Control Of Grid-connected Converters Research Articles

Model Predictive Direct Power Control of Grid-connected Converters Considering Unbalanced Filter Inductance and Grid Conditions

The grid-connected converter (GCC) is widely used as the interface between various distributed generations and the utility grid. To achieve precise power control for GCC, this paper presents a model predictive direct power control (MPDPC) with consideration of the unbalanced filter inductance and grid conditions. First, the characteristics of GCC with unbalanced filter inductance are analyzed and a modified voltage control function is derived. On this basis, to compensate for the power oscillation caused by unbalanced filter inductance, a novel power compensation method is proposed for MPDPC to eliminate the DC-side current ripple while maintaining sinusoidal grid current. Besides, to improve the control robustness against mismatched filter inductance, a filter inductance identification scheme is proposed. Through this scheme, the estimated value of filter inductance is updated in each control period and applied in the proposed MPDPC. Finally, simulation results in PSCAD/EMTDC confirm the validity of the proposed MPDPC and the filter inductance identification scheme.

PCC Voltage Perturbation Path Analysis and Compensation for Grid-Connected Voltage-Source Converter Under Weak Grid

Under weak grid condition, negative influence is brought in on system stability because the grid impedance and the grid-connected converter control system are coupled to each other. The key factor affecting the stability of the system is the perturbation of the voltage fluctuation at the point of common coupling (PCC) to the phase-locked loop (PLL) and the dc-link voltage loop. In this article, the impedance model of a three-phase grid-connected converter, including the current loop, PLL, and dc-link voltage loop in the dq frame, is adopted, because it includes coupling information of the converter and reflects the transferring relationship among grid-connected system variables. Based on the transferring relationship, this article analyzes the perturbation paths of PCC voltage and proposes the corresponding compensation methods, i.e., the PLL and dc-link voltage loop compensation terms, which are inserted into the q and d axes of the current loop controller, respectively. The impedance analysis results show that the proposed method can effectively improve the system stability. In addition, the experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed method.

OCNOP Control of Grid-Connected Converters for Single-Phase Open Circuit Fault

An open circuit fault-tolerant operation of a grid-connected inverter requires switching reconfigurations or redundant inverter leg to bypass the faulty device. Such measures led to increased cost and design complexities. In the absence of such reconfiguration mechanisms, a grid-connected inverter with an open circuit fault in one of the inverter legs injects negative sequence current into the grid. The negative sequence current causes oscillation in dc-link voltage and thereby injects lower order harmonic into the grid. The present work proposes a novel control strategy (open circuit nonoscillatory power) to deliver a steady power into a balanced three-phase grid with only two active legs of the grid-connected inverter. The current of these two active phases are controlled to be equal in magnitude and 60° phase apart. This inhibits the flow of negative sequence current and in turn limits the lower order harmonic injection within the permissible limit. A control strategy is also proposed to modify the power references during postfault operation so that the postfault current in two active phases are limited within the converter current rating. The performance of the proposed scheme is compared with the conventional vector control scheme through a simulation study. The simulation results for the proposed control scheme are verified through experimental study in a laboratory prototype.

H ∞-Control of Grid-Connected Converters: Design, Objectives and Decentralized Stability Certificates

The modern power system features high penetration of power converters due to the development of renewables, HVDC, etc. Currently, the controller design and parameter tuning of power converters heavily rely on rich engineering experience and extrapolation from a single converter system, which may lead to inferior performance or even instabilities under variable grid conditions. In this paper, we propose an $H_{\infty}$-control design framework to provide a systematic way for the robust and optimal control design of power converters. We discuss how to choose weighting functions to achieve anticipated and robust performance with regards to multiple control objectives. Further, we show that by a proper choice of the weighting functions, the converter can be conveniently specified as grid-forming or grid-following in terms of small-signal dynamics. Moreover, this paper first proposes a decentralized stability criterion based on the small gain theorem, which enables us to guarantee the global small-signal stability of a multi-converter system through local control design of the power converters. We provide high-fidelity nonlinear simulations and hardware-in-the-loop (HIL) real-time simulations to illustrate the effectiveness of our method.

Operating Cost Reduction of DC Microgrids Under Real-Time Pricing Using Adaptive Differential Evolution Algorithm

Virtual resistance-based droop control is widely adopted as secondary-layer control for grid-connected converters in DC microgrids. This paper presents an alternative usage of the virtual resistances to minimize the total operating cost of DC microgrids under real-time pricing. The total operating cost covers the running cost of utility grids, renewable energy sources (RES), energy storage systems (ESS), fuel cells, and power loss on the distribution lines. An adaptive Differential Evolution (ADE) algorithm is adopted in this paper to optimize the virtual resistances of the droop control for the grid-connected converters of dispatchable units, such that the power flow can be regulated. The performances of the proposed strategy are evaluated by the case studies of a 12-bus 380 V DC microgrid using Matlab and a 32-bus 380 V DC microgrid using a Real-Time Digital Simulator (RTDS). Both results validate that the ADE can significantly reduce the operating cost of DC microgrids and outperform the conventional Genetic Algorithm (GA) in terms of cost saving. Comparisons among the microgrids with different numbers of dispatchable units reveal that the cost saving is more prominent when the expansion of dispatchable units.

Tuning of Discrete Complex Proportional Integral Current Controller for Grid-Connected Converters Based on Critical Damping

Current control is of utmost importance for grid-connected converters to achieve a high level of performance. The complex proportional integral controller is usually employed for its excellent performance in terms of cross-coupling decoupling and stability. The pole/zero cancellation characteristic of the complex proportional integral controller, which is always valid in the continues-time domain, fails in the discrete-time domain, resulting in an oscillatory or even unstable response. The discrete complex proportional integral controller has thus been addressed in this work, which cancels the complex plant pole with a matching zero provided by the controller in the discrete-time domain. It has been proved that pole/zero cancellation of the discrete complex proportional integral controller is always valid, regardless of the variation of the excitation frequency. An important feature of performance independence from the pulse ratio is thus achieved. Also, to achieve the possible highest performance, a tuning method based on critical damping is developed in the discrete-time domain, which addresses the one-sample delay directly in the tuning process. In this manner, the minimum settling time and negligible overshoot for transient response can be achieved, along with the most enhanced stability and avoidance of closed-loop anomalous peaks. Experimental results have verified the effectiveness of the developed method.

Design Procedure Combining Linear Matrix Inequalities and Genetic Algorithm for Robust Control of Grid-Connected Converters

This article proposes a design procedure for $\mathcal {H}_{\infty }$ current controllers suitable for application in grid-connected converters (GCCs) subject to disturbances and uncertain parameters. Linear matrix inequalities provide robust state feedback controllers from a set of design parameters automatically tuned with the help of a genetic algorithm, oriented by a suitable objective function. Such strategy allows to overcome the problem of $\mathcal {H}_{\infty }$ high control gains, ensuring a good tradeoff between performance and robustness against uncertain grid parameters. The proposal contributes to improve the effective use of metaheuristics for automated control tuning in GCCs, avoiding time-consuming human–machine interaction in the design stage. A case study with experimental results is presented, demonstrating the viability and the superior performance of the proposed controllers with respect to similar techniques, and also the compliance with the IEEE 1547 Std for grid currents.

Integrated Common and Differential Mode Filter With Capacitor-Voltage Feedforward Active Damping for Single-Phase Transformerless PV Inverters

Recently, several publications have discussed the design of LCL filters and current controllers for grid-connected converters. However, the focus was the LCL filter used as a differential mode (DM) filter. Besides its conventional application as DM filter, a modified LCL filter configuration can be used as an integrated common mode (CM) and DM filter, which is particularly interesting for transformerless photovoltaic (PV) inverters, whereby the CM leakage current must be limited to comply with standards. This article proposes the joint design procedure for an integrated CM and DM filter plus the current controller used in single-phase grid-connected transformerless PV inverters. The increased number of components and resonance frequencies, compared to the conventional LCL filter, requires a careful analysis of the location of resonance and anti-resonance frequencies, taking into account parameters variation. The design procedure considers the extended region of stability for the DM resonance frequency allocation provided by the filter capacitor-voltage feedforward active damping with converter-side current feedback. It also considers continuous and discontinuous unipolar pulsewidth modulation. Design examples and simulation results for 1.5 and 10 kVA are presented. The effectiveness of the proposed design procedure is verified by experimental results obtained from a 1.5 kVA inverter.

Current Control of Grid-Tied LCL-VSI With a Sliding Mode Controller in a Multiloop Approach

Despite several studies and approaches, the design of current controllers for grid-connected converters via LCL filters is still a challenging task. This is mainly due to the parametric uncertainties and the grid background voltage distortions to which the system is submitted. In order to overcome these inherent system challenges, this paper proposes a multiloop control framework that simplifies the system dynamics of the overall circuit by splitting it in two equivalent sub-circuits. An inner loop is implemented as a fast sliding mode controller that controls the filter capacitor voltage in a fast and robust way. As a result, the plant, viewed by the outer loop, yields a voltage-controlled voltage source connected to the grid through an L filter. Thus, multiple resonant controllers are included in the state-space representation of this simplified system, allowing the design of a state-feedback controller in the outer loop for asymptotic tracking of the grid current with disturbance rejection of the grid background voltage. Here, a discrete linear quadratic regulator algorithm is used for designing the state-feedback gains. A simple design procedure is presented, as well as simulations and experimental results, to show the good performance of the proposed control scheme even under significant uncertainties and disturbances of the system.

Cost function‐based modulation scheme of model predictive control for VIENNA rectifier

Finite control set model predictive control (FCS-MPC) has been widely used in the control of grid-connected converters with the advantages of fast dynamics, multi-objective control, and easy implement. However, the conventional FCS-MPC bears with variable switching frequency, high current ripple and computational burden. An improved current model predictive with the cost function-based modulation scheme (CFM-MPC) is proposed for a three-phase three-level VIENNA rectifier to improve the power quality. First, the mathematical model and voltage vector are given according to the principle of deadbeat control. Then, the voltage vector of different voltage vectors are selected according to the location of the voltage vector reference, and the switching action time of the selected are directly calculated by the inversely proportional with cost function value of the selected vectors. It remains the merits of both the conventional MPC and space vector pulse width modulation schemes to track the optimum voltage vector without increasing the computational burden. Finally, a comparative study with the proposed CFM-MPC and conventional FCS-MPC has been conducted to verify the superiority of the proposed scheme. The results show the proposed CFM-MPC has the advantages of lower power ripple, fixed switching frequency, lower total harmonic distortion and neutral point potential balance.

Frequency Support Properties of the Synchronous Power Control for Grid-Connected Converters

Grid-connected power converters that integrate frequency support and inertia emulation functionalities have emerged in the last years. These features are promising for renewable energy-based power generation plants, as it permits such systems to contribute to enhance the power system stability. This paper discusses about three different active power control solutions that can be applied to grid-connected converters equipped with a synchronous power controller. For each solution, the detailed analytical relationship between the control parameters and the power loop dynamics is illustrated, and the local stability and dynamics are analyzed. The analysis and validation of frequency support functionalities are particularly addressed in this study, where the results obtained in a 10-kW experiment endorse the analysis; finally permitting to conclude that the synchronous power control offers good performance in terms of inertial response and droop characteristics.

An Implementation of Improved PLL for Control of Grid-Connected Converters Under Grid Perturbations

In this article, a new method is proposed for calculating electric angle using a phase-locked loop (PLL) to perform these calculations correctly under disturbance conditions of the grid voltage. This work is based on using the decoupled double synchronous reference frame PLL (DDSRF-PLL), and this kind of PLL has been expanded to solve its problems in the presence of harmonics and high disturbance in the input voltage. In the DDSRF-PLL, only components with a frequency of double the fundamental frequency will be deleted, but in the expanded PLL all the harmonics will be eliminated. The effect of this method was tested using both simulation and practical experimentation. The simulation of this method was performed in MATLAB software. It has been shown that this method is robust, precise and fast for calculating the electric angle under grid voltage perturbations. Subsequently, the developed PLL was implemented in a FPGA-based experimental setup and its function was studied.

Asymptotically Stable Predictive Control of Grid-Connected Converter Based on Discrete Space Vector Modulation

This paper presents an alternative implementation of the finite control set model predictive control (FCS-MPC) approach for a 3-O, two-level grid-connected power converter. The presented approach combines the concept of discrete space vector modulation into the FCS-MPC structure to improve the input current quality of the power converter. Further, with the help of Lyapunov's stability theory, it is shown that the predictive control law used in the present analysis ensures the stability of the overall system. However, the additional virtual voltage vectors can cause an unacceptable computational burden to the controller. To alleviate this issue, the proposed control approach uses a preselection technique that reduces the number of possible voltage vectors for the optimization process from 38 to 10, thereby allowing the proposed technique to be implemented in a standard control platform. Finally, measurement results are presented from an experimental prototype which demonstrates the efficacy of the proposed algorithm concerning the improvement of the input current quality.

Control Strategy for a Grid Connected Converter in Active Unbalanced Distribution Systems

The development in distributed energy resources technology has led to a significant amount of non-linear power electronics converters to be integrated in the power system. Although this leads to a more sustainable system, it also can have adverse impacts on system stability and energy power quality. More importantly, the majority of the distribution power systems currently are unbalanced (with asymmetrical voltages) due to load unbalance, while the most common fault types are unbalanced grid faults that can have many adverse effects on distributed resource operations. In that regard, proper control of the grid connected converters in active unbalanced distribution systems will become very important. This paper aims to present the behavior of the advanced grid connected converter control technique under different voltage states at the point of common coupling (according to the ABC classification). The main insufficiencies of the classical control technique will be highlighted, while the paper will propose an appropriate solution for mitigation of negative sequence currents under asymmetrical voltages at the point of common coupling. An extensive experimental verification of the proposed techniques is performed using an advanced laboratory prototype for research in grid integration of distributed resources. The experimental verification clearly demonstrates the benefits offered by the advanced control strategy.

Mitigation of Harmonic Voltage Amplification in Offshore Wind Power Plants by Wind Turbines With Embedded Active Filters

In this paper, active filters are embedded in the grid-connected converter control of Type-IV wind turbines to reduce harmonic voltage amplification in offshore wind power plants (WPPs). The extra element theorem is employed to tune the active filters and determine the stability margin. Active filter tunings that satisfactorily reduce harmonic voltage amplification yet provide sufficient stability margin in the augmented system are identified. A running example is performed on a 270 MW offshore WPP comprised of 3-MW wind turbines implemented in real-time simulation as hardware-in-the-loop. Both converter and grid voltage harmonics are suppressed at WPP buses to below the stipulated planning levels.

Generalized Multifrequency Current Controller for Grid-Connected Converters With LCL Filter

This paper presents a grid-side current controller for grid-tied inverters with LCL filter, including harmonic current elimination. The proposed controller only measures the grid current and voltage and it combines excellent dynamic characteristics with good robustness. Contrarily to previously proposed harmonic-current controllers, the presented solution offers a generalized method that gives a consistent (with minimal variation in the reference-tracking dynamics) and stable performance irrespectively of the number of current harmonics to be canceled and of the resonant frequency of the LCL filter (provided that it is lower than the Nyquist frequency). The response to reference commands is completely damped and fast. The response speed is set in accordance with the low-pass characteristic of the LCL filter so as to limit the control effort. Concerning the disturbance rejection, the controller offers an infinite impedance to any disturbances (such as grid voltage harmonics) at a set of arbitrarily specified frequencies. This allows the designer to eliminate all the undesired current harmonics with a simple design process. In addition, the performance of the presented controller is evaluated in terms of a fundamental tradeoff that exists between robustness to variations in the grid impedance and the number of frequency components rejected. Finally, simulation and experimental results that validate the proposal are presented.

Grid-connected converter control during unbalanced grid conditions based on delay signal cancellation

Grid-connected converter control during unbalanced grid conditions based on delay signal cancellation

Stability and Performance Assessment of Single-Phase <inline-formula> <tex-math notation="LaTeX">$T$ </tex-math> </inline-formula>/4 PLLs With Secondary Control Path in Current Sensorless Bridgeless PFCs

Bridgeless power factor correction (PFC) stages and the associated current shaping techniques require grid synchronization to ensure unity displacement power factor. In controllers with no current sensor, line current rebuilding algorithms are especially sensitive to synchronization issues. Phase-locked loops (PLLs) are used to synchronize the control of grid-connected converters, and may include a secondary control path to improve their dynamics in the case of grid disturbances. This paper presents linear models of $T$ /4 PLLs with secondary control paths. The $T$ /4 PLL structures are digitized and their performance evaluated. The stability conditions are determined. The PLL signal is utilized for current sensorless bridgeless PFCs, in the current estimation algorithm to reduce the estimation error, and as the reference in the current controller. The effect of the response of different PLLs on the PFC under grid disturbances is evaluated experimentally.

Enhanced Resonant Current Controller for Grid-Connected Converters With LCL Filter

Conventional resonant controllers (RCs) are commonly used in the current control of grid-tied converters with LCL filter due to their advantages, such as zero steady-state error at both fundamental sequences, easy design process, and straightforward implementation. Nevertheless, these traditional solutions do not permit to place the closed-loop poles of the system in convenient locations when dealing with a fourth-order plant model such as the LCL filter plus the computation delay. Therefore, the reference tracking and the disturbance rejection are deficient in terms of transient behavior and depend on the LCL filter. Furthermore, an additional active damping method usually has to be designed in order to ensure stability. This paper presents an enhanced current RC with stable and fast response, negligible overshoot, good disturbance rejection, and low controller effort for grid-tied converters with LCL filter. The developed solution uses a direct discrete-time pole-placement strategy from the classical control theory (using transfer functions), involving two extra filters, to enhance the performance of the RC. In this manner, the complexity of state-space methods from modern control theory is avoided. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.

A Review on Grid-Connected Converter Control for Short-Circuit Power Provision Under Grid Unbalanced Faults

As an increasing amount of converter-based generation on power electronics is connected to power systems, transmission system operators are revising the grid-connection requirements to streamline the connectivity of the devices to maintain security of supply. Converter-based generation can behave significantly different from the traditional alternators under grid faults. In order to evaluate the potential impact of the future converter-based power systems on protective relays, it is necessary to consider diverse current control strategies of voltage-source converters (VSCs) under unbalanced faults as the performance of converters primarily depends on their control objectives. In this paper, current control strategies of VSCs under unbalanced faults for short-circuit power provision are reviewed in two groups, namely, power-characteristic-oriented and voltage-support-oriented control strategy, respectively. As the fault current provided by converters should be restricted within secure operation limits considering semiconductor capabilities, converter current limit issue is also discussed.