Improving the Dynamic Performance of Distribution Electronic Power Transformers Using Sliding Mode Control

In recent years, electronic power transformer (EPT), which is also called solid state transformer, has attracted great interest and has been used in place of the conventional power transformers. These transformers have many important functions as high unity power factor, low harmonic distortion, constant DC bus voltage, regulated output voltage and compensation capability. In this study, proposed EPT structure contains a three-phase pulse width modulation rectifier that converts 800 Vrms AC to 2000 V DC bus at input stage, a dual active bridge converter that provides 400 V DC bus with 5:1 high frequency transformer at isolation stage and a three-phase two level inverter that is used to obtain AC output at output stage. In order to enhance dynamic performance of EPT structure, neuro fuzzy controllers which have durable and nonlinear nature are used in input and isolation stages instead of PI controllers. The main aim of EPT structure with the proposed controller is to improve the stability of power system and to provide faster response against disturbances. Moreover, a number of simulation results are carried out to verify EPT structure designed in MATLAB/Simulink environment and to analyze compensation ability for voltage harmonics, voltage flicker and voltage sag/swell conditions.

Power electronic transformer (PET) which has a big potential application value in smart grid is an electrical power transformer device adopting power electronic converter and high frequency switch transformer. A new PET with power factor correctors (PFC) is proposed in this paper. Due to high power level of PET, PFC should have a high power level as well. Therefore, the multi-phase interleaved PFC is employed. The paper describes the one cycle control principle, proposes a current synthesis method based on IGBT current, and then analyses the relationship between ripple current and duty cycle of IGBT. In addition, the whole PFC system is simulated completely by means of Matlab/Simulink. In order to verify the theoretical analysis and simulation analysis, a four-phase interleaved PFC with a rated output power of 8.0kW is designed and implemented based on the an analog control chip. The obtained results show that the interleaved PFC by means of one cycle control and current synthesis is feasible, capable of reaching a good suppression effect of ripple current.

Dynamic voltage restorer (DVR) is conventionally utilized in the electrical distribution system to mitigate the voltage sag and swell on point of common coupling of sensitive consumers. In this paper, a novel DVR based on power electronic transformer (PET) is proposed to mitigate the symmetrical and asymmetrical voltage sag and voltage swell. PET is connected between DC power supply and multilevel inverter to provide transitive DC voltage for DC-link of DVR. PET has effectually and flexibly created relevant DC voltage with required magnitude and low distortion. The performance of suggested DVR based on PET has been evaluated under occurrence of different disturbances in the electrical distribution system. Ultimately, the simulation results have confirmed that DVR-PET can significantly mitigate the symmetrical and asymmetrical voltage sag and voltage swell on sensitive consumers.

To solved the direct current (DC) link voltage fluctuation in Isolated shoot-through Z-source inverter (IST-ZSI). A switching controller based on sliding mode control and passivity-based control (PBC) for DC-link voltage of IST-ZSI is proposed. The IST-ZSI is assumed working in continuous conduction mode. At first, a DC link voltage sliding mode controller is designed, which uses inductor current error and DC link voltage error as state variables. The sliding mode control law is deduced, and the range of sliding coefficients is determined, which satisfy the existence and stable conditions. Secondly, the passivity based controller is designed by the dynamic equation of the IST-ZSI. In addition, a right switching function is selected to achieve the dynamic switching between the sliding mode controller and the passivity-based controller. This control method overcomes the chattering problem of sliding mode controller in steady state and accelerate the response speed of the system. And the system has strong robustness. Finally, the simulation results verify the effectiveness of the DC link voltage sliding mode controller.

Power Electronic Transformer (PET) is a new type of energy conversion device in power system. The electronic transformers used in PET play an important role in performance of the overall power electronic system, including efficiency, size, and cost. The transformers are high power medium frequency transformer according to present technology. Based on a proposed circuit topology of PET, the design method of the electronic transformer is presented. A 30KW medium frequency transformer is designed. The selection of core material was based upon the optimization among the specifications: transformer size, efficiency, and producing cost. Thermal analysis is made by ANSYS. The temperature rise is checked by calculating the distribution of temperature field. The design of electric insulation of the electronic transformer is proved to be good by simulation.

A new three-phase input, three-phase four-wire output, and with a high-frequency AC link matrix type convertor is discussed. The topology of the conventional matrix convertor is modified with an additional high-frequency transformer for three-phase four-wire electronic power distribution application. The proposed approach accomplishes high output voltage transfer ratio which is more than unity, galvanic isolation between both voltage sources and higher power density by employing a high-frequency transformer into the intermediate stage of the dual bridge matrix convertor. It has the same advantages as the conventional matrix convertor, such as bidirectional power flow capability, controllable input power factor and lower harmonic distortion on both input and output sides. Furthermore, with three dimensional space vector PWM (3D SVPWM) and modified control strategy, the input and output currents may keep low harmonic distortion under unbalanced input voltages or variable three-phase four-wire unbalanced loads. The proposed approach is a competitive solution to the electronic power transformer. Experimental results of a three-phase four-wire 220/380V 5KVA 50Hz electronic power transformer system based on DSP controller are presented and shown to demonstrate the advantages of the proposed system.

Compared with the line-frequency transformer (LFT), the emerging power electronic transformers (PETs) have gained wide concerns due to the significant merits of higher power density, higher reliability, more flexibility, and multiple functions. However, the need for bulky energy storage elements, multi-stage power conversion and reduced conversion efficiency, and the intrinsic twice-frequency pulsating power issue are the main disadvantages of the conventional single-phase PETs. To overcome the above shortcomings of conventional single-phase PETs, this paper develops a matrix-type single-phase AC-AC PET without bulky energy storage elements. The proposed PET consists of a line-frequency commutated rectifier, a half-bridge LLC resonant converter with a fixed switching frequency, a boost converter, and a line-frequency commutated inverter. The LLC operates efficiently with unity voltage gain and acts as a high-frequency isolated DC transformer (DCX). The boost converter provides AC output voltage regulation function and the line-frequency commutated inverter unfolds the output voltage of the boost converter to generate the sinusoidal AC output voltage. As a result, high power density, reduced power conversion stages, direct AC-AC power conversion without twice-frequency pulsating power, high conversion efficiency, and high reliability are achieved. The experimental results on a 1kW PET prototype show that sinusoidal input current and output voltage, ZVS of the LLC stage, and output voltage regulation capability are realized. The experimental results verify the correctness and feasibility of the presented methods.

A new optimal coordinated control strategy between an electronic power transformer (EPT) and generator excitation is proposed to improve power system dynamic performance and stability. The studied system is based on a single-machine multidouble-circuit transmission-lines (SMTL) system. In the SMTL system, the generator is connected to an infinite bus through a conventional power transformer with a double-circuit transmission line, and to another infinite bus with an EPT with a double-circuit transmission line. The mathematical model of the SMTL system is established, and the proposed optimal coordinated control strategy is upon linear optimal control theory. Numerical simulations were carried out for the SMTL system, and the simulation results showed that the proposed control strategy has good dynamical performance for a variety of disturbances, and can improve the system damping and voltage performance effectively.

Power electronics transformers (PETs) have better application in smart grid because they have more advantages than conventional transformers with a structure integrating the power electronic equipment and medium frequency transformers (MF transformers). As an important part of PET, the MF transformer undertakes significant functions of voltage conversion, energy transfer and electrical isolation. Its performance determines the efficiency and reliability of the overall system. This paper mainly discusses design procedure and efficiency optimization of MF transformers. The method of computing leakage inductance is also presented to achieve the series resonance. Finally the optimal MF transformer prototype is validated with finite element simulation.

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.

A novel scheme for the control of three-phase two-level grid-connected power converters is presented in this paper. A cascade-control structure, based on H ∞ control and sliding mode control (SMC), is proposed, which simultaneously achieves output voltage regulation and unity power factor under dc-load variations. The overall control strategy contains two main loops: first, a current tracking loop (internal loop) based on sliding mode control that guarantees the input currents to follow the desired references; second, a dc-link voltage regulation loop (external loop) based on an H ∞ controller integrated with an extended state observer (ESO), which regulates the output voltage and provides current references for the internal loop. Simulation results are provided to assess the efficiency of the proposed method.

The increase of sustainable energy sources and integration with the power distribution grid produced power quality (PQ) problems such as voltage sag and voltage swell. These problems are produced in the power distribution systems due to multiple reasons that affect sensitive loads. Voltage sag and voltage swell mostly occurring Power quality complications in the power system. These power quality issues in the power distribution system can be mitigated by using custom power devices, Dynamic Voltage Restorer (DVR) is the newly introduced device. Different techniques and algorithms have been used to mitigate these problems. In this research work, an ultra-capacitor-based DVR and a super-twisting sliding mode controller are used for the said problem. The addition of energy storage, such as an ultra-capacitor, to a dynamic voltage restorer, creates a voltage that can alleviate voltage issues such as sag and swell. For both the voltage sags and voltage swells, a high voltage is necessary for a short spell. A dynamic voltage restorer is an apparatus that is linked between the voltage source and the load side using an injection transformer to input voltage into the power distribution side to remedy a problem that has a significant impact on the load voltage. With the inclusion of a dynamic voltage restorer and sliding mode controller, the system will be able to fix PQ issues such as voltage sag and swell on its own, rather than relying on the grid as in past years. MATLAB/Simulink R2018a has been used to conduct simulation studies. Simulation studies have demonstrated that the proposed scheme is more effective, simple, robust, and more flexible and has a better mitigation time of 2.5 milliseconds and a total harmonic distortion of less than 5% as per IEEE standards.

This paper deals with a phase-locked loop (PLL) algorithm for both phase-angle and frequency detections in three-phase utility grid connected systems. The proposed algorithm uses non-autonomous adaptive filters (AFs) for extracting the fundamental components of the utility voltages, while a positive sequence detector (PSD) is employed for obtaining the positive sequence components at fundamental utility frequency. Both the AFs and the PSD operate in conjunction with a three-phase power-based PLL (3pPLL) yielding the scheme called AF-PSD-3pPLL. Therefore, power quality problems that interfere on the PLL performance are mitigated, such as voltage harmonics and unbalances. The performance of the proposed AF-PSD-3pPLL algorithm is evaluated by means of experimental tests considering utility disturbances, such as voltage harmonics, voltage unbalances, and voltage sags.

As a part of work performed on a government contract, magnet-wire materials were investigated for application in electronic power transformers and inductors. One of the most critical environments specified for the application was 1,000-hour life in the ambient temperature range - 65 C (degrees centigrade) to 500 C. This paper summarizes the planning and results of an ultrahigh-temperature heat-aging program designed to screen selectively the most promising magnet wires to meet this requirement.

In this paper, a predictive control strategy of power electronic traction transformer for fault tolerance condition is proposed to ensure the safe operation of the system after fault. The working principle and the fault-tolerant method for H-bridge and dual active bridge are introduced first as the basis. Then the predictive control strategy is illustrated. Based on the analysis, the predictive control combined strategy fault-tolerant method is proposed, which can effectively solve the problem of power distribution between the fault-tolerant module and the normal working module, maintain the voltage balancing and ensure the stable operation of the power electronic traction transformer. The experiment results are given to show the efficiency and correctness of the proposed strategy.