Large-signal impedance for the analysis of sustained resonance in grid-connected converters
01 Jul 2017-pp 1-8
TL;DR: In this paper, a large-signal impedance based calculation of the sustained resonance amplitude (interharmonics) is presented for a grid-connected three-phase voltage source converter (VSC), where hard nonlinearities such as PWM saturation dominate the sensitivity of the impedance response to the perturbation amplitude.
Abstract: This paper introduces large-signal impedance for the modeling and analysis of sustained resonance in grid-connected converters. The large-signal impedance captures the converter impedance response for different amplitudes of the injected perturbation. Large-signal impedance based calculation of the sustained resonance amplitude (interharmonics) is presented for a grid-connected three-phase voltage source converter (VSC). The paper shows that hard nonlinearities, such as PWM saturation, dominate the sensitivity of the impedance response to the perturbation amplitude. The paper also presents modeling of the large-signal impedance for VSC; modeling challenges in the absence of the small-signal approximation and their solutions are discussed.
Citations
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TL;DR: In this paper, a new power system stability classification framework is proposed, which not only maintains the inherent logic of the classical classification but also provides wide coverage and future adaptability of the emerging stability issues.
Abstract: This paper concerns with the emerging power system stability issues, classification, and research prospects under a high share of renewables and power electronics. The decades-old traditional power system is undergoing a fast transition with two most prominent features: 1) high-penetration of renewable power generators, utilizing intermittent renewable sources such as wind and solar, and 2) high-penetration of power electronic devices in the generation e.g., wind turbine converters and solar power inverters, transmission e.g., flexible ac or dc transmission system converters, and distribution/utilization systems e.g., electric vehicle and microgrid. The development of modern power systems with dual high-penetrations, i.e., high-penetrations of renewables and power electronic devices, influences the power system dynamics significantly and causes new stability issues. This paper first overviews equipment-level features and system-level stability challenges introduced under the dual high-penetration scenario of the modern power system. Next, the impacts of emerging stability challenges on various aspects of the classical stability issues and classifications are highlighted. Under this context, this paper discusses the validity and limitations of the existing classical and extended power system stability classifications proposed by different IEEE/Cigre Working Groups. Furthermore, a new power system stability classification framework is proposed, which not only maintains the inherent logic of the classical classification but also provides wide coverage and future adaptability of the emerging stability issues. Finally, various classification-oriented research prospects in the power system stability domain are highlighted.
94 citations
TL;DR: In this paper, a large-signal impedance-based modeling and mitigation of resonance of grid-connected voltage source converters is proposed to predict resonance-generated distortions in converter-grid systems.
Abstract: Large-signal impedance of grid-connected converters can be used to predict resonance-generated distortions in converter-grid systems. Note that the large-signal impedance of a network represents its impedance response for different magnitudes of perturbation injected at its terminals. This paper presents large-signal impedance-based modeling and mitigation of resonance of grid-connected voltage source converters. Challenges of large-signal modeling because of the inapplicability of the small-signal approximation are addressed by leveraging the dominating influence of hard nonlinearities (such as pulsewidth modulation saturation and limiters) over soft nonlinearities (such as Park's transformations and phase-locked loop (PLL)) in shaping the large-signal behavior of the converter. The paper develops large-signal gains of hard nonlinearities using different types of describing functions. The paper shows that the large-signal impedance of a voltage source converter (VSC) can be shaped to reduce resonance-generated distortions by inserting limiters in the control system of the VSC. Developed large-signal impedance models are validated using numerical simulations of a VSC with dq current control and PLL. Large-signal impedance measurements of a commercial 1 MW VSC-based inverter and a medium-voltage doubly-fed induction generator with approximate 4 MW rating are presented to experimentally demonstrate the influence of the injected perturbation magnitude on the impedance response.
35 citations
Cites background or methods from "Large-signal impedance for the anal..."
...8 are developed using harmonic linearization method [25], [12]....
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...As shown in [25], [26], large-signal impedance responses of a grid-connected converter for different perturbation ampli-...
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..., resonance magnitude) in grid-connected converters [25], [26]....
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TL;DR: Signal-flow graphs for linear time-periodic systems to streamline and visually describe the frequency-domain modeling of complex phase-locked loop (PLL) systems used in grid-connected converters are introduced.
Abstract: This article introduces signal-flow graphs for linear time-periodic systems to streamline and visually describe the frequency-domain modeling of complex phase-locked loop (PLL) systems used in grid-connected converters. Small-signal modeling using the proposed graphs is demonstrated for two commonly used single-phase PLL structures: SOGI-PLL and Park-PLL. Loop-gain models are developed for these PLLs to evaluate how an orthogonal signal generator (OSG), which is required in single-phase PLLs using the synchronous reference frame (SRF) architecture, modifies the PLL loop gain compared to that of a three-phase SRF-PLL, which does not require an OSG. It is shown that the OSG in the SOGI-PLL and Park-PLL introduces a significant phase lag in the PLL loop gain, limiting the maximum bandwidth for which either PLL can be designed. Slow-frequency adaptation (SFA) of OSG is proposed to mitigate the influence of the OSG dynamics on the PLL loop gain. Experimental results are presented to validate the developed loop-gain models and show that the proposed SFA-SOGI-PLL and SFA-Park-PLL have better transient performance, they do not suffer from the bandwidth limit, and they preserve the steady-state performance of the standard SOGI-PLL and Park-PLL.
33 citations
Cites methods from "Large-signal impedance for the anal..."
...[16]–[18]; modeling using the harmonic linearization method, however, is not straight-forward because it requires analytically tracing a sinusoidal perturbation through highly nonlinear circuit and control system of a converter such as a complex phase-locked loop (PLL) structure....
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TL;DR: A comparative insight is provided into the application of existing and emerging modeling and stability analysis approaches for SSCI investigations in large-scale wind farms and highlights the strengths and weaknesses of various modeling methods and analysis criteria.
Abstract: The subsynchronous control interaction (SSCI) occurs when the wind turbine converter (WTC) controls interact with the series-compensated or weak AC network. The mechanism and attributes of the emerging interaction phenomenon invilving wind turbine generators (WTGs) are quite different from the traditional subsynchronous resonance or oscillation (SSR/SSO) phenomenon in steam turbine-generators (STGs). The SSCI is characterized by various system-wide parameters, including the wind speed and its uneven distribution in a wind power plant, varying number of WTGs connected to the grid, type of the WTGs, parameters of the WTC controls, level of series compensation, stiffness of the grid, etc. Such system-wide parameters change over time and thus make the modeling and analysis rather challenging to conduct SSCI studies. An ideal modeling approach for the SSCI is expected to preserve the system topology as well as valid for a wide range of operating conditions and parameters of the system. On the other hand, an ideal stability analysis method is supposed to give key quantitative information, such as the magnitude, frequency, and origin of the oscillation, as well as the component level participation indices. This paper provides a comparative insight into the application of existing and emerging modeling and stability analysis approaches for SSCI investigations in large-scale wind farms. It highlights the strengths and weaknesses of various modeling methods and analysis criteria. Finally, the paper underlines the recent advancements and points out towards the research directions in small/large-signal impedance modeling and stability approaches for accurate and quantitative investigation of the SSCI.
32 citations
TL;DR: A nonlinear analytical approach based on the describing function and generalized Nyquist criterion is proposed to analyze the characteristics of SSO with wind farms to present describing function-based model reduction considering key nonlinear elements involved in SSO.
Abstract: Eigen-analysis is widely used in the studies of power system oscillation and small-signal stability. However, it may give inaccurate analyses on subsynchronous oscillation (SSO) when nonlinearity is not negligible. In this paper, a nonlinear analytical approach based on the describing function and generalized Nyquist criterion is proposed to analyze the characteristics of SSO with wind farms. The paper first presents describing function-based model reduction considering key nonlinear elements involved in SSO, and then uses a generalized Nyquist criterion for accurate estimation of SSO amplitude and frequency. The results are verified by time-domain simulations on a detailed model with different scenarios considering variations of the system condition and controller parameters.
28 citations
Cites background from "Large-signal impedance for the anal..."
...Compared with “hard” nonlinearities, small-signal models that linearize “soft” nonlinearities may be used without much sacrifice on the accuracy of oscillation or resonance analysis [30]....
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References
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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.
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TL;DR: In this paper, power quality problems associated with distributed power (DP) inverters, implemented in large numbers onto the same distribution network, are investigated, and a complete network simulation study on an existing residential network with large penetration of photovoltaics (PV) on rooftops of houses and commercial buildings is included.
Abstract: Power quality problems associated with distributed power (DP) inverters, implemented in large numbers onto the same distribution network, are investigated. Currently, these power quality problems are mainly found in projects with large penetration of photovoltaics (PV) on rooftops of houses and commercial buildings. The main object of this paper is to analyze the observed phenomena of harmonic interference of large populations of these inverters and to compare the network interaction of different inverter topologies and control options. These power quality phenomenons are investigated by using extensive laboratory experiments, as well as computer modeling of different inverter topologies. A complete network simulation study on an existing residential network with large penetration of PVs, is included.
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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.
765 citations
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
TL;DR: In this article, the authors investigated the stability of offshore wind farms integration through a modular multilevel converter-based high-voltage dc (MMC-HVdc) transmission system and applied an impedance-based analytical approach to analyze the stability and to predict the phase margin of the interconnected system.
Abstract: This paper investigates the stability of offshore wind farms integration through a modular multilevel converter-based high-voltage dc (MMC-HVdc) transmission system. Resonances or instability phenomena have been reported in between wind farms and MMC-HVdc systems. They are arguably originated due to interactions between the MMC and the wind power inverters. However, the nature of these interactions is neither well understood nor reported in the literature. In this paper, the impedance-based analytical approach is applied to analyze the stability and to predict the phase margin of the interconnected system. For that, analytical impedance models of a three-phase MMC in a compensated modulation case and a direct modulation case are separately derived using the small-signal frequency domain method. Moreover, the impedance models of the MMC take the circulating current control into account. The derived impedance models are then verified by comparing the frequency responses of the analytical model with the impedance measured in a nonlinear time-domain simulation model developed in MATLAB. The results show that the potential resonances or instability of the interconnected system can be readily predicted through the Nyquist diagrams. In addition, the analysis indicates that the circulating current control of the MMC has a significant impact on the stability of the interconnected system. Finally, the time-domain simulations validate the theoretical analysis.
194 citations