Voltage Transformer Smart Frequency Modeling and Output Prediction up to 2.5 kHz, Using Sinc-Response Approach.

The instrument transformers scenario is moving towards the adoption of a new generation of low-power instrument transformers. This disruptive change also requires that the modeling, characterization, and testing of those devices must be improved. Therefore, this study focuses on a smart approach developed by the authors in a previous study to estimate the output of low-power voltage transformers (LPVT). The approach—which is based on a sort of modeling in the frequency domain (the so-called sinc-response)—allows obtaining the behavior of the LPVT at rated and distorted conditions. Experimental tests performed on off-the-shelf devices confirm the applicability and effectiveness of the proposed approach when estimating the output response of LPVTs.

Smart Characterization of Rogowski Coils by Using a Synthetized Signal.

With the spread of new Low-Power Instrument Transformers (LPITs), it is fundamental to provide models and characterization procedures to estimate and even predict the LPITs’ behavior. In fact, distribution system operators and designers of network models are looking for all forms of information which may help the management and the control of power networks. For this purpose, the paper wants to contribute to the scientific community presenting a smart characterization procedure which easily provides sufficient information to predict the output signal of a Low-Power Current Transformer (LPCT), the Rogowski coil. The presented procedure is based on a synthetized signal applied to the Rogowski coil. Afterwards, the validity of the procedure is assessed within the Matlab environment and then by applying it on three off-the-shelf Rogowski coils. Simulations and experimental tests and results involving a variety of distorted signals in the power quality frequency range and by adopting a quite simple measurement setup demonstrated the effectiveness and the capability of the procedure to correctly estimate the output of the tested device.

Are Inductive Current Transformers Performance Really Affected by Actual Distorted Network Conditions? An Experimental Case Study.

The aim of this work is to assess whether actual distorted conditions of the network are really affecting the accuracy of inductive current transformers. The study started from the need to evaluate the accuracy performance of inductive current transformers in off-nominal conditions, and to improve the related standards. In fact, standards do not provide a uniform set of distorted waveforms to be applied on inductive or low-power instrument transformers. Moreover, there is no agreement yet, among the experts, about how to evaluate the uncertainty of the instrument transformer when the operating conditions are different from the rated ones. To this purpose, the authors collected currents from the power network and injected them into two off-the-shelf current transformers. Then, their accuracy performances have been evaluated by means of the well-known composite error index and an approximated version of it. The obtained results show that under realistic non-rated conditions of the network, the tested transformers show a very good behavior considering their nonlinear nature, arising the question in the title. A secondary result is that the use of the composite error should be more and more supported by the standards, considering its effectiveness in the accuracy evaluation of instrument transformers for measuring purposes.

A Simple Calibration Procedure for an LPIT plus PMU System Under Off-Nominal Conditions

This work deals with two paramount devices when Smart Grids are concerned: phasor measurement units (PMUs) and low-power instrument transformers (LPITs). In particular, a simple calibration procedure to test the measurement chain PMU + LPIT has been developed. The procedure involves steady-state conditions of the grid and off-nominal input signals for testing current, voltage, frequency, and rate of change of frequency (ROCOF). In addition, the procedure includes a preliminary testing of the PMU without the LPIT to understand the influence of the latter. The procedure is then applied to a specific case study: a low-cost PMU developed by the authors and one of the most common LPITs adopted in Italian medium voltage networks. The obtained results are promising for two reasons. First, the low-cost PMU works correctly even in off-nominal conditions of the grid; second, the presented calibration procedure demonstrated to be effective and applicable to very common equipment, using a rather simple test setup.

Evaluation of Low-Power Instrument Transformers for Generator Differential Protection

In hydroelectric power plants, which are characterized by particularly high short-circuit current levels and high time constants, conventional current transformer specifications can result in increased equipment dimensions, thus complicating their installation, especially during refurbishment projects. This paper presents a study to assess the potential of various low-power instrument transformers (LPITs) for generator protection applications. Two types of LPITs were evaluated: the optical current transformer (OCT) and the stand alone merging unit (SAMU). To test the OCT, a 65-kA-peak test source was developed using a real-time simulator. This source was used to inject symmetrical and asymmetrical currents into LPITs for the assessment of protection accuracy class. Low-current tests were also performed to assess their metering classes. This paper presents the design of a high-current test bench, measurement uncertainty analysis, and analysis algorithms used for device evaluation. Analysis results are then presented and discussed in terms of their applicability to generator differential protection using SAMUs and OCTs.

Low-power instrument transformer-based MV automation: lessons learned and future applications

In this study, the authors share the experience they have gained in the field of medium-voltage (MV) distribution automation, with particular focus on the use of low-power instrument transformers (a.k.a. sensors) and their implications from the perspective of safety, installation procedures, accuracy, standardisation, interoperability etc. The study presents the key learnings from several small pilots and one massive project, IBERDROLA's STAR project, a smart grid project aiming to provide automation solutions for a MV network of 90,000 secondary substations which is entering its final stage after 6 years.

Simplified Approach to Evaluate the Combined Uncertainty in Measurement Instruments for Power Systems

This paper presents the study of the propagation of the effects of uncertainties in a typical measurement system for modern power networks. Nowadays there is a strong demand from the electricity industry of simple and ready-to-use solutions for calculating uncertainty in such “new” measurement systems. To this purpose a simplified approach is presented whose results are shown to be in good agreement with those provided by conventional but more complex methods. Two applications, rather simple but very common, are considered in this paper. In the first one, the measurement of a voltage phasor is considered and the measurement chain is made by an low power instrument transformer (LPIT) and a converter used for adapting the level of signals from the LPIT to the inputs of intelligent electronic device. The second application deals with the measurement of active power of a system made by a low power voltage transformer (LPVT), a low power current transformer and a powermeter.