High Voltage Conversion Research Articles

A novel non-isolated high gain multiport DC-DC converter for integrating fuel cell/solar PV and battery energy storage system

ABSTRACT This paper presents a novel non-isolated capacitor-assisted quasi-Z-source (CA-QZS) multiport DC-DC boost converter, which operates as a dual-input single-output high gain boost converter. It consists of a bidirectional port for the smooth operation of the energy storage system, and a unidirectional port for the solar/fuel cell to deliver power to the battery and the load. The availability of common ground between input and output ports, high voltage conversion ratio, and continuous current of the unidirectional port are the main features of this novel design, making it suitable for a wider range of applications. A typical switching strategy is followed to regulate the output voltage as well as control input power from the sources. As a battery-incorporated system, the converter’s performance is determined by its selection between three operating modes. Therefore, the CA-QZS converter offers a viable interface for the management of battery energy storage systems. A comparative study is performed with other multi-port converters to highlight the advantages. It is observed that the voltage conversion ratio of 10, can be achieved at a low duty ratio of 0.3. To evaluate the effectiveness of the proposed topology, a prototype is developed with a nominal output power rating of 200W and an output voltage of 200 V.

Single‐switch coupled‐inductors‐based high step‐up converter with reduced voltage stress

AbstractThis paper proposes a non‐isolated DC‐DC high step‐up converter. To achieve a high voltage conversion ratio and overcome the problems related to near unity duty‐cycle of the conventional boost converter in high step‐up applications, coupled‐inductors along with switch‐capacitor techniques are merged in this topology. The proposed converter provides continuous input current with low ripple without using any current ripple cancellation method and the voltage across the switch is limited inherently. In order to reduce the control circuit complexity, the proposed converter uses only one switch. These features decrease the complexity of the proposed converter structure. The voltage stress of the semiconductor components is reduced significantly; thus, high‐quality elements (fast power MOSFET switches with low on‐state resistance and Schottky diodes with low forward voltage drop along with low power losses) can be used, which minimizes the switching and conduction losses and improves the converter efficiency. To confirm the converter operation and theoretical analysis, a 200‐W prototype converter with 24‐to‐200‐V input and output voltages operating at 100‐kHz is implemented in the laboratory.

Submodule Switching-State Based EMI Modeling and Mixed-Mode EMI Phenomenon in MMC

Modular multilevel converter (MMC) is widely used in high-voltage occasions for its good power qualities and flexible controls. However, the large number of semiconductor switches would lead to an extremely complicated electromagnetic environment. To clarify the detailed conducted electromagnetic interference (EMI) mechanisms of MMC, this article builds an EMI model of MMC considering different submodule switching states. Based on this model, the EMI paths of submodule switching- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> and switching- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> processes are analyzed. Then, this article analyzes the effects of submodule heatsink connecting patterns on EMI, and explains the mechanisms of mixed-mode (MM) EMI phenomenon that common-mode component brings in extra differential-mode component. Simulation and experiment results verify the proposed EMI mechanisms and MM phenomenon. The conclusions of this article have great reference significance for the EMI research of MMC and other high-voltage converters.

Fully Soft-Switched Non-Isolated High Step-Down DC–DC Converter With Reduced Voltage Stress and Expanding Capability

In this article, a fully soft-switched expandable high step-down dc–dc converter is presented. In order to achieve a high step-down voltage conversion ratio and low component count, the synchronous buck and Cuk converters are integrated. Lower switches’ voltage stress is realized by splitting the input voltage, which can considerably decrease the switches’ conduction loss. Moreover, switching losses along with the reverse recovery problems are mitigated due to the soft-switching operation of all semiconductor devices. The switch utilized for soft-switching operation is also used to replace the output diode as a synchronous rectifier, which reduces the conduction loss. The mentioned features have considerably contributed to the converter efficiency. Furthermore, the converter shares a common ground between the input and the output, which is desirable in many applications, while the converter output current is continuous without adding current ripple cancellation methods. Finally, the number of converter cells can be expanded or reduced; therefore, the converter can be applied to a wide range of loads. The operating principles and analysis of the proposed converter are presented, and the results from the implemented prototype are provided to verify the converter operation and performance.

Fully soft switched high step‐up/down bidirectional buck/boost converter with reduced switch voltage stress

AbstractA fully soft switched high step‐up/down bidirectional converter is presented in this paper. The high voltage conversion ratio is achieved by a pair of coupled inductors along with a switched capacitor while all active switches operate at zero voltage switching. To provide soft switching condition for the main switches and absorb the leakage inductance energy of coupled inductors, active clamp circuits are utilized for both operating modes. Furthermore, all active switches body diodes which are utilized as converter diodes turn off at zero current switching (ZCS) and thus, the reverse recovery problem is eradicated. Additionally, the voltage stress of the main switches is reduced in comparison with the basic bidirectional buck/boost converter and thus, their drain‐source resistance is relatively lower which leads to considerably lower conduction loss. All the mentioned advantages along with relatively low number of components contribute to the converter efficiency. Theoretical analysis of the proposed converter is presented in detail for both operating modes and the experimental results from the implementation of a 300 W prototype circuit confirm the validity of the converter performance. A comprehensive comparison table between the proposed converter and the recent counterpart bidirectional converters is presented to demonstrate the converter advantages.

A Common-Ground Bidirectional Hybrid Switched-Capacitor DC–DC Converter with a High Voltage Conversion Ratio

Electrical energy conversion and storage in DC systems, with increasing importance in industry, requires DC–DC power electronic converters with performances adapted to today’s requirements. In recent years, the applications of DC–DC converters have expanded, including energy storage management strategies, due to the use of supercapacitors for energy storage instead of—or together with—rechargeable batteries, in order to improve overall performance. This article presents a non-isolated, common-ground, bidirectional hybrid switched-capacitor DC–DC converter, which can be efficiently used for supercapacitor charging/discharging, due to its high voltage conversion ratio. The hybrid converter was obtained from the conventional bidirectional buck topology, inserting an “active” switched-capacitor cell. In addition to the high voltage conversion ratio, the switched-capacitor cell brings another important advantage: decreasing the values of all passive components without interrupting the input to the output ground path. All of these positive features were revealed through theoretical analysis and confirmed through digital simulations and experiments, proving that the hybrid converter performs well in both operating modes, with a smooth transition between them.

Flexible Interlinking Converter With Enhanced FRT Capability for On-Board DC Microgrids

In this paper, a flexible interlinking dc/dc converter with enhanced FRTC is proposed to satisfy the advanced requirements of modern on-board DC MGs; the topology uses a non-isolated switched capacitor-type multilevel converter in combination with a cascaded step up/down converter to facilitate the incorporation of various types of energy storage and/or production units in a DC MG. Overall, a high voltage conversion ratio between the dc bus of the MG and the power unit is achieved. The proposed configuration facilitates the CCM operation of both the power unit and the DC MG and it is characterized by bidirectional power flow capability, enhanced FRTC, zero switching losses for the multilevel converter, limited complexity of the control scheme (compared to the counterpart multilevel solutions) and low real-time communication demands, corresponding so to the increasing flexibility needs of the EMS of modern MGs. The mathematical analysis and the dynamic performance of the control scheme of the proposed topology concept are evaluated via MATLAB/Simulink simulations and real-time CHIL tests, with the use of a 1202 dSPACE platform and an external dsPIC30F4011 microcontroller.

Real-Time Monitoring Method for Thyristor Losses in Ultra High Voltage Converter Station Based on Wavelet Optimized GA-BP Neural Network

Real-Time Monitoring Method for Thyristor Losses in Ultra High Voltage Converter Station Based on Wavelet Optimized GA-BP Neural Network

SSR Stable Wind Speed Range Quantification for DFIG-Based Wind Power Conversion System Considering Frequency Coupling

The subsynchronous resonance (SSR) characteristics of doubly-fed induction generation-based wind power conversion system (DFIG-WPCS) varies with the wind speed. Obtaining the small signal SSR stable wind speed range is urgently important for wind farm dispatch planning. However, there are few methods to quantify the small signal SSR stable wind speed range of DFIG-WPCS. Moreover, the inherent frequency coupling when DFIG-WPCS is integrated to modular multilevel converter-high voltage direct current (MMC-HVDC) may lead to inaccurate stability assessment results. In this article, the frequency coupling mechanism between WPCS and MMC is revealed by analyzing the law of harmonic interaction in the interconnected system. The wind speed-frequency two-variable equivalent admittance model of DFIG-WPCS is established by taking the frequency coupling account. Relying on the two-variable open-loop transfer function, the multiple phase margin contours plot approach is proposed to quantify the small signal SSR stable wind speed range and damping properties. The effectiveness of two-variable admittance and analysis method are validated against electromagnetic transient (EMT) testbed with MATLAB /SimPowerSystems. The analysis results can provide a reference for the early planning and stable operation of DFIG-WPCS.

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.

Investigation of the High-Voltage Converter for Powering Electrostatic Precipitators

The article is devoted to the study of the power supply for air purification systems using electrostatic precipitators (ESP). The features of operation of the in-put thyristor regulator for changing the output parameters of the power source are considered. The nominal electromagnetic and design parameters of the elements of the ESP power supply circuit are presented. Mathematical expressions describing electromagnetic processes in the ESP are given. A computer simulation of a system including a power supply with an ESP was performed, taking in-to account the real parameters of the device elements. The characteristics of out-put parameters of the power supply are obtained for various firing angles of the input regulator and various connected numbers of turns of the high-voltage trans-former.

Centralized Reactive Power Controller for Grid Stability and Voltage Control

The integration of renewable energy sources like solar and wind energy into the transmission and distribution grid has increased gradually for quenching the increasing demand for alternative sources to fossil fuels. However, due to the intermittent nature of the renewable sources primarily solar and wind, the injection of the renewable power generation into the grid shall also be fluctuating which in turn will impact the voltage profile of the transmission and distribution grid. Also, in case of any major load disconnection or generator tripping in a weak grid, the voltage profile will be severely impacted in a weak grid. The aim is to control the sudden major voltage profile disturbance of a weak grid in case of variation of power injected into the weak grid from solar and wind energy and also due to sudden load tripping or generator tripping in the weak grid by controlling the reactive power in the weak grid. In this paper, a centralized reactive power controller has been proposed to control reactive power injection or absorption in the grid. By controlling the reactive power sources and sinks centrally via the centralized controller, any contingency can be met to prevent major disturbance of the voltage profile in the weak grid. This controller shall aim to control the connected reactive power sources &amp; sinks based on the voltage profile of the transmission grid and it shall also engage the Line Commutated Converter (LCC) High Voltage Direct Current (HVDC) Reactive Power Controller. Various cases have been analyzed in this paper for implementation of the centralized reactive power controller for voltage control in the transmission grid.

Design for a Four-Stage DC/DC High-Voltage Converter with High Precision and a Small Ripple

This paper presents a four-stage DC/DC converter with high precision and a small ripple utilized in an electronic power conditioner (EPC). The galvanically isolated four-stage topology contains four cascade connections: a buck circuit, a push–pull circuit, a power converter, and a voltage regulator. The push–pull switches, as well as the diodes in the output-side rectifier, operate in zero-voltage switching (ZVS) and zero-current switching (ZCS) modes at both switch off and switch on, which helps increase the efficiency. The maximum efficiency of the converter can reach 94.5%. The buck circuit and voltage regulator operate in a two-stage closed-loop condition and, thus, the precision is greater than 0.02%. Due to the voltage regulator, the ripple is less than 1 V when the output voltage reaches 7000 V.

A high step‐up DC‐DC converter based on three‐winding coupled inductor and voltage multiplier for renewable energy applications

AbstractA high step‐up DC‐DC converter for renewable energy applications is proposed. Based on a three‐winding coupled inductor and two voltage multiplier cells, the proposed converter obtains a high voltage conversion ratio. Through the passive clamp circuit, the voltage stress of the main switch is suppressed and the leakage energy of the coupled inductor is recycled. This leads to utilize a low on‐state power resistance and low voltage‐rating power switch that decreases the conduction losses. Meanwhile, the current stress of the diodes is minimized and the reverse recovery problem is alleviated. Thus, high efficiency can be achieved. Under continuous conduction mode (CCM) and discontinuous conduction mode (DCM), different operating principles and the mathematical derivations of the proposed converter are described in detail. To validate the feasibility of the proposed converter, a 320 W prototype with 25–38 V input voltage and 400 V output voltage is implemented. The measured maximum and full load efficiencies are 95.83 % and 94.96 %, respectively.

The Impact of Integration of the VSC-HVDC Connected Offshore Wind Farm on Torsional Vibrations of Steam Turbine Generators

For remote offshore wind farms, transmitting power to the main onshore grid via a Voltage Source Converter High Voltage Direct Current (VSC-HVDC) system is the mainstream of power transmission. It is not only cost-effective in long-distance transmission, but also can fully meet the grid side requirements such as black start, voltage support, fault ride through and frequency support. However, it still has some problems, such as the possible impact on the power grid needing to be paid attention to. In this paper, its impact on the torsional responses of turbine generator units neighboring to the onshore side of AC bus is studied by using the DIgSILENT PowerFactory software. It is found that the effects of the Sub-Synchronous Torsional Interaction (SSTI) with onshore controls and the generator de-rating operations can significantly affect the damping ratio of turbine torsional modes, whereas the effects of the machine configurations and the amount of wind farm power integrated can affect the electrical torque disturbance. The most noteworthy is that their effects can be superimposed on each other if these factors act simultaneously, which would lead to increased vibrations and reduce the turbine shaft’ s life. The findings will be helpful for avoiding accidents caused by torsional vibrations when it is going to integrate a VSC-HVDC connected wind farm into a power grid.

High Efficiency and High Voltage Conversion Ratio Bidirectional Isolated DC–DC Converter for Energy Storage Systems

In this paper, a novel high-efficiency bidirectional isolated DC–DC converter that can be applied to an energy storage system for battery charging and discharging is proposed. By integrating a coupled inductor and switched-capacitor voltage doubler, the proposed converter can achieve isolation and bidirectional power flow. The proposed topology comprises five switches and a common core coupled inductor that uses only a set of complementary pulse-width-modulated signals to control and achieve high voltage gain without requiring high turn ratios or excessive duty cycles. Moreover, the proposed topology can recover the leakage inductance energy to improve the conversion efficiency. The main switches exhibit zero-voltage switching, which reduces the switching losses. A 500-W bidirectional converter is used to verify the feasibility of the proposed bidirectional converter through theoretical analysis and experiments. The experimental results indicate that the highest efficiency of the proposed converter in the step-up and step-down modes is 97.59% and 96.5%, respectively.

Dynamic interaction analysis among different control loops and stability enhancement strategy for MMC-HVDC systems

This paper presents the dynamic interaction analysis method among different control loops of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems, and proposes an improved control strategy with feedback regulation loop and low-pass filter loop for stability enhancement. The small-signal cascade model of MMC with dc voltage control loop, ac current control loop and differential-mode ripple capacitor voltages loop is firstly established, which clearly shows the analytical interactions of inner capacitor voltage response and system input power’s dynamics with simplified closed/open-loop transfer functions. Then the small-signal dynamic analysis of MMC is conducted based on the derived closed/open-loop transfer functions, which reveals the interactions between the system stability and key control loops, as well as the main parameters of the MMC-HVDC system by both the Nyquist criteria and the pole-zero theory. Next, the feedback regulation loop and low-pass filter loop are proposed and introduced in the MMC control system to maximize stability enhancement performance during disturbances, and system dynamic response under the proposed control with different parameters are also explored through theoretical analysis. Finally, several cases on a benchmark two-terminal MMC-HVDC simulation model are carried out in PSCAD/EMTDC to verify the proposed control strategy.

Generation and Comparative Analysis of High-Voltage Gain Nonisolated DC–DC Converters With Ladder Switched Capacitor and Coupled Inductor

A design methodology for creating a set of high-voltage gain DC–DC converters is presented and discussed in this article. The proposed generation is based on ladder switched capacitor (SC) with coupled inductor. So, eight high-voltage converters with a single switch without increasing the number of components are proposed. Detailed evaluations about the derivation of each topology, comparative analysis among main features of each converter are presented. These converters are evaluated in different aspects, such as principle of operation, voltage gain, sensibility of the output voltage, total voltage stress (TVS), total current stress (TCS), ZCS, charging mode compared to the standard SCs cells, design guidelines, power density, power losses, and estimated efficiency. By these comparisons, the main feature and limitations of the analyzed converters are recognized. Thus, four topologies were chosen to be evaluated experimentally, considering 200 W, 30 V, 400 V, and 50 kHz. The combination of the techniques provides the best trend off on the comparative analysis carried out in this work. The results show that attention should be paid to choosing the best position to include a technique. In this case, the best choice was BCISC-V converter, which presented the best voltage gain, TCS, ZCS, and efficiency characteristics.

An Inrush Current Suppression Strategy for UHV Converter Transformer Based on Simulation of Magnetic Bias

The Ultra High Voltage (UHV) converter transformer may generate serious inrush current during closing. It will cause commutation voltage distortion and easily result in commutation failure of converters on the same commutation bus. Due to the change of flux amplitude and phase caused by closing resistance in the Ultra High Voltage Direct Current (UHVDC) system, it is difficult to determine the closing time of the converter transformer when considering the influence of residual flux. Therefore, this paper first analyses the existing closing resistance strategy, and constructs the magnetic bias function according to the influence characteristics of closing resistance on the flux and phase. Then an inrush current suppression strategy based on magnetic bias simulation is proposed for star connection and delta connection of UHV converter transformers, which can accurately calculate the ideal closing time of mutual cancellation between the residual flux and bias flux during closing. Finally, the no-load closing model of the UHV converter transformer is established by PSCAD/EMTDC for simulation analysis of the feasibility, real-time verification is carried out using the RTDS closed-loop experimental platform. The results show that the proposed inrush current suppression strategy can effectively suppress the inrush current under different residual flux conditions.

Grid-integrated single phase solar PV system employed with single switch high gain boost converter

• This paper deals with a grid-integrated single phase solar photovoltaic system (GISPSPS) employed with a single switched based high gain boost converter. The significant contributions in this paper are; • The dynamic model of HGBC including effective series resistance (ESR) of passive elements is developed for improving accuracy with slight increase in complexity of model. Then its stability analysis is carried out under different solar irradiations (i.e. 300, 500, 800 and 1000 W/m 2 ) using an eigenvalue plot. • The HGBC control is obtained using the sliding mode control (SMC) for PPE under change in solar irradiations. In proposed SMC only two sensing signals PV panel voltage and current (i.e. v pv and i pv ) are needed for PPE in comparison to three sensing signals PV panel voltage, current and DC link voltage (i.e. v pv , i pv and v dc ) require in SMC of ref. [10] . • The control of FBVSC is obtained by a F i OGI controller. The F i OGI provides noise-less extraction of synchronization signal, under presence of DC offset and noise in grid voltage. However, FOGI used in ref. [10] exhibits poor performance under presence of noise in grid supply voltage. This paper presents a grid-integrated single phase solar photovoltaic (PV) system (GISPSPS) with its modeling, design and control. The GISPSPS consists of a single switch based high gain boost converter (HGBC) and full bridge voltage source converter (FBVSC). The HGBC, boost the low PV panel voltage and extracts the optimum PV power. The HGBC allows high voltage conversion ratio and possesses less voltage stresses on power devices. The optimum PV power is extracted via a sliding mode control (SMC) strategy that generates the switching signal for the switch of HGBC. The FBVSC delivers PV power to the single phase utility grid, as well as load connected at point of interfacing (POI). A fifth order generalized integrator (F i OGI) is developed for FBVSC control to ensure harmonic suppression and DC offset rejection capability during extraction of fundamental grid voltage. Experimental validation of proposed GISPSPS and its control is also verified.