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With the wide application of voltage source converters (VSCs) in power system, dc-bus voltage control instabilities increasingly occurred in practical conditions, especially in weak ac grid, which poses challenges on stability and security of power converters applications. This paper aims to give physical insights into the stability of dc-bus volta...
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... the ac-bus voltage control and PLL dynamics in Fig. 4, the simplified second-order small-signal model of dc-bus voltage control is described in Fig. 5. Based on this model, the damping and restoring components are introduced. In addition, the dc-bus voltage stability is analyzed with investigating the damping and restoring components. Diagram for illustrating damping and restoring ...
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Citations
... Though this control is straight-forward, the performance of the PLL can be negatively impacted, specifically when VSCs are connected to the weak grids, leading to reduced tracking accuracy [16]. In more severe scenarios, the limitations in performance of the PLL due to weak grid conditions can result in instability in dc-link voltage control [17]. ...
While power electronic converters, such as voltage source converters (VSCs), are crucial for the operation of converter-dominated renewables and their integration with the electricity grid, their reliance on VSCs can pose a challenge. The limited inertia of these sources can lead to a deterioration of the rate of change of frequency, potentially causing power system stability issues. A grid-forming approach utilizing dc-link dynamics is one of the attractive alternatives to achieve grid synchronization and support grid frequency. Existing grid-forming control schemes, which assume a constant or virtually constant dc source, rely on a fixed physical dc-link capacitor. Nonetheless, the inertia support from such a capacitor is brief, owing to its limited energy storage capability. Consequently, enhancing inertia becomes imperative; otherwise, it may result in an increased rate of change of voltage on the dc side, potentially leading to issues with protection, undesirable interactions, and system instability. This paper proposes a new grid-forming control strategy that considers a virtual capacitor to achieve grid synchronization while simultaneously providing the network with inertia response services during power imbalances. Moreover, including a virtual resistor in the controller effectively attenuates power and dc voltage oscillations. Simulations using Simulink and small signal stability analysis are conducted to validate the efficacy of the proposed controller.
... In recent years, the penetration level of generated renewable energy such as wind and solar power into the power system has increased [1]. In grid-connected devices for renewable energy generation, inverters are widely applied [2][3][4]. Photovoltaic power stations and wind farms are usually located in remote places, and electricity generated by renewable energy sources needs to be transmitted over long distances [5]. For long-distance transmission lines, the line impedance and transformer impedance cannot be ignored, and various faults may occur throughout the line, which make the grid extremely sensitive to vulnerability and unbalance [6][7][8]. ...
As a common interface circuit for renewable energy integrated into the power grid, the inverter is prone to work under a three-phase unbalanced weak grid. In this paper, the instability of grid-connected inverters under the unbalanced grid condition is investigated. First, a dual second-order generalized integrator phase-locked loop (DSOGI-PLL)-based inverter under balanced and unbalanced conditions is modeled. A fourth-order impedance model is established to describe its impedance characteristics under the unbalanced grid condition. To analyze this multi-input multi-output system, a simplified stability analysis method based on the generalized Nyquist stability criterion and matrix theory is proposed. Then, the influences of circuit and control parameters on the stability of the grid-connected inverter system under the unbalanced grid condition are investigated. Finally, the accuracy of the derived frequency-coupled impedance model is verified via simulations, and the effectiveness of the proposed simplified stability analysis method on the system stability analysis is verified via both simulations and hardware experiments.
... Hence, this simplification can still capture the frequency-power dynamics of the converter. Additionally, this type of quasi-stationary model has also been implemented to investigate the stability of DC link voltage control for GFL converters [61,62] and compare the dynamics of GFM converters under stand-alone and SG-connected operations in [63]. ...
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.
... Furthermore, the weaker the grid, the stronger the interaction effect. [78][79][80] Additionally, the interconnected system of PMSG-based wind farms and converter stations also have resonance risk at the middle and low-frequency range. Generally, the PMSG-based wind farm behaves as "inductive negative impedance" below the fundamental frequency and as "capacitive negative impedance" for f 0 -2f 0 . ...
Recently, the large-scale integration of power electronic-based renewable energy power plants has changed the operation and response mechanism of the power system, resulting in several emerging oscillation issues that have seriously been threatening the system's stability. It helps us to recognize the similarities and differences among the triggering causes and formation mechanisms of oscillation scenarios. Following several typical oscillation events in the real world and the timescale decomposition method, this paper comprehensively reviews the wide-bandwidth oscillation study from the aspects of the analysis methods, possible cause, mechanism, and mitigation solution. The paper provides a perspective to classify the oscillations in the modern power systems on the basis of the oscillation frequency and the main oscillation module. This classification framework involves not only emerging oscillations in the power system with large-scale renewable energy sources integration but also includes typical oscillations in traditional power systems. It also systematically presents the relative relationship, development process, and inner influence between emerging oscillations and typical oscillations. Based on this review, the future research is suggested to focus on the relationship between different analytical methods or oscillation mechanisms, as well as the stability risk assessment of hybrid alternating current and direct current power systems.
... In order to suppress the oscillation phenomenon of grid-connected inverter systems, domestic and foreign scholars have carried out a lot of research on its instability mechanism and high-frequency oscillation suppression strategy. References [12][13][14] compared the Solving equivalent admittance model of weak grid with compensation capacitor Admittance remodeling oscillation suppression strategy Second -order generalized integrator Negative third -order differential element ...
... Electronics 2023, 12 Figure 2, the Kirchhoff voltage equation is written for the inverter control loop and the Park transformation is performed. The small signal model of the inverter in the d-q-0 coordinate system can be obtained: In the formula, u dc is the DC side voltage of the inverter. ...
As the penetration of renewable energy increases year by year, the risk of high-frequency oscillation instability increases when a three-phase, four-wire split capacitor inverter (TFSCI) is connected to the grid with complementary capacitors in weak grids. Compared to the three-phase, three-wire inverter, the TFSCI has an additional zero-sequence current loop. To improve the accuracy of the modeling and stability analysis, the effect of the zero-sequence loop needs to be considered in the impedance-based stability analysis. Therefore, a correlation model considering multi-perturbation variables is first established, based on which the inverter positive, negative, and zero sequence admittance models are derived, solving the difficult problem of impedance modeling under small perturbations. Secondly, an admittance remodeling strategy based on a negative third-order differential element and a second-order generalized integrator (SOGI) damping controller is proposed, which can improve the stability of positive, negative, and zero-sequence systems simultaneously. Finally, the effectiveness of the oscillation suppression strategy is verified by simulation and experiment.
... unique circuit structure of doubly fed converters, which is not common in other types of converters (Huang et al., 2016;Zhang et al., 2020). For the stability issues in PV systems, Yang et al. (2019) analyzed the impact of parallel compensation on the control stability of inverters and discovered that parallel compensation decreases the phase margin of inverter current control. ...
The voltage and frequency control of photovoltaic (PV) systems are influenced by coupled nonlinear factors. It has been discovered that frequency control stability is threatened by voltage regulation methods in PV systems. However, the frequency instability caused by voltage regulation methods has not been fully investigated. This paper investigates the voltage and frequency stability problems in PV systems connected with weak power grids. The voltage problems caused by grid impedance, comprising inverter AC voltage and DC voltage, are first analyzed. Then, methods for improving voltage stability, such as reactive power compensation, and the benefits and drawbacks of various compensation methods are investigated. Finally, the effect of reactive power compensation on frequency control stability is investigated and resolved. Simulations and experiments are used to validate the theory’s correctness.
... Excellent work has been done to improve the stability by modifying the control strategies, e.g. phase compensation [13], controller optimization [14], virtual impedance design [15], etc. Dc voltage control is also very important in system stability in addition to the current and synchronization control [16], [17]. The small signal stability has been widely discussed according to the power equation of dc-link capacitors [18]. ...
... Moreover, for the DVL parameters design, most of literatures regard the inner loop as a delay link (by ignoring the frequency coupling), and design the parameters according to a typical low-order system [20]- [22]. For instance, the DVL was regarded as a typical second-order link in [20]- [21], and the DVL parameters were simply tuned according to a typical third-order system in [22]. Note that the above studies of parameters design are all carried out under the stiff grid. ...
In the grid-connected inverter, both the PLL (phase-locked loop) and DVL (DC-voltage loop) can lead to the frequency coupling in the weak grid. Instabilities caused by PLL frequency coupling have been widely discussed. For DVL, past studies have analyzed the effect of DVL parameters on the degree of frequency coupling, but it is quite unclear how DVL parameters affect the inverter stability and how DVL can be designed under the weak grid. Hence, by fully considering the abovementioned frequency coupling, an accurate admittance model of PV (photovoltaic) grid-connected inverter is established in this study. By deriving the modified admittance matrix, the effects of grid impedance and DVL parameters on the frequency coupling are clearly studied. In addition, by modeling the equivalent inverter output impedance, the impact of DVL on the inverter stability under the weak grid is revealed. As proved, the increase of DVL bandwidth
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and grid impedance will aggravate the frequency coupling, but the current distortion does not necessarily increase. Meanwhile, the increase of
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helps to improve the inverter stability under the weak grid. Accordingly, a novel design for DVL in the weak grid is proposed. Experiments verify the correctness of the analysis and design.
... THE voltage source converter (VSC), functioning as the renewable energy grid-connected interface device, has been widely used in the PV and wind power generators [1][2][3][4][5][6][7]. In order to ensure the reliability of PV and wind power generation, VSC needs to have the capability of riding through low voltage situation [8,9] in case of voltage sag introduced by grid-side fault. ...
As the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation
... Although it was pointed out in [17] that the active power control was coupled with the reactive power control through the point of common coupling (PCC) voltage under the weak AC grid, the power coupling characteristics were not analyzed in detail, and the corresponding decoupling strategy was not proposed to tackle the issue. Huang et al. built a small-signal model of the VSC with DC voltage control, phase-locked loop (PLL) and AC voltage control, and the impact of dynamic interactions on DC voltage control dynamics was analyzed [18][19][20]. Egea-Alvarez et al. analyzed the coupling characteristics of active power and AC voltage, and proposed a decoupled and gain-scheduling controller [21]. However, the active and reactive power coupling characteristics were not discussed in [18][19][20][21]. ...
... Egea-Alvarez et al. analyzed the coupling characteristics of active power and AC voltage, and proposed a decoupled and gain-scheduling controller [21]. However, the active and reactive power coupling characteristics were not discussed in [18][19][20][21]. Meanwhile, the advanced vector control in [21] requires four more control loops and repeated designing procedure for all possible operating points, which results in mass calculation and complex implementation. ...
... (13) and (20) represent the linearized relationship between the power and dq-axis currents without and with considering the dynamics of the PLL, respectively. The dynamics of the PLL affect Δu sq from Fig. 2, and then affect Δi d from (18). Therefore, by comparing (13) and (20), we know that the PLL has a great impact on the coupling relationship between the power and the d-axis current, and has a negligible effect on the coupling relationship between the power and the q-axis current. ...
Conventional vector control (VC) is widely used in the voltage source converter (VSC) due to its simplicity and power decoupling ability. However, this paper reveals that the VC may result in strong active and reactive power coupling under weak AC grid conditions. The linearized relationship between the power and dq-axis currents is built. On this basis, the power coupling mechanism is revealed, and the power coupling process is presented. Finally, an improved decoupled power control is accordingly proposed based on the feedforward compensation method. The improved decoupling control method can realize more accurate decoupled active and reactive power regulation. Moreover, its design is simple, and thus is easy to be implemented in practice. The power coupling characteristics analysis and the improved power decoupling control are verified through PSCAD/EMTDC simulations and experiments.
