Grid-tied Voltage Source Converters Research Articles

A Partially Rated LC Trap Type AC Filter for Grid-Tied Voltage Source Converters

A Partially Rated LC Trap Type AC Filter for Grid-Tied Voltage Source Converters

Asymmetrical Fault Ride-Through and Power Oscillation Characterization for Grid-Tied Voltage Source Converters

As the number of converter-interfaced distributed energy resources connected to the power system continues to increase rapidly, recent grid codes require these grid-tied converters to maintain grid connection during fault to ensure power supply security and reliability. In this work, we analyze the oscillation in the injected real and reactive power that a grid-tied voltage source converter (VSC) introduces as it attempts to contribute to the power quality and improve the voltage at the point of common coupling (PCC) during an asymmetrical fault. We propose a new double sequence current reference generation method that can be utilized to derive a closed-form quantification of the peak value of both the real and reactive power oscillation during asymmetrical fault ride-through (AFRT) analytically. Effect of the resulting bus voltage oscillation and ripple current requirement at twice the grid frequency, corresponding to the real power oscillation, on the input DC bus capacitor and upstream converter is analyzed for facilitating system component sizing. Furthermore, the proposed current reference generation formulation helps control the negative-sequence current injection during fault to assist with fault identification. The theoretical analysis is validated through simulation in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PLECS</i> for both asymmetrical and symmetrical fault scenarios. Experimental results for a three-phase, grid-tied VSC operating under both asymmetrical and symmetrical faults are provided to evaluate the performance of the proposed current reference generation method and validate the analysis presented.

Artificial Neural Network-Based Constrained Predictive Real-Time Parameter Adaptation Controller for Grid-Tied VSCs

This article proposes a real-time algorithm for identifying the grid parameters, which is concurrently used for online tuning of the predictive controller in each iteration, in a grid-tied active front end (AFE) voltage source converter (VSC) applications. The algorithm is designed by inspiring from the concepts of the extended Kalman filter (EKF) and the model predictive control (MPC). The performance of the algorithm highly depends on the weighting factors of the algorithm. The artificial neural network (ANN)-based algorithm is used to find the optimal set of weighting factors among the ones in a parameter search block. An offline particle swarm optimization (PSO) is run to provide the data source for the parameter search block. The algorithm identifies not only the inductance but also the resistance of the grid. In addition, the hard constraints on the amplitude of the input and output variables are guaranteed. The validation of the proposed approach is performed experimentally and compared with the state-of-the-art conventional methods. The experimental results show that the proposed method could effectively stabilize the system in weak grid conditions and under wide impedance variations. In addition, the accuracy of the proposed impedance identification method is 96%.

Artificial Neural Network-Based Pole-Tracking Method for Online Stabilization Control of Grid-Tied VSC

To cope with the weak grid stability issue of grid-tied voltage source converters (VSCs), this article proposes an artificial neural network (ANN) based approach for online stabilization control of the grid-tied VSC with the pole-tracking feature. First, an ANN is adopted to establish the mapping between the control parameters and the closed-loop poles of the grid-VSC system, serving as a computationally light model surrogate that is favorable for real-time control applications. Then, an online parameter search algorithm enabling simultaneous tuning of multiple controllers and parameters is developed, by which the system's poles under a new grid condition can be pulled to the reference ones, i.e., achieving the pole-tracking-based stabilization control of this article. Finally, the efficacy of the proposed method along with its stabilization effect is verified by MATLAB simulations and experimental results.

Synchronization Stability of Grid-Following Converters Governed by Saturation Nonlinearities

Saturation nonlinearities caused by control limiters will be activated during large disturbances, complicating the stability of grid-tied voltage source converters (VSCs). This letter investigates the large-disturbance synchronization stability of the grid-tied VSC considering two typical control limiters, i.e., the hard limiter of the reference current and the circular limiter used to avoid overmodulation. A unified differential equation (UDE) model is proposed to quantify the transient responses and stability boundaries of the system under different saturation conditions. It is revealed that the VSC behaves as a current source when the hard limiter is activated; while the interaction between the two limiters drives the VSC to work as a voltage source, making it immune to synchronization instability.

Adaptive Multi-Parameter-Tuning for Online Stabilization Control of Grid-Tied VSC: An Artificial Neural Network-Based Method

Voltage Source Converter (VSC) for grid integration of renewable energies are prone to have small-signal stability issues when connected to weak AC grids. Such stability issues largely arise from the lack of VSC control adaptivity to the varying grid condition (e.g., grid impedance). To address this issue, this letter presents an adaptive multi-parameter tuning method using the Artificial Neural Network. Innovative aspect of the proposal lies in that it enables the VSC to simultaneously tune multiple controller parameters online, which brings about a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pole-tracking</i> -based stabilization control feature for the VSC. Experimental results demonstrate that the proposed method can effectively and adaptively stabilize the VSC when the grid impedance is varied.

A Probabilistic Framework for the Robust Stability and Performance Analysis of Grid-Tied Voltage Source Converters

This paper proposed a probabilistic framework that could be used for the sensitivity assessment of grid-connected voltage source converters (VSCs), where uncertainties in the grid short circuit ratio (SCR) and operating point conditions, as well as control-loop interactions, were considered. The proposed method tried to broaden the available knowledge on the small-signal stability analysis of VSCs and provide a probabilistic point of view of this subject. It considered the probability of different operational conditions in order to obtain less conservatism and more accurate results. Based on uncertain inputs and the employed stability model, the proposed model produced statistical distributions of the critical mode and its damping factor and ratio, which were not accessible by existing deterministic methods. Crucial statistical information measures how much system stability and performance are maintained or changed over the system uncertainties and disturbances, as well as provides a clear insight into the system stability problem. For instance, as concluded in this paper, for the conventional control system design, fast dynamic parts of a VSC, such as the current controller and control delay, significantly impact the minimum damping ratio. Furthermore, slow dynamic parts, such as outer voltage control loops and the synchronization block, influence the maximum damping factor. For strong grids, the AC voltage magnitude controller (AVC) significantly impacts the maximum damping factor due to its lower bandwidth among all control loops. For weak grids, the damping factor of the critical mode is highly affected by interactions between the VSC, the power grid, and different control loops due to the synchronization mechanism. The other contributions of this paper were the introduction of robust stability and performance definitions and indices; explanations of the pros and cons of probabilistic assessment methods and their applicability; interpretation of the obtained results; and, finally, a link was provided between system stability and reliability, which will be crucial for future power system design.

A Nonlinear Adaptive Stabilizing Control Strategy to Enhance Dynamic Stability of Weak Grid-Tied VSC System

Large scale renewable energy systems interfaced with voltage source converters (VSCs) experience several challenging issues when integrated to weak grids. One of the specific issue i.e. control loop interaction of vector-controlled VSC under high grid impedance leads to several instability mechanism ultimately resulting in unwanted tripping of the VSC unit. As a countermeasure, a unique nonlinear adaptive stabilizing control (NASC) technique has been proposed in this paper to enhance the dynamic stability of the VSC during such scenarios. A small-signal model is developed first to examine the instability mechanism of VSC connected to weak grids under abnormal operating conditions and varying set points of the power controller. The dominant modes are identified from the analysis and a stabilizing controller is developed based on adaptive dynamic surface control philosophy for enhancing its stability. The proposed controller has been proved to be uniformly ultimately bounded using Lyapunov analysis. Besides, the stabilizing mechanism of VSC is analyzed with the proposed controller by performing small-signal stability analysis of the entire system. The efficacy of the proposed control scheme is demonstrated by performing numerical simulations in MATLAB 2017a and conducting experimental validations in controller-hardware-in-the Loop (CHIL) platform.

Optimal Periodic Variable Switching PWM for Harmonic Performance Enhancement in Grid-Connected Voltage Source Converters

Variable switching frequency PWM (VSFPWM) modulation can be advantageously implemented in industrial applications, such as renewable energy, motor drives, and uninterrupted power supply (UPS) systems, to reduce the injected current harmonic amplitudes, to suppress audible noise, and to improve semiconductor power efficiency. In this article, the usage of periodic VSFPWM methods in a voltage source converter (VSC) is proposed, analyzed, and benchmarked in terms of harmonic spectrum spreading, following the IEEE-519 current harmonic standard for the connection to the distribution grid. Particular attention is paid to the influence of VSFPWM on the ac filter design. First, the analytical model of the voltage harmonic spectrum generated by a three-phase three-wire two-level VSC implementing several periodic VSFPWM methods is derived. Subsequently, a design guideline for the commonly used <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter in the grid-tied VSC application is proposed, which minimizes the size requirement of the necessary components. The voltage spectrum models of the proposed VSFPWM method and the optimal switching profiles are verified by MATLAB/Simulink simulations and a 5-kW three-phase two-level VSC hardware demonstrator. The study shows that the ac filter power density for the studied VSFPWM methods can be greatly increased when compared with the conventional and widely employed constant switching frequency continuous PWM strategies.

Describing Function Analysis of Sustained Oscillations in Grid-Tied Voltage-Source Converter With Double Saturation Limiters

Recently, with the fast development of renewable energy, various power-electronic-based devices have been widely incorporated in power systems, and controller saturation limiters have been broadly utilized, such as in voltage-source converter. It has been well recognized that these saturation limiters make sustained oscillations possible. To study the impact of saturation limiters, by considering the dynamical response of double saturation limiters in both the d-axis and q-axis of the alternating current control, this paper establishes single-input-single-output models for two novel phenomena including the double-clipped oscillation and single-clipped oscillation, based on the describing function. Then the describing-function-based Nyquist criterion is used to obtain the amplitude and frequency of the sustained oscillations. The model accuracy is verified by the electromagnetic transient simulation, and the influences of control parameters are extensively studied. All these findings clearly demonstrate that the saturation limiters play an active role in sustained oscillations in power-electronic-based power systems.

Unified SISO Loop Gain Modeling, Measurement, and Stability Analysis of Three-Phase Voltage Source Converters

Frequency-domain modeling is an effective technique in the dynamic analysis of power electronic converters-based power systems. In this paper, a unified single-input single-output (SISO) loop gain modeling for the three-phase grid-tied voltage source converters (VSCs) under both symmetric and asymmetric AC grids is presented, which facilitates the physical measurement and stability analysis. Based on the linear-time-periodic (LTP) modeling technique, the harmonic admittance model of the three-phase grid-tied VSC is developed in the stationary ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">αβ</i> )-frame, which accurately captures the coupling dynamics. This model can be transformed into other existing models through coordinate transformations. According to the idea of mathematical induction, a two-by-two recursive admittance matrix (RAM) model is derived, which is easy to include frequency coupling components of arbitrary order. It is illustrated that the VSC admittance is coupled with the grid admittance due to the frequency coupling effect (FCE). Furthermore, the RAM is converted to its equivalent SISO models following the concept of loop gain. The system stability is thus assessed by the SISO stability criteria (e.g., Nyquist stability criterion). In addition, the loop gain allows the traditional SISO perturbation and measurement scheme to be used for detecting the stability margin information. Finally, simulation results verify the feasibility and correctness of the theoretical analysis presented above.

Large-Signal Grid-Synchronization Stability Analysis of PLL-Based VSCs Using Lyapunov's Direct Method

Grid-synchronization stability (GSS) is an emerging stability issue of grid-tied voltage source converters (VSCs), which can be provoked by severe grid voltage sags. Although a qualitative understanding of the mechanism behind the loss of synchronization has been acquired in recent studies, an analytical method for quantitative assessment of GSS of grid-tied VSCs is still missing. To bridge this gap, a dedicated Lyapunov function is analytically proposed, and its corresponding stability criterion for GSS analysis of grid-tied VSCs is rigorously constructed. Both theoretical analysis and simulation result demonstrate that the proposed method can provide a credible GSS evaluation compared to the previous EAC/EF-based method. Therefore, it can be applied for fast GSS evaluation of the grid-tied VSCs as is exemplified in this letter, as well as an analytical tool for some GSS-related issues, e.g., GSS-oriented parameter design and stabilization control.

Stability Analysis of the Grid-Tied VSC Considering the Influence of Short Circuit Ratio and X/R

The grid impedance ratio X/R (GIR) and the short circuit ratio (SCR) are the indexes that widely applied to the stability analysis of power systems integrated with voltage source converters (VSC). Based on the assumption that GIR is large enough, the impacts of grid resistances are neglected in most studies. In this brief, the impact mechanisms of grid impedance are analyzed by single-input and single-output (SISO) transfer functions under different GIR conditions. It is found that the decrease of GIR has both “enhancement effect” and “reduction effect” on SISO transfer function magnitudes, while the “reduction effect” is stronger than the “enhancement effect”. When <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {GIR} \in [f_{0}/f_{\mathrm{ pll}}, \infty$ </tex-math></inline-formula> ), the interaction behaves between the grid impedance and phase-locked loop (PLL), and it mainly has impacts on the magnitude-frequency curve. When <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {GIR} \in [0, f_{0}/f_{\mathrm{ pll}}$ </tex-math></inline-formula> ), the interaction behaves among the phase of grid impedance, PLL and current loop, and it mainly has impacts on the phase-frequency curve. Simulations are carried on MATLAB to verify the effectiveness of the proposed analysis.

Coordinated Control System between Grid–VSC and a DC Microgrid with Hybrid Energy Storage System

Direct current microgrids (DCMGs) are currently presented as an alternative solution for small systems that feed sensitive electrical loads into DC. According to the scientific literature, DCMG maintains good voltage regulation. However, when the system is in islanded mode, very pronounced voltage variations are presented, compromising the system’s ability to achieve reliable and stable energy management. Therefore, the authors propose a solution, connecting the electrical network through a grid-tied voltage source converter (GVSC) in order to reduce voltage variations. A coordinated control strategy between the DCMG and GVSC is proposed to regulate the DC voltage and find a stable power flow between the various active elements, which feed the load. The results show that the control strategy between the GVSC and DCMG, when tested under different disturbances, improves the performance of the system, making it more reliable and stable. Furthermore, the GVSC supports the AC voltage at the point of common coupling (PCC) without reducing the operating capacity of the DCMG and without exceeding even its most restrictive limit. All simulations were carried out in MATLAB 2020.

Modeling and stability analysis of grid-tied VSC considering the impact of voltage feed-forward

The single-input and single-output (SISO) transfer functions are proposed in this paper to analyze the impact of voltage feed-forward (VFF) loop on the stability of grid-tied voltage-source converter (VSC). Based on the SISO transfer functions, the VFF’s impacts are analyzed by considering the influence of current control integral time constant (ITC) and the grid impedance ratio X/R. To be specific, the VFF can improve the stability when the VSC has a large ITC, while it can decrease the stability when the VSC has a small ITC. The impedance ratio X/R is another affecting factor to the VFF when the phase-locked loop (PLL) dynamics are included in the transfer function model. The closed-loop poles of the VSC without the VFF loop are sensitive to the decrease of X/R, while it is not very susceptible to the variation of X/R when the VFF loop is applied in VSC. The proposed SISO transfer functions and stability analysis are both verified through simulations.

An Uninterruptable PV Array-Battery Based System Operating in Different Power Modes With Enhanced Power Quality

This article aims to develop a solar- battery energy storage (BES) based system, which ensures an uninterruptable supply to loads irrespective of availability of the grid. This system comprises of a solar photovoltaic (PV) array, a BES, the grid and local residential loads. A new control is implemented such that the active power demand of residential loads, is fed from the PV array, a BES unit and the utility grid. In this system, the power control operates in different power modes, which delivers the benefits to the end users with an integration of BES and an excess of PV array power, which is sold back to the grid. For this, an effective control logic is developed for the grid tied voltage source converter. Moreover, this system deals with the issue of an integrating power quality enhancement along with the power generation from the solar PV source. The cascaded delayed signal cancellation based phase locked loop is implemented for grid synchronization during the grid voltage distortion. The developed control is easily implemented in a real time controller (dSPACE-1202). Test results validate the performance of the implemented control in different operating conditions such as varying solar power generation, load variations and unavailability of the grid.

Design of a Linear Time-Varying Model Predictive Control Energy Regulator for Grid-Tied VSCs

This article presents an energy regulator based on a Model Predictive Control (MPC) algorithm for a Voltage Source Converter (VSC). The MPC is formulated to optimise the converter performance according to the weights defined in an objective function that trades off additional features, such as current harmonic distortion, reactive power tracking and DC bus voltage oscillation. Differently from most approaches found in the research literature, the MPC proposed here considers the coupling dynamics between the AC and DC sides of the VSC. This study is focused on the example case of a single-phase VSC, which presents a nonlinear relationship between its AC and DC sides and a sustained double-line frequency power disturbance in its DC bus. To reduce the burden of the MPC, the controller is formulated to benefit from the slow energy dynamics of the system. Thus, the cascaded structure typically used in the control of VSCs is kept and the MPC is set as an energy regulator at a reduced sampling frequency while the current control relies on a fast inner controller. The computational burden of the algorithm is further reduced by using a linear time-varying approximation. The controller is presented in detail and experimental validation showing the performance of the algorithm is provided.

Instantaneous Complex Power Control of a Grid-Tied VSC Supplying a Constant Power Load

In this article, a nonlinear controller for a grid-tied voltage-source converter (VSC) supplying a constant power load is introduced. The control law combines a feedback loop and a feedforward compensator. The feedback loop strategy uses a feedback linearizing method for compensating nonlinearities appearing in the VSC model. The performance in the presence of changes between two different values of constant power is improved by including a fast feedforward compensation. This compensator uses estimates of power load and its time derivative obtained from a nonlinear observer. The resultant strategy is validated via simulation and experimental tests. These experiments show that the proposed scheme performs in a very good way.

Emulating Rotational Inertia of Synchronous Machines by a New Control Technique in Grid-Interactive Converters

Integration of renewable energy sources (RESs) into power systems is growing due to eco-friendly concerns and ever-increasing electricity demand. Voltage source converters (VSCs) are the main interface between RESs and power grids, which have neither rotational inertia nor damping characteristics. Lack of these metrics make the power grid sensitive to frequency disturbances and thereby under frequency, to load shedding activation or even large-scale collapse. In this regard, the contribution of this paper is to develop a new control technique for VSCs that can provide virtual inertia and damping properties with the DC-link capacitors inhered in the DC-side of grid-tied VSCs. The applied VSC is controlled in the current controlled model, with the capability of injecting extra active power with the aim of frequency support during perturbations. The dynamics assessment of the proposed platform is derived in detail. It is revealed that the control scheme performs in a stable region even under weak-grid conditions. Finally, simulations are conducted in MATLAB to depict the efficacy and feasibility of the proposed method. The results show that frequency deviation is mitigated under step up/down changes in the demand, and the rate of change of frequency is improved by 47.37% compared to the case in which the synthetic inertia loop is canceled out.

Investigation on Dynamic Response of Grid-Tied VSC During Electromechanical Oscillations of Power Systems

This study focuses on the dynamics of a grid-tied voltage source converter (GVSC) during electromechanical oscillations. A small-signal model with GVSC port variables (DC voltage and AC power) as the outputs and a terminal voltage vector as the input is derived to reveal the passive response of the GVSC on the basis of the power equation in the d–q coordinate system. An input–output transfer function matrix is constructed according to the proposed model. The frequency response of this matrix in the electromechanical bandwidth is described to reflect the dynamic behavior of the GVSC. The effects of the operation parameters, i.e., the grid strength, reference value of the control system, and grid voltage, on the dynamic behavior of the GVSC in the electromechanical bandwidth, are investigated using frequency domain sensitivity. Analysis results show that the GVSC generates responses with respect to the electromechanical mode. These responses have different sensitivities to the operation parameters. The IEEE 10-machine power system simulation is performed, and the power hardware-in-the-loop platform with the GVSC was applied to validate the analysis.