Grid-tied Converters Research Articles

Grid Impedance Estimator for Active Multisource AC Grids

This paper proposes an online grid impedance estimation method for AC Grids under the presence of grid-tied converters. A novel modelling approach in the α,β,0 reference frame is proposed, including the relationship with the instantaneous power theory. The proposed modelling framework considers both balanced and unbalanced conditions and operation in the four quadrants. For the impedance identification, a Pulsed Signal Injection (PSI) approach together with a Recursive Least Square (RLS) algorithm is used. The proposed modelling and identification method is thoroughly evaluated by simulation and experimental results. The work presents a contribution for the optimization of the distribution grid operation with high-dynamic impedance changes due to the integration of distributed resources.

Revisiting frequency coupling effects in grid-tied converter: A phase shifting transformer based circuit modeling framework

Due to asymmetric control algorithms and topology, the frequency coupling effect (FCE) extensively exists in the grid-tied converter (GTC), increasing the order of traditional impedance models and sacrificing their inherent physical insight. The two problems can be solved using the recently reported order reduction techniques. However, FCE is roughly regarded as a coupling phenomenon in the existing studies, which is not rigorous because a GTC system will present various degrees of FCE in different situations. Therefore, this paper revisits the FCE from the viewpoint of the complex circuit and categorizes the GTC systems into three types for analysis, i.e., strongly, weakly coupled, and decoupled systems. Firstly, the plausibility of using the complex-valued vectors to depict FCE is illustrated, and thus FCE is clarified as resulting from the rotation of complex-valued vectors and the admittance mismatching. Based on this new clarification, the phase-shifting transformer-based circuit modeling framework is proposed. Then GTC system can be intuitively presented by the proposed circuit model, whose mathematical model is still order-reduced. More importantly, coupling paths of strongly, weakly coupled, and decoupled systems can be flexibly modeled by adjusting the ratio of the phase-shifting transformer. Finally, simulations and experiments prove the analysis and proposed modeling framework.

Grid Current Control for Active-Front-End Electric Propulsion Systems in AC Ship Microgrids

This article proposes a finite control set (FCS) current controller for grid-tied converters, specifically tailored to meet the characteristics of ac ship microgrids in all-electric ships (AESs). Contrary to a pulsewidth modulator (PWM)-based design, which often employs an LCL filter, the proposal is able to meet applicable harmonic regulations using an L filter with the same total inductance as the LCL filter and the same switching frequency required by PWM-based solutions. Moreover, the proposed design provides a low sensitivity in a wide frequency range from dc to the switching frequency of the power converter. This low sensitivity permits to quickly attenuate low-order disturbances and it is particularly convenient in marine applications with a weak grid, where the frequent connection and disconnection of high power nonlinear loads can significantly affect the voltage waveform quality.

Reduced-Order Modeling and Transient Synchronization Stability Analysis of Multiple Heterogeneous Grid-Tied Inverters

This article outlines a generalized reduced-order modeling approach for assessing the transient synchronization stability of multiple grid-tied converter systems operating in grid-feeding mode and having heterogeneous characteristics. The analytical approach utilizes the nonlinear quasi-static dynamics of the phase-locked loop (PLL) for obtaining the reduced-order models (ROM) of multiple heterogeneous grid-feeding converters having same and different point of synchronization (POS). This is a significant contribution in contrast to the prior effort where reduced-order modeling has been carried out only for homogeneous grid-tied multi-converter systems. In addition to this, an innovative methodology has been proposed in this article for analyzing the transient synchronization stability of the multi-converter systems leveraging the Lyapunov's direct method. The Lyapunov functions are constructively synthesized by invoking an assumption that the reduced-order models are conservative in nature. Following this, the well-known Lyapunov's second theorem is applied for deriving the sufficient conditions required for obtaining the transient stability boundary of the heterogeneous grid-feeding converter systems. Numerical simulations are performed in MATLAB 2017a platform to demonstrate that the ROM are highly accurate and computationally efficient. Besides, the efficacy of the proposed methodology for analyzing the transient synchronization stability of heterogeneous grid-feeding multi-converter systems is verified numerically.

Synchronization stability of power-grid-tied converters.

Synchronization stability is one of central problems in power systems, and it is becoming much more complicated with the high penetration of renewable energy and power electronics devices. In this paper, we review recent work by several nonlinear models for renewable-dominated power systems in terms of multiple timescales, in particular, grid-tied converters within the DC voltage timescale. For the simplest model, a second-order differential equations called the generalized swing equation by considering only the phase-locked loop (PLL) is obtained, which shows a similar form with the well-known swing equation for a synchronous generator in the traditional power systems. With more outer controllers included, fourth-order and fifth-order models can be obtained. The fourth-order model is called the extended generalized swing equation, exhibiting the combined function of grid synchronization and active power balance on the DC capacitor. In addition, a nonlinear model for a two coupled converter system is given. Based on these studies, we find that the PLL plays a key role in synchronization stability. In summary, the value of this paper is to clarify the key concept of the synchronization stability in renewable-dominated power systems based on different nonlinear models, which still lacks systematic studies and is controversial in the field of electrical power engineering. Meanwhile, it clearly uncovers that the synchronization stability of converters has its root in the phase synchronization concept in nonlinear sciences.

Spectrum Estimation of Input Current Ripple on a Wide Class of Multilevel Grid-Tied Converters

Multilevel (ML) converters are frequently used to implement grid-tied ac–dc conversion systems. Their design may benefit from multiobjective optimization techniques, which typically involves time-consuming circuit simulations in order to obtain input current estimations suitable for input inductor and electromagnetic interference filter design. Herein, a closed-form expression of the input current ripple is derived to ease harmonic content estimations. The proposed approach separates the fundamental grid-current component from its ripple and models the latter like an amplitude modulation, where the modulating signal is its envelope and the carrier is the triangular current waveform. First, a general waveform analysis of ML converters is performed to derive the voltage across the grid-side inductor, then the associated current ripple is modeled. Experimental results on an ML converter prototype are reported to validate the analytical results.

State-Space Modeling and Control of Grid-Tied Power Converters With Capacitive/Battery Energy Storage and Grid-Supportive Services

Advances in the fields of renewable generation, electric vehicles, and energy storage systems push forward the research on ac–dc and dc–ac grid-tied power converters. However, the variabilities of power converters create new challenges in modeling and control. Existing state-space models fail to accurately describe various types of grid-tied converters (GTCs), particularly those with grid-supportive services, which are increasingly required by upcoming grid codes. As such, this article first proposes to classify GTCs into four basic types according to their ac and dc characteristics. Subsequently, corresponding detailed state-space models of GTCs are introduced, which serve as a useful tool for stability analysis. On top of that, this article further proposes control implementations of grid-supportive services related to active and reactive power control, including droop, inertia, and oscillation damping. Finally, simulation and experimental results demonstrate the effectiveness of the proposed models and grid-supportive services.

Topological Overview of Auxiliary Source Circuits for Grid-Tied Converters

This paper reviews different types of capacitors and auxiliary source circuit topologies and presents an introduction to control strategies used for circuit applications reducing DC-BUS capacitance. The paper argues in favor of replacing bulky electrolytic capacitors in capacitor-supported power electronic systems with auxiliary source circuits. DC-BUS capacitors are widely used in grid-tied power converters (rectifiers) and utilized for power balance, voltage ripple limitation, and short-term energy storage. The electrolyte capacitor is the Achilles heel of any rectifier and power converter due to its higher rate of failure than other circuitry components. Auxiliary source circuits are key components to qualitatively improve the reliability of the DC links, where they divert the instantaneous pulsating power into extra reliable storage components. Unlike previous work, this review serves to provide a clear picture of an auxiliary source circuit design, in favor of optimal solution selection according to the specific application. Therefore, energy storage components (capacitors), topologies, and control strategies of auxiliary source circuits are comprehensively reviewed in this paper. Additionally, detailed explanations, comparisons, and discussions of auxiliary source circuits are offered.

Assessment of the protection system in response to different grid-tied converters

The power system is migrating to 100% penetration of the converter-interfaced generation (CIG). The grid-forming converter-based CIG has been proposed to be a solution to replace the synchronous generation (SG) in order to form the grid and achieve the 100% penetration target. However, besides the generation, whether the protection system can coordinate this transition is still an open question. Although the grid-forming converter operates as the same manner as the SG, unlike SG, it physically has constraints on its current output which in transient leads to different operation to the SG. This paper aims to investigate whether these different transients have effects on the protection system, specifically, investigate if the protection system remains reliable when the system migrates to 100% penetration. The investigation is based on the hardware in-the-loop experiment, considering different converter control, different type of the fault, different length of the line and different fault location. The result shows that the grid-forming converter has a reliable coordination with the protection system in response to the fault.

Design of a Repetitive Control for a Three-Phase Grid-Tied Converter under Distorted Grid Voltage Conditions

The paper presents a design of repetitive control (RC) in the current control system of a three-phase grid-tied converter. The goal of the control system is to provide sinusoidal input filter currents under the conditions of distorted and asymmetrical grid voltage. A novel design of the RC is presented, in which the repetitive part is not excited by sharp and non-periodic changes of the reference signal, but it enables high-quality performance under periodic disturbance conditions. In the proposed system. RC cooperates with a discrete state feedback controller. An innovative approach to tuning is proposed in which parameters of the repetitive, as well as the state feedback controller, are selected as a result of the optimization process with the use of a particle swarm algorithm. The proposed control system is verified experimentally on a laboratory test bench. The achieved results confirm the high-quality system performance.

Modified Passivity-Based Control for LCL-Filtered Grid-Tied Inverter with Output Admittance Reshaping

As a nonlinear control method, the Euler–Lagrange- (EL-) based passivity-based control (PBC) has been studied for grid-tied converters based on energy function to achieve better performance. However, the EL-based PBC method is dependent on an accurate mathematical model. In the traditional EL-based PBC research for LCL-filtered grid-tied inverter, the effect of the digital control delay is rarely considered and the stability under the grid impedance uncertainties is not discussed, especially in the capacitive grid or complex weak grid. To address these concerns, this study proposes a modified EL-based PBC method based on the output admittance reshaping for LCL-filtered grid-tied inverters. The system’ passive region is expanded by adding capacitor current feedback loop up to the Nyquist frequency. The potential resonance is thus eliminated irrespective of the grid impedance. Additionally, the stable region and control parameters design methods of the modified EL-based PBC method with inverter-side control are also carried out. To verify the correctness of the theoretical analysis, both simulation and experimental results are investigated from a 3 kW grid-tied inverter prototype.

Performance-Based Tuning for a Model Predictive Direct Power Control in a Grid-Tied Converter With L-Filter

Model predictive control (MPC) is a powerful and widely used technique to address the control challenges in power converters as the grid interface for renewable energy systems. This technique combines closed-loop control with error and control effort minimization; however, its design is challenging, and we know little about how the controller parameters affect the closed-loop performance of grid-connected voltage source converters. In this study, we applied an MPC direct power control with modulation for a grid-connected power converter with an inductive filter. For the controller design, we proposed an initial set based on the power converter’s nominal setup. Then, we define the range of settings to guarantee stability by analyzing the closed-loop poles of the system. The fine-tuning to improve the performance can be identified visually using the performance maps built from simulations of the control system, simultaneously sweeping the time horizons of the predictive model and the weight factors of the cost function. Experimental results on a low-power bench demonstrate the excellent performance of the designed controller, following and even outperforming the classical proportional-integral (PI) controller and other advanced control techniques.

Space Vector Modulated Model Predictive Control for Grid-Tied Converters

This article proposes a space vector modulated model predictive control (SVM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> PC) for voltage source converters. It overcomes limitations of standard finite control set model predictive controller, ensuring fast dynamic response, fixed switching frequency, and low computational burden. It combines control and modulation in a convex optimization problem with affine inequality constraints to minimize the tracking errors of the output variables. First, the sector of the space vector diagram where the unconstrained solution lies is identified. Then, using the Karush–Kuhn–Tucker conditions, feasible duty cycles are computed for both linear and overmodulation regions. The proposed SVM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> PC is extended to multilevel converters. Hardware in the loop results are presented for three different case studies: 1) Grid-forming inverter with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter; 2) two-level grid-following inverter with LCL filter; and 3) grid-following neutral point clamped inverter with LCL filter. Experimental results demonstrate that the proposed SVM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> PC provides good performance, fast response, and low computational burden when compared with previously reported alternatives.

Improved Model Predictive Control for Single-Phase Grid-Tied Inverter With Virtual Vectors in the Compacted Solution-Space

In this letter, an improved model predictive control (MPC) has been proposed for single-phase grid-tied converter with virtual vectors in a reshaped and compacted solution space, while improving ac power quality, without increase of the virtual vectors number. Combining linear solution space reconstruction method, the global optimization solution space can be shrunken and reshaped in real-time, only relying on grid angle information. Therefore, MPC can select the optimal virtual-vector in a finer and closer solution subspace, then, much finer control effect can be obtained. Implemented by finite-control set MPC with an integrated difference modulator, equivalent fixed double switching frequency at the output can be achieved. Experimental results validate the effectiveness of the proposed methods.

DC-Link Voltage Stability Analysis of Grid-Tied Converters Using DC Impedance Models

With the integration of renewable energy sources into the power grid, a number of power electronic converters need to be connected together in parallel. Due to this interconnection among the power converters with a common DC bus, the equivalent impedance of the DC network, i.e., DC network impedance (DCNI) of these parallel converters, may vary and can cause oscillations in the DC link voltage (DCLV). In the literature, impedance models of grid-tied converters (GCs) based on the AC side are well reported without including these variations in DCNI. In addition, the dynamics of a phase-locked loop (PLL) play a significant role in GC system stability. To evaluate these stability issues, this paper proposes small signal impedance models viewing from the DC side of a three-phase GC operating under different control modes considering the PLL dynamics and the DCNI variations. Using the proposed DC impedance models (DCIM), DCLV stability analysis is evaluated for a GC. It is verified through bode plots that the interaction between the proposed DCIM and DCNI leads to unstable operation of the closed-loop converter near the PLL bandwidth when the phase difference between DCIM and DCNI is more than 180 degrees. Finally, the analytically developed models are validated using hardware in-the-loop (HIL) testing.

A Direct Discrete-Time Output Feedback-based LS-RMRAC for Grid-tied Current Control Loop of a Static 3-Wire Converter under Unbalanced Grid Voltage Conditions

Nowadays, renewable energy generation is a worldwide tendency. These clean sources are generally tied to the grid by voltage-fed static converters. In this work, a direct discrete-time Output Feedback-based LS (Least Squares)-RMRAC (Robust Model Reference Adaptive Control) is proposed for grid-side current loop control of a static 3-wire converter connected to the grid through LCL filter. The proposed control method mitigates the LCL filter resonance,rejects grid disturbances without needing Resonant Controllers implementation, and deals properly with grid uncertainties and parametric variations, being suitable for grids with unbalanced voltage conditions. Simulation results of proposed controller are presented. Keywords: Robust Adaptive Control. Grid-tied converter. LCL filter.

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.

Control and Optimization Algorithms for Lattice Power Grids With Multiple Grid-Forming Converters

The growth of renewable energies, together with their power converter interfaces, reshapes power systems into more-electronics power systems. Along with this paradigm shift is the requirement of grid formation by grid-tied power converters. However, existing grid architectures only allow parallel operation of grid-forming converters, which excludes high-voltage applications. This article proposes novel lattice power grids that combine the advantages of multilevel converters and power grids, thereby allowing both serial and parallel connectivity with modularity and scalability. Further, we propose control and optimization algorithms for lattice power grids by use of graph theory. In particular, we investigate H-bridge-based lattice power grids and achieve several objectives, including desired voltages and currents between any two selective nodes in lattice power grids as well as efficiency optimization by minimizing switching actions. To achieve these objectives, this article details control and optimization methodology for square lattice power grids. Finally, the proposed algorithms and lattice power grids are validated via simulation results.

IoT Communication for Grid-Tie Matrix Converter with Power Factor Control Using the Adaptive Fuzzy Sliding (AFS) Method

In this work, we propose and encourage the sending of an IoT-based domestic systems conditions small scale programmable framework for grid-tie matrix converter that can efficiently be executed with low-control equipment plan with quick execution capacity. The proposed framework for matrix grid-tie converter has been performed based on an adaptive fuzzy sliding technique and furthermore monitors the grid status through an IoT server. The adaptive fuzzy sliding method was connected to the framework, and a unity power factor was fulfilled alongside the wanted sinusoidal input and output results. The proposed AFS-based matrix converter has been validated through simulation using MATLAB software. The control clock signals are generated for the positive and negative clock pulses of the sequence. Here the sinusoidal reference signal is compared with the switching frequency of the triangular carrier frequency. The hardware results were also verified to validate the simulation. When diverged from other standard techniques, the proposed AFS technique has achieved 97.23% efficiency in full load conditions.

Droop Control-Based Grid Forming Inverters with an Auxiliary Grid Support Control for Frequency Regulation in a Low Voltage Microgrid

Aim: Improving the frequency regulation in a Low Voltage (LV) microgrid during grid mode transition using Grid Forming Grid Support (GFGS) converters with inertia emulation. Background: The ever-increasing demand for energy and the depleting fossil fuels has increased the proliferation of converter-fed Renewable Sources (RES) in the conventional grid. The reduced system inertia results in frequency regulation issues which are further aggravated by the unpredictable nature of RES and dynamic switching of loads. This scenario results in blackouts, unwanted load shedding, power quality issues, and discrepancies in protection schemes. Objective: A control scheme is presented on the principle of swing equation-based synchronization to regulate the active power output of a grid-tied voltage-source converter operating in a grid forming grid supporting mode in a low voltage PV-based microgrid. The Virtual Synchronous Machine control (VSM) is implemented for controlling the active power output, which follows the grid frequency deviations, ensuring the safe State of Charge (SoC) limits of the Battery Energy Storage System (BESS). Methods: The stability issues due to the absence of inertia in an inverter-fed LV microgrid are staved off by giving auxiliary grid support by implementing VSM control in a BESS inverter in the grid forming mode. The LV microgrid model with all the converters in the GFGS converter is developed in the MATLAB/Simulink environment. Particle Swarm Optimization (PSO) is used to fine- tune the control parameters. The effectiveness of the control strategy has been investigated for unintentional islanding events. Results: The droop control-based GFGS grid-tied converters with power balance equation-based VSM control gave frequency regulation within the range stipulated by the grid codes during the islanding events. Conclusion: The work demonstrates the effectiveness of VSM control in providing the auxiliary grid support for droop-based grid forming converters in an LV microgrid.