Instrument Transformers Research Articles

Enhanced PMU-Based Line Parameters Estimation and Compensation of Systematic Measurement Errors in Power Grids Considering Multiple Operating Conditions

Several monitoring, protection, and control applications designed for modern power grids are based on detailed grid models and thus require accurate knowledge of line parameters. A synchronized monitoring infrastructure based on phasor measurement units (PMUs) may be of great support to the task of line parameters estimation, but the accuracy of the estimated parameters may be largely affected by the uncertainty of all the elements of the PMU-based measurement chain. Thus, an accurate parameters estimation must appropriately consider the metrological behavior of all these elements, in particular that of instrument transformers (ITs). To address this challenge, this article proposes an enhanced multibranch method for accurate estimation of the line parameters and the systematic measurement errors introduced by the ITs when measurements for multiple operating conditions are considered. Indeed, multiple operating conditions are dealt with properly due to an in-depth analysis of the problem modeling within the framework of Tikhonov regularization. The validity of the proposed approach is confirmed by the results obtained on the IEEE 14-bus test system.

Estimation of Line Parameters, Tap Changer Ratios, and Systematic Measurement Errors Based on Synchronized Measurements and a General Model of Tap-Changing Transformers

A primary requirement for the transmission system operator is an accurate knowledge of grid parameters. Moreover, the availability of effective and accurate monitoring tools allows the proper operation of power transmission grids. However, in spite of the now widespread possibility of having monitoring systems based on synchronized measurements, the monitoring applications can be affected not only by the inevitable uncertainty sources but also by the simplified or incomplete modelling of the network components. For this reason, the impact on power system monitoring and control applications of tap-changing transformer models is a key point. In this scenario, this paper presents a method to estimate simultaneously line parameters, tap changer ratios and systematic measurement errors associated with the instrument transformers. The method exploits a flexible model of the tap-changing transformer based on a parameter representing the ratio between the two winding impedances of the transformer. The proposal is based also on the suitable modelling of the measurement chain and on the constraints introduced by the equations of involved transmissions lines and transformers. The validation has been carried out by means of tests performed on the IEEE 14 Bus Test Case.

Phasor Measurement Unit With Digital Inputs: Synchronization and Interoperability Issues

In recent years, the electrical substations are experiencing a rapid transition towards a fully digital measurement infrastructure, in terms of data acquisition, storage and processing. The IEC Std 61850-9-2 defines the data format, also known as Sampled Value (SV), to be employed by instrument transformers, merging units, and controller devices. In this scenario, it is reasonable to expect that also Phasor Measurement Units (PMUs) will be directly connected with digital instrument transformers and will be required to process directly the SV data stream. In this paper, we present the design and development of a stand-alone PMU based on digital inputs. The prototype has been characterized in terms of estimation accuracy and enriched with a novel functionality that allows for monitoring the time quality of the SV data stream. In this way, the device truly becomes an Intelligent Electronic Device (IED) that guarantees higher interoperability as well as a more robust management of sporadic synchronization issues. The proposed results confirm the potential of SV-based PMUs, and highlight the possibility of further enhancement for synchrophasor measurements based on digital inputs only.

Optimalno podešavanje rezolucije registra električne energije pametnog brojila

Technical losses in the transmission network are: losses of power, i.e. electrical energy, which are a consequence of the consumption of power, i.e. energy for heating the elements in the transmission network due to the existence of active resistance in these elements, losses due to hysteresis, losses due to eddy currents, losses due to drain currents in insulation, losses due to corona and dielectric losses. With the most frequent use of smart meters for indirect connection in operation in a plant set up to display electricity from the secondary side, the active electricity will be read on the meter screen with an additional error. Then the transformation ratios of voltage and current instrument transformers are set in the smart meter as 1:1. In order to obtain the actual value of energy, it is necessary to multiply the measured value of electricity from the meter by the transformation ratios of instrument transformers. Another way is to adjust the actual transformation ratios of voltage and current instrument transformers in the smart meter, so that the actual value of electricity in megawatts can be read on the meter screen. In this paper, the errors of electricity reading are considered, taking into account: transformation ratios of measuring transformers, resolutions and units of electricity, methods of setting smart meters, internal constants of meters, nominal secondary currents of electric meters and the prescribed maximum energy that is registered on the screen of electric meter. It investigates how much larger the errors of secondarily set electric meters for indirect connection with the usual resolution for displaying electricity in three decimal places and the unit of measurement in kilowatt hours compared to other possible settings for reading meters. Errors in the calculation of technical losses of electricity on high-voltage transmission lines and cables and power transformers are noticeable.

Time Series Forecasting With Volatility Activation Function

Time series forecasting is the method of predicting future values of a model by reviewing its past data. Various models like traditional approaches, statistical methods, moving average, ARIMA, RNN’s, or XGBoost may also be applied. Activation functions are the primary most important hyper parameters or artificial neural networks that decide whether a neuron will be active or not. However, the most widely used sigmoid and ReLU activation functions include the problems of linearity, gradient protection, vanishing gradient and data convergence. Solution of these problems is significant for the development of activation functions which are one of the most important hyper parameters for artificial neural networks. A new activation function is suggested to generate a solution for the problems existing in the activation functions within the scope of this study. A hybrid model has been created from the RNN and LSTM algorithms and applied on different time series data in order to implement this suggested activation function called as the volatility activation function. The volatility activation function has been compared with the literature studies, the conditions for being a function have been proved and its applicability has been demonstrated by means of financial instrument and electrical transformer temperature data. As a result of the study, the characteristics of the volatility activation function have been presented; additionally its applicability, validity and proof has been performed. Furthermore, it has been proved that the problems found in the activation functions used in the literature are not existing in the volatility activation function. It has been verified that the volatility activation function is functional on time series. To demonstrate the feasibility of the volatility activation function, it has been applied to three different time series problems. As a result of the study conducted on the electrical transformer temperature estimation, MSE=0.362 and MAE= 0.448 were obtained; namely these results are similar to the literature studies. In the test results with ounce gold data, the accuracy rate increased by 0,1. In the test results with Australian rain forecast data, the accuracy rate increased by 0,2. As a result, a new activation function is suggested for the deep learning.

Algorithms for automatic of metrological characteristics of transducers

The algorithms for automatic correction of metrological characteristics of high-voltage instrument transformers in the absence of benchmark measurement signals at the source input are proposed. The investigated algorithm can be applied for automatic correction of metrological characteristics of measuring channels in the case of multiplicative distortions caused by the nonlinear dependence of the transmission coefficient on the input value.

A Computer-Based IEC 61850 Sampled Values Analyzer for Parallel Power Quality Analysis

This paper introduces a system capable of characterizing on-site digital instrument transformers from the analysis of the IEC 61850/IEC 61869-9 frames injected by the Stand Alone Merging Units (SAMU) located in digital substations. Besides, the second objective of this work is to implement a suitable computation architecture to monitor distributed Power Quality (PQ) nodes. For that aim, capacity of calculating PQ from the information sent by the SAMU will be leveraged. In a first step, this approach relies on the implementation of an IEC 61000-4-30 class-A PQ library on an industrial-based computer. Concerning the first objective, several experimental tests were driven for a solid timestamping characterization of the received data. After that, different PQ tests were carried out to verify the suitability of the proposed solution by comparing the system against one traceable generator. The calculation of normalized error indexes (En) revealed that the developed system is compatible with respect to the reference generator. In this regard, the system can be used to calibrate digital instrument transformers (via SAMUs) against one analogue reference transformer. Later, the library is migrated to a more constrained hardware to identify the maximum number of parallel nodes that the analyzer can process. On this basis, several tests were conducted to stress the low-cost platform until reaching the thermal limit on CPU. In light of the results, the maximum number of nodes was 20. This fact indicates that the proposed low-cost analyzer meets the stringent class-A requirements of IEC 61000-4-30 for real-time parallel evaluations.

The Use of Voltage Transformers for the Measurement of Power System Subharmonics in Compliance With International Standards

The measurement of subharmonics in distribution systems requires instrument transformers to reduce voltage and current to levels fitting with the low voltage input of the power quality instruments. The in force international standards establish algorithms and methods for detecting, measuring and reporting subharmonics. In particular, the IEC 61000-4-7 suggests to perform the Discrete Fourier Transform over basic time frames of 10-cycles (12-cycles) for the 50 Hz (60 Hz) power frequency. Considering the case of 50 Hz constant power frequency, the spectral analysis is performed with a fixed spectral resolution of 5 Hz; thus, subharmonics with a frequency not integer multiple of 5 Hz could introduce inaccuracies in the measurements because of the spectral leakage. In this framework, this paper investigates the additional error contributions that can be introduced by voltage transformers used, at the input of power quality instruments, to measure subharmonics in compliance with international standards. The analysis is conducted through numerical simulations and experimental tests on two commercial voltage transformers based on different operating principles. Results show that the use of a voltage transformer to measure subharmonics, in compliance with international standards, could introduce higher additional errors compared to the ratio errors of the same device evaluated at subharmonic frequencies.

Research on Self-Adaptive Algorithm of Transient Performance Analysis for DC Electronic Instrument Transformer Calibration

DC electronic instrument transformers (DCEIT) are widely used in the voltage source converter based high voltage direct current transmission (VSC-HVDC) system to measure the bus voltage or current, and should be of accurate and quick transient response. The transient performance of the DCEIT need to be calibrated using a calibrator. The existing schemes are difficult to recognize different transient step responses automatically, and usually require human intervention, which limits their flexibility and applicability. Besides, the accuracy of the calculated results is affected by human factors. A self-adaptive algorithm of transient performance analysis is proposed. It recognizes the characteristics of the transient step response based on wavelet analysis, and distinguishes the steady-state and step intervals automatically. Then it evaluates and finds the stable data segments in the steady-state intervals adaptively to calculate the high and low steady-state values. Finally, the data in the step interval are interpolated to search the feature points of the waveform, and then the transient response parameters are calculated according to the coordinates of feature points. In the validation stage, six kinds of step response waveforms are generated by MATLAB to test the performance of the proposed algorithm. Compared with the boundary window method, the proposed algorithm could recognize the characteristics of the tested waveforms automatically and achieve a more accurate analyses of transient performance. In addition, a calibrator prototype based on the proposed algorithm is developed, and some tests are carried out at the National Center for High Voltage Measurement (NCHVM) in China, whose results verify the feasibility of the algorithm.

Theory and Experimental Validation of Two Techniques for Compensating VT Nonlinearities

Inductive instrument transformers (ITs) are still the most used voltage and current sensors in power systems. Among the numerous applications that require their use, one of the most important is surely represented by harmonics measurement. In this case, the recent literature shows that, since they suffer from both a filtering behavior due to their dynamics and from nonlinear effects produced by their iron core, they can introduce errors up to some percent. This article wants to deeply investigate, in the very same experimental conditions, about the performance of two digital signal processing techniques, recently introduced for the improvement of harmonics measurements performed through ITs, namely, SINusoidal characterization for DIstortion COMPensation (SINDICOMP) and compensation of harmonic distortion through polynomial modeling in the frequency domain (PHD). These methods have been applied to two different voltage transformers, having different specifications, by using two measurement setups based on different architectures. The impact of the voltage generator employed during the identification on the achieved accuracy is theoretically and experimentally evaluated. Modified versions of SINDICOMP and PHD compensation, which are more robust against nonidealities of the measurement setup, are presented. The performances of the techniques are evaluated by adopting voltage waveforms similar to those that can be encountered during the normal operation in a real distribution grid.

To the Problem of Protection of Medium Voltage Instrument Transformers with Fuses: Analytical Research

Introduction. In the medium voltage power grid of 6–35 kV, there is a problem of protecting voltage instrument transformers. This is due to the insufficiently effective level of their protection with fuses. Recently, there have been more and more reports of accidents associated with the failure of voltage instrument transformers not only in Ukraine, but also abroad. The issue of conducting an analytical study of the problem of protection of medium voltage instrument transformers by fuses is relevant. Goal. Investigation of designs and characteristics of fuses for medium voltage instrument transformers to improve the efficiency of their protection. Results. The article shows that protection of medium voltage instrument transformers with epoxy insulation is often provided by fuses, in which the nominal current of the fuse-link is significantly higher than the maximum permissible long-term current of the primary winding of the instrument transformer. A comparative analysis of the current values of the primary winding of medium voltage instrument transformers with the values of the rated currents of the fuse-links of fuses of various manufacturers presented on the Ukrainian market is carried out. The design features and technical characteristics of fuses for medium voltage instrument transformers have been investigated in order to increase the efficiency of their protection. The advantages and disadvantages of the designs of fuses from various manufacturers have been investigated and it has been established that the design of fuses requires further improvement in order to increase the efficiency of protection of medium voltage instrument transformers. Discussion and prospects for further development. Since in Ukraine, the need for fuses to protect medium voltage instrument transformers is provided mainly by supplies abroad, a promising direction for further development in this direction is the creation of a domestic competitive design of a fuse and its introduction into production.

A new fault resistance compensation scheme for distance relays of parallel lines with untransposed conductors

Line to ground (L-G) faults with a highly-resistive fault current path can be considered as the main reason for the distance relays underreach. This paper presents a new scheme to compensate commercial distance relays for the high fault resistance, without making any changes in its conventional structure, and is applicable to the under-operation relays. It operates on relay feeding signals measured by instrument transformers and corrects them in a way that the relay is enabled to calculate the desired fault impedance in case of high-resistance faults (HRFs). For this objective, a straightforward time-domain formula is creatively obtained first for a fast and accurate evaluation of the fault distance in case of L-G faults. The evaluated fault distance is then utilized to generate the signals correcting the relay feeding signals. The proposed approach is developed for the parallel lines with untransposed conductors on both circuits, hence it is intended for the lines at the HV and MV levels. Being readily implementable, preserving the relay security, insignificantly affecting the relay tripping time, being insensitive to the noise and fully eliminating the HRF-caused underreach are the features the proposed approach includes, which are verified by comprehensive simulation studies in PACAD/EMTDC for a variety of fault and system loading conditions.

Estimation peculiarities of external polymer insulation reliability of gas-filled instrument transformers

In the article, the authors propose a method for estimating the parameters of theoretical distributions for calculating the indicators of operational reliability. In the article, the authors propose a method for estimating the parameters of theoretical distributions for calculating the indicators of the operational reliability of a solid insulating structure of high-voltage devices, which is a supporting insulating cover for high voltage instrument transformers filled with gas as an insulating liquid. This technique makes it possible to estimate the parameters of a new distribution law, which is chosen on the condition that it does not contradict the existing distribution law with its known parameters. The developed technique makes it possible to obtain the values of the indicators of the operational reliability of high-voltage equipment by determining the parameters of theoretical distributions, if the developer is the data of experimental studies or statistical information as a result of monitoring the operation of insulating structures, taking into account the actual operating conditions of such high-voltage devices. This makes it possible to take into account the influence of external factors and performance characteristics inherent in instrument transformers, both current and voltage. In the proposed methodology, as an example, a supporting insulating casing is considered, which is during operation in the most unfavorable conditions, such as external pollution, humidification, overvoltage, etc. The theoretical conclusions are confirmed by the results of calculations using the example of the design of a current transformer of the ТОГ-362 series. A more accurate determination of the effectiveness of the proposed method for predicting the parameters of theoretical distribution laws can be achieved by performing an additional series of calculations and experimental tests of specific insulating structures. Thus, it was concluded that it is possible to use the results obtained to assess the operational reliability of both gas-filled instrument transformers and similar high-voltage equipment.

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.

Quality Improvement of Instrument Transformer Using Taguchi Experiment Method in PT. XYZ

Up to this year, PT. XYZ has been experienced of increasing sales of instrument transformer products, along with increasing consumer demand. Quality is the most important thing in the manufacturing product. But on the production floor of PT. XYZ faces quality problems. Based on the 2018 production report from January to December the number of defective products is more than 2 %. The high defective products indicate that there needs to be an improvement in the production process as a form of effort to improve quality on an ongoing basis. However, to achieve these objectives the company must know which control factors have an effect on improving the quality of the product. This research is intended to assist companies in solving problems of multiresponse cases that occur in the case of instrumentation transformer defect products, it will be analyzed using Taguchi Experiment combined with the PCR- TOPSIS method to improve product quality. Based on the results of this research, the results obtained that using the PCR-TOPSIS method optimum conditions can be achieved in the oven temperature control factor 110°C, oven time 240 minutes, mixing temperature 60°C, mixing time 130 minutes, clamping pressure 3 bars, and time clamping 20 minutes. By using the optimum treatment settings can be achieved quality improvement in each response with the characteristics of the quality is larger is better, where the increase in power frequency response increases by 11.11% and the response of partial discharge increases by 39.17%.

Calibration of a Digital Current Transformer Measuring Bridge: Metrological Challenges and Uncertainty Contributions

In this paper, we consider the calibration of measuring bridges for non-conventional instrument transformers with digital output. In this context, the main challenge is represented by the necessity of synchronization between analog and digital outputs. To this end, we propose a measurement setup that allows for monitoring and quantifying the main quantities of interest. A possible laboratory implementation is presented and the main sources of uncertainty are discussed. From a metrological point of view, technical specifications and statistical analysis are employed to draw up a rigorous uncertainty budget of the calibration setup. An experimental validation is also provided through the thorough characterization of the measurement accuracy of a commercial device in use at METAS laboratories. The proposed analysis proves how the calibration of measuring bridges for non-conventional instrument transformers requires ad hoc measurement setups and identifies possible space for improvement, particularly in terms of outputs’ synchronization and flexibility of the generation process.

Impact of optical current transformer on protection scheme of hybrid transmission line

Continuity of power transmission is important to ensure the reliability of the electricity supply. As most system faults are temporary, the auto reclose (AR) scheme has been used extensively to minimise the outage duration, prevent widespread outages, thus increase system stability. Meanwhile, the hybrid transmission line (HTL) combining overhead line (OHL) and high voltage cable has been introduced to provide an inexpensive solution for an urban power grid. Protecting HTL with a conventional protection system would forbid the operation of the AR scheme due to difficulty to ensure whether the fault occurred on the OHL or cable section. Therefore, the circulating current protection (CCP) scheme is used in the cable section to ensure the fault location and block the AR scheme. The technology of an optical current transformer (OCT) as one of the non-conventional instrument transformers (NCIT) has emerged to provide a solution to drawbacks on the conventional current transformer (CCT). Consequently, this paper investigated the impact of using OCT over the CCT for CCP of the HTL. The result shows that OCT could be used for CCP on much longer cable sections thus increase its reliability as the AR scheme can be used on longer or multiple cable section.

Optical voltage sensor based on a piezoelectric thin film for grid applications.

Continuous monitoring of voltages ranging from tens to hundreds of kV over environmental conditions, such as temperature, is of great interest in power grid applications. This is typically done via instrument transformers. These transformers, although accurate and robust to environmental conditions, are bulky and expensive, limiting their use in microgrids and distributed sensing applications. Here, we present a millimeter-sized optical voltage sensor based on piezoelectric aluminum nitride (AlN) thin film for continuous measurements of AC voltages <350kVrms (via capacitive division) that avoids the drawbacks of existing voltage-sensing transformers. This sensor operated with 110μW incident optical power from a low-cost LED achieved a resolution of 170mVrms in a 5kHz bandwidth, 0.04% second harmonic distortion, and a gain deviation of +/-0.2% over the temperature range of ~20-60°C. The sensor has a breakdown voltage of 100V, and its lifetime can meet or exceed that of instrument transformers when operated at voltages <70kVrms with capacitive division. We believe that our sensor has the potential to reduce the cost of grid monitoring, providing a path towards more distributed sensing and control of the grid.

Identification and correction of transmission line parameter errors using SCADA and synchrophasor measurements

An efficient operation of a power system relies on the use of a huge amount of information. Among those are the transmission line parameters. It is crucial that such parameters, which are stored at the control center’s database, are as accurate as possible. This paper proposes a robust and efficient method for identifying and correcting erroneous transmission line parameters, using a dedicated parameter estimator, based on measurements from both SCADA and phasor measurement units (PMUs). Besides, the parameter estimator is part of an integrated framework for the evaluation of erroneous transmission line parameters considering all lines in the network, namely, those with PMUs at both ends, PMU at one end, as well as transmission lines without any PMUs at their ends. The proposed method proved to be effective for identifying and correcting line parameters, even in situations where relatively large measurement noise is present, because of the low accuracy of measuring devices and instrument transformers. Simulations with the IEEE 14-, 118- and 300-bus systems were carried out to evaluate the performance of the proposed method.

Reliability modelling of capacitor voltage transformer using proposed Markov model

Capacitor voltage transformer (CVT) is one of the most important instrument transformers widely used to prepare the voltage signal for control and protection equipment. The measuring accuracy of CVT plays an important role in the proper operation of the protection system. Therefore, maintaining the accuracy of CVT throughout its lifetime at the desired level is of great significance, and proper maintenance activities must be implemented regarding the equipment conditions. Evaluating the reliability of the CVT over its lifetime is necessary to determine the proper maintenance. Until now, no proper model has been suggested to evaluate the reliability of CVT. Therefore, this paper proposes a new Markov model to evaluate the reliability of CVT. To obtain the model, initially, a Markov model is presented for each subsystem of CVT including capacitor voltage divider (CVD), electromagnetic unit (EMU), and high voltage bushing. Then, by integrating these models, a 10-state extended Markov model is proposed for CVT. Finally, by combining similar states, a 3-state model including healthy, low-quality and failure states is obtained. The simulation results show that the second capacitive group in the CVD subsystem, and the compensating reactor in the EMU subsystem play a major role in the CVT performance.