Active Front End Converter Research Articles

Controller Design of an Active Front-End Converter Keeping in Consideration Grid Dynamic Interaction

This paper presents a systematic control design procedure for an active front-end (AFE) converter connected to a power electronics embedded grid. A parametric model of the grid is firstly identified using the AFE itself to excite the system. This model is then integrated with AFE one in order to obtain a global system representation. Subsequently, a globally optimized local controller is synthesised for the AFE keeping in consideration interaction between the converter and the grid. The proposed method is finally tested experimentally and its performance is compared with a traditional Proportional-Integral (PI) controller in a dq synchronous reference frame.

A Gen-3 10-kV SiC MOSFET-Based Medium-Voltage Three-Phase Dual Active Bridge Converter Enabling a Mobile Utility Support Equipment Solid State Transformer

The emergence of medium-voltage silicon carbide (SiC) power semiconductor devices, in ranges of 10&#x2013;15 kV, has led to the development of simple two-level converter systems for medium-voltage applications. A medium-voltage mobile utility support equipment-based three-phase solid state transformer (MUSE-SST) system, based on Gen3 10 kV SiC MOSFETs, is developed to interconnect a three-phase 4160 V/60 Hz grid to a three-phase 480 V/60 Hz grid to provide a shore-to-ship power interface for naval vessels. The MUSE-SST system consists of three power conversion stages, namely, MVac/MVdc stage (MV: active front-end converter), MVdc/LVdc stage (dual active bridge converter), and LVdc/LVac stage (LV: active front-end converter). The galvanic isolation is introduced in the MVdc/LVdc stage using MV/LV high-frequency transformers (HFTs). This article demonstrates the operation of the three-phase Y&#x2013;<inline-formula> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> connected dual active bridge converter used in the MVdc/LVdc stage of the MUSE-SST system. Equations for phase currents, power flow, and zero-voltage switching (ZVS) boundaries are derived for all possible modes for the three-phase Y&#x2013;<inline-formula> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> configuration. A detailed parasitic simulation model is derived by measuring and experimentally verifying the parasitic elements of the HFT. A brief discussion regarding the design considerations required for the hardware development of the medium- and low-voltage sides of the three-phase dual active bridge converter is also provided. Successful tests demonstrating the operation and feasibility of the medium-voltage dual active bridge converter, at medium-voltage levels (7.2 kV dc-link voltage), are shown. The results indicate that these devices can accelerate the growth and deployment of the medium-voltage SiC-based converter for isolated and bidirectional medium- to low-voltage dc systems.

Hybrid Modular Multilevel Converter Design and Control for Variable Speed Pumped Hydro Storage Plants

Hybrid modular multilevel converter control and design for pumped hydro storage plants is presented, addressing application-specific shortcoming of conventional back-to-back half-bridge modular multilevel converter: high common-mode-voltage machine stress. Common-mode-voltage-free operation in the entire or partial frequency range is enabled by variable DC link voltage control, through introduction of minimal full-bridge submodule share in hybrid active front-end converter stage. The developed generalized converter design approach for arbitrary DC link voltage range operation and additional internal energy balancing control layers enable down-to-zero DC voltage control. The results are verified through high-fidelity switched-model simulations of 6 kV converter, with 10/6 ratio of full-bridge to half-bridge submodules in active front-end. The analyzed hybrid active front-end stage benefits from lower converter losses for equal machine operation flexibility compared to full-bridge design, while trade-off between grid-side power factor range and full-bridge submodule share is offered within the design stage. Compared to the state-of-the-art, zero-to-rated DC link voltage operation is possible at lower full-bridge submodule share (62% against 75 %), at the penalty of reduced grid-side power factor. Alternatively, operation at higher full-bridge submodule share (62% against 50% existing solution) enables grid-side reactive power support over wide speed range, without branch current overload.

Simulink implementation of three-phase PWM rectifier using five-level cascaded H-bridge MLI configuration

Ability of multilevel inverters (MLI) to operate with matured medium power electronic devices has enhanced their prominence for active front converters in various applications such as drive control, battery energy storage systems and PV/Grid integrated systems. In short, Active front end (AFE) converters are regenerative rectifiers incorporated to, simultaneously regulate dc-link voltage and ensure unity power factor at source terminals. Realizing the significance of MLI based active front end converter, this paper presents Simulink implementation of MLI based active front end converter, using most popular classical MLI i.e., cascaded H-bridge. Imposing level shifted and phase shifted carrier arrangements, implementation PWM for CHB is demonstrated and MATLAB simulation results for both open loop and closed systems are presented.

Single-Phase to Three-Phase Power Converter With Reduced DC-Link Voltage Ripple and Grid Current Harmonics

This article presents a control technique to improve the power quality and performance of grid-connected single to three-phase power converter. The conventional five-leg (5L) topology is modified to parallel five-leg (P5L) topology that has fault tolerance capability and operates even either of the front end converter leg is open-circuited. In such topologies, lower order harmonics (3rd, 5th, 7th, 9th, 11th, 13th, and 15th) are dominant, which are eliminated using a proportional plus resonant controller in the inner current control loop of active front end converter (AFEC). The control technique implemented over single-phase AFEC controls the dc-link voltage and provides unity power factor operation at the grid side and also balances the individual capacitor voltages (Vc1 and Vc2) even under faulty conditions of power semiconductor switches. The dc-link voltage oscillation is the inherent problem of such converters, therefore, it is proposed to use advanced bus clamp PWM technique at the inverter side to reduce such ripples and also improves the inverter output voltage profile. The comparative improvement under operating conditions is presented through the simulation and verified on hardware setup to validate the efficacy of the control algorithm.

THD Reduction in Distributed Renewables Energy Access through Wind Energy Conversion System Integration under Wind Speed Conditions in Tamaulipas, Mexico

In this article, a technique for the reduction of total harmonic distortion (THD) in distributed renewables energy access (DREA) composed of wind turbines is introduced and tested under the wind speed conditions presented in Tamaulipas, Mexico. The analysis and simulation are delimited by a study case based on wind speeds measured and recorded for one year at two highs in the municipality of Soto La Marina, Tamaulipas, Mexico. From this information, the most probable wind speed and the corresponding turbulence intensity is calculated and applied to a wind energy conversion system (WECS). The WECS is composed of an active front-end (AFE) converter topology using four voltage source converters (VSCs) connected in parallel with a different phase shift angle at the digital sinusoidal pulse width modulation (DSPWM) signals of each VSC. The WECS is formed by the connection of five type-4 wind turbines (WTs). The effectiveness and robustness of the DREA integration are reviewed in the light of a complete mathematical model and corroborated by the simulation results in Matlab-Simulink®. The results evidence a reduction of the THD in grid currents up to four times and which enables the delivery of a power capacity of 10 MVA in the Tamaulipas AC distribution grid that complies with grid code of harmonic distortion production.

A DC Power Exchange Highway Based Power Flow Management for Interconnected Microgrid Clusters

In this paper, the concept of dc power exchange highway (DCPEH) is introduced to interconnect a cluster of ac microgrids (MGs) to facilitate an efficient and resilient energy system. A power flow controller is proposed to manage the power flow between an MG and DCPEH, based on local load demand, surplus power, and power shortfall of an individual MG. This control integrated with DCPEH will improve the resiliency and reliability of MG clusters by utilizing the maximum available power from uncertain distributed energy resources (DERs) like solar or wind energy conversion systems in individual MGs. Each MG is connected to the DCPEH through an active front-end converter and a storage capacitor. The converter holds the dc voltage at a pre-defined value. A dynamic droop control for the DCPEH is adopted to effectively distribute the power demand between MGs. In this paper, it has been assumed that the cluster operates autonomously with the presence of a utility grid. It has been further assumed that the MGs are dominated by converter interfaced DERs, and hence, the power sharing is performed using angle droop control. To verify the efficacy of the interconnected MG clusters using DCPEH, various simulation studies are conducted using power system computer aided design electro magnetic transient design and control.

Voltage Sensorless Control of VIENNA Rectifier in the Input Current Oriented Reference Frame

The VIENNA rectifier is increasingly being considered as the active front-end converter in many applications that do not require regeneration or controllable input reactive power. In such applications, this rectifier is normally operated at unity source displacement factor, which derates its nominal voltampere (VA) rating. Moreover, in the “source voltage oriented” reference frame that is normally used for control, this rectifier has complex operating limits on the rectifier terminal voltage (imposed by the operating conditions), which needs to be computed online to avoid the line currents from going through zero-crossing distortions. Also, the source voltage has to be either measured or estimated, which further increases the cost and/or computational burden. This paper proposes a control method that eliminates the requirement of the input voltage sensors (or estimators) and makes the limits on the rectifier terminal voltage far simpler to compute. The control method is implemented in the input line current oriented reference frame, considering unity displacement factor at the rectifier input terminals, which minimizes the VA rating of the rectifier (for a given rated load power). The proposed sensorless current-oriented control approach is compared with the conventional voltage-oriented control strategy through experimental results.

The reactive power demand in DEMO: Estimations and study of mitigation via a novel design approach for base converters

In the present pre-conceptual design, the base converters to supply the main DEMO poloidal magnets are rated for 45 kA and about 10 kV. If the traditional design approach, based on thyristor bridges, was adopted, this would result in very large reactive power exchanged when low voltage values are required by the load. To satisfy the limitations imposed by National Grid Operators, the reactive power should be compensated inside the DEMO Plant Electrical System by means of a Reactive Power Compensation and Harmonic Filtering system. This could represent an additional cost and takes up a large area of the plant. This paper shows that the reactive power demand, estimated with an analytical model starting from the current/voltage waveforms foreseen in DEMO magnets, is huge. Therefore, an innovative approach, based on Active Front End converters, is proposed and compared with the traditional solution for coil power supplies.

Design and Controller-In-Loop Simulations of a Low Cost Two-Stage PV-Simulator

A PV-Simulator is a DC power source in which the current-voltage (I-V) characteristics of different PV arrays can be programmed. With a PV-simulator, the operation of the solar power conditioning systems can be validated at a laboratory level itself before actual field trials. In this work, design, operation and controls for a two-stage programmable PV-simulator required for the testing of solar power conditioning systems are presented. The proposed PV-simulator consists of a three-level T-type active front-end converter in the first stage and a buck-chopper-based DC-DC converter in the second stage. An active front-end rectifier using a three-level T-type IGBT-based converter is used at the input stage to help in operating the system at unity power factor. A DC-DC converter at the output stage of the simulator is regulated to obtain the I-V characteristics of the programmed PV-Array. Hardware-In-Loop simulations are carried out to validate the proposed system and the associated controls implemented in the controller. As a case study, this PV-simulator is programmed with electrical parameters of a selected PV-array and the characteristics obtained from the PV-simulator are compared with the actual PV-array characteristics. The dynamic response of the system for sudden changes in the load and sudden changes in irradiance values are studied.

THD Reduction in Wind Energy System Using Type-4 Wind Turbine/PMSG Applying the Active Front-End Converter Parallel Operation

In this paper, the active front-end (AFE) converter topology for the total harmonic distortion (THD) reduction in a wind energy system (WES) is used. A higher THD results in serious pulsations in the wind turbine (WT) output power and several power losses at the WES. The AFE converter topology improves the capability, efficiency, and reliability in the energy conversion devices; by modifying a conventional back-to-back converter, from using a single voltage source converter (VSC) to use pVSC connected in parallel, the AFE converter is generated. The THD reduction is achieved by applying a different phase shift angle at the carrier of digital sinusoidal pulse width modulation (DSPWM) switching signals of each VSC. To verify the functionality of the proposed methodology, the WES simulation in Matlab-Simulink® (Matlab r2015b, Mathworks, Natick, MA, USA) is analyzed, and the experimental laboratory tests using the concept of rapid control prototyping (RCP) and the real-time simulator Opal-RT Technologies® (Montreal, QC, Canada) is achieved. The obtained results show a type-4 WT with a total output power of 6 MVA, generating a THD reduction up to 5.5 times of the total WES current output by Fourier series expansion.

Shifting Resonances in Wind Farms to Higher Frequencies due to TUPF Converters

The increasing penetration of wind power in power systems is an ongoing trend. In wind farms, harmonic distortion and resonances concern both wind turbine manufacturers and power system operators. The frequency of occurrence and amplification factors of resonances are affected by the number of wind turbines in operation, their connection in series or in parallel through the collecting grid and the type of active front-end converter employed. This paper focuses on the latter and proposes the use of the True Unity Power Factor converter as an alternative, in which the resonance avoidance feature is built-in the wind turbines and is independent of external circumstances. Besides suppressing current harmonics below the 50th order without using filter capacitors, the TUPF converter modifies the parallel resonance characteristics of the wind farm by pushing the frequencies of their occurrences to higher values, making them less likely to be excited. The nearly sinusoidal output current behavior is demonstrated via simulation and experimental results and the variation in resonance peak characteristics is corroborated by modal analysis in a case study wind farm. It is concluded that there is a significant advantage in employing these converters in wind power generation, especially at the buses identified as the most prone to being the center of resonances.

Design and Implementation of Sensorless Voltage Control of Front-End Rectifier for Power Quality Improvement in Telecom System

In this paper, a three-phase three-switch three-level boost (Vienna) type pulse-width modulation rectifier is proposed as an active front-end power factor correction (PFC) rectifier for power quality improvement in telecom load. A sensorless voltage control technique is proposed and it does not rely on any input voltage information results in reliable and robust operation. A brief description on the principle of operation and the most advantageous modulation method of the proposed system are discussed. The feasible switching states are identified for the proposed active front-end converter resulting in reduced switching stress and dc ripples. A triangular carrier based control logic is applied and the input current reaches sinusoidal shape without the need of sensing the input voltage. The detailed analysis of front-end PFC converter is carried out by equivalent circuit analysis. Also, the complete loss and efficiency calculation of the converter is explicitly carried out with the help of hardware design guidelines. The performance of the proposed system and its capability of operating satisfactorily in the event of failure of one phase of the mains is verified through MATLAB Simulation, and the results obtained are presented. The experimental setup rated for 9 kW is developed in the laboratory to validate the simulation results. From the simulation and hardware results, it is observed and recorded that the power quality parameters are improved and are well within the IEEE and IEC standards.

Modeling and Control of Marine Diesel Generator System With Active Protection

On-board dc grid systems have been proposed in the marine industry to reduce specific fuel consumption and to improve dynamic performance and manoeuvrability. The primary energy sources of an on-board dc system are variable frequency operating diesel engine generators with an active front-end converter system. Such systems might suffer disturbances from the engine side as well as from the electrical load side. These dynamic conditions result in torque overloads in the mechanical system, resulting in failures to sensitive driveline components. To understand the dynamics associated with disturbances and to propose a robust and reliable system, a detailed modeling approach is required. This paper presents complete modeling of a diesel generator system, including cylinder pressure, mechanical driveline system, electrical generator characteristics, and control system of an active front-end converter. The detailed model is used to develop a novel power electronic-based controller to reduce elevated load levels induced during disturbances. The controller increases the damping ratio of the first natural frequency by using the speed difference between a generator and a flywheel. The generator speed is estimated while fly wheel inertia speed is measured from an existing speed sensor used for monitoring and electronic control, and their difference is used to improve the reliability under disturbance conditions. Simulation with the detailed model has indicated excessive drivetrain loads and potential damage to critical components. The proposed controller enhances the damping of the system by transferring the torsional energy to the electrical system, reducing the shaft torque amplitude during resonance. This paper presents simulation results demonstrating the fidelity of the diesel generator model as well as the effectiveness of the proposed controller.

Finite set model predictive control with on-line parameter estimation for active frond-end converters

This paper proposes a finite set model predictive control (FS-MPC) system for active front-end (AFE) converters in combination with an extended Kalman filter (EKF) for on-line parameter estimation to overcome the issues of parameter uncertainty and measurement noise. The FS-MPC performance of such converters is largely affected by variations in the model impedance (filter impedance and grid impedance), especially for systems with low short circuit ratios. Therefore, an EKF is used to estimate these model parameters on-line. Moreover, the EKF is used to filter out measurement noise in the feedback variables. For implementation of the FS-MPC, the discrete-time models of the AFE converter and the filter are derived using two discretization methods (forward Euler method and Taylor series expansion). The observability matrix of the linearized model is computed, and the observability is checked for each time instant to verify that the EKF is working properly. Moreover, the delay time due to the digital calculation is compensated. The performance of the proposed method is illustrated via simulation results.

Neutral Point Potential Control for Three Phase 3 - level Neutral Point Clamped Active Front End Converter

Neutral Point Potential Control for Three Phase 3 - level Neutral Point Clamped Active Front End Converter

Single-current-sensor-based active front-end-converter-fed four quadrants induction motor drive

Induction motor (IM) is a workhorse of the industry, whose dynamics can be modified close to that of a separately excited DC machine by field-oriented control technique, which is commonly known as vector control of induction machine. This paper presents a complete performance of the field-oriented control of IM drive in all four quadrants with a single-current-sensor-based active front end converter whose work is to regulate DC link voltage, draw pure sinusoidal currents at unity power factor and to facilitate bi-directional power flow between the grid and the drive. The entire system is completely modelled in MATLAB/SIMULINK and the results are discussed in detail. The vector control analogy of the back to back converters is highlighted along with the experimental results of field-oriented control of induction machine using a dsPIC30F6010A digital signal controller.

Harmonic Analysis and Controller Design of 15 kV SiC IGBT-Based Medium-Voltage Grid-Connected Three-Phase Three-Level NPC Converter

Cascaded converters are generally used for medium-voltage (MV) grid-connected applications due to the limitation in the voltage rating of available silicon (Si) power devices. These converters find application in active power filters, STATCOM or as the active front end converters for solid state transformers at the distribution voltage levels. The high voltage wide bandgap semiconductor devices have enabled the grid connected operation of noncascaded converters. This results in high power density, less number of switching devices, and high efficiency for three-phase MV grid interface. This also results in control simplicity without the need for complex dc bus balancing algorithms otherwise needed for cascaded converters. However, such noncascaded, grid-connected converters introduce challenges in maintaining power quality at low currents. This paper investigates the harmonic performance and current distortion of the grid-connected, three-level neutral point clamped converter using 15 kV silicon carbide Insulated Gate Bipolar Transistor (IGBTs). A suitable control scheme for stable harmonic compensation is proposed. The challenges and control performance are explained through frequency domain analysis, simulations, and experimental validation on a developed prototype of the three-phase converter up to 4.16 kV, three-phase MV grid-connected operation.

A Hidden Block in a Grid Connected Active Front End System: Modelling, Control and Stability Analysis

An LCL filter utilized in an active front end (AFE) converter can generate significant resonance, which can affect quality and stability of the system. Thus, a proper active or passive damping technique and/or a suitable controller is required to be designed for the converter. The duty cycle constraint—though inevitable in practical inverter systems—is often ignored in most of the existing literature. The effects of measurement noise on filter control system performance must be considered and evaluated under different conditions. Finally, almost all inverter control systems are implemented digitally using microcontrollers. Consequently, the effects of sampling on control system stability and performance should be evaluated as well. Thus, this paper presents stability analysis of an AFE converter-based filter design with a complete practical system configuration, including saturation and sampling blocks and measurement noises. Mathematical analysis and simulations have been carried out to validate the proposed method.

Harmonics and Interharmonics Compensation With Active Front-End Converters Based Only on Local Voltage Measurements

The current grid codes for distribution networks impose on operators to provide ancillary services, like fault ride through capability and reactive power compensation. In this context, generating units with power electronics interfaces could offer as an additional service the active compensation for harmonic and interharmonic currents introduced by other converters or distorting loads. Typically, the converters of these generating units do not have information on the distortion of either other loads or of the grid current. Thus, this paper presents a control algorithm for grid harmonics and interharmonics compensation that relies only on the measurement of the voltage at the point of connection of the unit. The reference for the compensating current is calculated from the harmonic components of grid voltage in the synchronous reference frame. The paper also addresses the influence on the compensation performance of the line impedance between the generating unit and the point of connection. Experimental tests on a laboratory setup fully validate the proposed compensation method.