Electronic Transformer Research Articles

The averaged-value model of flexible power electronics substation in hybrid AC/DC distribution systems

The concept of flexible power electronics substation (FPES) has been applied firstly in the Zhangbei dc distribution network demonstration project. As a multi-port power electronics transformer (PET) with different ac and dc voltage levels, the FPES has adopted a novel topology integrating modular multi-level converter (MMC) and four-winding medium frequency transformer (FWMFT) based multiport dc-dc converter, which can significantly reduce capacitance in each sub-module (SM) of MMC and then save space and cost. In this paper, in order to accelerate speed of electromagnetic transient (EMT) simulation of FPESs based hybrid ac/dc distribution systems, an aver-aged-value model (AVM) has been proposed for efficient and accurate representation of FPES. Assume that all SM capacitor voltages are perfectly balanced in the MMC, the MMC behavior can be modeled using controlled voltage sources based on modulation voltages from control systems. In terms of the averaged current transfer characteristics among the windings of the FWMFT and consider that all multiport dc-dc converters are controlled with same dynamics, a lumped averaged model using controlled current and voltage sources has been developed for these four-port dc-dc converters connected to the upper or lower arms of MMC. The presented FPES AVM model has been tested and validated by comparison with a detailed IGBT-based EMT model. Results show that the AVM is significantly more efficient while maintaining its accuracy in EMT simulation.

A New Modular Multilevel AC/AC Converter Using HF Transformer

This paper proposes a new modular multilevel AC/AC converter using high-frequency (HF) transformer, which has the advantages of modular design, easy expansion, high power density, no circulation current, good output voltage waveform quality and low cost. This converter has broad application prospects in the fields of high-voltage and high-power wind power generation, fractional frequency power transmission (FFTS) and power electronic transformer. The proposed converter topology can connect two three-phase AC systems with different frequencies and amplitudes directly. By introducing HF transformer, the direct series connection of input and output modules can be realized simultaneously, and the expensive industrial frequency transformer with large volume and weight can be removed. In order to achieve HF electrical isolation, the HF inversion of the output pulse is carried out at the inverter side to realize the HF output of the fundamental wave. After passing the HF transformer, the pulse is restored to the low-frequency output wave by the cycloconverter. Aiming at the problem of freewheeling of HF link, the method of common state conduction is introduced, which can effectively reduce the switching loss. The modulation scheme, control strategy and typical parameter design are developed. Furthermore, the feasibility of the proposed converter is verified by simulation results.

Capacitor Voltage Ripple Minimization of a Modular Three-Phase AC/DC Power Electronics Transformer With Four-Winding Power Channel

Power electronic transformer (PET) with modular cascaded structure has become a flexible interface between AC and DC grids. Nonetheless, the notable 2nd-order ripple power introduced from the AC source requires considerable energy storage in the module capacitors, which is likely to degrade the overall power density and increase system cost. In this paper, a power channel based four-winding PET (PC-PET) is proposed to address such an issue. Owing to the phase-crossing power channels, the low-frequency ripples can cancel each other through magnetic flux counteracting. To improve the accuracy of power decoupling, the effect compromised by inconsistent windings is considered. Accordingly, a proportional-resonant (PR) control based power decoupling strategy for PC-PET incorporated with power balance is proposed to decouple the three-phase low-frequency ripple powers, and minimize the voltage ripple in module capacitors. Additionally, the improvement of power density is discussed based on analyses on power losses, capacitor volume and transformer volume. Simulation and experimental results verified the validity of proposed control strategies.

An improved digital integral algorithm to enhance the measurement accuracy of Rogowski coil-based electronic transformers

In order to promote the practical application of the Rogowski coil-based electronic transformers, it is of great necessity to handle the issues associated with the measurement error and interference in the integral process of the electronic transformer. In this paper, an improved digital integral algorithm with frequency self-adaption ability that aimed at overcoming the above-mentioned issues is proposed. Based on the electronic transformer calibration and harmonic measurement requirements, two specific applications of the proposed integral algorithm are carried out to validate the practical feasibility of the proposed algorithm. Experimental results reveal the improved performance of the Rogowski coil-based electronic transformer equipped with the proposed digital integral algorithm. Errors in harmonic currents measurement (up to 50th order) can be significantly reduced to a level less than 0.01% at a sampling frequency of 12.8 kHz by employing the proposed algorithm. For calibrating electronic transformers, results show that the performance of the proposed algorithm is better than the conventional trapezoidal integral algorithm and Simpson algorithm. Furthermore, the proposed algorithm has the ability to eliminate the impact of frequency fluctuation around the power frequency.

Hybrid Regenerative System on Power Electronic Transformer for Electric Traction Applications

Regenerative braking has been playing a significant role in electric locomotives to overcome dissipation of the kinetic energy as heat. For high-speed rail topologies, Power electronic transformer based locomotive has the only solution to achieve it. For the isolation purpose and to reduce the weight of the locomotive, a feasible method by substituting loco transformer with a transformer with high frequency design. With the increasing awareness of energy consumption more electrified locomotives now moving to "Green Energy ". This paper aims to describe the importance of hybrid electric locomotive system when compared to conventional one. By integrating the regenerative braking on power electronic transformer with a storage medium will be a promising solution for the future high-speed rails. The simulation of IGBT based dc to dc converter with traction inverter with storage medium has simulated from MATLAB/ SIMULINK platform.

Power Electronic Transformer based IGBT Converters for Electric Locomotive Applications

Electric locomotives up gradation is one of fastest growing technology on railway research. Modern locomotive traction drives operate for high speed rail networks. This paper proposes locomotive drives directly from the mains without line frequency locomotive transformer. Power electronic transformers (PETs) offer the advantage of the reduction in size and weight compared to conventional line frequency transformers. In this article discussed about high power converter using IGBT with high-frequency transformer for electric locomotives is utilized for the power converter side. The catenary feeding front end of the locomotive is connected directly to IGBT based cascade H bridge converter and conventional IGBT based three phase inverter act as load end converter for driving induction motor. The MATLAB simulation results of PET based high frequency transformer with IGBT power converter integrated with traction inverter has justified .

Solid-State Transformers in Locomotives Fed through AC Lines: A Review and Future Developments

One of the most important innovation expectation in railway electrical equipment is the replacement of the on-board transformer with a high power converter. Since the transformer operates at line-frequency (i.e., 50 Hz or 16 2/3 Hz), it represents a critical component from weight point of view and, moreover, it is characterized by quite poor efficiency. High power converters for this application are characterized by a medium frequency inductive coupling and are commonly referred as Power Electronic Transformers (PET), Medium Frequency Topologies or Solid-State Transformers (SST). Many studies were carried out and various prototypes were realized until now, however, the realization of such a system has some difficulties, mainly related to the high input voltage (i.e., 25 kV for 50 Hz lines and 15 kV for 16 2/3 Hz lines) and the limited performance of available power electronic switches. The aim of this study is to present a survey on the main solutions proposed in the technical literature and, analyzing pros and cons of these studies, to introduce new possible circuit topologies for this application.

Power-Linked Predictive Control Strategy for Power Electronic Traction Transformer

Power electronic traction transformer (PETT) is a module-cascaded converter with high-frequency-link technology to realize high-power conversion. A typical structure of PETT contains a single-phase cascaded H-bridge rectifier in the first stage and several dual active full-bridge (DAB) dc-dc converters in the second stage. For PETT adopting such a complex topological structure, the control strategy also becomes complicated and should be carefully designed. In this article, a power-linked predictive control (PLPC) strategy for PETT is proposed based on the predictive controls of the H-bridge rectifier and DAB dc-dc converter. A power link is brought into the control strategy to connect the H-bridge rectifier with a dc-dc converter, through which the input power of PETT can match the output power. Moreover, the voltage balance control and power balance control are also combined with predictive control by the PLPC strategy. Using such a strategy, the fluctuation of the H-bridge rectifier dc voltage is suppressed, the circumfluence among modules is reduced, and the dynamic and steady-state performances are both improved compared with traditional strategy. Simulation and experimentation platforms with both resistance load and motor load are built to show the effectiveness and correctness of the PLPC strategy.

Electronic transformer performance evaluation and its impact on PMU

The electronic transformer of smart substations, which transfers primary signals, is important for measurements and controls. In this study, the transformation characteristics of a Rogowski electronic current transformer (ECT) are analysed theoretically. Then, experimental platforms are established to verify the analysis and study the impact of the transformation characteristics of the ECT and an electronic voltage transformer (EVT) on a phasor measurement unit (PMU) using standard test signals. The analysis and the testing results show that this ECT can transfer the phasor well under both steady and dynamic conditions. However, the harmonics introduced by the ECT may lead to the measurement errors of frequency and the rate of the change of the frequency (ROCOF) from the PMU. Therefore, a phasor algorithm with powerful DC offset and harmonics immunity is suggested for PMUs connected to the output of this type of ECT. For the EVT, the ROCOF measurements are easily affected as well. The EVT was also shown to be more sensitive to temperature than the potential transformer (PT).

Reliability evaluation method for AC/DC hybrid distribution power network considering cascaded multiport power electronic transformer

With the advantage of high-power supply capacity, low loss in power transmission and distribution process, strong power flow control ability, direct current (DC) distribution network has been one hotspot of the future distribution network. The cascaded multiport power electronic transformer (PET) has been proposed as key equipment for future distribution grid with multi-voltage level and alternating current (AC)/DC hybrid feature. Based on the typical topological structure and engineering reliability theory, a reliability evaluation model for PET considering the device redundancy is established. After that, the reliability evaluation method of the AC/DC hybrid distribution network with a cascaded multiport PET is proposed. Finally, case studies are provided in a ‘hand in hand’ AC/DC hybrid power distribution network structure. Case 1 analyses the PET reliability level under different design patterns and the redundancy level. Case 2 gives the reliability evaluation results of the AC/DC hybrid distribution network with PETs, which proves the correctness and validity of the proposed algorithm. Case 3 compares the reliability level of the AC and DC system under different scenarios and proposes the reliability promotion advice for the DC power distribution network.

A novel low voltage ride through strategy for cascaded power electronic transformer

In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a low-voltage ride through(LVRT) strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.

Travelling wave fault location principle for hybrid multi-terminal LCC-VSC-HVDC transmission line based on R-ECT

This paper investigates the fault location issue of hybrid multi-terminal high voltage direct current (HVDC) overhead transmission line based on the line commutated converter (LCC) and voltage source converter (VSC). There exists two issues that should be addressed: (i) the travelling wave (TW) fault location principle’s adaptability to multi-terminal network topology and converter structures; (ii) the transfer method of high frequency TW signal. Therefore, a novel TW fault location principle for the hybrid multi-terminal LCC-MMC-HVDC transmission line is proposed based on the Rogowski coli based electronic current transformer (R-ECT). Firstly, the high frequency TW transfer capability of the R-ECT is studied. Secondly, a novel TW fault location principle based on the secondary differential current travelling wave (D-CTW) is proposed. Eventually, in order to verify the high accuracy of the proposed principle, a typical ±400 kV hybrid three-terminal LCC-VSC-HVDC transmission system is used for the case study based on PSCAD/EMTDC. Furthermore, its robustness against different fault locations, different fault resistances and different fault types is also investigated.

Harmonics in Power Systems and Filter Applications

With the advancing technology, loads in the system have been varied. These loads are always asked to be linear. However, with the effect of progressive semiconductor technology, an increase in nonlinear loads has been observed in the system. Nonlinear loads are loads whose current and voltage characteristics are nonlinear. Nonlinear loads cause harmonic currents and voltages in the system. Major loads causing harmonics are power electronic components, transformers, uninterruptible power supplies (UPS), converters and high-power induction motors. Harmonics cause additional energy losses in the system, heating and damage to the insulation. For these reasons, it is important to reduce or eliminate harmonics after formation or to block them before the formation. The most important method for eliminating harmonics is the filtering of harmonics with filters. In this study, firstly, it is given basic information about harmonics, their effects on energy system, filtering methods. Then, a sample system model is performed in MATLAB program and the effect of passive filters is examined.

Impact of Coreless Current Transformer Position on Current Measurement

Magnetic sensors have been widely used for current measurements in diverse industries. In order to achieve a large current measurement without an iron core structure, magnetic sensors are often arranged to be a circular array for designing a new electronic current transformer (ECT). However, this ECT with a circular array of magnetic sensors usually suffers from tradeoffs between the number of sensors and the measurement accuracy. This paper visualizes this tradeoff and proposes a 3-D magnetic-field model to analyze the impact of the magnetic sensor circular array position on current measurements. We develop an analytical expression of the current measurement errors to estimate the minimum number of required magnetic sensors and point out the acceptable deviations of the ECT position under various accuracy classes. Based on the estimation results, we design a new circular array with eight magnetic sensors and build a three-phase current measurement system. The test results demonstrate the efficiency and the robustness of the designed class-1.0 ECT, which successfully maintains accuracy class 1.0 under various position deviations in balanced or unbalanced three-phase power delivery systems.

A Power Electronic Traction Transformer Configuration With Low-Voltage IGBTs for Onboard Traction Application

Solid-State transformer for onboard traction application is also addressed as power electronic traction transformer (PETT), which is used to replace the line frequency traction transformer and four-quadrant converter to further improve efficiency, save installation space, and reduce weight of the onboard equipment. However, based on the conventional PETT configuration with the modular series–parallel structure, it is hard to guarantee the high efficiency and high power density at the same time due to the limitations caused by the switching characteristics of the semiconductor devices, transformer design, and high-voltage insulation design. To deal with the mentioned problem, a PETT configuration and the parameters’ design method for onboard traction application are proposed in this paper to replace the conventional dc/dc stage with the low-voltage insulated gate bipolar translator (IGBTs). By using the proposed configuration, the high-voltage IGBTs can be replaced by the low-voltage IGBTs, which can help reduce switching loss. Meanwhile, zero-current switching or zero-voltage switching for all the IGBTs can be guaranteed, and the number of the medium-frequency transformers (MFTs) will not increase. Therefore, a multiple optimization of PETT, including switching losses, the number of MFTs and switches, and costs, can be finally achieved. Finally, an experimental prototype is built to verify the performance of the proposed PETT configuration.

An Analytical Loss Model of Litz-Wire Windings for Transformers Excited by Converters With Winding Configurations Considered

An accurate analytical model for litz-wire (LW) winding loss calculation of power electronic transformers (PETs) was deduced. First, the skin effect and proximity effect losses at bundle levels of a single straight LW were introduced. Then, the proximity loss of an air-core LW winding was studied by adopting the coordinate transformation technique (CTT) considering helical winding (HW) configurations and twisting configurations (TCs). Last, a 2-D analytical model for LW windings considering magnetic materials, LW configurations, and winding configurations (WCs) was proposed with the help of CTT, Dowell method, and Roth’s method. This model is helpful for the design of LW windings considering more details such as the choices of WCs and LW configurations.

A Multiport Power Electronic Transformer Based on Modular Multilevel Converter and Mixed-Frequency Modulation

Power electronic transformer (PET) is a key apparatus in smart grid system, due to the advantages of power controllability and ac/dc grids interconnection. The four-stage PETs have multiple output ports, good controllability, and flexibility, but are limited by multiple power conversion stages, complex structure, and low efficiency. Therefore, a novel modular multilevel converter (MMC)-based PET topology with mixed-frequency modulation is proposed to reduce the power conversion stage. The power decoupling circuits are designed to realize multiple output ports, including medium voltage ac (MVAC) port, medium voltage dc (MVDC) port, and low voltage dc (LVDC) port. The equivalent circuit and controllability of proposed PET are analyzed. The proposed PET topology provides the same multiple output ports by only three power conversion stages, therefore, the PET structure is simplified. In addition, simulation results are provided to verify the effectiveness of proposed PET.

A Smart Substation Training Platform based on Primary Equipment Simulator

A training scheme based on combination of virtual substation primary equipment real secondary devices is proposed for smart substation accident management. The primary equipment simulator is implemented through communication between 3D cartoon server and simulated switch controller. Electronic current and voltage transformer is simulated by EMTDC transient model and FPGA+DSP terminal controller. A smart substation on-site training environment is established for the substation O&M personnel.

TravellingWave Fault Location System based on IEC61850

Travelling wave fault location (TWFL) system has been matured in recent years due primarily to the availability of high speed sampling techniques, fast communications technology and GPS time synchronization. However, it has not yet been fully integrated into the digital substation based on IEC61850 standard. Proposal for modeling the TWFL system by the IEC61850 standard has already been proposed in another paper. This paper focuses on the communication requirements of the TWFL system in a digital substation, both at the station level and the process level. The application of GOOSE messages and the need for high speed sampling merging unit are discussed. The paper also reports on the latest research into the electronic transformer technology which can satisfy the high speed sampling requirements of TWFL.

Real-time implementation of electronic power transformer based on intelligent controller

Real-time implementation of electronic power transformer based on intelligent controller