Weak Grid Research Articles

Novel Active Damping Design Based on a Biquad Filter for an LLCL Grid-Tied Inverter

LLCL filters for grid-tied inverters have been adopted to get better performance for the harmonics near the switching frequency than commonly used LCL filters. However, the resonant peak caused by a pair of unstable resonance poles of the LLCL filters is introduced and makes the system become unstable. In this paper, a biquad filter composed of a notch filter and a resonator is introduced to restrain the resonant peak. In this method, the resonance point and the notch point of the biquad filter are placed at the appointed frequency, and the resonant peak is transferred to the stable area by phase transformation, so that the system does not cross −180° at the resonant peak. This method makes the system have higher control bandwidth and stronger robustness even in a weak power grid. Meanwhile, a proportional-integral multiresonant repetitive controller is used to restrain low-frequency current harmonics and improve the steady-state and dynamic performance of the control system. Furthermore, based on the active damping of the biquad filter, the stability criterion of the control system under a weak power grid is given. Finally, the accuracy of the analysis and the effectiveness of the method is verified by simulations.

Equivalent modeling and multi-parameter coupling optimization for DFIG-based wind farms considering SSO mode

As a low-carbon and environmentally friendly renewable energy source, wind power has been globally recognized as the best solution to achieve energy saving and emission reduction and promote low-carbon economic growth. With the increase of wind power penetration, wind power has a great impact on sub-synchronous state stability and dynamic characteristics of the grid-connected system. Aiming at the fact that the correlation between clustering indexes and sub-synchronous oscillation (SSO) mode and the difference of the contribution to the clustering results are seldom considered in the current equivalent modeling of doubly-fed induction generator (DFIG)-based wind farm, this paper proposes a clustering method based on the index dimension reduction and weighted fuzzy C-means (WFCM) clustering algorithm. Besides, for the SSO study of the grid-connected system without sufficiently considering the coupling effects between controller parameters, a multi-parameter coupling optimization design strategy combining orthogonal experiment method (OEM) and response surface method is proposed. Firstly, the dominant variables of SSO mode of the DFIG-based wind farm connected to weak grid by series compensation system are taken as the initial clustering indexes. After dimension reduction by principal component analysis, the WFCM algorithm is utilized to cluster the wind farm. Then, the proportional and integral coefficients of the grid-side controller, rotor-side controller and phase-locked loop are optimized to achieve the simultaneous optimization of the SSO characteristics and dynamic characteristics of the system. Finally, the interaction between control parameters and the influence degree and trend on the system performance are quantitatively evaluated, and the optimal parameter combination is obtained. The proposed strategy can mitigate SSO more effectively while improving anti-interference than the particle swarm optimization based on OEM.

Ultra-low frequency oscillation analysis considering thermal-hydro power proportion

The Ultra-low Frequency Oscillation (ULFO) has attracted much attention due to its field test capture in China, and it is believed that the ULFO is related to the hydraulic generator. To clarify the cause and innate essence of ULFO, the develop of the ULFO under different thermal-hydro power proportions is investigated. A small-signal model of two-machine system considering different thermal-hydro power proportions is established. Based on the small-signal model, the influence of hydropower proportion on the oscillation characteristics is analyzed in different conditions, and it is found that a higher proportion of hydro power results in a lower oscillation frequency, though the hydropower proportion is not the crucial cause in ULFO. Then Based on the eigenvalue analysis, it is observed that the essential cause of ULFO is the weak grid condition rather than the high hydropower proportion. The system connection strength determines whether the system is prone to ULFO. In addition, the participating factors of the generators’ significant state variables are obtained, which indicates that the governor plays a more vital role in ULFO in the case of weak grid connection. To verify the theoretical analysis, the simulations of the two-machine system and six-machine nine-node system are implemented, respectively. The results reveal that system with all thermal power is still prone to ULFO when the connection is weak. Conversely, when the connection is strong, the ULFO does not occur even if all the generators are hydraulic. Furthermore, in a weak grid condition, the oscillation frequency and damping ratio decrease with increasing hydropower proportion, which indicates that a high hydropower proportion deteriorates the ULFO problem.

Dynamic Modeling and Analysis of a Virtual Synchronous Generator with Supercapacitor

With the continuous integration of new energy sources, the power system gradually begins to present the characteristics of a weak power grid. The system’s inertia decreases, leading to problems in the stability of the power grid. In this paper, a virtual synchronous generator model with a supercapacitor is analyzed. The supercapacitor provides additional virtual inertia to the system and suppresses system frequency disturbances more quickly. Bifurcation theory is used to analyze the nonlinear dynamics of the model. The bifurcation diagram of input active power is given in this paper, and the phase portraits and sequence diagrams of the frequency and power angle are presented to verify that, if the initial value of the system falls inside the stability region, the system can remain stable. If the initial value of the system falls outside the stability region, conversely, the system will lose stability. Finally, the simulation verifies the influence of the supercapacitor on the system inertia. The results show that the recovery speed of a small capacitance system is faster than that of a large capacitance system when disturbance occurs. It is concluded that, the smaller the supercapacitor is, the greater the virtual inertia it can provide.

Assessment of Flicker Emission in a Grid Connected Wind Farms

Wind farms in Tamil Nadu's Coimbatore district have identified several issues, including flickering emissions, frequent generator tripping, and power evacuation issues caused by a weak grid. Taking into consideration the scarcity of research on flickering emissions, this work focuses on the causes of short-term flicker severity (Pst) in wind farms that export generated power to industrial loads. To identify the scenarios that cause flickering, simulation models of fixed speed wind farm and variable speed wind farm with controllers were developed using DIgSILENT power factory software. The flicker emissions were measured at the wind farm substations using Fluke and Dranetz PX5.8 power quality analyzers in accordance with the IEC 61400-21 standard. To validate the simulation model, the results from the flickermeter during the simulation and the field measurements were compared. According to the results of this research, both fixed and variable speeds produce flicker emissions that exceed the IEC standard limit, that causes the power electronics-based industrial drives to fail to operate. The controllers were developed to improve the performance of wind farms that will benefit the current and future wind energy-efficient conversion systems.

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.

Control Loop Stability Criterion and Interaction Law Analysis for Grid-Connected Inverter in Weak Grid

The study of stability criterion and interaction analysis for grid-connected inverter under weak grid is of great value. Different from traditional impedance stability criterion, this paper proposes control loop stability criterion for grid-connected inverter from the perspective of controller bandwidth overlap. Firstly, the system d-q overall small-signal model G0 considering phase-locked loop (PLL) of grid-connected inverter under weak grid is given and split into three multiplied independent parts: grid impedance, phase-locked loop and current controller. Then using the equivalent loop ratio expression obtained by combining PLL and grid impedance together and then divided by the current controller, the control loop stability criterion is proposed. The proposed stability criterion can not only maintain the independence of each single loop, but also can analyze the overall system stability. So, further analysis of the interaction law among the three parts under this control loop stability criterion is carried out by deducing the expression of the bandwidth ratio n of the PLL and current controller. And it is found that under the weak grid, the interaction between the links will be generated when the bandwidth ratio is greater than the threshold of n. This phenomenon can be represented by the overlapping area of amplitude-frequency curves in the bode diagram for G0. And the weaker the grid or the closer the bandwidth of PLL is to current controller, the larger the overlapping area of amplitude-frequency curves, and the more likely it is to lead the closed-loop gain to infinity. Furthermore, when the bandwidth of the current controller is fixed, the threshold of n varies in the regions of less than 1 as well as more than 1 with the change of the grid strength. Therefore, the system can remain stable no matter what the bandwidth ratio of the phase-locked loop to the current controller is greater or less than 1. Accuracy of the proposed control loop stability criterion and interaction analysis is verified through simulation and experimental results.

A New Type of Weak Grid IBR Oscillations

This letter presents the discovery of a new type oscillations associated with inverter-based resources (IBRs) in weak grids. While typical weak grid oscillations become worse when the grid strength decreases and the power exporting level increases, this new type of oscillations appear when the grid becomes weaker and the power exporting level decreases. Electromagnetic transient (EMT) simulation results are presented, along with eigenvalue loci for a varying power. The notable signatures of such oscillations are also remarked. In further analysis, a feedback system consisting of two blocks is derived with one block associated with the phase-locked-loop (PLL) and the other associated with the rest of the system. This feedback system quantitatively demonstrates why insufficient damping in the PLL may be an issue for stability. Open-loop analysis shows that a larger phase lag in angle tracking in the low-frequency range due to high power exporting level at the weak grid condition is beneficial to the open-loop poles introduced by the PLL. And this is the reason why such oscillations may appear when power is ramping down.

Control Performance Characterization and Monitoring Scheme for Power Converters in Weak Grids

Control Performance Characterization and Monitoring Scheme for Power Converters in Weak Grids

An Adaptive Hybrid Control of Reduced Switch Multilevel Grid Connected Inverter for Weak Grid Applications

Grid-connected inverters have a very significant role in the integration of renewable energy resources with utility grids. However, in recent studies, it is revealed that grid-connected inverters are vulnerable to instability when the nature of the grid changes from strong to weak, which produces uncertainty and performance degradation. An increase in grid impedance decreases stability margins, tremendously increases total harmonic distortion after a certain limit, and amplifies the voltage harmonics in the grid. A cascaded reduced switch symmetrical multilevel inverter along with an adaptive hybrid control technique is proposed for injecting power generated from distributed energy resources efficiently and stably to the utility grid. This research contributes twofold: a multilevel inverter topology and the other is its control method. The multilevel inverter reduces total harmonic distortion and size of the filter while increasing power handling capability. The control unit of the proposed system further consists of two parts: one is the synchronous frame current controller, and the other is stationary frame adaptive harmonic compensators. The grid current controller which is working in a synchronous reference frame ensures regulated current injection to the grid. It is not favorable to implement a harmonic compensator in a synchronous reference frame due to computation complexities. Therefore, the stationary reference frame controllers are used for harmonic compensations. But the resultant harmonic compensators have narrow bandwidth. Thus, these are not robust against variation in grid frequency. In this research, this problem is resolved by adding the adaptive features within the harmonic compensators which shift its passing band according to the frequency of the grid while remaining with the same bandwidth. The proposed design of the hybrid frame controller is validated by considering a nine-level inverter connected with a weak grid.

Small-signal Stability Analysis and Improvement with Phase-shift Phase-locked Loop Based on Back Electromotive Force Observer for VSC-HVDC in Weak Grids

Voltage source converter based high-voltage direct current (VSC-HVDC) transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources. However, connecting a voltage source converter (VSC) to an AC weak grid may cause the converter system to become unstable. In this paper, a phase-shift phase-locked loop (PS-PLL) is proposed wherein a back electromotive force (BEMF) observer is added to the conventional phase-locked loop (PLL). The BEMF observer is used to observe the voltage of the infinite grid in the stationary <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha\beta$</tex> frame, which avoids the problem of inaccurate observations of the grid voltage in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$dq$</tex> frame that are caused by the output phase angle errors of the PLL. The VSC using the PS-PLL can operate as if it is facing a strong grid, thus enhancing the stability of the VSC-HVDC system. The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL, which makes it easy to implement. In addition, because it is difficult to obtain the exact impedance of the grid, the influence of short-circuit ratio (SCR) estimation errors on the performance of the PS-PLL is also studied. The effectiveness of the proposed PS-PLL is verified by the small-signal stability analysis and time-domain simulation.

Linear Active Disturbance Rejection Control and Stability Analysis for Modular Multilevel Converters Under Weak Grid

Linear Active Disturbance Rejection Control and Stability Analysis for Modular Multilevel Converters Under Weak Grid

Analysis of Sub-Synchronous Oscillation of Virtual Synchronous Generator and Research on Suppression Strategy in Weak Grid

Analysis of Sub-Synchronous Oscillation of Virtual Synchronous Generator and Research on Suppression Strategy in Weak Grid

Investigating the Effects of Slack Bus Selection in Load Flow Studies: A Comparative Analysis of Robust and Weak Grids

Investigating the Effects of Slack Bus Selection in Load Flow Studies: A Comparative Analysis of Robust and Weak Grids

A Comparative Study of Grid-Following and Grid-Forming Control Schemes in Power Electronic-Based Power Systems

Abstract Along with de-carbonisation, the penetration of power electronic converters has increased, and the power system has become a power electronic-based power system. In such a situation, the stability of the power system faces great challenges. In the event of a large disturbance, the power grid will lack the ability to maintain a stable voltage and frequency. In order to improve the stability of the power grid, the traditional grid-following (GFL) control is needed to be converted to the grid-forming (GFM) control. This paper analyses the control schemes of the GFL and GFM converters by investigating their state-space models, and the eigenvalue trajectories of both control schemes are shown to analyse the stability of the systems. Moreover, a case study is exemplified to compare the performance of the two control strategies while responding to frequency disturbances. Finally, a time-domain simulation model of a 15 kW grid-connected converter is built in Matlab/Simulink to benchmark the performance of the GFL and GFM converters under different working conditions. The result reveals that the GFL converter may encounter some instabilities when applied in power electronic-based systems, while the GFM converter is more suitable for the weak power grid.

Modeling, Stability, and Design of the Single-Phase SOGI-Based Phase-Locked Loop Considering the Frequency Feedback Loop Effect

For single-phase grid-connected inverters, the second-order generalized integrator-based phase-locked loop (SOGI-PLL) is widely used for good harmonics attenuation and frequency adaptability. However, existing studies usually neglect the frequency feedback loop in the modeling and design process, and thus, the potential instability caused by the frequency feedback loop is not well explained. Besides, the improper design of SOGI-PLL can lead to the inverter instability both under ideal and weak grid conditions. Therefore, by fully considering the coupling of the frequency feedback loop, this article establishes an accurate model for SOGI-PLL. Furthermore, effects of different PLL parameters on behaviors of both the SOGI-PLL itself and the inverter are studied, based on which a graphical method for PLL parameters design is proposed. As proved, the frequency feedback loop can deteriorate the stability and performance of both SOGI-PLL itself and inverter system, especially if rapid transient is expected. The modeling method is also extended to the low-pass filter-SOGI-PLL and the improved parameters design is discussed. Experimental results verify the correctness of the proposed PLL model and the abovementioned analysis results. The proposed model is helpful to deeply reveal the instability mechanism and provides a reference for the PLL design under nonideal grid conditions.

An Alternative Control Structure for Grid-Following Converters of Inverter-Based Resources

In this paper, an alternative control structure of grid-following control is proposed and applied in inverter-based resources (IBRs). An IBR’s grid-following control normally employs Power/Voltage (P/V) control strategy to control the output real power and the voltage at the point of common coupling bus. Compared with the conventional cascade structure, the proposed alternative control structure follows the principle of robust cascade structure. It has the same inner current control for current order tracking while the outer loop is redesigned. This new design leads to power order and voltage order tracking without use of integral control. This point alone is advantageous since an integral control has the known issue of saturation and anti-freezing has to be implemented. The purpose of such structure is to be robust against the uncertainty in the inner feedback system. We implement the robust cascade control and examine its capability of accommodating choke filter parameter variation. Besides this capability, the control’s weak grid stability performance and low voltage ride through performance are also examined. Using nonlinear analytical model-based eigenvalue analysis and nonlinear dynamic simulation, we find that the robust cascade control structure can be designed to accommodate choke filter parameter variation, improve weak grid stability, and avoid overvoltage after fault clearance.

Stability Analysis of a Weak-Grid-Connected Voltage-Sourced Rectifier Considering the Phase-Locked Loop Dynamics

The high bandwidth of a phase-locked loop (PLL) can deteriorate the small-signal stability of a voltage-sourced converter (VSC) under a weak grid condition. This instability can occur for a VSC operating as both a rectifier and an inverter, but the root causes can differ for the two. This study, through modal analysis, demonstrates that different approaches are required depending on the power flow direction to resolve the instability. Subsequently, an analysis of the rectifier dynamics is set as the primary target, and the instability characteristic is investigated via a new approach that utilizes the open-loop transfer function of the power control loop. Consequently, it is discovered that the high-bandwidth PLL reduces the reactive power compensation for the AC system and results in a severe voltage drop. A novel parallel voltage filter technique is proposed to overcome this instability. The performance of the proposed method is verified in the frequency- and time-domain analyses. The results indicate that the proposed method effectively improves stability without major modifications to the control system.

Self-reliant solar PV based microgrid with seamless transition capabilities at weak grid conditions

• The extended proportional complex filter (EPCF) control allows a fast estimation of the quadrature and in-phase fundamental load current components with reduced harmonics within the prescribed limit. • The EPCF-based control has extracted the fundamental component of load current with high rate of convergence and better system response. The variations on load, utility, and solar array radiation are well established in simulation and test results. The PQ is enhanced at unbalancing of load, change in solar insolation and grid adverse conditions of voltages unbalances. The performance of the system is found satisfactory and its response is enhanced at adverse grid conditions. Experimental results have demonstrated that the THD are as recommended by the IEEE-519 standard even during weak grid conditions. • A comparative analysis is included, which validates the superior filtering capability and improved power quality during weak grid conditions. In addition a tabular comparative analysis is included to support its effectiveness while addressing abnormalities of the grid. • The EPCF-PLL grid synchronization and phase angle estimator scheme has provides fast mode transition, frequency tracking and uninterrupted supply to the load during non availability of the grid. This has given precise phase angle estimation. Conventional PLL controllers are non-adaptive towards large frequency. EPCF-PLL has provided effective frequency adaptability. This makes the synchronization process immune from frequency variation and voltage unbalance. • The proportional resonant controller with harmonic compensation (PRHC) controller provides satisfactory performance under all the intermittent conditions overcoming the tradeoff between improved steady-state and transient performance. This paper presents a solar photovoltaic (PV) based energy conversion system with seamless transfer to the grid. To provide satisfactory performance in grid connected mode, a control based on extended proportional complex filter (EPCF) is used. The EPCF is used to estimate the fundamental current component (LCFC) of local loads At the linkage point (LP), the power quality (maintaining IEEE-519 standard) is improved using this control. The EPCF maintains balanced grid currents whether PV array generation is available or not. It provides effective mitigation of harmonics and monitors the flow of active power, which improves the grid current quality.This control provides versatile actions such as extraction of LCFC, harmonics elimination, DC offset removal, robust performance during voltages unbalance etc. The same control technique (EPCF) along with phase locked loop is used for smooth synchronization; it reduces the use of another algorithm, which improves the dynamic response of the system. This control estimates exact phase and frequency variation for the seamless mode switching. It provides frequency adaptation during mode shifting. The comparative results validate that the control based on EPCF has a faster detection of grid in comparison with existing controls. During the grid-outage, the proportional resonant (PR) controller with harmonics compensation (HC) based control algorithm enhances performance of the microgrid. This provides successful solution for harmonics compensation and minimizing steady state errors. Simulated and experimental results, in various scenarios, have verified the effectiveness of the controllers and demonstrate the favourable solution in controlling the unfavorable grid conditions.

Power Decoupling Strategy for Voltage Modulated Direct Power Control of Voltage Source Inverters Connected to Weak Grids

The grid voltage modulated direct power control (GVM-DPC)-based inverter is an attractive solution to regulate the instantaneous real and reactive powers injected into power grids. However, the power coupling in GVM-DPC, i.e., interactions between the real and reactive power loops, may deteriorate its transient performance. In this paper, a power decoupling strategy for GVM-DPC is proposed based on a dynamic feedforward power compensation (DFPC) algorithm. It is proved that the proposed strategy has better decoupling performance than that of the traditional virtual impedance method (VIM). Firstly, power coupling characteristics in GVM-DPC are analyzed, indicating that the ignorance of voltage angle variation at the point of common coupling (PCC) will result in severe power coupling. Then, power coupling magnitudes are derived according to relationships among the real power, reactive power and PCC voltage. Next, the coupling magnitudes are compensated into the power control loop of the GVM-DPC for the better decoupling performance. The transient performance of inverter using DFPC is studied based on the impedance analysis. Also, the relationship between coupling magnitude and grid side impedance is studied, which indicates the limited power decoupling capability of VIM. Finally, the proposed method is validated by the simulations and a hardware-in-loop system.