arXiv subjects

Hua Geng

Publications and source records attributed to Hua Geng.

23 records · Page 2Linked to original sources

arXiv2021

Transient Stability of Hybrid Power Systems Dominated by Different Types of Grid-Forming Devices

This paper investigates the transient stability of power systems co-dominated by different types of grid-forming (GFM) devices. Synchronous generators (SGs and VSGs) and droop-controlled inverters are typical GFM devices in modern power systems. SGs/VSGs are able to provide inertia while droop-controlled inverters are generally inertialess. The transient stability of power systems dominated by homogeneous GFM devices has been extensively studied. Regarding the hybrid system jointly dominated by heterogeneous GFM devices, the transient stability is rarely reported. This paper aims to fill this gap. It is found that the synchronization behavior of the hybrid system can be described by a second-order motion equation, resembling the swing equation of SGs. Moreover, two significant differences from conventional power systems are discovered. The first is that the droop control dramatically enhances the damping effect, greatly affecting the transient stability region. The second is that the frequency state variable exhibits a jump at the moment of fault disturbances, thus impacting the post-fault initial-state location and stability assessment. The underlying mechanism behind the two new characteristics is clarified and the impact on the transient stability performance is analyzed and verified. The findings provide new insights into transient stability of power systems hosting heterogeneous devices.

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arXiv2021

Mode Clustering Based Dynamic Equivalent Modeling of Wind Farm for Small-Signal Stability Analysis

Dynamic equivalent models (DEMs) are necessities for the small-signal stability (SSS) analysis of the power system with large-scale wind farms (WFs). This paper proposes a mode clustering based dynamic equivalent modeling method of WFs for the SSS analysis. It is deemed that a DEM can be used to represent the whole WF to evaluate its impact on the SSS of power systems, as long as the frequency response of the DEM adequately matches that of the detailed WF model around the frequency of oscillation modes of concern. Focusing on the concerned oscillation modes in the small-signal model of the whole WF, closely distributed modes of them on the complex plane are classified into a cluster and then represented by a single mode. By the linear superposition principle, the modal participation factor (MPF) regarding each of modal clusters are superimposed, generating a feature vector for each wind turbine (WT). Based on the feature vectors, the oscillation feature similarity of the WTs as well as their aggregation can be evaluated and performed easily. Taking the DC-link voltage control (DVC) mode of full-power WT converters as an example, the aggregated DEM is verified by frequency-domain analyses and time-domain simulations.

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arXiv2021

Synchronization Instability of Inverter-Based Generation During Asymmetrical Grid Faults

The transient stability of traditional power systems is concerned with the ability of generators to stay synchronized with the positive-sequence voltage of the network, whether for symmetrical or asymmetrical faults. In contrast, both positive- and negative-sequence synchronizations should be of concern for inverter-based generation (IBG) under asymmetrical faults. This is because the latest grid codes stipulate that IBG should inject dual-sequence current when riding through asymmetrical faults. Currently, much less is known about the synchronization stability during asymmetrical faults. This significantly differs from the positive-sequence synchronization alone because the coupled dual-sequence synchronization is involved. This paper aims to fill this gap. Considering the sequence coupling under asymmetrical faults, the dual-sequence synchronization model of IBG is developed. Based on the model, the conditions that steady-state equilibrium points should follow are identified. The conditions throw light on the possible types of synchronization instability, including the positive-sequence dominated instability and the negative-sequence dominated one. For different types of instability, the dominant factors are analyzed quantitatively, which are reflected by the limit on the current injection amplitude. Exceeding the limit will lead to the loss of both positive- and negative-sequence synchronizations. The model and the analysis are verified by simulations and hardware-in-the-loop experiments.

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arXiv2020

Transient Stability of Power Systems Integrated With Inverter-Based Generation

The fundamental mechanism of transient stability of the conventional power system can be physically understood with the rotor swing dynamics of synchronous generators (SGs). The basic factor affecting the transient stability is the power-angle relationship among the SGs. The integration of a large number of inverter-based generations (IBGs) into the power system inevitably changes the original power-angle relationship, therefore leading to a more complicated transient stability mechanism. In this letter, the impact of IBG on the power system transient stability is analytically investigated. It is shown that the connection of IBG changes the original power-angle relationship into a nonsinusoidal curve, introduces a limit on the peak of the rotor swing, and therefore brings about a new bottleneck for the transient stability.

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arXiv2020

Synchronization Stability Analysis and Enhancement of Grid-Tied Multi-Converter Systems

Synchronization instability of grid-tied converters during grid faults is a serious concern. For phase-locked loop (PLL) synchronized converters, synchronization stability during grid faults requires that PLL can resynchronize with the power grid. It is of great significance for meeting grid code specifications such as reactive current support. However, the synchronization in the context of high-impedance weak grid is not always readily achieved. On one hand, the terminal voltage of converters is changeable due to being susceptible to the output current. One the other hand, there is interaction between multiple converters interconnected to the grid. Therefore, loss of synchronism (LOS) of converters during grid faults likely occurs. Previous research has investigated the LOS issue of single-converter infinite-bus (SCIB) system, but mostly overlooked the interaction between multiple converters. Previous efforts to address the issue of LOS are also in doubt for multi-converter systems. This paper develops a multi-converter infinite-bus (MCIB) system model and analyzes the root cause of LOS with considering multi-converter interaction during grid faults. Moreover, a feedforward compensated PLL (FFC-PLL) method applicable for multi-converter systems is developed to address the LOS issue. Simulations results verify the effectiveness of the method.

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