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

The contribution of this paper has been focused on investigating a new microgrid architecture that integrates the solid-state transformer with zonal dc microgrids. By utilizing the dc and ac links of the solid-state transformer, both ac and dc networks can access the distribution system, which renders the coordinate management of the power and guarantees high power supply reliability. In addition, the presented system together with the proposed power management strategy can minimize the effect of newly established zonal dc microgrid to the existing power grid, of which promises a better stability. To this end, this paper takes the photovoltaic and battery as the typical renewable energy resource and energy storage device, and develops a simulation platform for system study. Simulation results are shown to demonstrate the feasibility of the proposal.

On Integration of Solid-State Transformer With Zonal DC Microgrid

Differing from synchronous generators, there are lack of physical laws governing the synchronization dynamics of voltage-source converters (VSCs). The widely used phase-locked loop (PLL) plays a critical role in maintaining the synchronism of current-controlled VSCs, whose dynamics are highly affected by the power exchange between VSCs and the grid. This article presents a design-oriented analysis on the transient stability of PLL-synchronized VSCs, i.e., the synchronization stability of VSCs under large disturbances, by employing the phase portrait approach. Insights into the stabilizing effects of the first- and second-order PLLs are provided with the quantitative analysis. It is revealed that simply increasing the damping ratio of the second-order PLL may fail to stabilize VSCs during severe grid faults, whereas the first-order PLL can always guarantee the transient stability of VSCs when equilibrium operation points exist. An adaptive PLL that switches between the second-order and the first-order PLL during the fault-occurring/-clearing transient is proposed for preserving both the transient stability and the phase-tracking accuracy. Time-domain simulations and experimental tests, considering both the grid fault and the fault recovery, are performed, and the obtained results validate the theoretical findings.

Design-Oriented Transient Stability Analysis of PLL-Synchronized Voltage-Source Converters

With an increasing capacity in the converter-based generation to the modern power system, a growing demand for such systems to be more grid-friendly has emerged. Consequently, grid-forming converters have been proposed as a promising solution as they are compatible with the conventional synchronous-machine-based power system. However, most research focuses on the grid-forming control during normal operating conditions without considering the fundamental distinction between a grid-forming converter and a synchronous machine when considering its short-circuit capability. The current limitation of grid-forming converters during fault conditions is not well described in the available literature and present solutions often aim to switch the control structure to a grid-following structure during the fault. Yet, for a future converter-based power system with no or little integration of synchronous machines, the converters need to preserve their voltage-mode characteristics and be robust toward weak-grid conditions. To address this issue, this article discusses the fundamental issue of grid-forming converter control during grid fault conditions and proposes a fault-mode controller which keeps the voltage-mode characteristics of the grid-forming structure while simultaneously limiting the converter currents to an admissible value. The proposed method is evaluated in a detailed simulation model and verified through an experimental test setup.

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Current Limiting Control With Enhanced Dynamics of Grid-Forming Converters During Fault Conditions

Grid-connected converters exposed to weak grid conditions and severe fault events are at risk of losing synchronism with the external grid and neighboring converters. This predicament has led to a growing interest in analyzing the synchronization mechanism and developing models and tools for predicting the transient stability of grid-connected converters. This paper presents a thorough review of the developed methods that describe the phenomena of synchronization instability of grid-connected converters under severe symmetrical grid faults. These methods are compared where the advantages and disadvantages of each method are carefully mapped. The analytical derivations and a detailed simulation model are verified through experimental tests of three case studies. Steady-state and quasi-static analysis can determine whether a given fault condition results in a stable or unstable operating point. However, without considering the dynamics of the synchronization unit, transient stability cannot be guaranteed. By comparing the synchronization unit to a synchronous machine, the damping of the phase-locked loop is identified. For accurate stability assessment, either nonlinear phase portraits or time-domain simulations must be performed. Until this point, no direct stability assessment method is available which consider the damping effect of the synchronization unit. Therefore, additional work is needed on this field in future research.

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An Overview of Assessment Methods for Synchronization Stability of Grid-Connected Converters Under Severe Symmetrical Grid Faults

The virtual synchronous generator (VSG) is emerging as an attractive solution for interconnecting converter-based resources with the power grid. However, due to the nonlinear power-angle relationship, VSGs are, similar to synchronous generators, prone to the loss of synchronization (LOS) under large grid disturbances. This article, thus, analyzes the large-signal synchronization stability, i.e., the transient stability of grid-connected VSGs, and then proposes a mode-adaptive power-angle control method for enhancing the transient stability. In this approach, the positive-feedback mode of the power-angle control of the VSG is detected and adaptively switched to the negative-feedback mode after large disturbances. Thus, the LOS of the VSG can be avoided when there are equilibrium points after the disturbance. Moreover, during severe grid faults without any equilibrium points, a bounded dynamic response of the power angle can be obtained with the mode-adaptive control, and the VSG can still be stabilized even if the fault-clearing time is beyond the critical clearing time. These superior features prevent the VSG-based system from collapsing due to the delayed fault clearance or the malfunction of protective relays. Finally, the time-domain simulations and experimental tests are performed to verify the effectiveness of the control method.

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A Mode-Adaptive Power-Angle Control Method for Transient Stability Enhancement of Virtual Synchronous Generators

This article discusses the mathematical relationship between gridvoltage-modulated (GVM) direct-power control (DPC) and vector-current control (VCC) for three-phase voltage-source converters (VSCs). GVM-DPC is equivalent to VCC at the steady state, and it presents a superior transient performance by removing the need for the phase-locked loop (PLL). This means that GVMDPC solves the disadvantage of conventional DPC, such as poor steady-state performance. Moreover, GVM-DPC will reduce the computational burden in comparison with VCC due to the absence of Park transformation and PLL. Consequently, we can expect that the GVM-DPC method will be capable of plug-and-play for VSCs. Finally, the experimental results match the theoretical expectations closely.

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Control of Grid-Connected Voltage-Source Converters: The Relationship Between Direct-Power Control and Vector-Current Control

Increased penetration of converter-based power generation has enforced system operators to require ancillary services from distributed generation in order to support the grid and improve the power system stability and reliability. Recent and next-generation grid codes require asymmetrical current provision during unbalanced faults for optimal voltage support. To address this, based on the highly used flexible positive- and negative-sequence control method for current reference generation, this paper presents a general current reference strategy for asymmetrical fault control, where a direct and explicit method is proposed to calculate power references and controller gains while simultaneously complying with converter current limitation and fulfilling the next-generation grid code requirements. The proposed method is tested for three distinct asymmetrical grid faults considering the requirements for dynamic voltage support of the recently revised German grid code as well as the next-generation grid codes. It is shown that the proposed method can improve the fault ride-through performance during asymmetrical faults compared with conventional solutions and comply with modern grid code requirements in a general and flexible manner.

Xiongfei Wang

Papers

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

An Overview of Assessment Methods for Synchronization Stability of Grid-Connected Converters Under Severe Symmetrical Grid Faults

A Mode-Adaptive Power-Angle Control Method for Transient Stability Enhancement of Virtual Synchronous Generators

Control of Grid-Connected Voltage-Source Converters: The Relationship Between Direct-Power Control and Vector-Current Control

Current Reference Generation Based on Next-Generation Grid Code Requirements of Grid-Tied Converters During Asymmetrical Faults