Conventional 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.

Challenges of Accurate Measurement of Distorted Current and Voltage in the Power Grid by Conventional Instrument Transformers

Power grids are a combined system where the electrical energy produced by the power plants is transmitted to consumers. This forms a specific interdependence where the recipients have a significant impact on the power quality. Therefore, the nonlinear loads connected by households and industrial customers cause current and voltage distortion in the power networks. This creates the need for accurate measurement of nonsinusoidal voltage and current composed not only from the fundamental component but also containing higher harmonics, interharmonics, and subharmonics. In order to ensure high transformation accuracy of distorted current and voltage, the inductive instrument transformers have to be tested in these conditions. Many papers describe their behavior during the transformation of sinusoidal current or voltage. Nowadays, the scientific field in this scope is focused on the evaluation of their exploitation properties for distorted signals. The common problem of inductive instrument transformers is the self-generation of low-order higher harmonics to the secondary current or voltage. In the case of the inductive VTs, an additional problem results from the resonance caused by the parasitic capacitance of the primary winding. The proposed solutions to compensate for the values of current or voltage errors and phase displacement of inductive instrument transformers are also analyzed.

Reducing the Dimensions of the Ship’s Main Switchboard—A Contribution to Energy Efficiency

Energy efficiency generally implies the efficient use of energy in all sectors of final consumption—industry, services, agriculture, households and transport. Shipping accounts for nearly 3% of global greenhouse gas emissions, making it the sixth largest CO2 producer in the world. This is a result of inefficient ship design, lack of planning and optimal use of resources. As the transport sector expands, so does the pressure for a greener and cleaner maritime industry. Reducing fuel consumption is a major driver of the need for energy efficiency on ships. In this paper, due to the importance of maritime transport, we observed the impact of reducing the dimensions of the main switchboard as a contribution to energy efficiency. This contribution is not of great importance as is the case with the optimization of the navigation route, etc., but it certainly affects the weight and, thus, the fuel consumption, which contributes to energy efficiency in the designed system. The aim of this paper is to optimize the design of the main switchboard by using 2D simulations of possible bus topologies, in order to develop six different busbar models and find one that best meets the requirements. The simulation results indicate the optimal location and dimensions of the busbars in the main switchboard in accordance with the switchgear parameters. Apart from the change in layout and dimensions of the busbars, the replacement of conventional instrument transformers with new current/voltage sensors contributes to a significant reduction in the weight and size of the switchboard, which ultimately benefits energy efficiency.

A temperature-independent optical voltage transformer based on FBG-PZT for 13.8 kV distribution lines

Instrument Optical Voltage Transformers (OVTs) have been presented by the electric power industry as an alternative to the conventional iron and copper inductive instrument transformers from the last years. OVTs based on fiber Bragg gratings and piezoelectric (PZT) crystals present two well know limitations: maximum voltage that can be applied on the PZT due to their limited electric field and the cross-sensitivity of the FBG on temperature that influence measurement accuracy. This paper presents an FBG-PZT based OVT with a novel thermally compensated method, capable to measure and monitor distribution voltages of 13.8 kV class. The presented results show that the developed system satisfies the 0.2 Accuracy Class of IEC standards, meaning a 0.2% accuracy.

Notice of Removal: NCIT Enabled OPNET Based Design of a Digital Substation for IEC 61850-9-2 Implementation

Reliable protection and communication are the key features in a digital substation from protection, control and automation perspective. Implementation of digital protection in high voltage (HV) substations could provide increased reliability, optimized maintenance cost and improved communication capabilities. The application of IEC 61850-9-2 process bus technology in a utility substation has reduced the fault diagnosis downtime due to decreased number of nodes likely to be tested for investigation. Additionally, manufacturers are offering new smart primary plants that have intelligent features built within to makes it a fit case to replace Conventional Instrument Transformers (CITs). These Non-Conventional Instrument Transformers (NCITs) coupled with Merging Units and Intelligent Electronic Devices (IEDs) provide better accuracy, transient response and wider bandwidth when implemented in a process bus system guided by IEC 61850-9-2 standard. It is seen that use of NCIT could mitigate issues arising out of CIT failures, provide better safety, reliability, and increased asset life. At present, majority of the CITs operating in the air insulated and gas insulated high voltage (HV) substations are either oil filled or SF6 type which has few environmental issues in the event of a catastrophic failure or leakage. NCITs have better performance as compared to CITs. However, before these high voltage NCITs could find their way in a mass implementation in smart grids and digital substations, there is a need for modelling and simulation at the design stage for its application as per IEC61850-9-2 process bus and its successful implementation. Optimized Network Engineering Tool (OPNET) or Riverbed Modeler is a popular simulation software for such design and modeling of Ethernet networks. This paper presents a model of OPNET software for the simulation of a NCIT based on digital substation in a process bus environment.

Industrial Comparator for Smart Grid Sensor Calibration

Smarter sensors and transducers are required in the new smart grid scenario with increased performances, in terms of high accuracy and wide frequency bandwidth. Traditional calibration systems, developed for conventional instrument transformers are not suitable for performance verification of the new types of non-conventional instrument transformers, both in terms of input range and frequency bandwidth. Therefore, a new industrial comparator for smart grid sensor calibration is presented in this paper, which is intended for the calibration of conventional and non-conventional voltage and current instrument transformers, in a frequency range from dc to tens of kilohertz. Its accuracy performances allow for the calibration, in sinusoidal and non-sinusoidal conditions, of class 0.1 sensors.

A Computer-Controlled Calibrator for Instrument Transformer Test Sets

In recent years, test sets for both conventional and analog nonconventional instrument transformers have been made commercially available. Reference transformers are still of the conventional type. Therefore, test sets need to compare an analog signal of a conventional level with another analog signal of a different order of magnitude or, in the case of current-to-voltage transformers, even a different quantity. Traditional designs based on bridge techniques are not suitable for this application. This paper describes a new calibrator that reaches uncertainties of the order of $10\times 10^{-6}$ while being suitable for the calibration of test sets for nonconventional analog instrument transformers and fully computer-controlled.

High Frequency Modeling of Smart Sensor for Precise Power Quality Assessment Using Fuzzy Logic

Power Quality Assessment (PQA) is a critical issue both in transmission and distribution networks. Therefore, it is necessary to precisely classify the disturbances in shortest possible time to prevent the malfunction or increase of losses in the electrical equipment through appropriate remedial techniques. This paper proposes a highly accurate method of PQA through data acquisition using smart sensors, the Rogowski coils (RCs). RCs with wide band width and linear characteristics allow faithful reproduction of high-frequency (HF) signals. In the proposed method, simulated disturbance signals are applied to RC. The output signals are subjected to multilevel wavelet decomposition and then computation of the energy difference in the detailed components between the disturbance signal and the pure sinusoidal waveform is performed to design a fuzzy logic Power Quality Classifier. The classifier is tested by varying the magnitude, frequency and duration of the disturbance and found to be accurate to 98.38%. The classification accuracy depends mainly on the performance of sensors at HFs. Thus, with RCs as sensors instead of conventional instrument transformers, it is found that the precision of power quality classification is greatly improved.

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.

Research on Key Technology of Smart Substation with Intelligent Materials

Smart substation is the intelligent node of the smart grid, which plays a fundamental part in the construction of the smart grid. This paper presents the concept and the technical characteristics of the smart substation and some important aspects in the development of Substation Automation System (SAS): the meaning, technical characteristics and the development of IEC61850 which is the international and the best standard for the construction of SAS; different Substation Communication (SC) architecture to ensure the reliability, availability and survivability of the geographically distributed complex power; the characteristics of Non Conventional Instrument Transformer(NCIT), the Intelligent Switchgear as well as their implement.

Wide-area Protection Research in the Smart Grid

The smart grid brings opportunities and challenges to the power system protection and control. Fast advancement in communication and measurement techniques accelerates the development of wide-area protection, based on the wide-area measurement system. In this paper, the background of smart grid is described. In the smart grid, advanced technologies such as the non conventional instrument transformer, clock synchronization and data synchronization etc offer great opportunities for the development of wide-area protection. According to that, the concept as well as the content and development of wide-area protection is discussed. Some key technologies, like wide-area measurement system, and wide-area communication system are analyzed. Finally, the development trends of wide-area protection in smart grid are prospected.

Experimental Comparison of Conventional and Optical Current Transformers

Several manufacturers offer optical current transformers (OCTs) to replace the conventional magnetic current transformers (CTs). This paper compares the performances of some commercially available optical and conventional CTs. The steady state and transient responses of the two systems are compared in a laboratory. Field records are also obtained and evaluated to verify that the optical systems can replace conventional instrument transformers. In addition, OCT step response as well as the impact of ambient temperature on the OCT performance has been investigated. Results show that optical CTs are more than suitable for replacing conventional CTs. Further, the results also show that OCT has better frequency response than conventional CT. Differences found in the field and laboratory measurements are within experimental errors. Unlike the conventional CT, the OCT can have a digital output according to the IEC 61850 standard. The digital output of the OCT is tested for its frequency-amplitude characteristic. Results confirm that digital output has a narrower bandwidth than the low energy analog output of the OCT due to the limited sampling rate prescribed in a UCA guide to IEC-61850-9-2.

APPLICATION OF COMBINED CURRENT AND VOLTAGE SENSOR FOR PROTECTION AND METERING IN GIs SYSTEMS.

Gas insulated switchgear (GIS) is an ideal system for installations in a limited space, if a high availability is required. The new combined sensors are a perfect base for a wide range of applications in protection, metering and control in the field of gas insulated substations. The two most important functions performed in this combined sensor are the measurement of current and voltage for protection, metering and conrrol. The principle of Regowski coil is used for current measurement and of the capacitive voltage divider for voltage measurement. Signals are converted to digital values and forwarded through fiber optical cable to local control cubicles. This data acquisition is an important part of an intelligent control and protection technique for GIS system. This paper describes the design of a combined sensor, block diagram and Accuracy features of such sensors. Good properties of the sensor permit its use for measurement, protection and control. Moreover, insulating gas density and temperature, also enables evaluating and recording the signals. The simple layout of this combined sensor significantly improves dielectric capability in comparison with conventional designs. Measuring techniques have been developed to asses and improve measuring and dynamic accuracy. Results show that the combined sensor is of better quality than known for conventional inductive instrument transformers. Combined current and voltage sensors for GIs fulfill the requirements for protection, metering and control which add several advantages for the user.

A Comparison of Instrumentation for Measuring the Losses of Large Power Transformers

The results of a comparison of several different types of equipment and techniques for measuring the short circuit (copper) and open circuit (iron) losses of a large power transformer are presented. The measurements were made on a 233-MVA, 735-kV, single phase, 60-Hz power transformer with a short circuit power factor of 1.3%, at currents up to rated, and open circuit voltages to 115 percent of the rating. The short circuit tests were performed using two current comparator type high-voltage capacitance bridges, a thermal wattmeter, an electro-dynamic wattmeter system with conventional instrument transformers and a new semi-automated digital wattmeter system. For the open- circuit tests only the three wattmeters were employed. The average discrepancy between the five measuring systems used in the short circuit tests was less than 1% and between the three systems in the open circuit tests less than 0.5 %.