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A robust optimized active disturbance rejection control (ADRC)-based grid voltage sensorless current controller is developed for an LCL-filtered grid-connected inverter (GCI) via a predictive control approach under various sources of disturbance including the model uncertainties, the LCL inherent resonance phenomenon, and non-ideal grid environment. Aiming to improve the sinusoidal reference tracking performance as well as to reject lumped sinusoidal disturbances in the control practice, a resonant extended state observer is integrated in the ADRC structure, which guarantees a rigorous stable operation of inverter for bounded filter parameter uncertainties and adverse grid voltage conditions. The grid frequency adaptability is thoroughly considered in controller design process and synchronization technique, offering an extra capability for GCI to operate under different grid voltage frequency levels or even the frequency deviation caused by grid fault events. Robustness against parameter uncertainty and system stability are analyzed through the discrete-time frequency analysis and pole-zero map approaches. Simulation and hardware experiments are conducted for GCI with LCL filter parameters designed for two typical regions (i.e. the resonance frequency is less and greater than 1/6 of switching frequency) to validate the theoretical analysis and the effectiveness of the proposed control method.
Thuy Vi Tran; Kyeong-Hwa Kim; Jih-Sheng Jason Lai. Optimized Active Disturbance Rejection Control With Resonant Extended State Observer for Grid Voltage Sensorless LCL-Filtered Inverter. IEEE Transactions on Power Electronics 2021, 36, 13317 -13331.
AMA StyleThuy Vi Tran, Kyeong-Hwa Kim, Jih-Sheng Jason Lai. Optimized Active Disturbance Rejection Control With Resonant Extended State Observer for Grid Voltage Sensorless LCL-Filtered Inverter. IEEE Transactions on Power Electronics. 2021; 36 (11):13317-13331.
Chicago/Turabian StyleThuy Vi Tran; Kyeong-Hwa Kim; Jih-Sheng Jason Lai. 2021. "Optimized Active Disturbance Rejection Control With Resonant Extended State Observer for Grid Voltage Sensorless LCL-Filtered Inverter." IEEE Transactions on Power Electronics 36, no. 11: 13317-13331.
An assessment of the stability and performance of current controllers with harmonic compensators is presented for an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter under distorted weak grid conditions. By using two typical current control schemes which are the direct current controller with the capacitor current-based active damping and integral-resonant state feedback current controller, the closed-loop system stability and current control performance are investigated in the presence of both uncertain grid impedance and distorted grid. Even though the controller stability has been investigated under weak grid in several studies, the stability assessment of the entire current control scheme, including the harmonic resonant controllers, still needs a further comprehensive investigation. The system stability is analyzed by obtaining the movement of the closed-loop poles in the discrete-time domain when the grid impedance varies. To fully study the impact of distorted weak grid condition on the LCL filters, three LCL filter parameter sets giving the resonance frequency in different frequency bands are chosen for the purpose of evaluating the system robustness and grid-injected current quality. In order to support the presented theoretical analyses, comprehensive simulation and experimental results based on 32-bit DSP TMS320F28335 to control 2 kVA grid-connected inverter are presented in terms of grid current quality and control stability in the environment of both uncertain grid impedance and distorted grid.
Seung-Jin Yoon; Thuy Vi Tran; Kyeong-Hwa Kim. Stability Assessment of Current Controller with Harmonic Compensator for LCL-Filtered Grid-Connected Inverter under Distorted Weak Grid. Applied Sciences 2020, 11, 212 .
AMA StyleSeung-Jin Yoon, Thuy Vi Tran, Kyeong-Hwa Kim. Stability Assessment of Current Controller with Harmonic Compensator for LCL-Filtered Grid-Connected Inverter under Distorted Weak Grid. Applied Sciences. 2020; 11 (1):212.
Chicago/Turabian StyleSeung-Jin Yoon; Thuy Vi Tran; Kyeong-Hwa Kim. 2020. "Stability Assessment of Current Controller with Harmonic Compensator for LCL-Filtered Grid-Connected Inverter under Distorted Weak Grid." Applied Sciences 11, no. 1: 212.
A high reliability of a grid-connected inverter (GCI) system at reasonable cost is a critical requirement for maximizing renewable energy potential in the electrical energy market. Several grid voltage sensorless control approaches have been investigated not only to eliminate the vulnerability of faulty sensors but also to further reduce the GCI commercial price. In this paper, a frequency adaptive integral-resonant full-state feedback current control scheme with the facilitation of a full-state observer is adopted for a grid-connected inductive–capacitive–inductive (LCL) filtered inverter without sensing the grid voltages. The proposed scheme actively damps the filter resonance and ensures the robustness of the inverter system against unexpected severe grid conditions with low cost and simplified hardware construction. The synchronization of the inverter with the main grid is accomplished by the proposed current controller-based grid voltage estimator, in which the grid frequency and phase angle can be detected effectively. In addition, the actual grid voltages are precisely regenerated to ensure the stable performance of the full-state observer. A safe start-up procedure is also presented for the grid voltage sensorless control of the LCL-filtered inverter to avoid a critical overcurrent and long settling time during the start-up instant, offering a stable and reliable inverter system operation with low computational burden. The effectiveness and feasibility of the proposed voltage sensorless current control scheme are validated by the simulation and experimental results under non-ideal grid conditions such as the harmonic distortion, grid frequency variation, and sudden grid phase angle jump.
Thuy Vi Tran; Kyeong-Hwa Kim. Grid Voltage Estimation Based on Integral Resonant Current Controller for LCL-Filtered Grid-Connected Inverter without AC Voltage Sensors. Electronics 2020, 9, 2051 .
AMA StyleThuy Vi Tran, Kyeong-Hwa Kim. Grid Voltage Estimation Based on Integral Resonant Current Controller for LCL-Filtered Grid-Connected Inverter without AC Voltage Sensors. Electronics. 2020; 9 (12):2051.
Chicago/Turabian StyleThuy Vi Tran; Kyeong-Hwa Kim. 2020. "Grid Voltage Estimation Based on Integral Resonant Current Controller for LCL-Filtered Grid-Connected Inverter without AC Voltage Sensors." Electronics 9, no. 12: 2051.
This paper presents a frequency adaptive grid voltage sensorless control scheme of a grid-connected inductive–capacitive–inductive (LCL)-filtered inverter, which is based on an adaptive current controller and a grid voltage observer. The frequency adaptive current controller is constructed by a full-state feedback regulator with the augmentation of multiple control terms to restrain not only the inherent resonance phenomenon that is caused by LCL filter, but also current harmonic distortions from an adverse grid environment. The number of required sensing devices is minimized in the proposed scheme by means of a discrete-time current-type observer, which estimates the system state variables, and gradient-method-based observers, which estimate the grid voltages and frequency simultaneously at different grid conditions. The estimated grid frequency is utilized in the current control loop to provide high-quality grid-injected currents, even under harmonic distortions and the frequency variation of grid voltages. As a result, the grid frequency adaptive control performance as well as the robustness against distorted grid voltages can be realized. Finally, an inverter synchronization task without using grid voltage sensors is accomplished by a fundamental grid voltage filter and a phase-locked loop to detect the actual grid phase angle. The stability and convergence performance of the proposed observers have been studied by means of the Lyapunov theory to ensure a high accuracy tracking performance of estimated variables. Simulation and experimental results are presented to validate the feasibility and the effectiveness of the proposed control approach.
Thuy Vi Tran; Kim; Myungbok Kim; Kyeong-Hwa Kim; Tran. Frequency Adaptive Current Control Scheme for Grid-connected Inverter without Grid Voltage Sensors Based on Gradient Steepest Descent Method. Energies 2019, 12, 4266 .
AMA StyleThuy Vi Tran, Kim, Myungbok Kim, Kyeong-Hwa Kim, Tran. Frequency Adaptive Current Control Scheme for Grid-connected Inverter without Grid Voltage Sensors Based on Gradient Steepest Descent Method. Energies. 2019; 12 (22):4266.
Chicago/Turabian StyleThuy Vi Tran; Kim; Myungbok Kim; Kyeong-Hwa Kim; Tran. 2019. "Frequency Adaptive Current Control Scheme for Grid-connected Inverter without Grid Voltage Sensors Based on Gradient Steepest Descent Method." Energies 12, no. 22: 4266.
This paper presents a voltage sensorless control design for an LCL-filtered grid-connected inverter in discrete-time domain. The proposed scheme comprises a frequency adaptive current controller and a discrete current-type observer based on the linear quadratic regulator (LQR) approach, which is robust against the harmonic distortion and frequency variation of grid voltage. A frequency adaptive observer is studied in detail by means of Lyapunov stability theory to ensure high accuracy tracking performance of estimated system variables and grid voltages under different grid conditions. Since the proposed method also generates precise grid frequency and phase angle to facilitate the synchronization task, the conventional phase-locked loop (PLL) structure can be effectively eliminated. Simulation and experimental results are presented to validate the feasibility of the proposed control scheme.
Thuy Vi Tran; Kyeong-Hwa Kim. Frequency Adaptive Grid Voltage Sensorless Control of LCL-Filtered Inverter Based on Extended Model Observer. IEEE Transactions on Industrial Electronics 2019, 67, 7560 -7573.
AMA StyleThuy Vi Tran, Kyeong-Hwa Kim. Frequency Adaptive Grid Voltage Sensorless Control of LCL-Filtered Inverter Based on Extended Model Observer. IEEE Transactions on Industrial Electronics. 2019; 67 (9):7560-7573.
Chicago/Turabian StyleThuy Vi Tran; Kyeong-Hwa Kim. 2019. "Frequency Adaptive Grid Voltage Sensorless Control of LCL-Filtered Inverter Based on Extended Model Observer." IEEE Transactions on Industrial Electronics 67, no. 9: 7560-7573.
This paper presents a grid voltage-sensorless current control design based on the linear quadratic regulator (LQR) approach for an LCL-filtered grid-connected inverter. The proposed scheme relies only on the information from the grid-side current sensors to implement the control algorithm as well as to synchronize the inverter system with the utility grid. Basically, the construction of current controller consists of a full-state feedback regulator augmented with an integral control term for achieving control objectives, and a frequency-adaptive observer to estimate the system state variables and grid voltage parameters even under a non-ideal grid environment with frequency variation. A systematic design method based on the LQR approach is introduced to obtain optimal gains for the controller as well as the adaptive observer. The effectiveness of the proposed scheme is validated through the simulation results.
Thuy Vi Tran; Kyeong-Hwa Kim. A Voltage-sensorless Current Control of Grid-connected Inverter Using Frequency-adaptive Observer. IFAC-PapersOnLine 2019, 52, 63 -68.
AMA StyleThuy Vi Tran, Kyeong-Hwa Kim. A Voltage-sensorless Current Control of Grid-connected Inverter Using Frequency-adaptive Observer. IFAC-PapersOnLine. 2019; 52 (4):63-68.
Chicago/Turabian StyleThuy Vi Tran; Kyeong-Hwa Kim. 2019. "A Voltage-sensorless Current Control of Grid-connected Inverter Using Frequency-adaptive Observer." IFAC-PapersOnLine 52, no. 4: 63-68.
This paper proposes a frequency-adaptive current control design for a grid-connected inverter with an inductive–capacitive–inductive (LCL) filter to overcome the issues relating to both the harmonic distortion and frequency variation in the grid voltage. The current control scheme consists of full-state feedback control to stabilize the system and integral control terms to track the reference in the presence of disturbance and uncertainty. In addition, the current controller is augmented with resonant control terms to mitigate the harmonic component. The control scheme is implemented in the synchronous reference frame (SRF) to effectively compensate two harmonic orders at the same time by using only one resonant term. Moreover, to tackle the frequency variation issue in grid voltage, the frequency information which is extracted from the phase-locked loop (PLL) block is processed by a moving average filter (MAF) for the purpose of eliminating the frequency fluctuation caused by the harmonically distorted grid voltage. The filtered frequency information is employed to synthesize the resonant controller, even in the environment of frequency variation. To implement full-state feedback control for a grid-connected inverter with an LCL filter, all the state variables should be available. However, the increase in number of sensing devices leads to the rise of cost and complexity for hardware implementation. To overcome this challenge, a discrete-time full-state current observer is introduced to estimate all the system states. When the grid frequency is subject to variation, the discrete-time implementation of the observer in the SRF requires an online discretization process because the system matrix in the SRF includes frequency information. This results in a heavy computational burden for the controller. To resolve such a difficulty, a discrete-time observer in the stationary reference frame is employed in the proposed scheme. In the stationary frame, the discretization of the system model can be accomplished with a simple offline method even in the presence of frequency variation since the system matrix does not include the frequency. To select desirable gains for the full-state feedback controller and full-state observer, an optimal linear quadratic control approach is applied. To validate the practical effectiveness of the proposed frequency-adaptive control, simulation and experimental results are presented.
Rizka Bimarta; Thuy Vi Tran; Kyeong-Hwa Kim. Frequency-Adaptive Current Controller Design Based on LQR State Feedback Control for a Grid-Connected Inverter under Distorted Grid. Energies 2018, 11, 2674 .
AMA StyleRizka Bimarta, Thuy Vi Tran, Kyeong-Hwa Kim. Frequency-Adaptive Current Controller Design Based on LQR State Feedback Control for a Grid-Connected Inverter under Distorted Grid. Energies. 2018; 11 (10):2674.
Chicago/Turabian StyleRizka Bimarta; Thuy Vi Tran; Kyeong-Hwa Kim. 2018. "Frequency-Adaptive Current Controller Design Based on LQR State Feedback Control for a Grid-Connected Inverter under Distorted Grid." Energies 11, no. 10: 2674.
In order to alleviate the negative impacts of harmonically distorted grid conditions on inverters, this paper presents a linear quadratic regulator (LQR)-based current control design for an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter. The proposed control scheme is constructed based on the internal model (IM) principle in which a full-state feedback controller is used for the purpose of stabilization and the integral terms as well as resonant terms are augmented into a control structure for the reference tracking and harmonic compensation, respectively. Additionally, the proposed scheme is implemented in the synchronous reference frame (SRF) to take advantage of the simultaneous compensation for both the negative and positive sequence harmonics by one resonant term. Since this leads to the decrease of necessary resonant terms by half, the computation effort of the controller can be reduced. With regard to the full-state feedback control approach for the LCL-filtered grid connected inverter, additional sensing devices are normally required to measure all of the system state variables. However, this causes a complexity in hardware and high implementation cost for measurement devices. To overcome this challenge, this paper presents a discrete-time current full-state observer that uses only the information from the control input, grid-side current sensor, and grid voltage sensor to estimate all of the system state variables with a high precision. Finally, an optimal linear quadratic control approach is introduced for the purpose of choosing optimal feedback gains, systematically, for both the controller and full-state observer. The simulation and experimental results are presented to prove the effectiveness and validity of the proposed control scheme.
Thuy Vi Tran; Seung-Jin Yoon; Kyeong-Hwa Kim. An LQR-Based Controller Design for an LCL-Filtered Grid-Connected Inverter in Discrete-Time State-Space under Distorted Grid Environment. Energies 2018, 11, 2062 .
AMA StyleThuy Vi Tran, Seung-Jin Yoon, Kyeong-Hwa Kim. An LQR-Based Controller Design for an LCL-Filtered Grid-Connected Inverter in Discrete-Time State-Space under Distorted Grid Environment. Energies. 2018; 11 (8):2062.
Chicago/Turabian StyleThuy Vi Tran; Seung-Jin Yoon; Kyeong-Hwa Kim. 2018. "An LQR-Based Controller Design for an LCL-Filtered Grid-Connected Inverter in Discrete-Time State-Space under Distorted Grid Environment." Energies 11, no. 8: 2062.