Control Of Power Converters Research Articles

Intelligent control of grid‐connected AC–DC–AC converters for a WECS based on T–S fuzzy interconnected systems modelling

Control of power converters is undoubtedly an essential issue in wind-energy systems. This study focuses on the development of a control scheme dedicated to a wind-energy conversion system (WECS) based on Takagi-Sugeno (T-S) fuzzy model. The treated system includes a wind turbine followed by a permanent magnet synchronous generator and power electronic converters. After describing the elements constituting the WECS, the main objectives of control are to extract the maximum amount of available power and to control the DC-bus voltage. The proposed approach is based on the concept of decentralised stabilisation. First, as to reduce the complexity of the system, the WECS is divided into two interconnected non-linear subsystems. Then, each subsystem is represented by a T-S fuzzy model. Using the parallel distributed compensation concept, a decentralised fuzzy controller is synthesised for each subsystem. Sufficient conditions are derived from Lyapunov theory and the local feedback gains for each controller are calculated by using linear matrix inequalities tools. Therefore, the whole closed-loop fuzzy system is stable using the elaborated fuzzy control scheme. Simulation results are included to show the performance of the proposed algorithm.

Grid-forming control for power converters based on matching of synchronous machines

We consider the problem of grid-forming control of power converters in low-inertia power systems. Starting from an average-switch three-phase power converter model, we draw parallels to a synchronous machine (SM) model and propose a novel converter control strategy which dwells upon the main characteristic of a SM: the presence of an internal rotating magnetic field. In particular, we augment the converter system with a virtual oscillator whose frequency is driven by the DC-side voltage measurement and which sets the converter pulse-width-modulation signal, thereby achieving exact matching between the converter in closed-loop and the SM dynamics. We then provide a sufficient condition asserting existence, uniqueness, and global asymptotic stability of a shifted equilibrium, all in a rotating coordinate frame attached to the virtual oscillator angle. By actuating the DC-side input of the converter we are able to enforce this condition and provide additional inertia and damping. In this framework, we illustrate strict incremental passivity, droop, and power-sharing properties which are compatible with conventional power system operation requirements. We subsequently adopt disturbance-decoupling and droop techniques to design additional control loops that regulate the DC-side voltage, as well as AC-side frequency and amplitude, while in the end evaluating them with numerical experiments.

Emulation of a Permanent-Magnet Synchronous Generator in Real-Time Using Power Hardware-in-the-Loop

Permanent-magnet synchronous generators (PMSGs) are used in several applications of traction electrification such as power supplies for auxiliary systems and vehicle on-board range-extenders. In this paper, a PMSG is emulated using power hardware in-the-loop. This emulation scheme provides a method to eliminate risks and costs associated with the testing, prototyping, and validation of controllers for power converters and other parts of the traction system. The proposed scheme can be used for testing of motor drives’ different parameters, thereby enabling the testing of a variety of electrical machine drives where the machine prototype is unavailable. The voltage output of the proposed emulator system replicates the voltage generated within the real-time model. The terminal voltage from the real-time model depends on the load connected to the emulator terminals; the current drawn from the emulator output terminals is sensed and fed back to enable the emulator to replicate the PMSG characteristics in steady state and transients. The performance of the system for nonlinear loads, such as a rectifier with a capacitor filter and transient conditions, is also experimentally verified and presented along with validation against a physical PMSG coupled to a dynamometer.

Distributed Coordination Control Based on State-of-Charge for Bidirectional Power Converters in a Hybrid AC/DC Microgrid

This paper proposes a distributed coordination control for multiple bidirectional power converters (BPCs) in a hybrid AC/DC microgrid with consideration of state-of-charge (SOC) of storages. The researched hybrid AC/DC microgrid is composed of both AC and DC subgrids connected by multiple parallel BPCs. In the literature, the storages of a hybrid microgrid are considered to allocate in only the AC subgrid or DC subgrid, which reduces the reliability of the whole system, especially during the islanded mode. Besides, the SOC management has not been considered in BPCs’ operating strategy. This paper considers a hybrid microgrid topology which has energy storages in both AC side and DC side. This ensures the reliability while increasing the complexity of the control strategy at the same time. Further, a distributed coordination control method for multiple BPCs based on SOC was proposed to enhance the reliability of hybrid microgrid. Finally, the performance of the proposed control methods was verified by real-time hardware-in-loop (HIL) tests.

Decentralized Optimal Servo Control System for Implementing Instantaneous Reactive Power Sharing in Microgrids

Active power is dispatched among distributed generation (DG) units in microgrids (MG) by means of f/P droop control loop, which controls the frequency set-point of voltage source converter (VSI). Since the frequency is a global variable, active power sharing is implemented well proportional to droop coefficients. However, the reactive power is not shared accurately, through V/Q control loop and according to the droop gains, as the voltage is a local variable. Furthermore, considering the small scale of DG units, reactive power sharing should be implemented instantaneously to prevent DG units from overcurrent or even blackout of the MG. This paper deals with reactive power sharing issue in droop control-based MGs as well as stability and dynamic performance concerns of V/Q control loop. A servo control system is designed to control power converters in MGs, by which droop-based VSIs are converted to servo VSIs (S-VSIs). A novel decentralized method is proposed to obtain the reactive power set-points of S-VSIs according to their droop coefficients, and fuzzy particle swarm optimization method is used to optimize the S-VSI's parameters, so that, in addition to securing stability of the V/Q loop, the desired (fast) response in reference tracking is achieved. The simulation results show that the proposed strategy is effective and its performance is not affected by delay or interruption of the existing low bandwidth communication link.

Optimal Control Solutions for PMSM Drives: A Comparison Study With Experimental Assessments

In this paper, a general optimum control for power converters and drives is proposed. The proposed optimum control will select an optimum voltage vector from the whole hexagonal plane, leading to the best control performance fulfilling a predefined performance index. With the proposed concept, two subsolutions, i.e., both continuous and discrete solutions, are derived and unified in the frame of optimum control . The continuous solution utilizes the averaged continuous-time model of the system and is capable of dealing with multiple system constraints, showing good performance with less calculation efforts, while the discrete solution takes the finite set of the power converter switching vectors into consideration and the state transition of the system can be predicted with a chosen vector. Both methods require less calculation efforts compared with the well-known finite-control-set model predictive control method, which makes it very suitable for practical realizations. Finally, as a case of study, the proposed concept is tested at a current-controlled 3-kW surface-mounted permanent magnet synchronous motor drive under different scenarios. The experimental results validate the effectiveness of both solutions.

Intelligent control strategy for a grid connected PV/SOFC/BESS energy generation system

In this paper, an intelligent control strategy for a grid connected hybrid energy generation system consisting of Photovoltaic (PV) panels, Fuel Cell (FC) stack and Battery Energy Storage System (BESS) is proposed. Firstly, the dynamic modeling of the electrical energy resources is carried out. Then, the local controllers of DC-DC power converters are designed to regulate the operating points of the energy generation units. An online-trained Elman Neural Network (ENN) based controller is developed to perform the tracking of the optimal operating point of the PV source. Moreover, a Takagi-Sugeno-Kang based fuzzy gain tuner is adopted for the adjustment of the PI parameters of the FC and BESS controllers. Besides, the Virtual Flux Oriented Control (VFOC) scheme based on neuro-fuzzy gain tuners is adopted to control the power flow between the hybrid system, the local loads and the utility grid. Furthermore, a centralized power supervisor is established to determine the levels of the power references for the local controllers according to the supply and demand conditions. The simulation results obtained in the Matlab/Simulink environment demonstrate the efficiency of the proposed control scheme.

A PLL-Based Controller for Three-Phase Grid-Connected Power Converters

The current control of three-phase grid-connected converters is typically carried out by using a proportional-resonant controller or synchronous reference frame (SRF) proportional-integral regulator. The implementation of these controllers often requires knowledge of the grid voltage frequency/phase angle, which is typically provided by a synchronization unit. It implies that dynamics and possible inaccuracies of the synchronization unit have a considerable impact on the current controller performance. The aim of this letter is to design an adaptive current controller by using a conventional SRF phase-locked loop. In this way, the current controller and synchronization part are merged into a single unit, which results in a simpler and more compact structure. The effectiveness of the proposed controller is verified using experimental results.

Sensorless force control for dielectric elastomer transducers

Multilayer stack-transducers made of dielectric elastomers generate considerable tensile forces and deformations when they are electrically stimulated. Thus, due to their capacitive behavior, they are for example energy efficient substitutes for conventional electromagnetic drives and enable various further completely new applications. Within this contribution, we present a sensorless force control for dielectric elastomer stack-transducers driven by a bidirectional flyback-converter. This force control for example can be used as interface for superimposed application-oriented controls. First of all, the properties of dielectric elastomer stack-transducers and the flyback-converter as well as its control structure are summarized. Due to the characteristic behavior of the power converter, the design of a sliding mode force controller is carried out based on an analytical model of the coupled electromechanical system containing the transducer and driving electronics. Using this model, the transducer force can be determined with the measured driving voltage and deformation so that an explicit force measurement is not required. In a first step, a two-point controller is developed for the variable structure control that is afterward extended in order to improve the control quality, for example, by adapting the inner power converter control. Finally, the experimental validations with a prototype stack-transducer and bidirectional flyback-converter demonstrate the feasibility as well as the precision and dynamics of the developed force control.

Machine Learning Based Adaptive Prediction Horizon in Finite Control Set Model Predictive Control

In this paper, an adaptive prediction horizon approach based on machine learning is presented for the finite control set model predictive control (FCS-MPC) of power converters. Usually, in FCS-MPC, the prediction horizon is kept constant. A large prediction horizon improves performance, however, it significantly increases the computational cost. The prediction horizon is typically chosen to be just large enough to give the required performance. We present a novel technique, where the prediction horizon adapts to the states of the converter. We define a cyber-physical objective function that penalizes both the error in converter performance and computational complexity. We perform several offline simulations to find the optimal prediction horizon based on the instantaneous state of the converter, based on a cyberphysical objective function. An artificial neural network is trained to calculate the optimal prediction horizon in run time. The proposed scheme allows a varying prediction horizon that reduces the overall computational complexity, while guaranteeing the required physical performance. The simulations and experimental results of the proposed technique justify the usefulness of our approach.

Performance evaluation of a DC-AC inverter controlled with ZAD-FPIC

Introduction: Power converters are used in microgrids to transfer power to the load with a regulated voltage. However, the DC-AC converters present distortions in the waveform that can be improved with the help of real-time controllers.Objective: Evaluate the response in alternating current of the buck converter controlled using ZAD-FPIC technique.Methodology: Based on the differential equations that describe the buck power converter, the ZAD and FPIC controllers are designed, simulations of the complete controlled system are made in Simulink of MATLAB, the system is implemented experimentally, and the controller is executed in real-time with the help of a DS1104 from dSPACE. In the end, several tests are carried out to check the effectiveness of the controller.Results: The results show that the controller allows good stability against different variations in the system and in the load.Conclusions: The ZAD-FPIC technique controls the variable and tracks changes in the waveform, magnitude, and frequency of the reference signal. The controller presents good stability to different tests, tracking the reference signal after each event.

Smart Transformer-Fed Variable Frequency Distribution Grid

The smart transformer (ST), a solid-state transformer with control and communication functionalities, that interfaces medium voltage (MV) and low voltage (LV) grids, enables the control of the frequency in the LV grid independently from the MV one. In a ST-fed distribution grid, the ST can interact with the droop controllers of local generators and loads frequency characteristic to control the LV power demand. However, most of the existing controllers for power converters cannot guarantee good harmonic control under variable frequency condition. To address this issue, a frequency-adaptive control scheme based on the fractional-order repetitive control and the frequency-adaptive phase-locked loop are proposed in this paper. The proposed scheme provides fast online parameter tuning capability in order to be highly adaptive to variable frequencies, and it can be easily implemented in the power converters controllers of a ST-fed distribution grid. Moreover, the stability analysis of the frequency-adaptive system considering the effect of synchronization has been investigated in this paper. Simulation and experiments have been carried out to verify the effectiveness of the proposed scheme as well as the considered scenarios.

FPGA based Hardware-in-the-Loop Simulation for Digital Control of Power Converters using VHDL-AMS

This paper presents a new approach for complex system design, allowing rapid, efficient and low-cost prototyping. Using this approach can simplify designing tasks and go faster from system modeling to effective hardware implementation. Designing multi-domain systems require different engineering competences and several tools, our approach gives a unique design environment, based on the use of VHDL-AMS modeling language and FPGA device within a single design tool. This approach is intended to enhance hardware-in-the-loop (HIL) practices with a more realistic simulation which improve the verification process in the system design flow. This paper describes the implementation of a software/hardware platform as effective support for our methodology. The feasibility and the benefits of the presented approach are demonstrated through a practical case study of a power converter control. The obtained results show that the developed method achieves significant speed-up compared with conventional simulation methods, using minimum resources and minimum latency.

Hierarchical control for parallel bidirectional power converters of a grid-connected DC microgrid

The DC microgrid is connected to the AC utility by parallel bidirectional power converters (BPCs) to import/export large power, whose control directly affects the performance of the grid-connected DC microgrid. Much work has focused on the hierarchical control of the DC, AC, and hybrid microgrids, but little has considered the hierarchical control of multiple parallel BPCs that directly connect the DC microgrid to the AC utility. In this paper, we propose a hierarchical control for parallel BPCs of a grid-connected DC microgrid. To suppress the potential zero-sequence circulating current in the AC side among the parallel BPCs and realize feedback linearization of the voltage control, a d-q-0 control scheme instead of a conventional d-q control scheme is proposed in the inner current loop, and the square of the DC voltage is adopted in the inner voltage loop. DC side droop control is applied to realize DC current sharing among multiple BPCs at the primary control level, and this induces DC bus voltage deviation. The quantified relationship between the current sharing error and DC voltage deviation is derived, indicating that there is a trade-off between the DC voltage deviation and current sharing error. To eliminate the current sharing error and DC voltage deviation simultaneously, slope-adjusting and voltage-shifting approaches are adopted at the secondary control level. The proposed tertiary control realizes precise active and reactive power exchange through parallel BPCs for economical operation. The proposed hierarchical control is applied for parallel BPCs of a grid-connected DC microgrid and can operate coordinately with the control for controllable/uncontrollable distributional generation. The effectiveness of the proposed control method is verified by corresponding simulation tests based on Matlab/Simulink, and the performance of the hierarchical control is evaluated for practical applications.

Message from Editors

Electrified transportation in automotive, railway, aviation and navigation has received more and more attention in recent years. Innovations in energy efficiency, speed and performance of all kinds of transportation vehicles are hot topics both in academia and industry. The corresponding researches relate to the design and control of electrical machines, power converters and energy storage systems.

HW/SW Co-Simulation System for Enhancing Hardware-in-the-Loop of Power Converter Digital Controllers

Digital controllers of power converters are more and more implemented in FPGAs due to the increasing complexity of current control algorithms, higher switching frequencies, and concurrence requirements. System behavior depends not only on the control algorithm but also on the implementation issues. Thus, closed-loop controller evaluation at early design stages is a main concern. In this paper, a new hardware-in-the-loop method is proposed. It profits from FPGAs and their design tools in order to validate the closed-loop power converter before prototyping the power stage. The proposed solution presents a general architecture that does not depend on specific vendors or CAD tools, but it uses those utilized for the final implementation of the controller. A case study is presented with a given implementation of the proposed solution. Comparisons with existing alternatives show the advantages of our approach.

Modeling and Design of $df/dt$ -Based Inertia Control for Power Converters

Power systems with high penetration rates of inverter-based generation units exhibit reduced system inertia. Faults like plant outages or fault-induced system splits then cause an increased rate of change of frequency and may lead to frequency instability. One of the proposed schemes to provide synthetic inertia with power electronic converters is the frequency-derivative-based ( $df/dt$ ) approach. Converters with this method inject an active current that is proportional to the derivative of the grid frequency to mimic the inertia of synchronous plants. Specification of synthetic inertia is currently being discussed for future grid codes. In this paper, we show that the frequency derivative in the $df/dt$ controller is subject to a severe bandwidth limitation. Considerable low-pass filtering is indispensable to avoid instability due to the excitation of local oscillation modes in the harmonic frequency range. We have found a first-order lag with a time constant around 1 s to be required for a robustly stable system with common parameters. A new discrete-time linear model that accurately represents sampling effects is introduced and comprehensively described. A $df/dt$ control design is proposed that leads to a robustly stable system. Model and control design are validated by laboratory experiments, especially regarding mechanisms of instability. The main finding of this paper—applying a $df/dt$ controller harbors the risk of merely shifting the initial danger of system-wide frequency instability to local stability problems—is important for power plant manufacturers as well as system operators.

NPC Multilevel Photovoltaic Inverter with Nonlinear MPPT Using Boost Converter

This work proposes an advanced control of power converters used in photovoltaic solar energy. The boost converter and NPC three level inverter are usually associated with photovoltaic cells. A maximum power point tracking algorithm is adopted in this case to maximize the output power by using the sliding mode control approach. For three level inverter, five command strategy is applied in order to compare their performances: PDPWM (Phase Disposition PWM), PODPWM (Phase Opposition Disposition PWM), APODPWM (Alternative Phase Opposition Disposition PWM), COPWM (Carrier Overlapping PWM), PSPWM (Phase Shift PWM). The total harmonic distortion (THD) is evaluated for the various modulation index. Finally, validity and effectiveness are verified through a simulation in Matlab/Simulink.

Optimization for Customized Power Quality Service in Multibus Microgrids

Power quality requirements for different consumers and electric equipment are distinctive. Conventionally, common power quality standards are applied to the whole power grid inducing debate between technical commissions. Customized power quality standards for different consumers become a widely accepted solution. The main challenge is on the proper regulation of power quality in different areas. This paper considers a multibus microgrid system, where the power quality in each area has individual adjustable standard. Voltage unbalance, induced by connection of single-phase loads, is taken as the example. Distributed generators are utilized to reduce the negative-sequence voltage on consumer side. An optimization-based strategy is proposed and implemented with the power converter control system. A general mathematical model is established, which can be used in the optimization problem for evaluating the objective function. Peak current of each distributed generation is considered as the main constraint. Simulation is conducted to demonstrate the effectiveness of the method.

Corrective frequency compensation for parasitics in boost power converter with sensorless current mode control

For boost power converter with Sensorless Current Mode (SCM) control, the inductor current is acquired by current estimator instead of current sensing. Without consideration of parasitics, the estimator is low-accurate, which affects system Small Signal Model (SSM) and degrades the control performance. In this paper, the issue is studied in frequency domain, and solved by a Corrective Frequency Compensation for Parasitics (CFCP) strategy. First, transfer functions for power converter, current estimator and current mode controller are given for frequency-domain analysis. Second, without consideration of parasitics, conventional proportional integral compensation is used to improve the system stability. Furthermore, with consideration of parasitics, converter main pole is replaced by two poles while a zero emerges at the origin. In order to cancel out the influence, the proposed CFCP strategy adopts a second-order transfer function to correct zero/pole variation induced by parasitics. Finally, steady state performance and transient response of the converter are improved, which are verified by simulations and experiments.