Primary Winding Of Transformer Research Articles

Improved Duty Compensation Control-Based Bidirectional Resonant DC−DC Converter With Reduced Input-Current Ripple for Battery Energy Storage Systems

This paper proposes an improved duty control-based resonant bidirectional dc–dc converter (BDC) to remarkably minimize input-current ripple for battery energy storage systems. The proposed converter offers a resonant-boost operation using a pulse-width-modulation (PWM) full-bridge circuit with a push-pull transformer under the forward-mode. Whereas, in the backward-mode, a resonant-buck operation is implemented with a PWM neutral-point-clamped (NPC)-type circuit. Unlike conventional current-fed BDCs, the most appealing characteristic of the proposed converter is the NPC-type structure with a push-pull transformer, which can significantly mitigate the input-current ripple without using extra input-inductors. Moreover, the proposed BDC minimizes the conduction-loss of the low-voltage side bottom switches by alleviating unbalanced DC offset-currents via proposed duty-compensation control under mismatched magnetizing inductances of the transformer's primary-windings. The designed circuit operation is analyzed in detail and its practicability has been fully confirmed with the proportional-integral and spider-monkey-optimization control techniques-based experimental verifications using a prototype 1-kW test-bed.

A series inductance based three-port isolated hybrid DC–DC converter for microgrid applications

The paper proposes a series inductance based three-port isolated hybrid converter (3PIHC) for wide change in voltage transformation ratio (VTR) which supports DC microgrid applications. Three-ports in the proposed converter is integrated by means of a three-winding high frequency transformer (HFT). The leakage inductance of three-winding HFT acts as an energy transfer element between the ports. The factors which need to be considered for designing the leakage inductance are power transfer flexibility and limits of current transfer at wide change in VTR. In a feasible scenario, the intricate aspects such as inconsistency of leakage inductance, requirements of power flow variability and current restrictions necessitates auxiliary inductors in series with the three-winding HFT. This paper focuses on selecting appropriate value of series inductance for 3PIHC based on the application demand and minimized converter losses. Study and analysis are carried out to evaluate the converter performance at certain values of series inductance which is the sum of leakage inductance of HFT and auxiliary inductance. A hybrid configuration is adopted in the proposed converter where the primary port holds multi-level half bridge structure which reduces the voltage stress across the transformer primary winding and semiconductor switches. The secondary and tertiary port occupies H-bridge circuit topology. Converter analysis is presented for both conventional phase-shift modulation (CPM) and dual phase shift modulation (DPM). A simple computational method for obtaining the phase-shift variables at minimum value of current stress is developed. Evaluated the converter current stress for various values of series inductance and at wide change in VTR. A prototype of the proposed converter is implemented and examined. The experimental results demonstrated authenticates the accuracy of theoretical analysis.

Determination of Power Transformer Winding Natural Frequencies and Damping Factors from the Measured Voltage Transfer Functions

The problem of damage inflicted to power transformers by high-frequency switching overvoltages occurring in 6–35 kV electric networks is solved by using protective RC circuits and detuning the windings natural frequencies from the frequencies of network voltage transient oscillations. To select the optimal parameters of protective RC circuits, simulation of transients in the cable network together with the transformer primary winding is required. To this end, it is necessary to have a reliable high-frequency model of the transformer. For verifying this model, measured values of natural frequencies and damping factors of free oscillations are required. To detune the transformer winding natural oscillation frequencies, their values have to be measured together with measuring the resonance overvoltage ratios in the transformer windings. Based on this information, the frequency ranges of network voltage transient oscillations that are dangerous for the transformer windings can be estimated. An approach to estimating the natural frequencies and damping factors based on measurements and analysis of voltage transfer functions at accessible intermediate points of power transformers windings is proposed. The article gives a practical example of estimating the natural frequencies and damping factors of free oscillations in the high-voltage winding of a dry transformer using the proposed approach, as well as by approximating transient voltages and analyzing the power transformer primary winding frequency responses.

Novel distribution transformer design for data center applications resilient to transient voltage stresses generated by VCB operation

Dry-type distribution transformers in modern data centres are usually protected by medium voltage (MV) vacuum circuit breakers (VCB). The interaction between VCB, MV cables, and dry-type transformers were studied extensively due to some catastrophic failures of the transformer primary windings, which have had a significant impact on data center up-time. It was observed that VCB switching events can initiate series and parallel resonances in transformer windings, as well as prestrike/restrike overvoltages at transformer terminals. In this work, a novel transformer design concept is presented, based on the shifting of natural transformer series resonance frequencies above the range of the expected frequencies common to operation of VCBs. This novel design has been confirmed by standard manufacturing tests, as well as novel special testing procedures replicating the real-life equipment installations in data centres.

Assembly Automation Technology of the Stand Type Transformer Using Power System Gripper Cable

The cold crimping process of the lead wire of the vertical transformer of the clamp cable is a commonly used manufacturing method in the current transformer manufacturing process. Its advantages are: clean, simple, reliable, and convenient, which satisfies the manufacturing needs of high-voltage electrical products. However, due to the diversity of the lead cross-sectional area, in actual operation, it is often necessary to fill between the lead and the terminal, and there are many optional specifications for the filling line, which may easily lead to inconsistent filling methods and unclear filling standards. Aiming at a kind of vertical transformer primary winding structure and primary cable lead assembly automation research. The improved cold crimping of the lead includes two parts: the tightening and rounding of the lead and the filling of the lead copper tube as the preparatory process before the cold crimping. The post-assembly paper uses machine learning algorithms to simulate and model the wiring reliability of the connected vertical transformer, and evaluate its assembly automation level. The research results found that the wiring method proposed in the paper can improve the reliability of the wiring and the assembly efficiency is higher.

Computational and experimental study of air-core HTS transformer electrothermal behaviour at current limiting mode

The paper provides the results of the experimental and computational study of the processes occurring in high temperature superconducting transformer windings while secondary winding is short-circuited. The obtained mathematical simulation matches closely with the experimental results. The temperature variation curves for superconducting windings were analysed, and conclusions were made on the necessity of changes in HTS transformer design, namely the necessity of windings heat-insulation from each other and adding a high-resistance coating material for HTS wire in HTS transformer primary winding.

Novel Method for Discrimination of Transformers Faults from Magnetizing Inrush Currents Using Wavelet Transform

Discrimination of power transformer inrush from fault current is most important to proper operation of differential relays. Most differential relays discriminate the inrush current from short circuit fault according to high content of second harmonic of inrush. Unfortunately, in some power system operation, it has been seen that this algorithm is not accurate enough due to the presence of some kinds of loads. On the other hand, current transformer (CT) saturation due to large inrush or fault current also can cause the discrimination algorithms mal-operation. This paper presents a novel algorithm for power transformer differential protection which differentiates internal faults from those of magnetizing inrush currents in the presence of CT saturation. In order to eliminate the impact of CT saturation on the performance of the proposed technique, at first the currents are compensated by a CT saturation compensation method. Furthermore, a technique based on wavelet transform is used for discrimination of transformer internal fault from magnetizing inrush currents. Using discrete wavelet transform (DWT) features of primary and secondary differential currents and those of flux measured at transformer primary winding, an algorithm was proposed. Discrimination criterion is based on synchronization of maximum or minimum points between wavelet coefficient of differential current and flux of the transformer. The efficiency of the algorithm was evaluated using different scenarios of inrush and internal fault current of a power transformer simulated in MATLAB/Simulink software environment.

Аппроксимация переходных резонансных напряжений и токов в обмотках силовых трансформаторов для определения собственных частот колебаний и коэффициентов затухани

Transient interaction between power transformers and power cable lines may give rise to resonance overvoltages in the transformer primary windings. To develop protection measures against resonance overvoltages and to design transformers resistant to resonance overvoltages, it is necessary to know the natural frequencies of the transformer windings. Recent years have seen very rapid development of transformer windings high-frequency models. However, the mathematical models used in practice, which came from calculations of impulse overvoltages in transformer windings, reproduce the frequency dependences of losses and damping at natural frequencies with insufficient accuracy. To verify and improve the mathematical models used for analyzing high-frequency processes in transformer windings, it is necessary to have sufficient experimental data on the values of natural frequencies and damping factors. Methods for experimentally determining the natural frequencies and damping factors of power transformer windings are considered. Theoretical principles and analytical expressions for transient voltages and currents obtained for simplified equivalent circuits of windings with lumped parameters are given. An approach is proposed, according to which the transient voltages and currents in the winding are represented as the sum of steady-state and free components. The free component is then approximated using the theoretical expressions obtained for the equivalent circuits of the windings. The results of applying the approach to approximating the transient voltage at the midpoint and the current in the neutral of a dry-type transformer’s high-voltage winding are presented.

Энергосберегающая методика изучения физической модели высоковольтного инвертора на основе электронно-лучевого клапана

The schemes of inverters applied in static reactive power compensators (SRPC) are as a rule developed with the use of power semiconductor devices (PSD); however, attempts to develop industry-grade prototypes of high-voltage inverters on the basis of serially produced vacuum-tube devices (VTD) are also made. High-voltage electron beam valves (EBV), which feature—owing to beam recovery at the anode — the minimum possible voltage drop across the anode-cathode section in the conductive state among the VTDs and a high steepness of the impulse front and trailing edges in comparison with PSDs, seem to be promising for such applications. An EBV was simulated in the EWB circuit engineering computer program environment to determine the minimum permissible load resistance at which the EBV has the maximum electrical efficiency up to 0.99. To determine whether it is advisable to construct such inverters, it is necessary to conduct not only theoretical, but also experimental investigations on physical models using dedicated test benches. A high-voltage energy-saving bench for testing EBVs operating within the SRPC circuit in an inverter mode has been developed. The bench takes power supply from the 3x380 V, 50 Hz network. The bench comprises a three-phase high-voltage transformer, using which the power supply voltage can be increased to a 40 kV level. This voltage is rectified according to the Larionov bridge scheme, and, owing to an artificial midpoint, the 0 to +20 kV and 0 to -20 kV voltages with the maximum current up to 1 A are obtained. The voltage amplitude can be smoothly varied using a three-phase electromechanical regulator installed in the high-voltage transformer primary winding. A 50 Hz sine-wave voltage is produced in the inverter mode by means of pulse-width modulation (PWM) with a clock frequency smoothly variable in the 1...3 kHz range and is applied to a water equivalent of load. To increase the bench power level during the test and achieve energy saving, a gating logic device (GLD) was introduced, which allows the EBV control system to operate in an intermittent PWM generation mode. The GLD produces pulses using which three cycles of a 50 Hz sine-wave voltage (with the total duration equal to 60 ms) repeating at approximately 1.2 s intervals are generated. The use of the GLD producing packs of control pulses makes it possible to study the EBV operation modes on the bench at a short-term power of up to 200 kW in pulse-periodic mode, which is significantly (by 10.20 times) higher than the average power of the bench constant-voltage network power supply source. The GLD output voltage waveforms and the load voltage and current front and trailing edge waveforms are given.

THE EFFECT OF THE ACTIVE RESISTANCE OF THE PULSE TRANSFORMER WINDINGS ON THE PARAMETERS OF VOLTAGE PULSES GENERATED ON A CAPACITIVE LOAD

Goal. Analysis of the influence of the active resistance of the primary and secondary windings of a pulse transformer on the voltage at the load capacitance based on the developed methodology for the analysis of transients caused by the discharge of the storage capacitance in the primary winding. Methodology. A model for calculating transients is developed using the Laplace transform. Transient modeling is carried out in the MATLAB software package. The results of transient calculations are compared with experimental results. Results. A method for calculating transients in test installations with pulse transformers has been developed, which allows taking into account the effect of power losses in the primary and secondary windings on the voltage at the load capacitance. The calculated relations are obtained, allowing to take into account the influence of the active resistance of the primary and secondary windings of the transformer on the voltage at the load capacitance, the currents in the primary and secondary windings of the transformer, as well as on the voltage drop on the inductance of the primary winding of the transformer. Scientific novelty. A mathematical model is developed for calculating transients in the primary and secondary windings of a pulse transformer, taking into account the influence of the active resistance of the windings when it changes over a wide range of possible values. Practical value. Using the proposed technique, it is possible to determine the parameters of the discharge circuit at which test voltage pulses are formed on the load capacitance without distorting the shape of the pulse front.

Hybrid Filter Control for Improvement Quality in a Distribution System

With the dynamic filtering method in the distribution scheme, a fresh converter transformer using the inductive dynamic filtering technique is recommended in this paper to prevalent issues of the traditional converter transformer. The distribution network offers energy for multiple types of non-linear charges such as three-phase rectifier fed engine drives and high-power industrial electrolysis. In the rectifier and inverter station, a converter transformer is generally used. The harmonic elements from the nonlinear load flow readily into the transformer winding, which reduces the transformer's lifetime and also causes the distribution network (DN) and the supply system to have Power Quality issues. An inductive filtering technique was suggested in this article to resolve this issue. A tap is connected with the filter circuit at the linking point of the secondary continuous winding and the secondary prevalent winding. When the harmonic current enters the secondary extended winding, the new converter transformer's common winding will create the contrasting harmonic current to balance it with the zero impedance layout of the secondary common winding, so that there will be no induced harmonic current in the new converter transformer's primary winding. Compared to the traditional shunting active power filtering technique, the efficiency of the inductive active filtering technique is very efficient

99% Efficient Isolated Three-Phase Matrix-Type DAB Buck–Boost PFC Rectifier

Three-phase power factor correction rectifiers are an essential area of power electronics, supplying a direct current load with tens of kilowatts, or more, from the public three-phase mains and achieving sinusoidal input currents. In many applications, isolation is required between the mains and the load, for example, due to safety reasons or different grounding schemes. This paper describes the modulation, design, and realization of a buck–boost-type, unity-power-factor, isolated matrix-type, dual-active-bridge, three-phase rectifier. It uses a circuit similar to a conventional dual-active-bridge converter, but employs a direct matrix converter to connect the high-frequency transformer's primary winding to the mains. A soft-switching modulation scheme is proposed and comprehensively analyzed, deriving closed-form solutions and numerical optimization problems to calculate switching times that achieve minimal conduction losses. Based on this analysis, the design of an 8-kW 400-V rms three-phase ac to 400-V dc prototype is discussed, striving for the highest possible efficiency. Using 900-V SiC mosfet s and a transformer with an integrated inductor, a power density of ${\text{4}}\; {\text{kW}\cdot \text{dm}^{-3}}$ ( ${\text{66}}\; {\text{W}\cdot \text{in}^{-3}}$ ) is achieved. Measurement results confirm an ultrahigh full-power efficiency of 99.0% at nominal operating conditions and 98.7% at 10% lower input voltage.

Effect of Capacitance Variation on Physical Characteristics of Ferroresonance Severity and Jump Phenomenon Based on Rudenberg Graphical Method

This paper performed the ferroresonance generation experiment with variation of capacitance based on Rudenberg graphical method. The voltage and current measurements were conducted on the transformer primary windings, source voltage, and capacitor. The trend of transformer voltage waveform was then analyzed by using bifurcation diagram. Moreover, the fast Fourier transform (FFT) was applied on the transformer primary voltage. The harmonics spectrum would show the ferroresonance mode. Then, its total harmonics distortion (THD) was calculated. The level of ferroresonance severity involving the values of transformer voltage and overvoltage level as well as THD was mapped based on the quadrant system. The results showed that the appearances of jump phenomenon and different ferroresonance severities could be explained more clearly by using Rudenberg graphical method. In addition, the proposed mapping of ferroresonance responses based on the quadrant system was successful not only to highlight the impact of given variables on ferroresonance but also to categorize more easily its danger level.

Hybrid control scheme for mitigating the inherent DC‐current in the transformer in buck‐boost full‐bridge converter for an all‐electric motor drive system

A buck-boost full-bridge topology is preferred for low-input voltage, high gain, high-power battery fed front-end converter of an all-electric motor drive system. Variants of this topology feature high direct current (DC)-gain, soft switching, continuous input and output currents with a phase modulation/asymmetrical pulse width modulation (PWM) scheme. These variants exhibit a high-magnitude DC-current in the transformer primary winding for low-input voltage, high-power application leading to poor core utilisation and low-power density of the system. The operation and analysis of the converter are presented to highlight the DC-current in the transformer and to mitigate this, a hybrid control scheme (HCS) is proposed. The proposed HCS consists of a DC-current compensation (DCCC) loop to mitigate the DC-current in the transformer without altering the DC-gain with no additional components and output regulation (REG) loop to regulate the output. The output REG and DCCC loops are independent of each other. Soft switching is retained with this proposed scheme. The necessity to mitigate the DC-current, analysis, and implementation of the HCS is discussed. The verification of the proposed HCS scheme in simulation and the experimental prototype is presented.

Trapezoidal Approximation of LCC Resonant Converter and Design of a Multistage Capacitor Charger for a Solid-State Marx Modulator

This paper describes the circuit of a capacitor charger for a solid-state Marx modulator. A high-efficiency LCC resonant inverter is proposed for the simultaneous charging of many capacitors in parallel using a multistage transformer and rectifier. Using a high-voltage insulation cable, the charging loop, which represents the primary winding of transformer, is implemented to transfer the power from the resonant inverter to each stage. In addition to the simultaneous charging of separate capacitors, the advantages of the proposed circuit and structure include compact design and reliable insulation performance against high-voltage pulses. Based on the relevant approximation, the simplified analysis of an LCC resonant converter with a trapezoidal shape of resonant current is provided and a 50-kW high-voltage capacitor charger is designed for a 40-kV solid-state Marx modulator. The power stage, which consists of a transformer with four voltage-doubled rectifiers, is designed, and six power stages are configured for generating a pulse output of 40 kV. The detailed implementation of the multistage transformer and rectifier is presented in accordance with the design requirements of a high voltage modulator. Finally, the developed charger achieves 96% of the maximum efficiency and 0.96 of the maximum power factor. The experimental results verify that the proposed circuit and structure can be effectively used for a solid-state Marx modulator.

Impact of Windings Configuration of Three-Phases Power Distribution Transformer on Ferroresonance

This paper dealt with the impact of ferroresonance on four common windings configurations of three-phases power distribution transformer, that were wye-wye (Y-Y), wye-delta (Y-∆), delta-delta (∆-∆), and delta-wye (∆-Y). The ferroresonance was initiated by the occurrence of open-phase faults of transformer primary windings, such as opened single-phase and opened two-phases (unbalanced system) as well as opened three-phases. The investigation of ferroresonance was based on simulation using ATPDraw. The results showed that the transformer windings configuration affected the ferroresonance responses. Subharmonic and quasi-periodic mode appeared on all configurations, whereas fundamental mode was only seen on Y-∆ configuration. The worst overvoltage on primary windings during opened single-phase and opened two-phases was observed on Y-Y configuration, that was about 554 % and 223 % at the R phase, respectively, while the worst overvoltage on primary windings during three-phases opened was obtained on Y-∆ configuration, that was about 150 % at the T phase. However, the primary three-phases voltage on Y-∆ configuration during opened single-phase and two-phases was generally the lowest compared to the other configurations. These results also revealed that Y-Y configuration was the most susceptible to overvoltage due to ferroresonance. On the other hand, Y-∆ configuration was the most robust to overvoltage due to ferroresonance.

Parallel operation of triple- and double-wound transformers with difference transformation ratios

During the operation of electric-power systems, there is often a problem of splitting up the capacity of transformers installed on substations, which, as a rule, entails mounting two or more transformers in parallel instead of a single one, the total capacity of which is the same. In such replacement, there is a problem of uniform distribution of the load between transformers. When using transformers that have the same capacity and are structurally similar, uniformity of load distribution is reached achieved owing to symmetry of parallel circuits. However, if transformers with of different designs and various transformation ratios are connected, then uniformity of a capacity distribution between transformers is violated. With an increase in the total load current, in the case of double-wound transformer I and triple-wound transformer II connected in parallel, the secondary current of transformer I increases, as does its primary current, while the primary and secondary currents of transformer II decrease. In addition, the total current from the mains is less than the current of the primary winding of transformer I. With an increase in the loading current, the secondary current of transformer I increases and the secondary current of transformer II decreases. At the highest value of the secondary current of transformer II, the vector of primary current of this transformer advances the voltage vector by more than by 90°. In addition, the secondary winding of transformer II consumes active power from transformer I and returns it to its primary winding.

A Wide Output Range HB-2LLC Resonant Converter With Hybrid Rectifier for PEV Battery Charging

This paper proposes an HB-2LLC resonant converter with hybrid rectifier to realize wide output voltage control range (100–420 V) for plug-in electric vehicle (PEV), on-board battery charging. The converter has two resonant tanks, each connected with one of the two primary windings of transformer. The two resonant tanks are alternately fed from 400 V dc source by operating two power switches, each with a half switching cycle. The hybrid rectifier connected at the secondary of transformer can be operated as a bridge rectifier or voltage doubler. It operates as a bridge rectifier to achieve output voltage range of 100–250 V for deeply depleted battery charging, and as a voltage doubler to achieve output voltage range of 250–420 V for normal battery charging. To validate the operation and performance of converter, a laboratory prototype rated at 1.5 kW output power and 400 V input dc voltage was designed and implemented. The presented results show that with combined mode changing and frequency control, the converter achieves an output voltage range of 100–420 V. The peak efficiency of converter was measured to be 95.21% at a peak power of 1.5 kW. Thus, the proposed converter is suitable for PEV battery charging application.

Transient current limiter for suppressing transformer inrush, motor starting and fault currents in power system

Transient currents (TCs) including transformer inrush, motor starting and fault currents cause mechanical, thermal and electrical stresses to the transformer windings and the power utility equipments. Of these, the inrush current suppression is crucial due to high second harmonic level, causing inadvertent operation of the protective relays. In this study, a TC limiter (TCL) comprising of two diodes and two reactors in each phase is proposed. The number of diodes in the proposed TCL is the half of those employed in the single-phase diode-bridge type inrush current limiter (ICL); thus, the proposed TCL topology offers much more reliability and lower cost in addition to lower current ripple. Comparing with the three-phase diode-bridge type ICL, the proposed topology is independent of the transformer primary winding connection type. The analysis and design procedure is provided and the performance is verified through simulation and an experimental scaled down prototype for transformer energising, motor starting and fault event.

Inrush Current Limiting of Transformer Primary Winding With Grounded or Nongrounded Y-Connection Using Diode Bridges

This paper presents two novel limiters for mitigating inrush current on primary grounded and nongrounded transformers. The simplicity of structure, reliability of operation with a low failure rate, and a lack of influence on steady-state operation of transformers are some benefits of the two proposed inrush current limiters (ICLs). In addition, they do not require any data for the switching angle of circuit breakers or measurement of residual flux. Additionally, the number of diodes and the inductance value of two proposed diode bridges decreases compared to a three diode bridge in three-phase structures and improve bridge type inrush current limiting for primary grounded and nongrounded transformers. The two proposed methods for inrush current limitation are applicable for all transformers, which have a Y of Yn connection in their primary side. The proposed methods can be used for Y-yn, Yn-yn, Y-delta, Yn-delta, and Y-zn connections and there is no limitation on the secondary side winding type. To reduce losses in the steady-state condition, the ICL should be bypassed. To bypass the ICL, at least three switches are used for the topology proposed in conventional methods. For the topology presented in this paper, only one switch is needed to perform the same function.