Intermediate Transformer Research Articles

An Isolated Single Input-Multiple Output DC–DC Modular Multilevel Converter for Fast Electric Vehicle Charging

This article proposes an isolated single input-multiple output (SIMO) dc–dc modular multilevel converter (MMC) for fast charging of multiple electric vehicles (EVs). The proposed topology provides galvanic isolation and facilitates a high-voltage conversion ratio from medium voltage direct current (MVDC) to LVDC (EV battery voltage levels) via intermediate medium-frequency transformer. The proposed topology, as compared to conventional MMC, has reduced number of arms, semiconductor devices and has inherent dc fault blocking capability. Operation of the converter with medium frequency reduces the size of passive components present in it. The output voltage waveshape of the converter results in nearly sinusoidal wave being impressed across transformer's primary with very less <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> . Analysis of the converter in steady state and during fault conditions have been included in this article. The proposed converter is validated in simulation model and scale down laboratory prototype.

Разработка модели переходных процессов в трансформаторе при емкостном характере нагрузки

Voltage transformers with capacitive loading during transients can cause significant distortion of the secondary voltage of the ship's network. For the analysis, a well-known nonlinear model of a single-phase two-winding transformer is used, in which the magnetic flux is divided into “working” and leakage fluxes coupled to individual windings that close outside the core. For practical purposes, when using such transformers, it is important to know both the conditions for the occurrence of autoparametric oscillations and the dynamics of the transient process. It is convenient to carry out a comparative evaluation of capacitive power take-off schemes on a computer, which makes it possible to reproduce non-linear characteristics and vary parameters over a wide range, which is difficult in physical modeling. The results of the study of a short circuit downstream of the reactor in the capacitor selection circuit proved that the short circuit at the terminals of the intermediate transformer does not affect the elements of the upper arm of the voltage divider, but significantly affects the lower arm of the divider and the compensating reactor, as a result, subharmonics appear. It is shown that the developed model is convenient for studying the influence of transient processes (short circuits) on the elements of a voltage divider. The applied model is universal for ship power networks and can be used for both capacitive and inductive dividers. A divider circuit with specific parameters has been modeled and relevant studies have been carried out. Mathematical results are verified on a physical model, which confirmed the correctness of the mathematical model. It has been inferred that a decrease in the phase voltage leads to a decrease in the overvoltage on the divider elements during a short circuit behind the compensating reactor of the power take-off circuit.

Control of Photovoltaic System Connected Directly To The Grid by a 3-Phase BOOST Inverter

For PV applications, many studies focus on maximum power extraction without evaluating losses in the conversion chain. As part of this research, it is proposed to evaluate a direct energy conversion solution from the panel to the network without intermediate stages or transformers. The structure is presented as a 3-phase boost inverter integrating the maximum power extraction function (MPPT) while ensuring control of the power fed into the grid. A precise assessment of the losses will be carried out and an evaluation on the different components will be compared. The operation will be studied in particular according to the power rating and the switching frequency.

An Improved Core Loss Model of Ferromagnetic Materials Considering High-Frequency and Nonsinusoidal Supply

With the emerging power electronic technologies, numerous electrical devices use high-frequency and nonsinusoidal supplies. Such sources generate complex harmonic fields that overburden the accurate prediction of core losses using the conventional finite element method (FEM). Thereby, to predict core losses under those high-frequency and nonsinusoidal supplies accurately, this study first built a test system for core losses measurement of ferromagnetic materials (e.g., soft ferrite, laminated steels, and nanocrystalline alloy) fed by the square and pulsewidth modulated (PWM) supplies. Then, the loss prediction problems of the classical core loss model under high-frequency and nonsinusoidal supplies are revealed. Afterward, an improved FEM-based core loss model is proposed, which includes a piecewise variable parameters based hysteresis loss model, an eddy current loss model considering different cross sections of magnetic circuit and supply waveforms, and improved excess loss coefficients. Experimental validations on different materials, the nanocrystalline alloy in an 11-kVA intermediate frequency transformer, the ultrathin silicon sheets in a 10-kW high-speed permanent magnet motor, and the nanocrystalline alloy in a 3-kW wireless power transfer system, are conducted systematically. The effectiveness and generality of the improved core loss model were verified.

Influence of CVT transient response on commutation process of DC transmission and improvement measures

Capacitor voltage transformers (CVT) have been widely used in 220kV, 500kV HVDC transmission lines. The transient characteristics of CVT mainly show that when the primary side fails,the secondary side voltage can no longer be obtained simply from the primary side voltage according to the intermediate transformer conversion ratio, which will affect the commutation process of HVDC transmission system and even the frequency of commutation failure. This paper first establishes a simulation model in PSCAD/EMTDC with the actual parameters of the 500kV fast-saturated CVT, verifies its transient characteristics, and proposes a built-in series low-voltage capacitor C3 in the CVT to eliminate the output voltage error. The principle of commutation failure predictive control (CFPREV) is analyzed, and its input voltage has an impact on the commutation process of the DC system. Based on the Yi-hua DC model of East China Power Grid, the output voltages of CVT and CVT with C3 are used as input voltages for CFPREV. Simulation results show that the commutation results of CVT with C3 can be more accurate reflects the impact of AC bus faults on the inverter side on the DC system, and improves the speed and effectiveness of DC system recovery.

Modular Multilevel DC–DC Power Converter Topology With Intermediate Medium Frequency AC Stage for HVDC Tapping

This article proposes an isolated dc–dc modular multilevel converter (MMC) topology intended for tapping power from the high-voltage dc (HVdc) link. The proposed converter operates with two stages of power conversion, i.e., HVdc to ac and ac to medium-voltage dc, using an intermediate medium-frequency transformer. The proposed topology reduces the number of arms in MMC to two, along with a series LC passive filter tuned at a medium frequency at the primary side of the transformer. A high-voltage transformation ratio is possible in the proposed converter. Operation at the medium-frequency ac ensures the reduction in the size of the transformer and other passive elements. A sinusoidal level-shifted pulsewidth modulation technique is implemented for modulating the arm voltages of the converter, which results in a voltage close to the sine wave with small dv/dt being impressed across the transformer primary. The design of the passive filters is also included in the article. The operation of the converter is validated by the simulation of a 400 kV, 10 MW system, and a downscaled experimental prototype of 1 kW, 400 V system.

Research on control strategy of MMC DC power electronic transformer based on DAB mode

High-power DC power electronic transformer (DC PET) is becoming the key equipment in the DC power network. Face-to-Face (F2F) DC transformers based on modular multilevel converter (MMC) have the modular structure and can flexibly design systems based on different voltage and power levels. Previous studies show that the control strategy of the F2F topology regulates the MMCs separately. Moreover, the voltage ratio of the DC sides depends on the intermediate AC transformer. In the current study, a new control strategy is proposed, which can change the DC voltage ratio by turning on/off submodules of each MMC bridge flexibly and set relevant parameters with different application scenarios. The algorithm enables the DC PET with F2F topology to operate in a dual active bridge (DAB) circuit mode. Thereby reducing the overall system size and meeting different demands of medium and high voltage application scenarios. According to the control strategy used, a submodule capacitor voltage equalization method is proposed to eliminate the capacitance voltage deviation and reduce the capacitance voltage stress. At the same time, the current characteristics of the AC conversion part in the DC transformer are analyzed. Finally, the feasibility of the proposed method is verified by simulation and experiments.

Investigation of Processes in the Measuring Part of Digital Devices of Relay Protection in the MATLAB Software Package

It was revealed that one of the main reasons for erroneous operation of relay protection devices (RPs) is it being set in a way that does not correspond with the real operating conditions, which in turn is determined by the design procedures of the RP settings. An analysis of modern methodologies of RP adjustment revealed that there are no significant changes as compared with the guidelines of the past decades and, accordingly, there are the same disadvantages. The development of new adjustment techniques using detailed mathematical models taking into account the features of specific RPs and the processes in the measuring converters are considered in the work. Within the framework of solving this problem, mathematical models of measuring part of digital relay protection for various types of intermediate current transformers (ICTs) (active and passive) and filters (of Butterworth and Chebyshev types) are synthesized and a comparative analysis is done of their influence on the output signal waveform in the MATLAB software package, including taking into account the magnetization of measuring current transformers. It is revealed that the simplest (explicit and implicit Euler methods) numerical methods are not suitable to solving the created mathematical models. A comparative analysis of calculation results by these methods and by the more advanced Runge–Kutta method is conducted. The presented theoretical and practical studies made it possible to reliably formulate the requirements for detailed mathematical models of RPs.

Система векторно-импульсного пуска высоковольтного синхронного двигателя с ограничением пусковых токов

This article deals with the implementation of the start of a high-voltage synchronous motor. At present, the problem of starting high-power alternating current motors is acute. Therefore, they are left in operation to minimize the number of starts. The main problem of starting a synchronous high-power motor is that starting currents are generated that exceed the rated current seven times. In addition, there are other problems that negatively affect other consumers of the network. It is proposed to implement the start-up of a high-voltage synchronous motor using a vector-pulse method in this article. A functional diagram of the developed electric drive is given. This scheme has several distinctive features. In the scheme, intermediate transformers are provided. Their use makes it possible to use IGBT-transistors with a maximum voltage of 600 to 1200 V in the circuit. The structural scheme of the module for determining the generalized voltage vector is given. The scheme of limitation of starting currents is considered. Experimental studies were carried out on the model developed in Matlab Simulink. From the results of the experiment it follows that the system fulfills the process of starting a synchronous motor. The influence of intermediate transformers is estimated. It was revealed that the intermediate transformers slightly worsen the process of synchronous motor acceleration. The acceleration time of the motor during the introduction of intermediate transformers increased by 0.7 s. On the other hand, the availability of intermediate transformers will reduce the cost of the system in 1.3...1.7 times depending on the motor power.

A Modular Multilevel HVDC Buck–Boost Converter Derived From Its Switched-Mode Counterpart

This paper begins by presenting a generalized methodology for conceptualizing modular multilevel converter (MMC)-based dc–dc topologies, which is predicated on the concept of harmonic power balance. A compelling implication is that MMC-based variants of conventional switched-mode converter structures can be realized. As an example case study, this paper introduces a new dc–dc MMC for HVdc applications, which is derived from the classical buck–boost dc–dc converter. This new topology, which is revealed to be an alternative option to the well-known dual active bridge (DAB) converter with an intermediate transformer, offers buck–boost functionality and bidirectional dc fault blocking, using only two quadrant switching cells. Comparative analysis shows the proposed topology has lower operating losses and a lower total magnetics rating in comparison to an MMC-based DAB solution for dc stepping ratios around unity. A dynamic controller is developed that regulates the converter dc power throughput while maintaining balanced capacitor voltages. The converter operating principle, dynamic controller performance, and dc fault blocking are verified by simulation.

A Study of the Measurement Section of Digital Relay Protection Units

The problem of the electric power systems (EPS) relay protection (RP) units&rsquo; adjustment, which does not correspond to the real operation conditions, was considered. It was found that the availability of the integral analysis of the key elements of different design RP units under the particular operation conditions is the basic condition for the problem solving, which is provided by the application of adequate RP mathematical models and modern EPS motivating factors. The existing EPS and RP modeling methods and tools were analyzed. On the basis of the RP detailed modeling concept, developed by the authors, the mathematical models of the complete diagram of the measurement sections of the digital differential transformer protection were developed for different types of intermediate transformers and low frequency filters. The comparative numerical analysis of their influence on the primary input signal was performed, including the analysis with consideration of nonlinearity of the measuring current transformer excitation characteristic. The theoretical and practical studies, presented in the paper, permit to define the requirements imposed on RP mathematical models, which will be used in the further studies aimed at developing a method for RP adequate adjustment.

Design and Performance Analysis of an Intrinsically Safe Ultrasonic Ranging Sensor.

In flammable or explosive environments, an ultrasonic sensor for distance measurement poses an important engineering safety challenge, because the driving circuit uses an intermediate frequency transformer as an impedance transformation element, in which the produced heat or spark is available for ignition. In this paper, an intrinsically safe ultrasonic ranging sensor is designed and implemented. The waterproof piezoelectric transducer with integrated transceiver is chosen as an energy transducing element. Then a novel transducer driving circuit is designed based on an impedance matching method considering safety spark parameters to replace an intermediate frequency transformer. Then, an energy limiting circuit is developed to achieve dual levels of over-voltage and over-current protection. The detail calculation and evaluation are executed and the electrical characteristics are analyzed to verify the intrinsic safety of the driving circuit. Finally, an experimental platform of the ultrasonic ranging sensor system is constructed, which involves short-circuit protection. Experimental results show that the proposed ultrasonic ranging sensor is excellent in both ranging performance and intrinsic safety.

Система векторно-импульсконого пуска синхронного двигателя с промежуточными трансформаторами и бездатчиковым способом определения углового положения ротора

Система векторно-импульсконого пуска синхронного двигателя с промежуточными трансформаторами и бездатчиковым способом определения углового положения ротора

A Load Test Research on Large Power Transformers

Due to the large capacity of large power transformers, normally the power capacity of a load test is difficult to meet the test requirement, which makes the ancillary devices such as induction regulator, intermediate transformer etc. difficult to meet the test demands too. The transformer factories with limited power capacity can only perform load test under less than fifty-percent of the rated load current, which not obeys the standards and technical protocols. Thus, a field load test method for large power transformers is proposed in this paper. It carries out the reactive power compensation with compensation capacitors, which reduces the capacity of test power supply and makes the field load test possible. This method also gives a guarantee for the safe and stable operation of power transformers.

FREQUENCY DETERMINATION OF HIGH-FREQUENCY LINK FOR PERCPECTIVE ELECTRIC ROLLING STOCK

Purpose. Total mileage of Ukrainian electric railways is distributed approximately equally between the areas of direct and alternating current. A double system of electric rolling stock is used to pass jointing places of different current kinds without train’s stop. Therefore introduction of such rolling stock of a new concept that is using an asynchronous traction drive is prospective for Ukrainian railways. Apart from advantages a rolling stock of similar concept has significant disadvantages, it is pulse energy consumption from the power supply, and it can affect the reliability of track automatic devices, and consequently, the train traffic safety. In addition the specific power of traction transformer is considerably inferior to the power density of other traction elements. The promising schemes using an intermediary link of increased frequency, which consist of a transformer and inverter, have been proposed for disadvantages amendments. The main task for the further introduction of prospective circuit is to determine the operating frequency for high frequency link. Methodology. The method of thermal parameters calculation of semiconductor devices has been used for determination switching transistors of maximum operating frequency. To obtain analytical expressions curves of energy, released during the IGBT (insulated-gate bipolar transistor) switching from its current load approximation method is used. Findings. The permissible frequency of lowfrequency link is determinated by load current of intermediate transformer. Operating frequency range of a link depending on load current has been determined. A comparative analysis of the switching characteristics of 65 class IGBT production by companies Infineon and ABB has been performed. Originality. The further determination method of the maximum operating frequency of intermediate link for circuit with high-frequency transformer has been developed. Practical value. The established operating frequency range of the highlevel link will develop further research of tractive circuit with highfrequency transformer use for double feed electric rolling stock with asynchronous tractive drive. It will evaluate the introduction effectiveness of similar configuration circuits.

New Step-Up/Step-Down DC–AC Converter

This paper introduces a new dc-ac converter with the feature that it produces an instantaneous output voltage higher or lower than the input dc voltage without an intermediate power stage or transformers. This feature is provided by using one switching cell including two switches, two diodes, one inductor, and one capacitor on each inverter leg. Validating the theoretical analysis, a prototype was designed, built, and tested for an output rated power of 1 kW, a dc input voltage of 96Vdc, and output voltage of 110 Vrms. Furthermore, the fundamental output frequency was established at 60 Hz and the switching frequency at 20 kHz.

The Identification Methods of Partial Discharge Test for Equipment

When the power transformer in partial discharge test, if the test equipment (voltage regulator, intermediate transformer, etc.) ontology partial discharge is big, it will severely affect the quality of testing, and even cause false identify.How to correctly identify partial discharge from the power transformer or from the test equipment itself, it is very important for the test personnel. This paper introduces to use the idle 220kV circuit breaker in parallel capacitor to cleverly find out in the whole process of partial discharge from the test equipment itself.

The Design of Power Supply with Feed-Forward Control Method for EBW

A kind of accelerating power supply device for electronic beam welder was introduced in this paper. The device adopts PWM modulation method, use IGBT as DC chopper. M, N two groups of H inverse bridge parallel with each other compose the inverter. Intermediate frequency AC produced by inverter was sent into intermediate frequency high voltage transformer, after the high voltage rectifying and filtering, the high voltage output running steadily. In order to improve the power supply devices output characteristics when power system fluctuates or load changes, feed-forward control strategy was introduced. Use the control method that feed-forward combining with feed-back. The experiments show that the adjusting speed of this power supply device is fast, the output voltage ripple is small, the technical indicators can meet high welding technology for electron beam welding machine.

Analysis of a Harmonics Neutralized 48-Pulse STATCOM with GTO Based Voltage Source Converters

Multi-pulse topology of converters using elementary six-pulse GTO - VSC (gate turn off based voltage source converter) operated under fundamental frequency switching (FFS) control is widely adopted in high power rating static synchronous compensators (STATCOM). Practically, a 48-pulse (6×8 pulse) configuration is used with the phase angle control algorithm employing proportional and integral (PI) control methodology. These kinds of controllers, for example the ±80MVAR compensator at Inuyama switching station, KEPCO, Japan, employs two stages of magnetics viz. intermediate transformers (as many as VSCs) and a main coupling transformer to minimize harmonics distortion in the line and to achieve a desired operational efficiency. The magnetic circuit needs altogether nine transformers of which eight are phase shifting transformers (PST) used in the intermediate stage, each rating equal to or more than one eighth of the compensator rating, and the other one is the main coupling transformer having a power rating equal to that of the compensator. In this paper, a two-level 48-pulse ±100MYAR STATCOM is proposed where eight, six-pulse GTO-YSC are employed and magnetics is simplified to single-stage using four transformers of which three are PSTs and the other is a normal transformer. Thus, it reduces the magnetics to half of the value needed in the commercially available compensator. By adopting the simple PI-controllers, the model is simulated in a MATLAB environment by SimPowerSystems toolbox for voltage regulation in the transmission system. The simulation results show that the THD levels in line voltage and current are well below the limiting values specified in the IEEE Std 519-1992 for harmonic control in electrical power systems. The controller performance is observed reasonably well during capacitive and inductive modes of operation.

Dc SQUID Readout Electronics With Up to 100 MHz Closed-Loop Bandwidth

High-speed readout electronics for sensors based on dc superconducting quantum interference devices (SQUIDs) are presented. The SQUID sensor involves a series array of 16dc SQUIDs and an intermediate transformer to enhance its current sensitivity. By using a highly gradiometric design and /spl les/5 /spl mu/m linewidth for the SQUID array, the device can be cooled down in fields of up to 85 /spl mu/T and be operated magnetically unshielded. A special feedback coil design minimizes the parasitic coupling between feedback and input coil. The SQUID sensor is directly connected to the room temperature electronics. A composite preamplifier is used consisting of a slow dc amplifier in parallel with a fast ac amplifier. A virtual 50 /spl Omega/ input resistance with negligible excess noise is realized by active shunting. Two types of high-speed readout electronics were developed. The first was designed for optimum dc performance, high flexibility, and user-friendliness. It is fully computer controlled. The white voltage and current noise levels are 0.3 nV//spl radic/Hz and 3 pA//spl radic/Hz, respectively, resulting in an overall system noise level of 0.4 /spl mu//spl Phi//sub 0///spl radic/Hz or a coupled energy sensitivity around 500h (/spl Phi//sub 0/ is the flux quantum and h is Planck's constant). The maximum flux-locked loop (FLL) and open-loop bandwidths are about 20 MHz and 50 MHz, respectively. The second readout electronics is an ultra-high-speed prototype which was designed for maximum speed at the expense of dc performance. A very low intrinsic signal delay of 1.7 ns and a high open-loop bandwidth of 300 MHz were measured. Using a novel FLL scheme, a very high signal bandwidth of 130 MHz was achieved with 0.8 m distance between SQUID and electronics.