This paper presents an overview of the synchronization stability of converter-based resources under a wide range of grid conditions. The general grid-synchronization principles for grid-following and grid-forming modes are reviewed first. Then, the small-signal and transient stability of these two operating modes are discussed, and the design-oriented analyses are performed to illustrate the control impact. Lastly, perspectives on the prospects and challenges are shared.

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Grid-Synchronization Stability of Converter-Based Resources - An Overview

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.

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Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review

This paper investigates the impact of power grid strength and phase-locked loop (PLL) parameters on small signal stability of grid-connected doubly fed induction generator (DFIG)-based wind farm. Modal analysis of the grid-connected DFIG wind turbine under different operating conditions and various power grid strengths are investigated at first. Modal analysis results reveal that the DFIG connected to a weak grid may easily lose stability under the heavy-duty operating conditions due to PLL oscillation. The object of this paper is to identify the PLL oscillation mechanism as well as influence factors and propose a damping solution for this oscillation mode. A simplified linear system model of the grid-connected DFIG wind turbine is proposed for analyzing the PLL oscillation. Through the complex torque coefficients method and using this model, the oscillation mechanism and influence factors including the power grid strength and the PLL parameters are identified. To suppress this PLL oscillation, a mixed $H_2/H_{\infty }$ robust damping controller is proposed and designed for the DFIG. Electromagnetic transient simulation results of both single-DFIG system and multiply-DFIG system verify the correctness of the analysis results and effectiveness of the proposed damping controller.

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Impact of Power Grid Strength and PLL Parameters on Stability of Grid-Connected DFIG Wind Farm

Power electronic converter (PEC)-interfaced renewable energy generators (REGs) are increasingly being integrated to the power grid. With the high renewable power penetration levels, one of the key power system parameters, namely reactive power, is affected, provoking steady-state voltage and dynamic/transient stability issues. Therefore, it is imperative to maintain and manage adequate reactive power reserve to ensure a stable and reliable power grid. This paper presents a comprehensive literature review on the reactive power management in renewable rich power grids. Reactive power requirements stipulated in different grid codes for REGs are summarized to assess their adequacy for future network requirements. The PEC-interfaced REGs are discussed with a special emphasis on their reactive power compensation capability and control schemes. Along with REGs, conventional reactive power support devices (e.g., capacitor banks) and PEC-interfaced reactive power support devices (e.g., static synchronous compensators) play an indispensable role in the reactive power management of renewable rich power grids, and thus their reactive power control capabilities and limitations are thoroughly reviewed in this paper. Then, various reactive power control strategies are reviewed with a special emphasis on their advantages/disadvantages. Reactive power coordination between support devices and their optimal capacity are vital for an efficient and stable management of the power grid. Accordingly, the prominent reactive power coordination and optimization algorithms are critically examined and discussed in this paper. Finally, the key issues pertinent to the reactive power management in renewable rich power grids are enlisted with some important technical recommendations for the power industry, policymakers, and academic researchers.

Reactive Power Management in Renewable Rich Power Grids: A Review of Grid-Codes, Renewable Generators, Support Devices, Control Strategies and Optimization Algorithms

国際会議報告 IEEE-Power Engineering Society Winter Meeting

This paper analyzes the power transfer limitation of the photovoltaic (PV) power plant under the ultra-weak grid condition, i.e., when the short-circuit ratio (SCR) is close to 1. It explicitly identifies that a minimum SCR of 2 is required for the PV power plant to deliver the rated active power when operating with the unity power factor. Then, considering the reactive power compensation from PV inverters, the minimum SCR in respect to power factor (PF) is derived, and the optimized coordination of the active and reactive power is exploited. It is revealed that the power transfer capability of PV power plant under the ultra-weak grid is significantly improved with the low PF operation. An adaptive reactive power droop control is next proposed to effectively distribute the reactive power demands to the individual inverters, and meanwhile, maximize the power transfer capacity of the PV power plant. Simulation results of a 200-MW PV power plant demonstrate that the proposed method can ensure the rated power transfer of PV power plant with the SCR of 1.25, provided that the PV inverters are operated with the minimal PF=0.9.

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Adaptive Reactive Power Control of PV Power Plants for Improved Power Transfer Capability Under Ultra-Weak Grid Conditions

This paper proposes a symmetrical phase-locked loop (PLL) that can eliminate the frequency-coupling terms caused by the asymmetric dynamics of conventional PLLs. In the approach, a concept of complex phase angle vector with both real and imaginary phase components is introduced, which enables to control the direct- and quadrature-axis components with symmetrical dynamics. The small-signal impedance model that characterizes the dynamic effect of the symmetrical PLL on the current control loop is also derived, which, differing from the conventional multiple-input multiple-output impedance matrix, is in a single-input single-output (SISO) form based on complex transfer functions. This SISO representation allows for a design-oriented analysis. Moreover, the undesired sub-synchronous oscillation caused by the conventional asymmetrical PLL can be avoided, and the classical SISO impedance shaping can be utilized to cancel the negative resistor behavior caused by PLL; thus can greatly enhance the grid synchronization stability under weak grid conditions. The effectiveness of the theoretical analysis is validated by experimental tests.

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Symmetrical PLL for SISO Impedance Modeling and Enhanced Stability in Weak Grids

This paper analyzes the sub-synchronous control interaction of droop-controlled grid-forming voltage-source converters (VSCs). The complex-valued impedance model of the droop-controlled VSC in the stationary frame is proposed first, based on which the grid-VSC interaction is characterized. It is revealed that the sub-synchronous resonances can be induced in stiff grids, owing to the interactions of inner voltage control loop and the outer power control loop. Experimental tests corroborate the theoretical analysis.

Sub-Synchronous Control Interaction in Grid-Forming VSCs with Droop Control

The Photovoltaic (PV) power plants are usually deployed in remote areas with the high solar irradiance, and their power transfer capabilities can be greatly limited by the large impedance of long-distance transmission lines This paper investigates first the power transfer limit of the PV power plant with the Short-Circuit Ratio (SCR) close to 1 It explicitly identifies that a minimum SCR of 2 is required for the PV power plant to deliver the rated active power when operating with the unit power factor Then, considering the reactive power compensation from PV inverters, the minimum SCR along with the different Power Factor (PF) is derived An adaptive reactive power droop control is further proposed to improve the power Transfer capability of the PV power plant Simulation results of a 20MW solar farm demonstrate that the proposed method can ensure the rated power transfer of PV power plant with SCR of 125, provided that PV inverters with PF min =09 is used

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Adaptive reactive power control of PV power plants for improved power transfer capability under ultra-weak grid conditions

This paper presents an adaptive Discontinuous Pulse Width Modulation (DPWM) method for three phase inverter. A common-mode (CM) signal is injected in the modulation wave to realize the DPWM and the waveform of the injected CM signal is modulated according to the modulation index of the inverter. This feature can lower the equivalent CM voltage source in the low modulation index case to avoid the oscillation without increasing the CM choke. In high modulation index case, the proposed DPWM can realize the same switching loss reduction effect as conventional DPWM methods. The implementation of the proposed DPWM method is explained. A detailed analysis of proposed DPWM method and conventional DPWM methods is presented. The simulated results shows that the proposed DPWM method achieves a balance between efficiency and stability in wide modulation index range.

Xin Kai

Papers

Adaptive Reactive Power Control of PV Power Plants for Improved Power Transfer Capability Under Ultra-Weak Grid Conditions

Symmetrical PLL for SISO Impedance Modeling and Enhanced Stability in Weak Grids

Sub-Synchronous Control Interaction in Grid-Forming VSCs with Droop Control

Adaptive reactive power control of PV power plants for improved power transfer capability under ultra-weak grid conditions

An adaptive Discontinuous Pulse Width Modulation (DPWM) method for three phase inverter