Short-circuit Impedance Research Articles

Metaheuristic Optimization Preliminary Design Process of Core-Form Autotransformers

Because of the strong competition in the equipment market, the aim of the manufacturers is to reduce the product price as much as possible, fulfill the high-quality standards, and improve the reliability and efficiency of the transformers. The capitalization of the costs enables to handle the different economic and technological parameters together in the same cost function. Large power autotransformers are usually designed for specific economic and technical conditions specified by the customer. At the tender design stage, the most economical solution has to be found in a limited time, and a great number of feasible transformer designs have to be compared to find the optimal solution. Nowadays, the solution of this design process is mainly based on the experience of the transformer designer, but mathematical programming models can facilitate the process. It is well known that the design of shell-type power transformers can be optimized by geometric programming. The constructed models of geometric programming can be solved accurately with the interior-point-method-based solvers. According to the results, this kind of modeling is not applicable for the core-form transformers. However, with branch-and-bound search, the geometric programming method can also be used for these cases. This paper introduces a new algorithm, which can also handle the autotransformer designs with different positions of the regulating winding by applying a more general metaheuristic routine to perform the calculation of the short-circuit impedance.

Development of a 1250-kVA Superconducting Transformer and Its Demonstration at the Superconducting Substation

A three-phase 1250-kVA/10.5-kV/0.4-kV high-temperature superconducting (HTS) transformer has been designed and fabricated. The windings of the HTS transformer are wound by copper-alloy-reinforced multifilamentary Bi2223/Ag tapes, and the structures of the primary and secondary windings are solenoid and double pancake, respectively. The three-phase three-limb core is made of domain-refined high-induction-type grain-oriented silicon sheet steel, and the cryostat is made of glass-fiber-reinforced plastics. According to the tests, the no-load loss is 2319.2 W at an exciting current of 0.254% of the rated level, the short-circuit impedance of the transformer is 5.6% of the rated value, and the average load loss is 249.6 W. In addition, the leakage current between the primary and secondary windings is about 12.6 mA under the condition of 35 kV/1 min/50 Hz, and the negative full-wave lighting impulse test with a peak voltage of $75 \mbox{kV}/1.0 \mu\mbox{s}/50 \mu\mbox{s}$ from the primary winding is carried out without breakdown. The experimental results show that the developed HTS transformer meets the requirements of the Chinese standards, and the demonstration of the HTS transformer at the superconducting power substation since September 9, 2014 has been reliable, showing its potential for long-term operation in the grid.

A study of the sweep frequency impedance method and its application in the detection of internal winding short circuit faults in power transformers

Undetected short circuit faults are a significant problem in power transformers and can eventually develop into catastrophic faults. At present, frequency response analysis (FRA) is one of the well-recognized diagnostic tools for the detection of winding faults, but it has some limitations, such as a low signal-to-noise ratio (SNR) and instability caused by changes in the measuring voltage. In this paper, a novel method called sweep frequency impedance (SFI) is proposed to address the difficulties that arise from FRA. Based on the mechanism of this new method, a nondestructive testing system was established to demonstrate the advantages of SFI measurements. The SFI test system has a better stability, repeatability, and SNR by comparing it with the FRA test system. Moreover, FRA and SFI curves obtained under the same conditions was symmetrical about a specific straight line above 10 kHz, and the SFI value at 50 Hz is equivalent to the short circuit impedance (SCI) value of a transformer. These results indicate that the existing criteria of FRA and SCI methods can be used in the SFI method to detect transformer faults. Finally, the experiments on a special oil-immersed testing transformer demonstrate that the SFI detection system is feasible, sufficiently sensitive to detect short circuit faults and able to quantify the level of the fault.

Novel LVRT Testing Method for Wind Turbines Using Flexible VSC Technology

Wind energy is one of the fastest growing renewable energy sources. For this reason, several countries have included requirements in their grid codes to guarantee proper operation of wind turbines under stable operations of the power systems as well as under grid disturbances. This paper presents a novel methodology for grid code testing of wind turbines based on voltage source converter (VSC) technology. In particular, this paper focuses on a low voltage ride through (LVRT) test of a full size converter-based wind turbine. The investigated testing setup consists of a 4-MW wind turbine and an 8-MW back-to-back VSC system, operated as test equipment. The control algorithm of the testing device is derived in detail and validated through time-domain simulations. The risk of poorly damped resonances and possible interaction between the testing equipment and the tested object is investigated through small signal analysis. The obtained results demonstrate the flexibility of the proposed approach in controlling the voltage at the wind turbine terminals, including the ability in emulating the short-circuit impedance of the grid at the connection point.

Double Earth Faults in Power Stations

Single-phase-to-ground faults in power stations sometimes cause a second phase-to-ground fault in another phase. If the phase-to-ground faults are located on different sides of the generator circuit breaker (CB), one has a double earth fault of a special and so far not analyzed kind with respect to the CB. It may lead to high stresses for the last CB pole to clear. The general expressions for the current, the rate of rise of recovery voltage, and the power frequency recovery voltage across this CB pole have been deduced as function of the sequence values of the short-circuit impedances and surge impedances, respectively. The corresponding results are discussed for typical cases. A detailed case analysis by means of the Electromagnetic Transients Program shows that at rated load, such a double earth fault, produces CB stresses comparable to out-of-phase conditions. A combination of double earth fault and out-of-phase conditions, however, causes a peak value of the transient recovery voltage, which will significantly exceed the values laid down in the standards.

Design and construction of a ±100 kV gas switch for linear transformer drivers

In this paper, the design, installation, and performance of a ±100 kV gas switch for linear-transformer-driver (LTD) are presented. LTD demands require gas switches that can be operated at a 200 kV DC voltage, and require trigger jitter and inductance as small as possible. A field distortion switch has been designed based on the L3 switch, the design criteria of switch electric field and trigger system are explained in detail. A two dimensional plasma model is developed to analyze the contribution of trigger system to the closing of gas switch. An experimental platform was built to test the self breakdown and triggered breakdown characteristics of the switch. When the switch is charged to ±100 kV and is triggered by a -120 kV pulse with a 30 ns rise time, the delay time is 78 ns and the jitter is 1.6 ns. A comparative test was done to research the function of UV preionization gap, the test results shown that the UV preionization gap can reduce the switch delay time and jitter, it can effectively improve the trigger performance of the switch. Meanwhile, using the short-circuit discharge, the inductance and impedance were measured, the switch inductance is about 30 nH, and the impedance is varied from 50mΩ to 30mΩ.

Application of 2D linear modeling for computing zero-sequence short-circuit impedances of 3-phase core-type YNynd transformers

This paper shows the application of 2D linear modeling for computing zero-sequence short-circuit impedances (Z0SC) of 3-phase core-type YNynd transformers. Firstly, a basic description of these impedances is shown. The proposed 2D model is based on two steps: (a) the search of the currents to satisfy the condition of induced voltage equal to zero in short-circuited windings; (b) the use of constants (KX and KP) to approximate 2D results to results of real 3D geometries. KX is related to the computed reactances, and KP is related to the computed values for the power losses during Z0SC tests. Induced currents in short-circuited windings are found by an iterative process, in order to apply constant current densities in the model of each winding. Results of the model are accurate in comparison with measured values in five transformers. Tertiary is the innermost winding in four units, and the outermost winding in one unit. Open-delta cases and closed-delta cases are considered, as well as cases with and without magnetic shunts on the tank walls. Equivalent permeability of tank steel has an influence on results but a reasonable accuracy is obtained with an intermediate value of this parameter; thus, non-linearity is not a concern in this case. A good accuracy is also obtained for a wide range of KX and KP; in fact, reactances can be estimated without considering that 2D geometry is not an exact representation of real 3D geometry (unlike power losses during the tests, where this fact must be taken into account).

Study on Technical Parameters for 220/20kV SF<sub>6</sub> Gas Insulated Transformers

Considering the requirement of Shenzhen power grid development, this paper investigates the parameters selection for 220/20kV gas insulation transformer, including capacity, short circuit impedance, cooling mode, insulation level, over-load capability, temperature rise tolerance, loss , noise level and bushing selection. For different capacity value, the investment and loss concerning substation, 220kV grid and 20kV grid are analyzed and compared, by which the final capacity value is determined. The short circuit current level, transformer loss, manufacturing capability and investment is investigated to determine the short circuit impedance for the gas insulation transformer. The characteristic and application condition of gas cooling as well as liquid cooling mode is studied and the cooling mode for the gas insulation transformer is determined. The insulation level, overload capability and temperature rise tolerance are determined by considering related standards and the requirement of Shenzhen. The loss and noise level for the gas insulation transformer is determined by considering related standard and parameters of the domestic gas insulation transformers in operation. The gas-gas bushing is used to connect cable box and the transformer.

Numerical Analysis and Optimization of Lobe-Type Magnetic Shielding in a 334 MVA Single-Phase Auto-Transformer

In this paper, the influences of the lobe-type magnetic shielding in a transformer in terms of its leakage fields, stray losses in structural parts, shielding losses, and short-circuit impedance are examined using 3-D non-linear frequency-domain finite element method. An ODFS-334 MVA/500 kV single-phase auto-transformer is used as the prototype model under study. The magnetic shielding geometry of the transformer is optimized with its stray losses minimized based on 3-D non-linear finite element analysis. To validate the feasibility of the proposed model and method, experimental data and simulation results are compared and reported.

Magnetic Integration Technology in Controllable Reactor of Transformer Type Constituted by Various Magnetic Materials

Eliminating the magnetic coupling among control windings and simplifying the complicated structure caused by current-limiting inductance cascading in the control winding circuit of a controllable reactor of transformer type (CRT) are quite important. Based on the technology of magnetic integration, a magnetic integration structure composed by a variety of magnetic materials is proposed. The formulas for short-circuit impedance and short-circuit current of each winding are deduced by the establishment of the equivalent circuit and are then verified by an example. The results of the calculation example show that the structure cannot only integrates control winding and current-limiting inductance but can also weaken the magnetic coupling among control windings; therefore, the structure meets the requirement of “high impedance” and “weak coupling.” Thus, the whole structure of CRT is simplified, and the no-load current is reduced. It can be drawn that the magnetic integration structure constituted by various magnetic materials could provide useful reference for the magnetic integration of similar multi winding electromagnetic equipment.

Bitter coil design methodology for electromagnetic pulse metal processing techniques

Electromagnetic pulse metal processing techniques (EPMPT) such as welding, forming and cutting have proven to be an effective solution to specific manufacturing problems. A high pulse magnetic field coil is a critical part of these technologies and its design is a challenging task. This paper describes a Bitter coil design using a newly developed methodology for a simplified analytical calculation of the coil and complementary finite element models (FE) of different complexity. Based on the methodology a Belgian Welding Institute (BWI) Bitter coil has been designed and tested by means of short circuit experiments, impedance and B-field measurements. A good agreement between the calculated and the experimental design parameters was found.

About the difference between zero-sequence magnetizing impedances of a 3-phase core-type transformer

This paper analyzes the difference between zero-sequence magnetizing impedances (Z0M) of a 3-phase core-type transformer. Z0M in per-unit is smaller for an external winding than for an internal winding. The difference between both Z0M values is similar to the positive-sequence short-circuit impedance between both windings for units without magnetic shunts on the tank wall as well as for units without tank. This fact can be observed in measured values, and in the results of 3D models. However, the results of simple 2D models do not show this behavior; therefore, this paper demonstrates that this fact only can be explained from 3D models. The results are shown for different cases: transformers with and without magnetic shunts on the tank wall, and transformers without tank. In transformers with magnetic shunts on the tank wall, or in magnetostatic simulations of transformers with tank, the difference between both Z0M values is not very similar to the positive-sequence short-circuit impedance between both windings.

Calculation of Leakage Magnetic Field and Short-Circuit Impedance of Power Transformer

The leakage magnetic field and equivalent circuit model of OLTC double-winding power transformer has been established by electromagnetic field Finite Element analysis software. The method of field-circuit coupled method has been used to calculate its magnetic field leakage, the short circuit impedance obtained by the calculation of formula and compared with the experimental data. At the same time, based on the calculation method for the Visual Basic language as a tool for MAGNET software for secondary development, obtain the software to calculate the short-circuit impedance of power transformer simulation software, and is verified by using the typical products, proves the validity of this software.

Research on Electrical Characteristics of a Novel Balance Transformer with Asymmetrical Winding

For problems of zero sequence current and negative sequence current of traction transformer in Electrified Railway, a novel balance transformer with asymmetrical windings was presented. By a systematic analysis method based on the magnetic force balance equations, winding connection equations, output-port equations and voltage transfer equations, the current relationships of the primary windings and secondary windings, the balance condition and the short circuit impedances (when the neutral current in the primary side is zero) were deduced, and the electric characteristics of the balance transformer was analyzed. Through the experimental model of balance transformer in simulation platform based on Matlab/Simulink, the electric characteristics of balance transformer and change law of zero sequence current and negative sequence current in different mode were researched. Correctness and feasibility of the theoretical analysis are verified through the simulation experiment.

Effects of Electrolytes on the Photoelectric Properties of TiO2-Based Dye-Sensitized Solar Cells

The unique photoelectric properties of TiO2, which make it a better choice of dye-sensitized solar cells with high efficiency, have attracted a lot of interest in recent years. Especially, single-crystal rutile TiO2 nanorod arrays have been extensively investigated. In this study, in order to improve the photoelectric properties of TiO2 and analyze the effects of different electrolytes on the performance of TiO2 based dye-sensitized solar cells, the solar cells were fabricated with iodine base electrolyte, sulfur based electrolyte and inorganic solid state electrolyte, CsSnI2.95F0.05, as electrolyte respectively, and Pt deposited on the conductive face of fluorine-doped tin oxide (FTO) glass as the counter-electrode. TiO2 nanorod arrays were treated with ethanol solution of cis-diisothiocyanato-bis-(2,2,-bipyridyl-4,4,-dicaboxylato)-ruthenium()bis(tetrabutyl-ammonium) (N719, DYESOL) for 18h before the cell assembly. Open-circuit voltage (V OC), short-circuit current density (J SC) and impedance of the solar cells with these three kinds of electrolytes were measured respectively, and then their conversion efficiencies were calculated. The TiO2 based solar cell fabricated with CsSnI2.95F0.05 showed the highest conversion efficiency of about 8.9%, which was much higher than those of the solar cells fabricated with the other two kinds of electrolytes.

Discussion Based on the Condition of Transformer Parallel Operation

When three-phase transformer runs parallel, power supply reliability can be improved effectively , and the number of transformer running parallel can be adjusted according to the size of the load ,which can improve the operation efficiency of the whole, in addition, it reduce the loss of transformer effectively, so as to improve the economy of power supply, but the two transformer works at the same time, they must meet the same ratio、connecting groups、per-unit value of short-circuit impedance and short-circuit impedance angle as the three conditions. This paper based on the principle of the parallel operation of transformers gives the corresponding theoretical basis. The conclusion shows that the parallel transformer which satisfies the above three factors can run normally.

Diagnosis Method Study of Transformer Windings Deformation Basing on Sweep Frequency of Short-Circuit Impedance

Combing the advantages of frequency response method with the advantages of short-circuit impedance method, put forward the sweep frequency short-circuit impedance method. Analyse the windings deformation basing on the mutual difference of short-circuit impedance at different frequencies. The validity of this method is preliminary verified at last.

Software System for Automotive Wiring Harness Testing Based on Virtual Instrument

Development of modern information technology provides a technical platform for the wide application of virtual instrument technology. Here we apply virtual instrument technology to test the faults of wiring harnesses, such as short circuit , broken circuit , wiring error , poor contact, poor insulation and unsatisfactory impedance and monitor the product quality at real-time. We use LabVIEW of the graphical programming language based on virtual instrument as the tool to develop the testing software. It realizes the functions such as the data acquisition, storage, analysis and display, single point wiring harness testing, all wiring harness testing, impedance testing, query and statistics, etc. We uses software instead of hardware so that functions can be added or cut down at random without adding any other hardware facilities.

Methods of Phasor Algorithm Applied on Substation Short-Circuit Impedance Calculation Analysis

Dynamic and real-time monitoring of substation, the analysis of the operating state of the substation through the phasor values of the substation. The phase of accurate measurement is a prerequisite to ensure accurate analysis Substation running state. This paper uses Fourier algorithm, recursive least squares algorithm and Kalman algorithm three methods of phasor calculation solving the acquisition voltage, current from the substation, get the results of calculation and error analysis. Come up with a more accurate method of phasor calculation. Provide a practical method for the accurate analysis of the operating state of the substation.

Al-Ru 이층 상대전극을 채용한 염료감응 태양전지의 물성

In order to improve energy-conversion efficiency using a Cu/Pt bilayer on a flat-glass substrate of a counter electrode (CE), a 0.45 cm dye-sensitized solar-cell (DSSC) device (with glass/FTO/blocking layer/ TiO2/N719 (dye)/electrolyte/50 nm-Pt/50 nm-Cu/glass) was prepared. For comparison, 100-nm thick Cu and Pt CEs were also prepared on flat-glass substrates using the same method. The photovoltaic properties of the DSSC device, such as short-circuit current-density (Jsc), open-circuit voltage (Voc), fill factor (FF), energy-conversion efficiency (ECE), and impedance, were checked using a solar simulator and potentiostat. The sheet resistance was examined using a four point probe. The phases of the bi-layered films were examined using X-ray diffraction. The measured energy-conversion efficiency of the DSSC devices with only Pt and Cu/Pt bilayer counter electrodes was 4.60% and 5.72%, respectively. The sheet resistance and interface (CE/electrolyte) resistance of a Cu/Pt bilayer were smaller than those of a Pt-only layer. The phases of the Cu/Pt bi-layered films were identified in pure Cu and Pt without any intermetallic layer. We concluded that the increase in the efficiency of DSSCs employing Cu/Pt, resulted from employing the low-resistive Cu layer. †(Received August 20, 2013)