Grid-feeding Converters Research Articles

Converter-based multi-frequency power transfer system for efficient energy packet transmission in the energy internet

The energy internet concept involves the transmission of energy in discrete packets, akin to the information internet's principle. The energy internet must adopt a decentralized configuration and enable bidirectional and simultaneous power transfer. This raises the problem of congestions during the energy packet dispatching process. To address potential congestion issues, we propose a three-phase multi-frequency power transfer system (MFPTS) where the positive-sequence fundamental frequency component is supplemented with the zero-sequence third and the negative-sequence fifth harmonic components. By using such signals with proper amplitudes, the harmonics are added to the voltage without changing the peak amplitude but increasing the rms value. To exert complete control over the different frequencies employed within the MFPTS, a decoupling method is necessary to separate these frequencies in the control system. This leads to creating three independent channels of power transfer by three frequencies without changing the existing infrastructure as these powers can be transferred within the same lines. This paper explores the feasibility of three-phase three-frequency power transfer by using three parallel power transfer channels, achieved through the utilization of three-phase grid-forming and grid-feeding converters. Using power converters provides the advantage of bidirectional power transfer and facilitates decentralization by serving as an interface between distributed power generation systems and the utility grid. These properties are essential for achieving optimal energy internet performance. The simulation and practical results prove that the proposed MFPTS could solve the energy packet dispatching congestion problem to have a reliable energy internet for the future.

Convolutional Neural Network With Reinforcement Learning for Trajectories Boundedness of Fault Ride-Through Transients of Grid-Feeding Converters in Microgrids

Convolutional Neural Network With Reinforcement Learning for Trajectories Boundedness of Fault Ride-Through Transients of Grid-Feeding Converters in Microgrids

Modelling and design of grid voltage oriented vector control scheme for DC railway recuperating system

The braking energy harvested by a railway vehicle can be restored to the utility grid with a power recuperating system. A grid connected voltage source inverter (VSI) is commonly used as a grid-feeding converter in the recuperating system. This paper proposes to integrate grid voltage oriented vector control (GVOVC) and third harmonic voltage injection pulse width modulation (THVI-PWM) technique for the VSI to ensure grid voltage and frequency synchronization. A simulation study is carried out to evaluate the feasibility of the proposed control and modulation schemes using MATLAB/Simulink. The results show that the proposed controller may reach steady-state operating mode within 7 ms by producing good quality AC voltages and currents waveforms. With the independent control of voltage quantities in dq reference frame, the regulation of active and reactive power could be realized.

A Detailed Assessment of the Power Quality Improvement of an Islanded AC Microgrid through Upgrading Conventional Grid-Feeding Current-Controlled Converters to Operate as Multifunctional Converters

The decarbonization of the global economy is one of society’s urgent objectives nowadays. Thus, the large-scale adoption of renewable energy sources, like solar and wind energy, seems to be the best pathway to achieving sustainable development. Thus, the microgrid concept has emerged as a solution to address concerns related to the intermittent nature and lack of inertia for such sources. Several studies involving microgrids have been published in the last few years. However, in most of them, power quality (PQ) disturbances and strategies to manage such disturbances are not considered, especially in the islanded operation mode. Hence, in this paper, a detailed assessment of the PQ of an islanded AC microgrid is conducted, considering the coexistence of a nonlinear load, voltage-controlled converters, and a current-controlled converter (CCC). The analyses consider two scenarios, depending on the operation mode of the CCC: (1) the CCC operating as a conventional grid-feeding converter and (2) considering a cost-effective upgrading of the conventional CCC to operate as a multifunctional converter. An experimental test bed is built to validate both scenarios. The presented results provide strong evidence that the AFF significantly improves the microgrid PQ and its suitability for dealing with PQ disturbances in real-world applications.

Modified Series Chain Link MMC for Offshore Wind Farms With Boosted AC Voltage: Frequency-Domain Modeling and Submodule Capacitor Voltage Ripple Optimization

Series Chain Link (SCL) converter, a derivative of Modular Multilevel Converter (MMC), offers a very pragmatic grid-feeding converter technology to form HVDC link for offshore wind farms. However, the SCL-MMC incurs high SubModule (SM) Capacitor Voltage Ripple (CVR), demands high current rating switches and needs High Turns-ratio Transformer (HTT). To address these issues, in this paper, bipolar voltage generator-SMs are integrated into the SCL-MMC to realize negative arm-voltage generation capability. The Modified-SCL MMC (MSCL) in conjunction with a modified-switching function utilizes negative arm-voltage to facilitate the boosted AC voltage and improves CVR distribution among SMs. A newly introduced variable &#x2018;<i>g</i>&#x2019;, representing AC voltage boosting, determines the AC voltage gain and HTT requirements. The boost enabled MSCL-MMC, considering its internal dynamics, is modeled using Harmonic State Space (HSS) approach. The developed HSS small-signal impedance model is capable of accurately predicting the resonance peak and frequency. The HSS large-signal model, on the other hand, predicts the presence of 3^rd Harmonic Circulating Current (3-HCC) in the DC-link, accurately, based on which a suitable suppression scheme is proposed. Hence, the proposed complete solution relieves the switch current rating upto 30&#x0025;. Simulation and experimental results are included to validate the theoretical claims.

On the stability of distributed secondary control for DC microgrids with grid-forming and grid-feeding converters

This paper investigates the stability problem of a single-bus DC microgrid with mixed grid-forming/grid-feeding converters and the constant impedance, constant current, and constant power (ZIP) loads. Considering the different output properties of grid-forming and grid-feeding converters, distributed secondary controllers with mixed V–I droop and I–V droop controllers are designed to achieve accurate bidirectional current sharing and voltage regulation. As few converters have access to the information of the common bus, an observer is designed for each local controller to estimate the bus voltage. Then, the observer state is used to generate voltage correction term in the feedback loop, which gets potential faster regulation compared with the existing approaches that use pinning gains. Based on the rank-one perturbation analysis, a stability criterion of DC microgrid is given for the case of ZI load, where global voltage regulation and current sharing are guaranteed. Then, the stability criterion is extended to the case of ZIP load, where local stability of the DC microgrid is guaranteed. Both numerical examples and experimental tests are given to verify the effectiveness of the theoretical results.

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.

Impact of Frequency Anti-windup Limiter on Synchronization Stability of Grid Feeding Converter

Impact of Frequency Anti-windup Limiter on Synchronization Stability of Grid Feeding Converter

Proposal of Grid-Forming and Grid-Feeding Converter Control for Transition of AC Microgrid Operating Modes

Proposal of Grid-Forming and Grid-Feeding Converter Control for Transition of AC Microgrid Operating Modes

Advanced Grid Synchronization Scheme Based on Dual eSOGI-FLL for Grid-Feeding Converters

In this article, an improved grid synchronization method for three-phase grid-feeding converters using a dual enhanced second-order generalized integrator (eSOGI) frequency-locked loop (FLL) is proposed. The eSOGI, of which gains can be adjusted based on the positive-sequence component of the grid voltage, is designed to estimate the grid voltage accurately. Moreover, to improve the grid frequency estimation under grid voltage sags, frequency rate limits is suggested in the FLL structure. The selection of the eSOGI-FLL parameters is described as well. After the estimated grid voltages are obtained using the dual eSOGI-FLL, they are used to synthesize the references in the dual-current controllers for positive- and negative-sequence components. Through the simulation and experimental results under grid voltage sags and distorted grid conditions, the improved power control performance of the proposed method has been demonstrated in comparison with those of the existing methods.

Impact of PLL Frequency Limiter on Synchronization Stability of Grid Feeding Converter

It is well known that grid-feeding converters that synchronize to the grid through a Phase-Locked Loop (PLL) can become unstable after a fault. An often-neglected element that plays an important role in the converter synchronization stability is the PLL frequency limiter. While it slows down the phase change during the fault, the frequency limiter also constrains the error of the PLL input, thus leading to a longer settling time. This letter investigates the mechanism of the converter synchronization stability caused by the frequency limiter and provides a taxonomy to evaluate its impact on the overall system dynamic response.

DC-Link Voltage Control Aided for the Inertial Support During Severe Faults in Weak Grids

Keeping the synchronization of a grid feeding converter (GFC) with a weak grid during deep voltage sags has been introduced as a serious challenge in converter-interfaced renewable energy source-dominated weak grids. To deal with this challenge, a simple yet effective solution based on the virtual inertia concept is proposed in this article. This method is realized by adding a correction term to the dc-link voltage controller, which adjusts the active and reactive current set points and enables the converter to remain synchronized to the grid during severe faults. Closed-loop dynamics of the system in the presence of the parametric uncertainty of the grid-side impedance has been studied, in both normal and fault conditions with different voltage drops. Along these, system performance has been investigated, and in comparison with previous methods, it is revealed that the direct inertial support gain may possibly cause instability and do not propose a stable synchronization process to the GFC under deep faults. The performance of the proposed method has been verified by real-time laboratory results for different resistive/inductive weak grids with various levels of voltage sags. Real-time verification demonstrates the effectiveness of the proposed control in stabilizing GFCs for inertia emulation and its role in a better synchrony process.

Impact of Current Transients on the Synchronization Stability Assessment of Grid-Feeding Converters

The synchronization instability in the presence of a fault is a main issue for the dynamic behavior and control of grid-feeding converters. In the literature, the synchronization stability assessment is carried out considering the dynamics of Phase-Locked Loops (PLL) but the transients of converter currents are neglected. The letter shows that such a simplification leads to inaccuracies and, thus, the current transients cannot be neglected. The letter proposes a model that captures the effect of such current transients on the converter synchronization. This model allows assessing the transient behavior and, hence, the stability, of power electronics converters with high accuracy, comparable, in fact, to EMT models. The fidelity of the proposed model is duly discussed in the case study.

Enabling Grid-Feeding Converters With a Dissonant-Resonant Controller for Negative-Sequence Voltage Elimination

The mitigation of the adverse effects of voltage unbalance in equipment and power quality can be performed by the power electronic converters that interface distributed generators to the grid. Inspired in a resonant controller, this article presents a dissonant-resonant controller for negative-sequence voltage elimination for a grid-feeding converter connected to the grid. The controller eliminates the negative-sequence voltage at the converter output with a regulable precision, it does not require knowing the grid impedance for successful operation, and it can be a good candidate for parallel operation because it operates not like an integrator, but like an “untuned” integrator. Using the stationary $\alpha \beta$ frame, a closed-loop model is developed in a complex space vector built from the complexification of the stationary components. This allows extracting stability conditions for safe closed-loop operation as well as deriving design guidelines for the controller parameters. Numerical and experimental results show the ability of the proposed controller to meet its design goals, thus, corroborating the theoretical approach.

Assessment of Grid-Feeding Converter Voltage Stability

This letter applies voltage stability analysis to grid feeding converters in the presence of the converter stability versus the grid state and its operation. By applying this analysis, it is shown that the converter may become unstable if the converter reference power or current exceeds the line capacity. This letter proposes to use a conventional PV curve to determine the stability of the dynamic response of grid-feeding converters considering both power and current limits.

Adaptive Slope Voltage Control for Distributed Generation Inverters With Improved Transient Performance

Reactive power injection in distributed generation inverters is an useful ancillary service for grid supporting purposes. For grid-feeding converters, the slope control method is the most common voltage regulation strategy used in local (communication-less) applications. Despite its simplicity, this method offers limited dynamic properties in scenarios with changing operation conditions. In this sense, this paper presents an adaptive slope voltage control that provides an improved transient performance against operating variations. To derive the control configuration, a control-oriented mathematical model is developed. The accuracy of the modeling and the performance of the proposed control are validated by selected experimental results.

Low-voltage Ride-through Methods for Grid-connected Photovoltaic Systems in Microgrids: A Review and Future Prospect

&lt;span lang="EN-US"&gt;Power quality is a concern for utility and grid operators due to a large penetration of intermittent and stochastic renewable power generation sources. One of the major concerns, when designing and controlling grid-feeding photovoltaic (PV) inverters is meeting the grid requirements. International grid requirements demand low-voltage ride-through (LVRT) capability and maintaining grid functionality during fault conditions. This paper presents a comprehensive review for several control techniques to assure the LVRT capability of grid-feeding converters as well as discussing their respective advantages and limitations in detail. Areas for further research are identified afterwards. Finally, the conclusion gives a brief summary and critique of the findings.&lt;/span&gt;

Flexible Grid Connection and Islanding of SPC-Based PV Power Converters

At the present time, distributed generation systems are required to disconnect from the main grid when there is an outage. In order to fulfill this requirement, photovoltaic (PV) power plants are equipped with anti-islanding algorithms, embedded in the converters controller, to avoid the island operation. However, the current trends in the development of the future electrical networks evidence that it is technically feasible and economically advantageous to keep feeding islanded systems under these situations, without cutting the power supply to the loads connected to the network. Nevertheless, commercial PV power converters are programmed as grid-feeding converters and they are unable to work in island mode if there is not an agent forming the grid. In order to overcome this problem, the synchronous power controller (SPC) is presented in this paper as a suitable alternative for controlling PV inverters. As will be further discussed, this controller permits PV plants to operate seamlessly in grid-connected and island mode, with no need of changing the control structure in either case. Moreover, the participation of SPC-based power converters integrating energy storage enables other grid-feeding systems to contribute to the grid operation in island conditions. The good results achieved with the SPC in different conditions will be shown in simulations, and also with experiments considering a real PV power plant combining SPC and commercial PV inverters.

Enhanced Control for Improving the Operation of Grid-Connected Power Converters under Faulty and Saturated Conditions

In renewable energy based systems Grid-Connected Voltage Source Converters (GC-VSC) are used in many applications as grid-feeding converters, which transfer the power coming from the renewable energy sources to the grid. In some cases, the operation of GC-VSC may become unstable or uncontrollable due to, among others: a grid fault or an inappropriate current-power reference, that give rise to fast electrical transients or a saturation of the controller. In this paper, an improved control scheme is proposed to enhance the controllability of GC-VSC in all these situations. This solution consists of two parts, on the one hand a new Proportional-Resonant (PR) controller with anti-windup capability to be used as current controller, and secondly a new current/power reference modifier, which defines the suitable reactive current/power reference to keep the system stable. It is worth to mention that the proposed scheme does not need information about the grid parameters as it only uses the converter current, and the voltage at the capacitors of Inductor-Capacitor (LC) output filter.

H2/H∞ control for grid-feeding converter considering system uncertainty

ABSTRACTThree-phase grid-feeding converters are key components to integrate distributed generation and renewable power sources to the power utility. Conventionally, proportional integral and proportional resonant-based control strategies are applied to control the output power or current of a GFC. But, those control strategies have poor transient performance and are not robust against uncertainties and volatilities in the system. This paper proposes a H2/H∞-based control strategy, which can mitigate the above restrictions. The uncertainty and disturbance are included to formulate the GFC system state-space model, making it more accurate to reflect the practical system conditions. The paper uses a convex optimisation method to design the H2/H∞-based optimal controller. Instead of using a guess-and-check method, the paper uses particle swarm optimisation to search a H2/H∞ optimal controller. Several case studies implemented by both simulation and experiment can verify the superiority of the proposed control strategy than the traditional PI control methods especially under dynamic and variable system conditions.