Weak Grid Research Articles

A Comprehensive Review: Mitigating Techniques for Power Variability Due to Integration of Renewable Energy Sources With Grid

Renewable energy sources like solar panels and wind turbines are becoming more important for generating power. The inconsistent and unreliable output of solar panels and wind turbines is due to factors like clouds, weather conditions, and wind speed that cause their energy production to fluctuate. Connecting intermittent sources to the utility grid creates difficulties in different technical areas, such as ensuring a steady and reliable power supply, protecting the system from disruptions, controlling the distribution of generated power, and maintaining a consistent level of power quality. In this situation, adjusting the amount of electricity produced by an intermittent energy source to keep the power grid stable is essential. This issue must be resolved as soon as possible. The extension to these locations diminishes the grid's strength. It leads to a scenario where a solar power plant is integrated into a subpar electric grid. However, integrating solar power into a shoddy AC infrastructure has problems with power quality (PQ) problems, which restricts penetration levels. This review article suggests various FACTS devices and various conventional, adaptive, and AI-based algorithms to reduce PQ issues brought on by a weak grid and increase renewable energy source (RES) penetration levels. This paper discusses the PQ issues that occur when RES like solar power plants (SPP) and wind power plants (WPP) or both as hybrid are connected to the electrical grid. It also explores different ways to reduce the fluctuation in power output from the solar panels. It analyzes the numerous power quality issues that arise with solar penetration and PQ mitigation methods using several FACTS devices and control algorithms, including traditional control, adaptive control, and AI-based control algorithms. For the benefit of engineers and academics working in this field of study, various research publications have been carefully evaluated, organized, and placed on this paper for convenient reference. It also briefly discusses the plan for controlling battery storage system for solve this problem.

Advanced adaptive algorithm controlled single‐phase DSTATCOM operation during weak grid conditions

SummaryThis paper presents the performance of a single‐phase utility‐tied distribution static compensator (DSTATCOM) with nonlinear load operating under weak grid conditions. The DSTATCOM is controlled by a complex kernel risk‐sensitive p‐power (CKRSP) algorithm. Ever‐increasing nonlinearity of the load, rapidly increasing renewable energy system installed capacity, and rising heating loads, especially during the winter season, lead to various power quality issues that worsen during weak grid conditions. An advanced adaptive control scheme, that is, CKRSP, enforces the system to perform various multifaceted operations, including harmonics suppression, handling rapid load variations, voltage sag and swell, and active and reactive power control at the point of common coupling (PCC) during weak grid conditions. The proposed CKRSP algorithm outperforms conventional control algorithms regarding steady‐state error, convergence speed, and accuracy. During weak grid conditions, the proposed system ensures stable DC bus voltage with a reduced second harmonic component. This further assists the CKRSP control in better extracting the fundamental load current component using a smoother loss component of current . The proposed system can handle various power quality issues during weak grid conditions while ensuring the overall system's stability. The laboratory prototype results show that the presented system performs satisfactorily under weak grid conditions per IEEE 519‐2014 standards.

Power Quality Enhancement of Remote Gas Field Generations with Smart Power Converters

Direct power generation near gas fields offers numerous benefits, including optimized economic efficiency and reduced environmental impact. Moreover, building on-site greenhouses emerges as a promising approach to further minimize carbon emissions and residual heat, greatly promoting resource utilization. However, such power plants generally have access to a weak grid due to their remote locations, and they also contain nonlinear local loads, such as the grow lights in the greenhouses. Consequently, the generation system is susceptible to power quality issues, manifested in overvoltage and harmonics. To address these issues, a smart back-to-back converter is employed to interconnect the gas turbine generator and the utility grid. This smart converter not only enhances power quality but also offers potential ancillary services that contribute to the dynamics of the gas generation system, such as damping low-frequency oscillation among parallel-connected generators. In this paper, three control configurations for the back-to-back converter are developed, enabling the coordinated regulation of exported active power, AC voltage, and DC-link voltage in either a grid-following or grid-forming manner. Furthermore, comparative studies are conducted to provide guidelines for selecting an appropriate control strategy that ensures stable operation under various short circuit ratios. A practical gas cogeneration system is chosen to evaluate the performance of the back-to-back converter, and real-time simulations based on RT-LAB are carried out to validate the effectiveness of the methodology.

Suppression Strategy of Subsynchronous Oscillation Based on Dynamic Voltage Compensation Control for Grid-Forming D-PMSG in Weak Grid

The virtual impedance can suppress the low-frequency oscillation effectively caused by the power coupling effect for the grid-forming direct-drive permanent magnet synchronous generator (D-PMSG). However, there are few studies on the influence of virtual impedance on the subsynchronous oscillation caused by the interaction between the grid-forming D-PMSG and the weak grid. In this paper, the mechanism of virtual impedance improving the stability of virtual synchronous generator (VSG) control for the converter is revealed from the perspective of power coupling, and the influence of virtual impedance on the system stability in subsynchronous frequency range was analysed based on the sequence impedance model. The analysis results show that the virtual inductance cannot effectively improve the oscillation suppression effect, but the virtual resistance can greatly improve the damping characteristics of the system which has a good oscillation suppression effect. However, excessive increase in virtual resistance will cause power coupling oscillation, even subsynchronous oscillation, which shows limitations in subsynchronous oscillation suppression. To solve the problem, a dynamic voltage compensation control method based on active disturbance rejection control (ADRC) was proposed, and the parameters are designed by bandwidth method. The method can suppress the subsynchronous oscillation effectively in a certain frequency range under different grid strength and disturbance conditions. And it shows strong adaptability and good system robustness in weak grid. Finally, the effectiveness of the proposed oscillation suppression strategy was verified by time domain simulation.

Analysis of Ocean Energy Integration in Ibero-American Electric Grids

The development of the marine renewable energy, as referenced in the objectives achievement of many energy roadmaps [1], is conditioned by the possibilities to integrate the marine generation power plants in the electric systems. Offshore wind, floating solar PV, tidal, waves and even saline gradient are the technologies considered. The success of these plans needs to fulfil, among others, the compliance of the electric grid codes, ensuring the stability of electric grids.
 The integration of marine renewable energies is still not a problem at the electric grids, since the penetration level is not so high. However, the objectives established in the roadmaps invite to analyse the expected effects of new power plants from the point of view of performance at the electric systems. Actually, some research works have identified an increase in the number of frequency and voltage events in electric systems due to higher penetration of marine energies [2].
 The connection of the marine renewable generation is usually taken place in nodes of the electric grids not very favourable in terms of power capacity and voltage drops. Moreover, many of the locations where the marine renewable energies result economically viable are linked to weak areas of the electric grid, meaning that frequency and voltage are more sensible. That is the case of remote isolated areas, as usually coastal areas, or islands. Additionally, some of the technologies considered, such as wave energy, present very oscillatory power profiles which produce an unbalance with the consumption profiles, leading to stability problems.
 More and more countries are putting an eye on new sources of energy, integrating them in their energy policies in order to search alternatives to face energy crisis. This is the case of many countries in Centre and South America, that have included in their roadmaps the renewable marine energies [3]. One of the main issues in these countries is the lack of robust electric grids or the condition of weak grids in coastal areas.
 This paper presents the result of the analysis of different electric grids in Ibero-American countries. Three different regions have been considered: Mesoamerica, South America and Iberian Peninsula and Macaronesia, and particular areas or countries have been selected from each region as representative of the critical conditions in terms of grid integration. Costa Rica, Peru, Brazil and Argentina, and the Canary Islands in Spain, are the selected case studies. The results presented will summarize: power limits, time response, energy storage and backup generation requirements, power ramps, power oscillation allowed, etc.
 This work has been accomplished in the frame of the activities carried out by the groups involved in the thematic network REMAR (Opportunities of Integration of Ocean Energies in Ibero-American Electric Grids), promoted and funded by the Ibero-American Programme of Science and Technology for the Development (CYTED).

Voltage Detection-Based Selective Harmonic Current Compensation Strategies for Photovoltaic Inverters

Voltage and current detection-based harmonic current compensation (VDB-HCC and CDB-HCC, respectively) strategies allow the use of photovoltaic (PV) inverters to enhance the grid power quality. CDB-HCC strategies require converter hardware retrofit by inserting an extra current sensor to measure load or downstream grid currents. On the other hand, VDB-HCC strategies are straightforward solutions employing only embedded measurements used for protection, control, and synchronization purposes. This paper provides a comprehensive comparison of three VDB-HCC strategies: VDB-HCC based on a single current loop (SCL); VDB-HCC based on dual parallel current control loops (DCL); and VDB-HCC based on parallel voltage and current control loops (VCL). The comparison includes the requirement of harmonic detection algorithms, PV array maximum power point tracking (MPPT) performance, stability over grid impedance variations and inverter power rating, steady-state harmonic compensation, enabling transient response, and computational burden. Experimental results using a 1.5-kW commercial PV inverter are conducted. Based on distributed HCC applications by PV systems, VCL has shown the best performance in terms of voltage distortion reduction under weak grids.

Novel Source-Storage Coordination Strategy Adaptive to Impulsive Generation Characteristic Suitable for Isolated Island Microgrid Scheduling

Wave energy generator (WEG) is a new renewable energy source with prominent advantages and disadvantages. Theoretically, it can move around the sea to harvest wave energy better and will not take up valuable land resources. Normally, WEG collects tiny wave energy continuously and generates electricity intermittently on the minute time-scale to enhance energy harvesting efficiency. Thus, the output of WEG shows prominent impulsive characteristics, which needs to be compensated by the battery energy storage system (BESS) and therefore brings about significant battery life losses. For the purpose of stable and economic operation of the weak power grid connected by WEG, a flexible control method enabling a quantifiable tradeoff between its energy harvesting efficiency and impulsive characteristics is proposed. Then, a novel source-storage coordination strategy and the corresponding energy exchange model of WEG with several optimizable control parameters is formulated to depict the energy flow within the microgrid. Subsequently, an optimization algorithm is proposed to schedule the WEG’s mobile route and optimizable control parameters simultaneously. Simulation results show that the proposed source-storage strategy and the energy management scheme can balance WEG’s energy harvesting and impulsive impact, and utilize its mobility to improve the isolated island microgrid’s operation economy and power quality.

Harmonic Resonance Analysis and Impedance Remodeling Method of Multi-Inverter Grid-Connected System

The harmonic and resonant characteristics of a multi-inverter grid-connected system can negatively affect power quality when weak grid conditions are present. In this paper, firstly, harmonic currents are modeled for the inverter, the correctness of low-frequency harmonics and high-frequency harmonics are verified in closed loop, and the characteristics of harmonic currents are analyzed when the parameters are varied. Secondly, the resonant characteristics of the inverter with feed-forward link are analyzed, and a multi-inverter grid-connected equivalent model based on the triple-decomposition conductance is developed and analyzed in conjunction with the resonant modal analysis method. Then, a harmonic resonance impedance reshaping method is proposed to suppress the background harmonics using improved weighted average current control (WACC), suppress the system resonance based on the point of common coupling (PCC) paralleling virtual conductance method, and improve the stability margin of the system combined with impedance reshaping resonance suppression method. Finally, simulations and comparison of results with different suppression methods verify the superiority and effectiveness of the proposed method.

Virtual synchronous generator‐based control of modular multilevel converter for integration into weak grid

AbstractIntegrating Power Electronics Converters (PECs)‐based renewable energy sources can establish a weak grid due to the substantial inertia reduction leading to severe frequency fluctuations. In this paper, a control strategy based on the Virtual Synchronous Generator (VSG) concept is designed for a three‐phase Modular Multilevel Converter (MMC) connected to a weak grid. The proposed VSG control approach improves the frequency response during transient mode, resulting in a 66.7% enhancement at the Point of Common Coupling (PCC). The proposed strategy is shaped within four control loops wherein the VSG‐dependent output current control possesses the duty of enriching the MMC output current tracking capability. Focusing on the MMC output current‐based transfer function, the effectiveness of this capability is guaranteed using the frequency domain analysis including the Nyquist and Bode diagrams. Moreover, a comprehensive discussion was carried out regarding the impact of VSG on the voltage of submodule capacitors and the circulating current. Simulation results in MATLAB/Simulink environment validate the accuracy and weak grid‐connected MMC performance under the proposed VSG‐based control technique. To further verify the superiority of the proposed strategy, two control techniques including Droop and SEBIR are compared with the VSG proposed controller.

Stability analysis of different control modes of grid-connected converters under different grid conditions

With the sustained popularity of renewable energy generation, high penetration of variable energies, e.g., wind and solar, is reshaping the form of power systems and weakening the strength of the grid. The stability mechanism of the grid-connected converter in a weak power network, however, has yet to be evaluated. This paper establishes impedance and transient models for Grid-Following (GFL) as well as Grid-Forming (GFM) converters through the impedance analysis method and equal area criterion analysis method. The stability of these two control methods is then comprehensively studied under small and large interference with different grid conditions. The analytical results show that the GFM control is more stable against small disturbances in a weak network. In contrast, it is prone to a significant disturbance stability problem in the strong grid due to the large grid impedance. The GFL control is more suitable for a vigorous power grid, whereas introducing oscillation in a weak power grid due to its negative damping. Simulation experiments have verified the accuracy of the analytical results.

Two-stage PV grid-connected control strategy based on adaptive virtual inertia and damping control for DC-link capacitor dynamics self-synchronization

Conventional DC-link voltage-controlled voltage source converter (VQ-VSC) controls DC-link capacitor voltage and reactive power output by using phase locked loop (PLL) for synchronous grid connection of new energy sources such as PV. However, the deterioration of dynamic performance in PLL can lead to instability in VQ-VSC control within a weak grid. To improve the stability of VQ-VSC grid connection, the DC-link capacitor dynamic self-synchronous grid-connected control strategy without using PLL is proposed in this paper. The converter virtual rotational inertia variability and virtual inertia damping frequency characteristics are analyzed based on the synchronous generator rotor rotation characteristics. The proposed control strategy provides adaptive virtual inertia and damping coefficients for DC-link capacitor self-synchronous units to dynamically match the inertia and damping requirements during frequency synchronization, and enables VQ-VSC to operate in grid-forming mode without changing the grid synchronization unit. Compared with constant virtual inertia-damping control and adaptive virtual inertia-damping control based on change rate of frequency, the simulation results demonstrate the effectiveness of the proposed control strategy, which significantly improves the grid connection stability of PV systems in weak grids.

Resonance suppression and quality improvement control strategy for a three-phase grid-connected voltage-source inverter under weak grid conditions

The three-phase voltage-source grid-connected inverters suffer from grid-connected current quality problems due to system resonance caused by the under-damping characteristics of the filter and grid impedance, on the one hand, and the grid-connected current distortion caused by the high content of low harmonics in the grid voltage, on the other hand. To resolve this problem, a current quality improvement control strategy, combining capacitor-current feedforward active damping and harmonic virtual impedance reshaping, is proposed by analyzing the mechanisms of system resonance and grid-connected current distortion. In addition, the proposed control strategy allows determining the virtual impedance parameters by using the root locus method, so as to achieve the purpose of improving the output grid-connected current quality of the three-phase voltage-source grid-connected inverter under weak grid conditions. Finally, the correctness and effectiveness of the proposed control strategy are verified by simulation and experiment.

Mitigation of power system oscillations in weak grids with battery energy storage systems: A real-world case study

Sub-synchronous oscillations are becoming commonplace in weak areas of power systems with high levels of renewable generation, affecting their operation. Moreover, there is a lack of methods and techniques that facilitate the understanding of such phenomena and the analysis of mitigation alternatives from a practical-knowledge point of view. In this manuscript, the combination of static and dynamic techniques is utilized and consolidated to derive general conclusions when mitigating sub-synchronous oscillations by means of grid-forming battery energy storage systems (GFM BESSs). The grid impedance shaping effect of GFM BESSs, which allows the mitigation of undesired oscillations, is demonstrated through small-signal (static) and frequency scanning (dynamic) analyses when studying an actual and extremely weak grid, the Australian West Murray Zone. These findings are further validated when performing detailed real-time EMT digital simulations of the grid. Furthermore, additional dynamic simulations demonstrate the ability of GFM BESSs to provide system strength and increment the hosting capacity of renewables in weak areas. The methods and techniques presented in this work can be applied to any power system, further supporting existing industry practices.

Control for Power Quality Improvement of Solar Photovoltaic-Distributed Static Synchronous Compensator Interfaced with Weak Grid Using Multi-Variable Filter Dual Second-Order Generalized Integrator Phase-Locked Loop

This study proposes a control method for transferring power generated by a solar photovoltaic system. The control algorithm, called MVF-DSOGI-PLL or Multi-Variable Filter Dual Second-Order Generalized Integrator Phase-Locked Loop), accurately estimates symmetrical components, phase angle, and positive sequence voltages of the Common Point of Interface (CPI) in distorted grid conditions. The approach creates a compensatory current to reduce PQ difficulties by extracting the size of the basic current load undergoing operation employing the SOGI method. These systems can operate in either modes of PV-DSTATCOM or DSTATCOM, depending on solar insolation’s availability. The simulation models of the grid-interfaced SPV systems are developed in Matlab/Simulink to evaluate the performances of the suggested control method. The results of simulation demonstrate that the suggested control systems efficiently transfers the power generated by PV arrays to a grid while performing tasks like load balancing, power factor correction (PFC), and harmonics reduction. Furthermore, the MVF-DSOGI-PLL-based control algorithm successfully mitigates PQ issues in distribution networks with weak grids, ensuring that PQ levels remain between a limited set by the IEEE-519 standard. A laboratory prototype of the proposed system is implemented to assess its steady states and dynamic efficient under fluctuating solar insolation, and load imbalance distortions.

A Derivative-Feedback Based Compensation Control to Enhance the Small-Signal Stability for LCC-HVDC System

The small-signal stability of the LCC-HVDC system connected to the weak AC grid is of great concern. This paper derives the SISO model of the LCC inverter station. On this basis, the influence mechanism of AC system strength on system stability is studied via the Bode stability criterion, and the coupling interaction between different control loops is quantitatively evaluated. Results reveal that, as the AC grid becomes weaker, the system stability margin is diminished characterized by the larger phase lag in the phase-frequency response. Also, the interaction between DC voltage controller (DVC) and phase locked loop (PLL) control loops is intensified. Then, a derivative-feedback based compensation (DFC) control is presented to improve the LCC system stability under the weak AC grid condition. Theoretical analysis and simulation results indicate that the presented DFC control can effectively improve the stability and dynamic performances of the LCC system, which allows the system to operate stably even connected to the extremely weak AC grid.

A Lyapunov-based nonlinear direct power control for grid-side converters interfacing renewable energy in weak grids

A Lyapunov-based nonlinear direct power control for grid-side converters interfacing renewable energy in weak grids

Impedance Reshaping Band Coupling and Broadband Passivity Enhancement for DFIG System

Doubly fed induction generator (DFIG)-based wind power systems incur a broadband negative resistive characteristic, resulting in resonance risks when connected with weak grid or diverse grid transmission infrastructure. The dominant factor and underlying mechanism of this broadband negative resistive characteristic are analyzed based on the passivity-based stability assessment. It is then noticed that the negative resistive region within low frequency, middle frequency and high frequency are closely linked to the current controller, phase locked loop and system delay, respectively. This article reveals the impedance reshaping band coupling phenomenon to facilitate some passivity-based designs, then puts forward a broadband passivity enhancement method for DFIG system. The experimental results validate the aforementioned conclusions and the effectiveness of the proposed passivity enhancement method.

An Adaptive Virtual Impedance Method for Grid-Connected Current Quality Improvement of a Single-Phase Virtual Synchronous Generator under Distorted Grid Voltage.

The proportion of distributed generation systems in power grids is increasing, leading to the gradual emergence of weak grid characteristics. Moreover, using voltage-sourced grid-connected inverters can enhance the stability of a weak grid. However, due to the presence of background harmonics in weak grids, the grid voltage can cause significant distortions in the grid-connected current, which adversely affects the quality of the grid-connected current. This paper begins by briefly introducing the principle of the virtual synchronous generator (VSG). Then, the output current of the voltage source inverter is analyzed to elucidate the mechanism of harmonic current generation. Considering the distortion in the grid-connected current of the voltage source grid-connected inverter caused by background harmonics in the grid voltage, a harmonic current suppression strategy based on an adaptive virtual harmonic resistor is proposed. The proposed strategy employs a signal separation module based on multiple second-order generalized integrators connected through a cross-feedback network. This module effectively separates the fundamental and harmonic currents from the grid-connected current, extracts the amplitudes of the fundamental and harmonic currents through coordinate transformation, and adaptively adjusts the virtual harmonic resistance magnitude through the negative feedback control of the harmonic content (the ratio of the harmonic current amplitude to the fundamental current amplitude). These measures are used to enhance the quality of the grid-connected current. Additionally, the stability of the system is analyzed using the root locus of the open-loop transfer function. Finally, the effectiveness of the proposed method is validated through a combination of MATLAB/Simulink simulations and experimental results.

Stability enhancement of grid side converter in PV-wind-BESS based microgrid under weak grid conditions

This article revisits the design of a feedforward controller to improve the stability of grid side converter (GSC) in a microgrid system with harmonic reinjection ability, and power transfer capability while integrating into a weak utility grid. The stability and injection/absorption of active power to the utility grid are greatly affected by grid conditions. In addition, the variable grid impedances because of the expanding utilization of renewable energy sources, long distance of transmission lines, power system faults, harmonic pollution, etc., interrupts the grid codes. In these uncertain grid circumstances, coupling between the phase-locked loop (PLL) of the d-q control scheme and the grid impedance might also lead to instability. An enhanced feedforward controller term is implemented during the proposed research to counteract this effect. The proposed controller adapts the changes in grid parameters and makes the system stable by increasing power transfer to the receiving end. Proficiency of the proposed control scheme is validated by comparing it with existing control schemes using simulation tools with different operating modes of the microgrid and weak grid conditions. It is established from the simulation results that the proposed novel feedforward controller gives substantiated results during stable operations and integration while maintaining the IEEE-1204 standards.

Grid Harmonics Suppression for Three Phase Dual-Frequency Grid-Connected Inverter Based on Feedforward Compensation

Using a low pulse ratio, the electromagnetic interference and switching loss of an inverter can be effectively reduced, particularly in high-power applications. However, due to variations in grid impedance, it is a challenging task to achieve stable operation of an LCL-type grid-connected inverter (GCI) using the active damping method with low pulse ratio. Thus, a novel three-phase dual-frequency GCI is presented to ensure the symmetry of the output power and the stable operation of the system address stability issues. The proposed inverter topology in this article is composed of two inverters in parallel, which are, respectively, a power inverter unit (PIU) and an auxiliary harmonic elimination unit (AHEU). To reduce the switching loss and improve the inverter efficiency, the switching frequency of the PIU is relatively low, injecting current into the grid. Moreover, the feedforward compensation method is used in AHEU. AHEU operates at high switching frequency to generate a current component that is symmetrical with the ripple com-ponent, improving the power quality, without extracting the current harmonic as the current reference. The operating principle of feedforward compensation is explained, and a proper parameter design procedure is presented in this paper. Since L filters are used for the proposed inverter, the system can operate stably where the ratio of switching frequency to fundamental frequency is low. A 10 kW laboratory prototype was built. The experimental results showed that the grid current ripple could be effectively eliminated and the THD of the grid current was 3.01%. The proposed inverter has good stability in a weak grid, and the efficiency of the proposed inverter is 95.98% at rated current, which is 0.81% higher than the traditional GCI, effectively increasing the efficiency of the system.