Impedance Modeling and Analysis of Grid-Connected Voltage-Source Converters
TL;DR: In this paper, a small-signal impedance modeling of grid-connected three-phase converters for wind and solar system stability analysis is presented, where a converter is modeled by a positive-sequence and a negative-sequence impedance directly in the phase domain.
Abstract: This paper presents small-signal impedance modeling of grid-connected three-phase converters for wind and solar system stability analysis. In the proposed approach, a converter is modeled by a positive-sequence and a negative-sequence impedance directly in the phase domain. It is further demonstrated that the two sequence subsystems are decoupled under most conditions and can be studied independently from each other. The proposed models are verified by experimental measurements and their applications are demonstrated in a system testbed.
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TL;DR: In this article, the small-signal impedance of three-phase grid-tied inverters with feedback control and phase-locked loop (PLL) in the synchronous reference ( d-q ) frame is analyzed.
Abstract: This paper analyzes the small-signal impedance of three-phase grid-tied inverters with feedback control and phase-locked loop (PLL) in the synchronous reference ( d-q ) frame. The result unveils an interesting and important feature of three-phase grid-tied inverters – namely, that its q–q channel impedance behaves as a negative incremental resistor. Moreover, this paper shows that this behavior is a consequence of grid synchronization, where the bandwidth of the PLL determines the frequency range of the resistor behavior, and the power rating of the inverter determines the magnitude of the resistor. Advanced PLL, current, and power control strategies do not change this feature. An example shows that under weak grid conditions, a change of the PLL bandwidth could lead the inverter system to unstable conditions as a result of this behavior. Harmonic resonance and instability issues can be analyzed using the proposed impedance model. Simulation and experimental measurements verify the analysis.
825 citations
Cites methods from "Impedance Modeling and Analysis of ..."
...Recently, smallsignal impedances and stability analysis of three-phase converters were also developed in the stationary frame using harmonic linearization on ac signals [5], [31], [35]....
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TL;DR: A systematic analysis of harmonic stability in the future power-electronic-based power systems reveals that the linearized models of ac–dc converters can be generalized to the harmonic transfer function, which is mathematically derived from linear time-periodic system theory.
Abstract: The large-scale integration of power electronic-based systems poses new challenges to the stability and power quality of modern power grids. The wide timescale and frequency-coupling dynamics of electronic power converters tend to bring in harmonic instability in the form of resonances or abnormal harmonics in a wide frequency range. This paper provides a systematic analysis of harmonic stability in the future power-electronic-based power systems. The basic concept and phenomena of harmonic stability are elaborated first. It is pointed out that the harmonic stability is a breed of small-signal stability problems, featuring the waveform distortions at the frequencies above and below the fundamental frequency of the system. The linearized models of converters and system analysis methods are then discussed. It reveals that the linearized models of ac–dc converters can be generalized to the harmonic transfer function, which is mathematically derived from linear time-periodic system theory. Lastly, future challenges on the system modeling and analysis of harmonic stability in large-scale power electronic based power grids are summarized.
703 citations
Cites background or methods from "Impedance Modeling and Analysis of ..."
...of the PLL and the outer control loops [11]–[13], [48]–[50]....
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...In the case that the sideband oscillations f1 ± fdq are both in the positive-sequence, they cannot be captured by the sequence-domain model [50]....
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...The complex equivalent of (3) is then simplified as [50]...
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...2) Harmonic Linearization Method: Alternatively, the LTI model of ac-dc converters can also be obtained by using the harmonic linearization method [27], [50]....
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...However, the cross-coupling dynamics between the sequence components are overlooked [50], which fails to characterize the frequency-coupling dynam-...
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TL;DR: In this paper, a unified impedance model of grid-connected voltage-source converters for analyzing dynamic influences of the phase-locked loop (PLL) and current control is proposed, which not only predicts the stability impact of the PLL, but also reveals its frequency coupling effect.
Abstract: This paper proposes a unified impedance model of grid-connected voltage-source converters for analyzing dynamic influences of the phase-locked loop (PLL) and current control. The mathematical relations between the impedance models in the different domains are first explicitly revealed by means of complex transfer functions and complex space vectors. A stationary ( αβ -) frame impedance model is then proposed, which not only predicts the stability impact of the PLL, but also reveals its frequency coupling effect. Furthermore, the impedance shaping effects of the PLL on the current control in the rotating ( dq -) frame and the stationary ( αβ -) frame are structurally comapred. The frequency-domain case studies on a three-phase grid-connected converter are next presented, and subsequently validated in time-domain simulations and experimental tests. The close correlations between the measured results and theoretical analysis confirm the effectiveness of the stationary-frame impedance model.
558 citations
Cites background or methods from "Impedance Modeling and Analysis of ..."
...However, the αβ-frame model in [10] neglects the frequency-coupling effect brought by the...
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...The second method is based on the principle of harmonic balance, where the harmonic linearization of sinusoidal signals is employed to formulate the impedance model directly in the stationary αβ-frame [10]....
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...Only a few works have recently been reported to include the effects of the grid synchronization and the outer power (dc-link voltage) control loops [7]–[10], [16], [17]....
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...To address the stability challenges with the grid-connected VSCs, the impedance modeling and control methods have been developed for dynamic characterization and active stabilization of VSCs [3]–[10]....
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TL;DR: In this article, the authors present an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived.
Abstract: The interconnection stability of a grid-connected voltage-source converter (VSC) can be assessed by the passivity properties of the VSC input admittance. If critical grid resonances fall within regions where the input admittance acts passively, i.e., has nonnegative real part, then their destabilization is generally prevented. This paper presents an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived. Design recommendations for minimizing the negative-real-part region are given as well.
493 citations
Cites background or methods or result from "Impedance Modeling and Analysis of ..."
...The modeling and analysis method in [10], [15], and [17] was generalized....
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...Similar conclusions are drawn in [10], [15], and [17], though using somewhat different motivations and a different linearization method....
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...This method is akin to that in [10], [15], and [17]....
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...has similarities to that suggested in [10], [15], and [17], but the impact of the DVC is included in addition to the impact of the PLL....
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TL;DR: A state-feedback quasi-static SRF-PLL model is proposed, which can identify and quantify the inherent frequency self-synchronization mechanism in the converter control system and explain the PLL instability issues and the related islanding-detection methods in early publications and industry reports.
Abstract: Synchronous reference frame (SRF) phase-locked loop (PLL) is a critical component for the control and grid synchronization of three-phase grid-connected power converters. The PLL behaviors, especially its low-frequency dynamics, influenced by different grid and load impedances as well as operation mode have not been investigated yet, which may not be captured by conventional linear PLL models. In this paper, we propose a state-feedback quasi-static SRF-PLL model, which can identify and quantify the inherent frequency self-synchronization mechanism in the converter control system. This self-synchronization effect is essentially due to the converter interactions with grid impedance and power flow directions. The low-frequency nonlinear behaviors of the PLL under different grid impedance conditions are then analyzed, which forms the framework of evaluating the impacts of the large penetration level of distributed generation units, weak grid, microgrid, and large reactive power consumption in terms of the frequency stability of PLL. Specifically, the PLL behavior of the converter system under islanded condition is investigated to explain the PLL instability issues and the related islanding-detection methods in early publications and industry reports.
482 citations
References
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TL;DR: An overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines is given and the possibility of compensation for low-order harmonics is discussed.
Abstract: Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control
4,655 citations
"Impedance Modeling and Analysis of ..." refers background in this paper
...Most grid-connected VSCs use current control in a rotating (dq) reference frame [8], which is synchronized to the fundamental component of the grid voltages by means of a phase-locked loop (PLL) [9]....
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TL;DR: In this article, power electronics, the technology of efficiently processing electric power, play an essential part in the integration of the dispersed generation units for good efficiency and high performance of the power systems.
Abstract: The global electrical energy consumption is rising and there is a steady increase of the demand on the power capacity, efficient production, distribution and utilization of energy. The traditional power systems are changing globally, a large number of dispersed generation (DG) units, including both renewable and nonrenewable energy sources such as wind turbines, photovoltaic (PV) generators, fuel cells, small hydro, wave generators, and gas/steam powered combined heat and power stations, are being integrated into power systems at the distribution level. Power electronics, the technology of efficiently processing electric power, play an essential part in the integration of the dispersed generation units for good efficiency and high performance of the power systems. This paper reviews the applications of power electronics in the integration of DG units, in particular, wind power, fuel cells and PV generators.
2,296 citations
"Impedance Modeling and Analysis of ..." refers background in this paper
...THREE-PHASE voltage-source converters (VSCs) are the basic building blocks for many applications in power systems, including grid integration of renewable energy [1] and energy storage [2], high-voltage dc transmission [3], as well as flexible ac transmission systems [4]....
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Journal Article•
TL;DR: In this article, power electronics, the technology of efficiently processing electric power, play an essential part in the integration of the dispersed generation units for good efficiency and high performance of the power systems.
Abstract: The global electrical energy consumption is rising and there is a steady increase of the demand on the power capacity, efficient production, distribution and utilization of energy. The traditional power systems are changing globally, a large number of dispersed generation (DG) units, including both renewable and nonrenewable energy sources such as wind turbines, photovoltaic (PV) generators, fuel cells, small hydro, wave generators, and gas/steam powered combined heat and power stations, are being integrated into power systems at the distribution level. Power electronics, the technology of efficiently processing electric power, play an essential part in the integration of the dispersed generation units for good efficiency and high performance of the power systems. This paper reviews the applications of power electronics in the integration of DG units, in particular, wind power, fuel cells and PV generators.
2,076 citations
TL;DR: An overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided in this paper, where a list of VSC-based HVDC installations worldwide is included.
Abstract: The ever increasing progress of high-voltage high-power fully controlled semiconductor technology continues to have a significant impact on the development of advanced power electronic apparatus used to support optimized operations and efficient management of electrical grids, which, in many cases, are fully or partially deregulated networks. Developments advance both the HVDC power transmission and the flexible ac transmission system technologies. In this paper, an overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided. Selected key multilevel converter topologies are presented. Control and modeling methods are discussed. A list of VSC-based HVdc installations worldwide is included. It is confirmed that the continuous development of power electronics presents cost-effective opportunities for the utilities to exploit, and HVdc remains a key technology. In particular, VSC-HVdc can address not only conventional network issues such as bulk power transmission, asynchronous network interconnections, back-to-back ac system linking, and voltage/stability support to mention a few, but also niche markets such as the integration of large-scale renewable energy sources with the grid and most recently large onshore/offshore wind farms.
2,023 citations
"Impedance Modeling and Analysis of ..." refers background in this paper
...THREE-PHASE voltage-source converters (VSCs) are the basic building blocks for many applications in power systems, including grid integration of renewable energy [1] and energy storage [2], high-voltage dc transmission [3], as well as flexible ac transmission systems [4]....
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TL;DR: In this paper, a new method to determine inverter-grid system stability using only the inverter output impedance and the grid impedance is developed, which can be applied to all current-source systems.
Abstract: Grid-connected inverters are known to become unstable when the grid impedance is high. Existing approaches to analyzing such instability are based on inverter control models that account for the grid impedance and the coupling with other grid-connected inverters. A new method to determine inverter-grid system stability using only the inverter output impedance and the grid impedance is developed in this paper. It will be shown that a grid-connected inverter will remain stable if the ratio between the grid impedance and the inverter output impedance satisfies the Nyquist stability criterion. This new impedance-based stability criterion is a generalization to the existing stability criterion for voltage-source systems, and can be applied to all current-source systems. A single-phase solar inverter is studied to demonstrate the application of the proposed method.
1,766 citations
"Impedance Modeling and Analysis of ..." refers methods in this paper
...In particular, it was shown in [6] that a grid-connected VSC used for grid integration of renewable energy can be modeled as a current source in parallel with an impedance, and the invertergrid system stability can be determined by applying the Nyquist stability criterion [7] to the ratio between the grid impedance and the VSC impedance....
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...Because of the decoupling between the two sequence subsystems, the stability criterion presented in [6] for grid-connected converters can be applied to each sequence impedance separately to determine overall converter-grid system stability....
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