Diego Perez-Estevez

The maritime transport sector is a major source of pollutant emissions, posing severe health risks especially in port cities. While shore-to-ship (S2S) power connections are an effective solution to decarbonize ports, their implementation presents significant technical challenges. Specifically, S2S connections must comply with the IEC/IEEE 80005 standard when powering vessels with diverse power ratings, voltages, and frequencies. Consequently, supplying these shipboard microgrids requires robust grid-forming (GFM) converters ensuring accurate voltage and frequency regulation, a superior dynamic response, and a wide stability range against severe grid impedance variations. To accommodate the highly variable power demands of different vessels, onshore facilities typically employ multiple parallel converters. Therefore, this paper proposes a GFM controller for parallel onshore voltage source converters (VSCs) in S2S applications. The scheme integrates an inner voltage control loop based on state-space theory, which maximizes the stability range against different impedance values. Furthermore, an outer droop control loop is implemented, using low-pass filters to provide virtual inertia, damp transient power fluctuations, and decouple the control loop dynamics. The proposed controller is validated by simulation and experiments under a standard S2S connection sequence, which encompasses the synchronization and parallel operation of two VSCs with the ship’s generator, and the subsequent transition to islanded mode. The results demonstrate zero steady-state error in voltage and frequency, a superior dynamic response, and robust disturbance rejection, thereby ensuring smooth connections, accurate power sharing, and stability throughout the entire sequence.

Voltage-controlled voltage-source inverters with an output $LC$ filter are used in a wide variety of power supply applications. Common control structures include single-loop and double-loop voltage control, typically employing a proportional-resonant voltage controller. This article focuses on the double-loop voltage control, where a proportional controller is employed in the inner current control loop. The stability region is defined as the frequency ratios between the sampling frequency ($f_{s}$) and the natural frequency of the $LC$ filter ($f_{n}$) with which the system can be stable. In previous publications, it has been proposed to place the current controller in the feedback path of the inner control loop, rather than in the forward path, with the aim of increasing the stability region. This article demonstrates that both control structures are equivalent, with the same stability region, although they require different controller tuning. This article presents, for the first time, the complete stability region of the double-loop voltage control, and demonstrates that it can be stable for any frequency ratio $f_{n}/f_{s}$. The stability region presented in previous publications, where $f_{n}=f_{s}/6$ is a critical frequency, is the stability region that allows obtaining a minimum-phase system, which should not be confused with the complete stability region. Finally, it is demonstrated that the unitary decoupling of the capacitor voltage allows obtaining a minimum-phase system across the complete stability region. Experimental results confirm the theoretical analysis conducted.

Grid-forming (GFM) converters facilitate voltage and frequency stability in grids with high penetration of inverter-based renewable energy technologies. Under nominal conditions, the primary function of GFM converters is to control the output voltage by operating in voltage-source mode (VSM). During off-nominal conditions, especially in the presence of severe grid disturbances, the primary objective is to ensure accurate and fast-response current limiting to protect the GFM converter from overcurrents by operating in current-source mode (CSM). A wide stability margin of the inner loops must be guaranteed in nominal and contingency conditions due to the high variability of scenarios that a GFM converter may encounter. This article proposes a double-loop controller with current-limiting capability for GFM converters, ensuring good GFM performance in stand-alone and grid-connected scenarios, including those with very weak and stiff grids. Additionally, the proposed controller provides an enhanced current-limiting response with precise current control during faults. Finally, the proposed controller demonstrates a wide stability range in both VSM and CSM modes. This is experimentally validated through tests of GFM performance in very weak and stiff grids, with sudden changes in the short-circuit ratio, as well as through current-limiting tests under grid-voltage bolted faults conducted across different grid-impedance conditions.

This article introduces an impedance model for a grid-forming (GFM) converter with droop control (Droop-GFM converter) connected to a weak power grid. The proposed model offers a more accurate and simpler alternative to the conventional minor-loop criterion and the generalized Nyquist stability criterion for studying the stability of a Droop-GFM converter, as it accounts for loading effects and uses only two transfer functions. Formulated in the $\alpha \beta$ frame, the model consists of only two balanced impedances, offering a clearer physical interpretation of the impedance asymmetry introduced by droop control, in contrast to dq-frame impedance matrix models. The model describes frequency coupling effects (the mirror frequency problem) across the entire frequency range, providing both a geometric and physical understanding of this phenomenon. It allows determination of the bandwidth of the frequency coupling region, and how this region changes based on the droop coefficients and the loading condition of the Droop-GFM converter. The theoretical findings are validated experimentally using a Droop-GFM converter in three distinct scenarios, including: weak power grid operation, a change in the droop coefficients, and a change in the power grid impedance.

Voltage-controlled voltage-source inverters with an output LC-filter are used in a wide variety of power supply applications. LC-filters allow for the attenuation of switching harmonic content, but their resonance can compromise the stability of the system. One of the most common control techniques in these systems consists of a single-loop voltage control with proportional-resonant (PR) or resonant (R) controllers. Numerous previous publications have shown that these systems are only stable for certain ratios between the resonant frequency of the filter ($f_{n}$) and the sampling frequency ($f_{s}$), leading to the proposal of different methods to widen this stability region. However, existing methods are based on a continuous-time domain stability analysis, where some approximations are used. This article, after presenting a discrete-time domain stability analysis without approximations that results in a stability region different from that presented in previous publications, demonstrates that the single-loop voltage control with PR controller and capacitor voltage unitary decoupling allows working with positive proportional gains when $f_{n}$ is either above or below $f_{s}/3$, improving the performance of existing methods. The article also demonstrates that unitary decoupling of the capacitor voltage is not as effective with an R controller. Experimental tests confirm the theoretical analysis conducted.

Loads affect the stability of grid-forming (GFM) converters, both in stand-alone and grid-connected modes. This has motivated the development of methods that study the stability of GFM converters using an impedance-based analysis. AC impedance-based stability analyses usually study the stability of GFM converters by applying the Nyquist criterion to an impedance ratio denoted as minor loop gain. A limitation of the previous approach when applied to GFM converters, which is not always clearly stated, is that the output impedance of a GFM converter depends on the impedance seen by the converter. This loading effect is not accurately modeled in the literature. This article proposes an impedance model that accounts for this loading effect on the GFM converter output impedance, without introducing approximations. As a direct consequence of the presented impedance model, the stability of the GFM converter when connected to different loads can be determined by checking the stability of a single transfer function, the sensitivity function. Using the sensitivity function, the impedance model presented also reveals a clear link between the voltage controller characteristics and the converter output impedance, which has been reported as a limitation of prior impedance-based analysis methods.

Grid-connected converters must remain coupled to the grid during a fault. They have to control the reactive currents injected to the grid because of its limited overload capacity. Particularly, grid-forming (GFM) converters ride through the fault either preserving grid-forming operation or switching from grid-forming to grid-following (GFL) mode. In GFM mode the current cannot be controlled therefore mechanisms as virtual impedances become necessary. This article proposes a converter with grid-forming and grid-following capabilities that demonstrates effective voltage tracking in GFM mode and current tracking in GFL mode, good dynamic disturbance rejection response and, wide stability range and robustness in both modes. During a fault, the proposed converter switches from grid-forming to grid-following mode in order to precisely control the injected currents. The current control is able to limit the current at the inception of severe faults. This characteristic is preserved for a wide range of grid impedances, thanks to its good robustness and wide stability margin. As a result, the controller can operate in stand-alone mode, connected to a weak grid, or connected to a stiff grid with a short-circuit ratio up to 50. The fault-ride through capability of the converter is tested under severe symmetrical and asymmetrical faults.

This article presents a voltage controller designed to operate grid-forming (GFM) converters under different grid impedance scenarios demonstrating good performance in conjunction with external power-synchronization loops (PSLs), e.g., droop control. GFM converters are required to have fast, accurate and stable voltage–frequency response, self-synchronization, power balance, grid supporting and fault ride-through (FRT) capabilities; either they are connected to the grid or to other generators. State-of-the-art voltage controllers are mainly focused on achieving either good voltage tracking and disturbance rejection or wide stability range and robustness. This proposal fulfils both tendencies as, first, it shows fast dynamic response, accurate voltage tracking, and total cancellation of output current disturbances at both positive-and negative-sequences of fundamental frequency because the controller presents zero-output impedance. Second, the controller shows a good sensitivity function and a wide stability range, which permit its operation in grid-connected and stand-alone mode. Finally, the proposed controller exhibits a good performance combined with a PSL under a wide range of grid impedance scenarios, showing seamless transition between grid-connected and stand-alone mode and vice versa, demonstrating also FRT capabilities preserving GFM operation under symmetrical and asymmetrical grid-fault events.

This article proposes a finite control set (FCS) current controller for grid-tied converters, specifically tailored to meet the characteristics of AC ship microgrids in all-electric ships (AESs). Contrarily to a pulse width modulator (PWM)- based design, which often employs an LCL filter, the proposal is able to meet applicable harmonic regulations using an L filter with the same total inductance as the LCL filter and the same switching frequency required by PWM-based solutions. Moreover, the proposed design provides a low sensitivity in a wide frequency range from DC to the switching frequency of the power converter. This low sensitivity permits to quickly attenuate low-order disturbances and it is particularly convenient in marine applications with a weak grid, where the frequent connection and disconnection of high power nonlinear loads can significantly affect the voltage waveform quality.