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To cope with the weak grid stability issue of grid-tied voltage source converters (VSCs), this article proposes an artificial neural network (ANN) based approach for online stabilization control of the grid-tied VSC with the pole-tracking feature. First, an ANN is adopted to establish the mapping between the control parameters and the closed-loop poles of the grid-VSC system, serving as a computationally light model surrogate that is favorable for real-time control applications. Then, an online parameter search algorithm enabling simultaneous tuning of multiple controllers and parameters is developed, by which the system's poles under a new grid condition can be pulled to the reference ones, i.e., achieving the pole-tracking-based stabilization control of this article. Finally, the efficacy of the proposed method along with its stabilization effect is verified by MATLAB simulations and experimental results. [ABSTRACT FROM AUTHOR]

The grid-connected converters are becoming a fundamental building block in the modern power grid, and the stability of grid-connected converters, especially when they are connected to weak grid, has drawn increasing attention. In order to test the interaction with various grid impedances and validate the stability behaviors, more accurate and advanced emulation of grid impedance is becoming an emerging need. However, the existing approaches are still difficult to flexibly shape the grid impedance under medium-to-high frequency range. In this article, a novel configuration for programable impedance emulation of power grid is proposed. The proposed grid emulator is composed of two power electronics converters with special control functions, i.e., the impedance forming converter (IFC) and power supporting converter (PSC). The IFC is designed with a higher switching frequency to virtually emulate different grid impedances under medium-to-high frequency band. On the other hand, the PSC is designed with a lower switching frequency to generate/absorb the power of the converter under test. Besides, a Norton-circuit-based virtual impedance control strategy is introduced to eliminate the derivation term in the control structure of virtual impedance. Finally, the performance of the proposed emulator is verified by various simulations and experimental prototype. [ABSTRACT FROM AUTHOR]

The high penetration of renewables has sparked new oscillatory concerns in hybrid AC/DC grids. Knowledge of the oscillation regarding its source and propagation mechanism, is crucial to system operation. In this respect, the frequency-domain modal analysis (FMA) provides a way (e.g., participation factor) to extract these oscillation properties, and has been widely applied in converter-based AC grids. Compared to AC grids, oscillation properties of hybrid AC/DC grids are much more complex. Such complexity mainly originates from the diversity of oscillation propagation behaviors in hybrid AC/DC grids, where a distinctive local-area oscillation will emerge. Therefore, a dedicated oscillation propagation analysis for hybrid AC/DC grids is desired, which is the main focus of this paper. First, the mechanism of the distinctive local-area oscillation is analyzed considering AC/DC coupled dynamics of converters. Then, a systematic method for the oscillation propagation analysis is presented, where indices for characterizing the local-area oscillation and its impact area are established. Finally, some propagation properties are discussed, by which a strategy for the online oscillation source locating is designed. Case studies are carried out in a typical hybrid AC/DC grid to validate the propagation analysis in this paper. [ABSTRACT FROM AUTHOR]

Insulated gate bipolar transistor (IGBT) modules with multiple chips have wide range of applications, and the correct estimation for the thermal behaviors inside IGBT modules is becoming crucial. Thermal impedance matrix is one of the most adopted approaches to describe the thermal-coupling effect of IGBT module. For simplicity of analysis, the heatsink or ambient temperature is typically chosen as the reference node for the thermal-coupling impedance term, while the case temperature is simplified or ignored. This letter provides a thermal coupling model enabling case temperature as reference node. This proposed model decouples the thermal coupling impedances of IGBT module itself and external cooling condition, so that the modeling of cooling conditions outside device and inside the power module, can be separately considered. The characterization method and advantages of the proposed model are verified by an experimental demonstration, and the potential applications are further discussed. [ABSTRACT FROM AUTHOR]

Impedance modeling of a permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS) usually ignores the machine-side dynamics, instead, the machine-side system is simplified as a constant power source, while merely keeping the detailed grid-side converter model. This simplification is mainly based upon the intuition that the dc-bus capacitor is large enough so that the machine- and grid-side dynamics can be decoupled. However, the simplification probably does not hold in practice since there is always a tendency to minimize the capacitor size to achieve a smaller package for installation. Therefore, the machine-side system should not be ignored, otherwise maybe leading to error for stability analysis. To address this issue, this paper developed a complete ac-side impedance model of the PMSG-based WECS. Then, impact factors about the coupling characteristics between machine- and grid-side systems of the PMSG-based WECS are discussed. Finally, a case study of sub/super synchronous oscillation is carried out on a practical offshore wind farm integrated with a modular multilevel converter based high-voltage dc (MMC-HVDC) transmission system in China. The results show that ignoring the machine-side system would lead to an optimistic stability judgement, which confirms the necessity of the complete model of PMSG-based WECS. Furthermore, the system can be stabilized by either changing the couplings between machine- and grid-side systems of PMSG-based WECS or implementing active damping in the wind farm side MMC. [ABSTRACT FROM AUTHOR]

Frequency-domain model reduction is a crucial concern in applying the prevailing impedance method for the stability analysis of complex systems, e.g., the modular multilevel converter (MMC). Recently, it has been shown that under symmetric conditions, a 2 × 2 matrix-based impedance model characterizing the two coupled frequencies of MMC are sufficient for its stability analysis. However, when the asymmetry occurs, principally, a much higher number of frequency couplings will appear in the MMC and thereby leads to a significant rise in the model dimension. Enlighted by this issue, there is an urgent need of finding a suitable frequency-domain method that can serve as a general criterion for model reduction. To this end, this article proposes a block diagonal dominance (BDD)-based model reduction method and applied it to the asymmetric MMC. Basically, the BDD can decompose an N-dimensional task to N one-dimensional tasks, via which a significant reduction in model dimension can be realized. It is shown that by properly shifting the impedance model from one domain to another (e.g., α-β domain to d-q domain), the BDD property can be achieved for most asymmetric scenarios. Finally, various case studies considering different asymmetry degrees are conducted to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

High penetration of distributed generations and active loads have enabled the power electronics converter to become a vital component in the modern power grid system, and the broad employment of grid-connected converters at various power levels is making the grid impedance and characteristics complicated. Consequently, in order to validate more advanced features such as the reliability and stability performances of the grid-connected converters, it is becoming an emerging need to emulate the grid behaviors from more aspects. This article serves to foster and investigate the state-of-the-art techniques in the field of ac grid emulation from the perspective of multiple spatial-scales and multiple time-scales. Four major concepts used for grid emulation with featured principles, including concept I (analog simulation with under-scaled components), concept II (grid characteristics in the real-time simulator), concept III (grid characteristics in the converters structure), and concept IV (grid characteristics in the converters controller), are summarized, respectively, in this article. The practical implementation regarding to the circuit topology and the power supply for the grid emulation system are also discussed. Finally, the future trends and conclusions in the field of ac grid emulation are provided. [ABSTRACT FROM AUTHOR]

Impedance modeling and stability analysis of a grid-voltage source converters (VSC) system under symmetric ac grids have been extensively discussed in the literature, where the dq domain impedances are usually adopted. As for asymmetric ac grids, impedance modeling is no longer straightforward in the dq domain due to the presence of negative sequence components, where the linearization will result in a linear-time-periodically-varying system, making the frequency-domain analysis intractable. One way to address this issue would be the harmonic-transfer-function (HTF) approach. Although this method is conceptually clear, its application to the stability analysis of an unbalanced grid-VSC system is still challenging and an effective model is missing here, therefore this paper aims to bridge this gap. First, the sequence impedances of an unbalanced grid-VSC system is modeled in the HTF framework. Then the HTFs are truncated into four-by-four matrices by exploiting the property of frequency couplings. Based on this, the equivalent source and load model for Nyquist-based analysis are established, and they are thoroughly verified by impedance measurements as well as the accuracy on stability analysis. Finally, several stability concerns of the unbalanced grid-VSC system, as well as the feasibility of symmetric models for asymmetric ac grid stability analysis are discussed and clarified. [ABSTRACT FROM AUTHOR]

This letter proposes a thermal characterization method of power semiconductor devices based on an H-bridge testing circuit, as well as its corresponding control and measurements. In the proposed method, the power semiconductor devices under test (DUTs) operate under the switching mode, which is closer to the practical use. Due to the presence of switching loss, similar ranges of junction temperature can be achieved with much lower heating current than the one in the conventional testing method. Besides, a current controller is used to cut off the heating current rapidly, so that the proposed method can approximate an ideal step power loss and thus contribute to more accurate estimation of thermal impedance. In addition, the proposed testing method enables multiple DUTs and repeated measurements, in order to take parameter distribution and uncertainty into account. The feasibility, control, and some electrical behaviors of the proposed method are verified through experimental tests. [ABSTRACT FROM AUTHOR]

Impedance modeling of a Type-IV wind turbine is usually associated with model reductions, e.g., the grid-side converter is modeled in detail, whereas the machine-side converter (MSC) is simplified as a constant power load (CPL). Meanwhile, the Nyquist-based stability analyses are normally conducted on the ac side, where the evaluation of the stability margin can be difficult due to the presence of multiple eigen-loci. Although some similar analyses regarding the high-voltage dc systems are performed on the dc side, a justification of the consistency between the ac and dc side analysis is lacking. Therefore, this paper aims to address these issues by first developing a detailed impedance model of the Type-IV wind turbine, and then providing a formal proof of the equivalence between the ac- and dc-side analysis. The detailed Type-IV wind turbine model is verified by the measured frequency responses from simulations, as well as its correctness in Nyquist-based stability analysis. The MSC modeling effects are further discussed, for which a thorough comparison of the CPL-based model and the detailed model with respect to the stability margin is conducted. As a result, the feasibility of the CPL-based model for stability analysis is clarified. [ABSTRACT FROM AUTHOR]

The small-signal impedance modeling of a modular multilevel converter (MMC) is the key for analyzing resonance and stability of MMC-based power electronic systems. The MMC is a power converter with a multifrequency response due to its significant steady-state harmonic components in the arm currents and capacitor voltages. These internal harmonic dynamics may have great influence on the terminal characteristics of the MMC, which, therefore, are essential to be considered in the MMC impedance modeling. In this paper, the harmonic state-space (HSS) modeling approach is first introduced to characterize the multiharmonic coupling behavior of the MMC. On this basis, the small-signal impedance models of the MMC are then developed based on the proposed HSS model of the MMC, which are able to include all the internal harmonics within the MMC, leading to accurate impedance models. Besides, different control schemes for the MMC, such as open-loop control, ac voltage closed-loop control, and circulating current closed-loop control, have also been considered during the modeling process, which further reveals the impact of the MMC internal dynamics and control dynamics on the MMC impedance. Furthermore, an impedance-based stability analysis of the MMC-high-voltage direct current connected wind farm has been carried out to show how the HSS-based MMC impedance model can be used in practical system analysis. Finally, the proposed impedance models are validated by both simulation and experimental measurements. [ABSTRACT FROM AUTHOR]