Control Of Grid-connected Converters Research Articles

Current-Oriented Phase-Locked Loop Method for Robust Control of Grid-Connected Converter in Extremely Weak Grid

Current-Oriented Phase-Locked Loop Method for Robust Control of Grid-Connected Converter in Extremely Weak Grid

Stability Control of Grid-Connected Converter Considering Phase-Locked Loop Frequency Coupling Effect

Stability Control of Grid-Connected Converter Considering Phase-Locked Loop Frequency Coupling Effect

Bidirectional Power Flow Control of Grid-Connected Converter Using Resistance-Emulating Approach

Bidirectional Power Flow Control of Grid-Connected Converter Using Resistance-Emulating Approach

Model-Free Predictive Control for Grid-Connected Converters With Flexibility in Power Regulation: A Solution for Unbalanced Conditions

Model-Free Predictive Control for Grid-Connected Converters With Flexibility in Power Regulation: A Solution for Unbalanced Conditions

Low-Complexity Model-Free Combined Control of Grid-Connected Converters Under Normal and Abnormal Grid Conditions

Low-Complexity Model-Free Combined Control of Grid-Connected Converters Under Normal and Abnormal Grid Conditions

Model reference adaptive controllers with improved performance for applications in LCL-filtered grid-connected converters

This work presents a control strategy combining a direct Model Reference Adaptive Control (MRAC) with a robust state feedback for current control of grid-connected converters with LCL filters operating under grid impedance variations. The MRAC is based on a gradient law, providing properly current reference tracking and voltage disturbance rejection. The inclusion of a state feedback inner loop provides robust active damping of the filter resonance for an entire range of grid impedances. A systematic control design procedure is proposed, and the combination of both control actions in a multi-loop approach does not add significant computational complexity. The proposal allows to increase the MRAC gains adaptation dynamics, providing suitable responses under grid current reference variations and under typical grid voltage disturbances, including grid faults. Performance indexes as the grid-currents total harmonic distortion and the mean squared error are also improved. In addition, robust stability analysis based on Lyapunov functions are carried out for the inner control loop and for the outer control loop, considering the grid impedance uncertain and with unmodeled dynamics. The strategy is implemented in a commercial digital signal processor, and the compliance of the grid currents with the IEEE 1547 Std. is verified with controller hardware-in-the-loop and experimental results in a 5.4 kW prototype.

The Effect of Grid-Connected Converter Control Topology on the Diagonal Dominance of Converter Output Impedance

The Effect of Grid-Connected Converter Control Topology on the Diagonal Dominance of Converter Output Impedance

Control of a Three-Phase Grid-Connected Voltage-Sourced Converter Using Long Short-Term Memory Networks

With the rise of inverter-based resources (IBRs) within the power system, the control of grid-connected converters (GCCs) has become pertinent due to the fact they interface IBRs to the grid. The conventional method of control for a GCC such as the voltage-sourced converter (VSC) is through a decoupled control loop in the synchronous reference frame. However, this model-based control method is sensitive to parameter changes causing deterioration in controller performance. Data-driven approaches such as machine learning can be utilized to design controllers that are capable of operating GCCs in various system conditions. This work explores a deep learning-based control method for a three-phase grid-connected VSC, specifically utilizing a long short-term memory (LSTM) network for robust control. Simulations of a conventional controlled VSC are conducted using Simulink to collect data for training the LSTM-based controller. The LSTM model is built and trained using the Keras and TensorFlow libraries in Python and tested in Simulink. The performance of the LSTM-based controller is evaluated under different case studies and compared to the conventional method of control. Simulation results demonstrate the effectiveness of this approach by outperforming the conventional controller and maintaining stability under different system parameter changes.

Positive-sequence virtual-flux control of grid-connected converter during unsymmetrical voltage dips

One of the major problems in direct power-controlled grid-connected voltage source converters is that during voltage dips, the converter current increases to compensate for the reduced grid voltage. The most common voltage dips are unsymmetrical, and they cause unbalance and distortion in the converter current. This paper introduces a new, simple but effective algorithm which limits the current in a direct power-controlled grid-connected voltage source converter during voltage dips. A positive-sequence virtual-flux based control scheme is employed and this makes the current balanced and sinusoidal during unsymmetrical voltage dips. The proposed control clearly demonstrates the performance which is illustrated through various simulations and experimental work. The current during unsymmetrical voltage dips is limited in magnitude and is balanced, with low distortion. The Implementation of this control scheme will enable voltage source converters to stay connected to the grid during voltage dips as required by most grid codes.

Power Loss Minimization of Parallel-Connected Distributed Energy Resources in DC Microgrids Using a Distributed Gradient Algorithm-Based Hierarchical Control

This paper presents a distributed hierarchical control to minimize the distribution power loss, including both line loss and converter loss, of distributed energy resources (DERs) in DC microgrids. The proposed control comprises a distributed gradient algorithm (DGA) at the top layer, a consensus control at the secondary layer, and a local droop and dual-loop control at the bottom layer. Since the distribution power loss of DERs is modelled a concave function of the output currents, the DGA with addition and subtraction operators can be directly used to derive the optimal current allocation coefficients based on peer-to-peer data exchange between the neighboring DERs. The derived optimal current allocation coefficients are adopted by the distributed secondary control to generate adaptive voltage terms for the local control. The local control consists of a droop control and a classic dual-loop control for grid-connected converters. Both simulation and experimental results have validated that the proposed control can reduce more distribution power loss of DERs than the conventional control by only considering line loss by various cases, including normal operation, power clip, plug-in-and-out of DERs, and communication failure. Experimental results also verify that the execution speed of the proposed control is faster than that of the conventional Lagrange multiplier-based centralized control.

Dynamic Grid Voltage-Based Impedance-Reshaped Control for the Stability Enhancement of Grid-Connected DC/AC Converter System under Bidirectional Power Flow

Considering the bidirectional three-phase DC/AC converter, it presents different impedance characteristics on AC side under different power flow directions, resulting in different stability margin. This may cause the system instability at high-power level. This directionally oriented stability difference has not been paid enough attention in the grid-connected converter control. To mitigate the stability variations under bidirectional power flow, a dynamic grid voltage-based impedance reshaping control is proposed in this paper. The proposed method extracts the grid voltage dynamic component, and correspondingly compensates the power output, which is capable of regulating the power output coordinated with voltage dynamics in both power flow directions, neutralizing the stability difference and enhancing the bidirectional power flow stability. Unlike the conventional unidirectional damping optimization control in the current loop, the proposed control method can maintain satisfied stability under bidirectional power flow through the grid side voltage, which can avoid increasing the current loop delay, and thus endow the converter flexible bidirectional power process capability. The effectiveness of the proposed method has been verified by both simulations and experiments.

A New Model Predictive Current Controller for Grid-Connected Converters in Unbalanced Grids

Distributed energy resources are often connected to low-voltage distribution networks where the grid voltages may be unbalanced. This leads to an unwanted ripple in the output active power at twice the fundamental grid frequency. In this article, a new model predictive current controller is proposed for unbalanced grids. The variable switching frequency of existing model predictive controllers is fixed and the power quality is improved. A Kalman filter estimator is used to extract the positive and negative sequence components. A new calculation time compensation technique is proposed, which offers superior accuracy to existing approaches. A grid voltage discretization compensation strategy is outlined, and its effectiveness is demonstrated. Finally, the system stability is verified theoretically. Simulation and laboratory results are included to prove the robustness of the proposed controller and support the theoretical analysis.

Plug-in disturbance observer assisted DC link voltage control of grid-connected converters to improve transient performance without deteriorating grid current quality

Trade-off between DC link voltage dynamics and AC-side current quality in grid-interfacing power converters limits the attainable DC link voltage loop bandwidth for a given desired value of grid-side current total harmonic distortion (THD). It was recently shown that adding a notch filter in series with the typically employed DC link voltage controller of PI or type-II structure releases the above trade-off, yet requires full controller re-design. The paper proposes to replace the notch filter by a plug-in disturbance observer, allowing to increase attainable DC link voltage loop bandwidth for given grid-side current THD without any modification of the existing DC link voltage controller by utilizing a special frequency-selective filter. Simulations and experimental results validate the proposed design methodology.

Power Control Strategy for Grid-Connected Inverters in Stationary Reference Frame

The control of grid-connected converters in stationary reference frame has been investigated in several works of literature. One of the challenges of this method is in the computation of the current computing current references as a function of power references. In some works, this issue has been solved based on instantaneous power theory equations described in the stationary reference frame. However, this approach can lead to steady-state errors in the injected power if there is an error in the current control loop, which is the focus of this investigation. Firstly, analytical expressions are derived to investigate the effect of current loop steady-state error in the active and reactive power injected into the grid. Then, this paper proposes a closed-loop power control for grid-connected inverters controlled in the stationary reference frame. This strategy is experimentally investigated in two power conversion system configurations for battery energy storage systems. The results indicate that the proposed scheme guarantees zero steady-state error in the injected power even when the current loop presents an amplitude and phase steady-state error.

Analysis and Design of Multiple Resonant Current Control for Grid-Connected Converters

Including “generalized integrators” for sinusoidal signals, multiple resonant control (MRSC) scheme which can accurately compensate selected sinusoidal signals, is widely used as a high-performance current control method for grid-connected converters. In this article, the plug-in digital plug-in MRSC scheme is proposed to provide a general framework for housing various MRSC schemes, which plugs the MRSC controller into a stable feedback control loop. It can take advantages of both controllers—the feedback controller provides fast response and good robustness, and the MRSC controller offers accurate compensation of the periodic signals. More critically, based on a new type of sinusoidal signal model, the sufficient stability criteria and the gain tuning rules are developed to offer a simple universal approach to the design of the digital plug-in MRSC schemes. An application case study on the digital plug-in MRSC of a 5-kVA three-phase four-wire grid-connected inverter with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter is provided. Experimental results demonstrated that the proposed MRSC scheme can force the inverter to produce high-quality currents with high accuracy, fast transient response, and good robustness.

Performance Analysis and Benchmarking of PLL-Driven Phasor Measurement Units for Renewable Energy Systems

Phasor measurement units (PMUs) are a key part of electrical power systems, providing the dynamic monitoring and control of electrical units and impacting overall operation and synchronization of a network if not properly designed. This paper investigates the use of a phase-locked loop (PLL)-based algorithm for PMUs (to accurately find the magnitude, phase, and frequency) in a three-phase system. Various PLLs are reported in the literature, ranging from the very basic to advanced, capable of dealing with normal and abnormal grid behavior and mainly used for the control of grid-connected converters. In this paper, a number of PLLs were utilized to perform PMU functions, and a benchmarking study has been investigated to analyze the developed PLL-driven PMUs under various grid conditions (such as unbalanced faults, harmonics, and frequency variations). The simulation and experimental results were provided to support the performance capabilities and suggested limitations. In addition, the best PMU in benchmarking was used in the Kundur’s two-area system to show the significance of PMUs in a power system.

An improved digital phase locked loop against adverse grid conditions

The phase-locked loop (PLL) is an essential synchronization technique to ensure stable operation and control of grid-connected converters. Nevertheless, under actual conditions, the grid voltage is often influenced by harmonics and frequency variation, resulting in unsatisfactory steady-state precision and dynamic performance of the PLL. Therefore, in this paper, an improved digital PLL (ID-PLL) including harmonics elimination module and angular frequency reference calculation module is proposed. The proposed ID-PLL can maintain steady-state precision and dynamic performance in the condition of voltage unbalance, voltage harmonics and voltage frequency variation. The influence of adverse grid voltage on PLL performance is analyzed in detail. The parameters design and discretization approach is presented accordingly. The effectiveness of the proposed approach is confirmed by compared experiments with DSOGI-PLL.

Power production strategies for two-stage PV systems during grid faults

The paper comprehensively addresses the control algorithm for the photovoltaic (PV) system. The system’s operation during both nominal and faulty grid conditions, most notably voltage unbalances, is addressed. The control of the boost converters and the inverter that secures reliable and maximized power production is proposed. Specifically, the paper offers a novel DC-link voltage stabilization algorithm that is not based on conventional approaches, such as droop schemes. Rather, the DC-link voltage rate of change is used to realize safe primary energy sources’ power management during transients caused by grid disturbances. This offers improved robustness, reliability and scalability. Moreover, the improved grid-connected converter control is proposed. It enables the production of arbitrary currents and powers profiles, during different working regimes, respecting the converter’s ratings. Consequently, higher-layer control functions can be realized by utilizing the proposed scheme. Also, the flexible power production prioritization feature is integrated into the overall solution. Such an extensive set of features was not demonstrated for PV systems in the literature previously. Different hardware topologies are regarded – centralized, decentralized and distributed. The proposed control schemes were tested using the hardware-in-the-loop (HIL) environment and were validated through the analysis of the obtained results.

Study on dynamic characteristic optimization of virtual inertia control for the grid-connected converters in DC micro grid

In order to reduce the transient time and voltage deviation due to voltage variation while increasing the system inertia of the DC microgrid, a new adaptive control method for the parameters of the grid-connected end is proposed. Firstly, a grid-connected converter control strategy mathematical model is established. Secondly, the influences of different control parameters on the dynamic characteristics of the system are studied. An adaptive equation based on the parameters of voltage deviation and voltage change rate of DC bus is proposed. Finally, through MATLAB/Simulink simulation, the simulation results of the traditional drooping control, the virtual inertial control with fixed parameters and the control strategy proposed in this paper are compared and analyzed. The result shows that the proposed control method has better dynamic characteristics.

An Improved Three-Stages Cascading Passivity-Based Control of Grid-Connected LCL Converter in Unbalanced Weak Grid Condition

This paper proposes an improved three-stages cascading passivity-based control (PBC) for a grid -connected LCL converter in unbalanced weak grid condition. In general, the traditional double-loop control based on positive and negative sequence transformations is used in grid-connected converter control in unbalanced weak grid condition. However, it is time consuming for second harmonic filtering, and positive and negative sequence currents need to be controlled separately. The PBC has strong robustness to interference, and the line voltage based PBC can deal with the voltage unbalance effectively and easily without negative sequence transformation. But the traditional PBC needs six variables and three damping coefficients for the grid-connected LCL converter in unbalanced grid condition, and it has the disadvantage of difficult implementation. The improved PBC can realize the same control effect with three-stages cascading PBCs of two variables and one damping coefficient, and it has the advantages of easy implementation, good performance and high stability. First, the modeling and controller design are detailed described. Then the SIMULINK simulation results demonstrate the benefits of the improved control strategy. Finally, a grid-connected LCL converter prototype of 5kW is built and the experimental results verify the correctness and effectivity of the improved three-stages PBC strategy.