Electromagnetic Transformer Research Articles

Modeling and Simulation of Wide-Frequency Characteristics of Electromagnetic Standard Voltage Transformer

To achieve the broadband applicability of standard voltage transformers in “dual high” power systems, an equivalent circuit model of the standard voltage transformer is first established. Using the complex magnetic permeability method and utilizing existing core parameters, the excitation impedance values are obtained. Next, based on the equivalent circuit model, the no-load error function of the standard voltage transformer is analyzed, and through simulation, the no-load error response curve of the standard voltage transformer in the frequency range of 20 Hz to 3000 Hz is derived. The simulation results indicate that within the 20 Hz to 700 Hz range, both the no-load ratio error and the no-load angular error meet the accuracy requirements, with the ratio error within ±0.05% and the angular error within 2′. Additionally, the derivation of the error transfer function demonstrates the correlation between the no-load error values and the number of turns and cross-sectional area of the standard voltage transformer. Simulation results, obtained by increasing and decreasing the number of turns and cross-sectional area by 10%, provide valuable insights for the error compensation and structural design of standard voltage transformers.

Определение факта насыщения магнитопровода трансформатора тока на основе непрерывного измерения сопротивления цепи намагничивания

Saturation of electromagnetic current transformers in transient modes of operation of an electric power system can lead to incorrect operation of relay protection devices. One of the most promising directions to improve the stability of the operation of relay protection devices in such modes is digital correction of secondary current. Currently, effective methods have been developed to restore the secondary current based on determination of magnetic circuit saturation. However, the practical application of these methods is impossible due to the imperfection of existing methods of secondary current segmentation. Thus, it is urgent to develop a method to determine saturation for implementation of digital correction algorithms for electromagnetic current transformers. To solve the stated problem, methods of mathematical modeling of electrical circuits containing transformers and the small-disturbance theory have been used. Verification of the research results is carried out in the Matlab software package. A mathematical criterion is formulated to determine the saturation of the magnetic circuit of an electromagnetic current transformer based on the inductance of the magnetization circuit. A method of segmentation of the secondary current has been developed for the implementation of digital correction algorithms based on the proposed criterion. The effectiveness of the developed method to determine the saturation of the magnetic circuit of an electromagnetic current transformer using computational experiments in the Matlab software package is demonstrated. It is planned to verify the developed method using a physical model of an electromagnetic current transformer.

Design of Optoelectronic Transformer for Its Application in Traveling Wave Protection of Power Grid

As an important foundational link of the power system, the reliable measurement and accurate data of the metering module are the important foundation and support for energy metering. As a new type of optoelectronic current transformer, magneto-optical crystal-based optoelectronic current transformer effectively avoids the defects of traditional electromagnetic current transformers and becomes an ideal equipment for measurement in power systems. This paper proposes an improvement method to enhance the temperature stability of optoelectronic current transformers, using a magneto-optical crystal sensor head based on Faraday effect to limit the fiber length to within 140 m. It considers the influence of a uniform temperature field on magneto-optical crystal materials to process data reasonably to ensure normal incidence of the incident beam. The magneto-optical crystal material in the non-uniform temperature field has non-uniform deformation due to thermal expansion and cold contraction. Here, a Karamay No. 45 cooling oil is used for filling and sealing. In the experiment, the temperature stability of the current transformer is tested. Firstly, the wiring method is designed. Then, the corresponding test system is designed. After improvement, there is no drift phenomenon in the ratio difference and angle difference of all points of the photoelectric current transformer within the testing temperature range, and the error difference is stable. The principle of identifying faults within the area is elaborated, and a directional longitudinal traveling wave protection method for direct-current transmission lines is designed based on the differential output characteristics of the designed photoelectric transformer. Through experiments, the transformer in this design can effectively identify potential traveling wave fault types and fault phases.

Operational failure mechanisms of electromagnetic potential transformer

In response to the problem of transformers blowing up or even burning up together with detuning devices in recent years, an assessment method based on the equivalent of transformer winding currents is proposed for potential transformers (Potential Transformer, PT) in low and medium voltage distribution networks. Firstly, the damage mechanism of potential transformers is analyzed and the thermal breakdown is identified as the main cause of damage to the winding insulation. At the same time, the winding currents under different faults are analyzed theoretically to identify the characteristics of various fault currents. Secondly, an operational failure indicator based on thermal equivalence, i.e. the fault current threshold indicator, is proposed in conjunction with the general voltage transformer specification. Finally, the proposed indicator is verified by means of ATP simulation software and combined with simulation tests. The results show that the proposed index can accurately reflect the winding failure of potential transformers, and the research results can provide a reference for the selection of potential transformers and the prevention of insulation breakdown.

Research on the mechanism and restraining measures of ferroresonance in distribution network based on ATP-EMTP

In order to monitor the three-phase voltage of the busbar to ground, the substation bus is usually connected to the electromagnetic voltage transformer by YN connection in the distribution network. Under certain conditions, the excitation characteristics are easy to saturate, and the inductance parameters match the system capacitance, excite ferromagnetic resonance, cause damage to fuses or voltage transformers, and seriously affect the safe and stable operation of the power system.By analyzing typical single-phase ground faults, this paper introduces the generation mechanism of ferromagnetic resonance and the main suppression measures at this stage. The electromagnetic transient analysis software ATP-EMTP is used to analyze the operation of a typical transformer, and the change of ferromagnetic resonance waveform after the neutral point of the voltage transformer is replaced by resonant eliminator and arc suppression coil grounding. The results show that when eliminating the single-phase ground fault, it is easy to cause the frequency division resonance of the system, and the use of these two measures can effectively suppress the ferromagnetic resonance of the system.

Monitoring in 6–35 kV power networks, location of single-phase ground fault and detection of fault feeder

To minimize the negative impact of arc overvoltage in three-phase electrical power system (ES) with the ungrounded or high impedance grounded neutral, it is proposed to monitor transient electromagnetic processes, which will allow evaluating the current condition of the insulation, and in addition, detecting the damaged feeder and a single-phase ground fault zone using telemetry systems. Detection of the damaged feeder (with a single-phase ground fault) using the parameters of the ground fault transient process can be incorrect due to low free oscillations frequencies of electromagnetic instrument voltage transformers (EMVT). The superposition of EMVT free oscillations on the ES oscillations leads to an incorrect determination of the power direction of the first high-frequency oscillations and, as a result, to incorrect detection of the damaged feeder. A more accurate detection of the damaged feeder can be done by the proposed method based on the processing the pre-breakdown voltage of power frequency in the damaged phase. It is shown that the method is suitable for networks with an isolated neutral, as well as grounded through an arc-suppression reactor (ASR) or a high-value resistor.

Current Transformer Saturation Detection Method Based on Artificial Neural Network

When current transformer is saturated, mainly due to the presence of an exponentially decaying DC component in the fault current, its secondary current has a distinctive distorted waveform which significantly differs from its primary (true) waveform. It leads to an underestimation of the secondary current value calculated by the relay protection compared to its true value. Thus, in its turn, results in trip time delay or even in a relay protection devices operation failure, since its settings and algorithms are calculated and designed on the assumption that the secondary current waveform is sinusoidal and proportional to the primary. And since, when using classical electromagnetic current transformer, it is impossible to exclude the possibility of its saturation, the detection of such abnormal condition is an urgent technical problem. The article proposes to use an artificial neural network for this purpose, which, together with the traditional method of saturation detection based on adjacent secondary current samples comparison, allows implementing a fast and reliable current transformer saturation detector. The article details the stages of the practical implementation of such an artificial neural network. The MATLAB-Simulink environment was used for assess the proposed saturation detector operation. The experiments that had been performed confirmed that proposed method provides fast and accurate saturation detection within the wide range of the power system and current transformer parameters change.

A Multistage Dry Insulated Current Transformer

The insulation method of traditional electromagnetic current transformers (CTs) is single-stage insulated, and the insulation materials include oil, SF6 gas, polytetrafluoroethylene (PTFE) film, and epoxy resin. It has to deploy multiple capacitive screens for voltage equalization to improve the electric field distribution, which results in complex insulation structure and high cost. A multistage dry insulated CT for measurement is proposed in this article, which is composed of multistage CTs in series and insulated by epoxy resin, so the line voltage is shared by the multistage CTs. Therefore, a five-stage prototype CT is analyzed and designed, and the structure, the measurement principle, the electric field distribution, the technical characteristics, and the error calculation and compensation of this CT are explored. The results show that the proposed dry CT meets the 0.2S class accuracy and has advantages of low insulation cost, high safety, environmental protection, and so on.

Разработка и исследование катушки Роговского для работы в условиях эксплуатации открытого распределительного устройства

The interest to use Rogowski coils (RC) as current sensors to design secondary systems increases due to application of microprocessor relay protection and automation devices, including those that support the reception and processing of information in a form of a digital code in the format of protocols defined in IEC61850, as well as the disadvantages of traditional electromagnetic current transformers. Despite the popularity of RC, there are many unstudied issues of their functioning under operating conditions of open switchgears of power facilities, especially when operating with modern relay protection and automation devices. The aim of the study is to analyze the effect of ambient temperature on the error of current conversion by Rogowski coils. Studies of Rogowski coils have been carried out using the experimental and industrial samples. During the research modern testing equipment has been used. The equipment is listed on the Unified Register of Measuring Equipment of the Russian Federation and has valid verification certificates. The analysis of the factors influencing the error of metrological channels has been carried out. The analysis is conducted based on traditional current transformers (CT). The factors influencing the measurement error of RC are systematized. Experimental data obtained as a result of the study of the RC conversion error in a wide temperature range are presented. It has been established that due to design solutions, it is possible to neutralize the influence of such factors as the position of the primary conductor, external electromagnetic fields, uneven winding, the number of layers of the secondary winding, and thermal expansion. The use of materials with a low coefficient of linear expansion to manufacture the core of the Rogowski coil makes it possible to obtain the metrological characteristics of the transducer that meet the requirements of normative technical documentation in a wide temperature range. Application of RC as a primary current sensor for relay protection and automation systems is a promising area that requires further comprehensive research, and specifically in the process of industrial operation.

Research on Suppression of Fuses of Potential Transformer Fuse

In this paper, through the analysis of an electromagnetic Potential Transformer (PT) fuse fault in the distribution network, it is found that the cause of this type of fault is the low-frequency oscillation and inrush current of the voltage transformer caused by the disturbance, and its occurrence mechanism is studied and explained. After analyzing the advantages and disadvantages of the common PT fuse-blowing fault control measures, this paper proposes a treatment plan for the PT neutral point to suppress the fuse blowing through a nonlinear flow-sensitive harmonic elimination device. The harmonic device has a remarkable suppression effect on the PT fuse blown fault.

Research on accuracy of electrical energy measurement in microgrid for mine dewatering

The object of the study is the current transformers of the electricity metering unit as part of the microgrid for mine dewatering, which includes a photovoltaic station. The need to pump mine waters is caused by the danger of flooding large areas. The introduction of a photovoltaic station to power underground pumps reduces electricity consumption from the central power grid. At the same time, it changes the operating conditions of the metering unit. There is a need to measure both the currents of the pumps in the absence of solar generation, as well as relatively small current flows in the case of commensurate capacities generated and consumed within the microgrid. A feature of electromagnetic current transformers is a decrease in the accuracy of operation if the primary current is several percent of the rated value. Based on the analysis of variance of the experimental data, it was established that there is a statistically significant connection between the secondary and primary currents of the measuring transformer at insignificant values of the primary current. Estimates of the values of the linear regression parameters that link the secondary and primary currents were determined. It was established that in the range of the transformer primary current from 0.22 % to 1.4 %, the current error reaches a value of 6.7 %. Taking into account the resulting dependence in the software of the electricity metering unit will make it possible to increase the accuracy of energy metering. Accordingly, the accuracy of financial calculations will be increased

Research on AC and DC leakage detection technology based on fluxgate principle

In view of the shortcomings of the traditional electromagnetic current transformer in the detection of pulsating DC and smooth DC, a leakage current detection method based on fluxgate magnetic field sensor is proposed in this paper. The leakage current is detected by the size of the magnetic field induced by the DRV425 fluxgate magnetic field sensor. At the same time, the magnetic core is designed to improve the sensitivity of the magnetic field sensor. Finally, the FFT is run in STM32 to identify the waveform of the leakage current from the frequency domain, and the tripping of the action mechanism is realized under different leakage current waveforms and different leakage current thresholds. The experimental results show that the method can effectively detect and identify the leakage current waveform, and can effectively improve the current status of the leakage protector.

Research on the traceability method based on half symmetric superposition

In order to achieve high-accuracy self-calibration and traceability of electromagnetic standard voltage transformers, a measurement method that can be used for voltage ratio standard error voltage coefficients is proposed. Using two auxiliary standard voltage transformers, the error variation of the voltage transformer under different voltages can be obtained through three-step measurement. Compared with the traditional error voltage coefficient measurement method and international comparison, the feasibility of the method and the rationality of the uncertainty evaluation of the measurement results are verified. The self-calibration ratio difference measurement uncertainty of the 110 kV standard voltage transformer is better than 5 × 10−6, and the angular difference measurement uncertainty is better than 5 µrad. The method can be used to trace the source of standard devices up to 1000 kV power frequency voltage ratio, which have characteristics such as high accuracy and good stability.

Magnetoelectric coil-free voltage transformer based on monolithic ferrite-piezoelectric heterostructure

Low-power voltage transformers using magnetoelectric (ME) effect in ferromagnetic-piezoelectric heterostructures occupy an intermediate position between electromagnetic and piezoelectric transformers. They have low input and high output impedance, their transformation ratio is controlled by magnetic field. The paper describes design and manufacturing technology of monolithic ME transformers based on the nickel-zinc ferrite - lead zirconate-titanate structure. A prototype of the transformer is fabricated, in which a bulk electromagnetic coil in the input circuit is replaced with a planar electrode, deposited directly on the ferrite layer. This results in simplification of the design and reduction in the size of the transformer. At operating frequency of 70 kHz, the input impedance of the transformer is 0.1 Ω, the output impedance depends on the direction of the piezoelectric polarization and is 6–300 kΩ, the transformation ratio is tuned in the range 0–43 by magnetic field up to 60 Oe. The coil-free magnetoelectric transformers are promising to match circuits with very different impedances.

Modeling Simulation and Experiment of Wide-range Current Transformer Based on New Nanocrystalline Alloy Material

In the traditional electromagnetic current transformer, when the operating current is too large or too small, the measurement error will fluctuate greatly, resulting in a sharp deterioration of the measurement accuracy. In order to solve this problem, the article first established a simulation model, measured the characteristic curve of the new nanocrystalline alloy material, and imported it into the simulation model. Compare the wide-range error conditions of the current transformer; then, use the new nanocrystalline alloy material as the new wide-range current transformer core, adjust the size of the wide-range current transformer core, and increase the diameter of the secondary winding for simulation; Finally, according to the simulation results, a prototype of a new wide-range current transformer is prepared and error tests are carried out. The test results of the experiment show that the primary side of the new wide-range current transformer is within the range of 0.1%-200% of the rated current. The limit of the ratio difference and the angle difference reaches ±0.2% and ±10’, which meets the measurement requirements of class 0.2 current transformers.

A Novel Anti-DC Bias Energy Meter Based on Magnetic-Valve-Type Current Transformer

The direct current will cause the electromagnetic current transformer (CT) saturation, which will lead to the measurement error of some energy meters using CT for current sensing. This article presents a novel anti-dc bias energy meter. In the proposed approach, the magnetic-valve-type current transformer (MVCT) is used as the current sensor in the meter instead of conventional CT. As a result, the distorted secondary current can be fully compensated within MVCT when the core gets saturated. The principle of the new technology is described in detail. The simulations and experiments are performed to verify the correctness and feasibility of this novel energy meter when measuring load power consumption with dc bias.

Comparison of Mathematical Methods for Compensating a Current Signal under Current Transformers Saturation Conditions.

Current measurements from electromagnetic current transformers are essential for the construction of secondary circuit systems, including for protection systems. Magnetic core of these transformers are at risk of saturation, as a result of which maloperation of protection algorithms can possibly occur. The paper considers methods for recovering a current signal in the saturation mode of current transformers. The advantages and disadvantages of methods for detecting the occurrence of current transformers core saturation are described. A comparative analysis of mathematical methods for recovering a current signal is given, their approbation was carried out, and the most promising of them was revealed. The stability and sensitivity of recovery methods were tested by adding white noise to the measured signal and taking into account the initial flux density (remanent magnetization) in the current transformers core. Their comparison is given on the basis of angular, magnitude, and total errors at a given simulation interval.

Effects of VA Rating on the Fault Diagnosis of Power Transformer Using SFRA Test

The electric power transformer is an essential part of an electrical power system since it is used to step up or down voltage levels to maintain the system performance as well as possible. Frequency response analysis (FRA) is one of the most widely used techniques for detecting various types of mechanical damage in transformers. The equivalent circuit of the transformer will be represented by a complex network of R, L, and C elements in the FRA technique. For transformer faults diagnosis, various calculation techniques and diagnostic techniques may be used, such as acoustic emission analysis, thermal images of electromagnetic radiation, transformer temperature, and humidity analysis. SFRA test is one of these techniques that could be used to determine the fault type based on its response over a wide frequency range. The main challenge of the SFRA test is that the functional interpretation requirement for this test is not universally accepted Also statistical features are defined for this SFRA response to be used in fault detection and classification. In this paper, the effect of the transformer rating on the fault diagnosis techniques using SFRA is tested. Also, the effect of the transformer VA rating on the statistical parameters and the classification rules of fault diagnosis is discussed. Finally, the features used in fault diagnosis are ranked according to its independence of the transformer rating resulting in a more accurate matching fault diagnosis technique.

Simulation Analysis of Ferro resonance of Generator-Transformer Group Shock Tripping

As a common self-excited oscillation in power systems, ferromagnetic resonance has a great threat to the insulation of electrical equipment due to overvoltage and overcurrent. The magnetizing inductance of electromagnetic voltage transformers has nonlinear characteristics, which tend to saturate the excitation and cause circuit resonance and generate resonance overvoltage. The ferromagnetic resonance overvoltage caused by the saturation of the transformer has greatly affected the safe and stable operation of the power grid. Therefore, it is very necessary to analyze, calculate and simulate the ferromagnetic resonance overvoltage, and to study the measures to eliminate resonance. Through theoretical calculation and simulation analysis of a hydroelectric power plant generator-transformer group reverse transmission impulse tripping fault, the cause of its ferromagnetic resonance was analysed, and relevant operation and maintenance suggestions were put forward to provide a basis for further research.

Analysis and Design of a Fully-Integrated High-Power Differential CMOS T/R Switch and Power Amplifier Using Multi-Section Impedance Transformation Technique

In this paper, a new topology for a high-power single-pole-double-throw (SPDT) antenna switch is presented, and its loss mechanisms are fully analyzed. The differential architecture is employed in the proposed switch implementation to prevent unwanted channel formations of OFF-state Rx switch transistors by relieving the voltage swing over the Rx switch devices. In addition to that, the load impedance seen by the Tx switch is stepped down to reduce the voltage swing even more, allowing the antenna switch to handle a high-power signal without distortions. To drop the switch operating impedance, two matching networks are required at the input and the output of the Tx switch, respectively, and they are carefully implemented considering the integration issue of the front-end circuitries. From the loss analysis of the whole signal path, an optimum switch operating impedance is decided in view of a trade-off between power handling capability and insertion loss of the antenna switch. The insertion loss of the proposed design is compared to the conventional design with electromagnetic (EM) simulated transformer and inductors. The proposed antenna switch is implemented in a standard 0.18 µm CMOS process, and all switch devices adopt the deep n-well structure. The measured performance of the proposed transmitter front-end chain shows a 1 dB compression point (P1dB) of 32.1 dBm with 38.3% power-added efficiency (PAE) at 1.9 GHz.