Grid-following Converters Research Articles

Modeling of Converter Synchronization Stability Under Grid Faults: The General Case

The synchronization dynamics of grid-connected power converters are known to have a tremendous impact on transient stability and fault ride-through performance under grid faults. Up till now, the modeling of converter synchronization stability, which has paved the way for numerous enhanced control methods, is developed around the assumption that the grid fault is symmetrical. However, this is rarely the case. Moreover, grid codes require dual-sequence current injection during asymmetrical faults, which implies that the previously developed models are no longer valid during unbalanced conditions. To address these issues, this article identifies the necessary stability conditions during asymmetrical conditions and presents a quasi-static large-signal reduced-order model of a grid-following converter for analyzing its synchronizing interaction with the external network during symmetrical and asymmetrical grid faults. The modeling approach is developed and tested for three different short-circuit faults: A single line-to-ground fault, a double line-to-ground fault, and a line-to-line fault. The accuracy of the proposed model is verified through detailed simulation studies and experimental tests. Thus, this model can be used to assess the transient synchronization stability of grid-following converters during any type of grid fault, and due to its low-order representation, it may be well applicable for large-scale power system studies.

Unified Virtual Oscillator Control for Grid-Forming and Grid-Following Converters

A unified virtual oscillator controller (uVOC) is proposed, which enables a unified analysis, design, and implementation framework for both grid-forming (GFM) and grid-following (GFL) voltage-source converters (VSCs). Oscillator-based GFM controllers, such as dispatchable virtual oscillator control (dVOC), offer a rigorous analytical framework with enhanced synchronization but lack effective fault handling capability, which severely limits practical application. The proposed uVOC facilitates synchronization with an arbitrarily low grid voltage and fast overcurrent limiting; this enables effective fault ride-through unlike existing GFM controllers which typically switch to a back-up controller during the fault. GFM operation with uVOC is achieved in both grid-connected and islanded modes with seamless transitions between the two. In GFL converters, bidirectional power flow control and dc bus voltage regulation are achieved with uVOC. No phase-locked loop (PLL) is required for either GFL or GFM operation circumventing the synchronization issues associated with PLLs in weak grid applications. Detail small-signal models for GFM and GFL operation have been developed, and the systematic design guidelines for controller parameters are provided. The proposed controller is validated through hardware experiments in a hybrid ac-dc microgrid.

Reduced-Order and Aggregated Modeling of Large-Signal Synchronization Stability for Multiconverter Systems

During severe grid faults, grid-following converters may become unstable and experience loss of synchronization when complying with requirements for low-voltage ride-through capability. This phenomenon is well described, understood, and modeled for single-converter systems but lacks a modeling framework when extended to multiconverter systems. To fill this gap, this work presents the necessary stability conditions and aggregated reduced-order models for different multiconverter configurations, which can be used to assess the transient synchronization stability of grid-following converters under symmetrical grid faults. The necessary conditions for transient stability and the aggregated models are verified through numerous simulation studies, which verify their high accuracy for large-signal synchronization stability assessment. To that end, the Anholt wind power plant is considered as a case study where the aggregated model is compared to the full operation of a wind farm string containing nine full-order grid-following converter models. High model accuracy is obtained, and the computational burden associated with the proposed model is reduced with a factor of 100 compared with a full-order representation on the tested system. Accordingly, the presented analysis and proposed modeling are attractive as a screening tool and a convenient approach for early-stage fault analysis of system design.

Grid Impedance Estimation Through Grid-Forming Power Converters

In more-electronics power systems, grid-forming power converters, which operate as ac voltage sources, regulate the grid frequency and voltages in replacement of synchronous generators. Notably, grid impedances greatly influence the small signal and voltage stability of grid-forming converters. As such, prior knowledge of grid impedances can be very helpful for controller design. However, grid impedance estimation schemes are normally designed for current-controlled grid-following converters. Moreover, they are either very complicated or only yield grid inductances in a generally intrusive way. To fill this research gap, an impedance estimation method well suited to grid-forming converters is proposed. The method consists of four operating modes, which work well in voltage and power control cases. In the voltage control case, the voltage amplitude perturbation or phase angle information is exploited. Subsequently, the grid inductance and resistance are derived from power measurement. Alternatively, the active or reactive power information serves to estimate the grid impedance in the power control case. The proposed method features an easy implementation without any harmonic distortion, safety concern, or dependence on control parameters. Moreover, the method operates nonintrusively in most scenarios. Furthermore, a novel Kalman filtering scheme is proposed to provide added incentives. Finally, simulation and experimental results validate the effectiveness and simplicity of the proposed method.

Robust Stability Investigation of the Interactions Among Grid-Forming and Grid-Following Converters

State-of-the-art grid-connected converters can be classified as “grid-following,” meaning that they require a dedicated synchronization unit in order to inject active and reactive currents into the grid. Recently, other converter control concepts have been proposed in the literature, such as the synchronverter, which can instead achieve synchronization without a dedicated unit and, within its physical limitations, make the converter behave as an ideal voltage source. Since it should be expected that the grid-connected converters having different control philosophies will coexist for many years, in this article, the interaction among the converters operating nearby are addressed. First, the component connection method (CCM) technique is introduced, as a means for obtaining the state-space representation of a complex system with several units operating nearby. Due to the complexity of the grid and the difficulty in obtaining its exact representation, $\mu $ -analysis is adopted in this article for assessing the robust stability of the converter under different operating conditions, according to a defined set of plant uncertainties. Simulation results and experimental tests in a laboratory environment by means of a power hardware-in-the-loop (PHIL) test bench are performed to demonstrate the validity of the presented analysis.

Using High-Bandwidth Voltage Amplifier to Emulate Grid-Following Inverter for AC Microgrid Dynamics Studies

AC microgrid is an attractive way to energize local loads due to remotely located renewable generation. The AC microgrid can conceptually comprise several grid-forming and grid-following power converters, renewable energy sources, energy storage and local loads. To study the microgrid dynamics, power-hardware-in-the-loop (PHIL)-based test setups are commonly used since they provide high flexibility and enable testing the performance of real converters. In a standard PHIL setup, different components of the AC microgrid exist as real commercial devices or electrical emulators or, alternatively, can be simulated using real-time simulators. For accurate, reliable and repeatable results, the PHIL-setup should be able to capture the dynamics of the microgrid loads and sources as accurately as possible. Several studies have shown how electrical machines, dynamic RLC loads, battery storages and photovoltaic and wind generators can be emulated in a PHIL setup. However, there are no studies discussing how a three-phase grid-following power converter with its internal control functions should be emulated, regardless of the fact that grid-following converters (e.g., photovoltaic and battery storage inverters) are the basic building blocks of AC microgrids. One could naturally use a real converter to represent such dynamic load. However, practical implementation of a real three-phase converter is much more challenging and requires special knowledge. To simplify the practical implementation of microgrid PHIL-studies, this paper demonstrates the use of a commercial high-bandwidth voltage amplifier as a dynamic three-phase power converter emulator. The dynamic performance of the PHIL setup is evaluated by identifying the small-signal impedance of the emulator with various control parameters and by time-domain step tests. The emulator is shown to yield the same impedance behavior as real three-phase converters. Thus, dynamic phenomena such as harmonic resonance in the AC microgrid can be studied in the presence of grid-following converters.