In applications of the dual-active bridge (DAB) converters, minimizing circulating current and maximizing zero-voltage switching (ZVS) capability are crucial to ensure high efficiency and reliable operation. To address these challenges, this article proposes an optimal triple-phase-shift modulation scheme for the DAB converter. The proposed scheme minimizes rms current and achieves ZVS with minimal increase in circulating current across the entire load range. Building on previous modulation strategies, this article introduces a comprehensive ZVS analysis. It establishes precise ZVS criteria by rigorously accounting for the nonlinear voltage characteristics of parasitic MOSFET capacitances and the dynamic circuit behavior during the dead time, factors often simplified or overlooked in prior approaches. Using nonlinear optimization constrained by these precise ZVS criteria, the proposed modulation scheme guarantees a wide ZVS range without requiring excessive circulating current for complete ZVS, significantly reducing both switching and conduction losses. For experimental validation, a 20 kW/50 kHz SiC-based DAB converter was built, where the converter achieved a peak efficiency of 98.8%, thereby confirming the effectiveness of the proposed method.

This letter presents a novel method to enhance the transient stability and grid forming (GFM) capability under current limiting conditions. The proposed method employs dual-loop vector voltage control with a back calculation anti-windup strategy, using adjustable gain to modify the X/R ratio of the equivalent virtual impedance. The method ensures seamless operation during transitions between normal and abnormal grid conditions while significantly improving transient stability and GFM capability. Theoretical analysis has been validated through 2 kVA scale experiments, demonstrating the effectiveness of the proposed method.

Virtual impedance (VI) is commonly implemented in a static manner, typically neglecting the derivative term. This letter investigates fundamental issues associated with algebraic virtual impedance (AVI), emphasizing its characteristics related to negative-sequence impedance. The analysis reveals that AVI exhibits capacitive behavior in the negative-sequence impedance, inducing significant harmonic currents near a particular negative resonant frequency. Furthermore, the influence of control delay is analyzed, illustrating that AVI can readily become non-passive within the negative-frequency region of the frequency domain, leading to harmonic instability. Experimental results are provided to validate the theoretical analysis.

The three-phase dual-active bridge (DAB3) converter is an isolated bidirectional dc-dc converter used in high-power dc microgrid. Without fast dynamic control of the DAB3, the required dc-link capacitance to maintain dc voltage of microgrid under transient conditions increases, resulting in a higher risk of excessive current of short-circuit accidents. Consequently, dynamic control methods for the DAB3 have been extensively studied, leading to the development of the state-of-the-art generalized instantaneous flux and current control (GIFCC). The GIFCC dynamically regulates the phase current and magnetizing flux-linkage of the transformer under both single phase-shift (SPS) and triple phase-shift (TPS) modulations. However, it applies different control laws depending on the transition type, leading to degradation of dynamic performance and complicating integration with outer-loop power or voltage regulators. To optimize dynamic performance under both TPS and SPS modulations, this article proposes the instantaneous pulse pattern control (IPPC) for the DAB3, which interprets TPS and SPS modulations as a dual-flux regulation problem. The IPPC handles both SPS and TPS modulations seamlessly based on a single control law, minimizing transient times to one-sixth of the switching period, regardless of the modulation scheme. Simulation and experimental tests validate significant reductions in transient times with the proposed IPPC in comparisons to the GIFCC.

In applications of the dual-active bridge (DAB) converters, minimizing circulating current and maximizing zero-voltage switching (ZVS) capability are crucial to ensure high efficiency and reliable operation. To address these challenges, this article proposes an optimal triple-phase-shift modulation scheme for the DAB converter. The proposed scheme minimizes rms current and achieves ZVS with minimal increase in circulating current across the entire load range. Building on previous modulation strategies, this article introduces a comprehensive ZVS analysis. It establishes precise ZVS criteria by rigorously accounting for the nonlinear voltage characteristics of parasitic MOSFET capacitances and the dynamic circuit behavior during the dead time, factors often simplified or overlooked in prior approaches. Using nonlinear optimization constrained by these precise ZVS criteria, the proposed modulation scheme guarantees a wide ZVS range without requiring excessive circulating current for complete ZVS, significantly reducing both switching and conduction losses. For experimental validation, a 20 kW/50 kHz SiC-based DAB converter was built, where the converter achieved a peak efficiency of 98.8%, thereby confirming the effectiveness of the proposed method.

This letter presents a novel method to enhance the transient stability and grid forming (GFM) capability under current limiting conditions. The proposed method employs dual-loop vector voltage control with a back calculation anti-windup strategy, using adjustable gain to modify the X/R ratio of the equivalent virtual impedance. The method ensures seamless operation during transitions between normal and abnormal grid conditions while significantly improving transient stability and GFM capability. Theoretical analysis has been validated through 2 kVA scale experiments, demonstrating the effectiveness of the proposed method.

Virtual impedance (VI) is commonly implemented in a static manner, typically neglecting the derivative term. This letter investigates fundamental issues associated with algebraic virtual impedance (AVI), emphasizing its characteristics related to negative-sequence impedance. The analysis reveals that AVI exhibits capacitive behavior in the negative-sequence impedance, inducing significant harmonic currents near a particular negative resonant frequency. Furthermore, the influence of control delay is analyzed, illustrating that AVI can readily become non-passive within the negative-frequency region of the frequency domain, leading to harmonic instability. Experimental results are provided to validate the theoretical analysis.

The three-phase dual-active bridge (DAB3) converter is an isolated bidirectional dc-dc converter used in high-power dc microgrid. Without fast dynamic control of the DAB3, the required dc-link capacitance to maintain dc voltage of microgrid under transient conditions increases, resulting in a higher risk of excessive current of short-circuit accidents. Consequently, dynamic control methods for the DAB3 have been extensively studied, leading to the development of the state-of-the-art generalized instantaneous flux and current control (GIFCC). The GIFCC dynamically regulates the phase current and magnetizing flux-linkage of the transformer under both single phase-shift (SPS) and triple phase-shift (TPS) modulations. However, it applies different control laws depending on the transition type, leading to degradation of dynamic performance and complicating integration with outer-loop power or voltage regulators. To optimize dynamic performance under both TPS and SPS modulations, this article proposes the instantaneous pulse pattern control (IPPC) for the DAB3, which interprets TPS and SPS modulations as a dual-flux regulation problem. The IPPC handles both SPS and TPS modulations seamlessly based on a single control law, minimizing transient times to one-sixth of the switching period, regardless of the modulation scheme. Simulation and experimental tests validate significant reductions in transient times with the proposed IPPC in comparisons to the GIFCC.

With the increasing integration of grid-connected inverter-based resources (IBRs) and the declining generation from fossil-fuel-based synchronous generators, the instability of grid voltage and frequency has become a growing concern. To address this issue, E-STATCOMs, which combine energy storage (ES) with static synchronous compensator (STATCOM), are gaining attention as a solution for enhancing grid stability. Conventional multilevel converter-based E-STATCOM topologies can be categorized into two types: those that distribute the ES within the multilevel converter and those that centralize it in the DC-link. Of the two, centralized ES installation is more advantageous in terms of management and maintenance. However, the inherent structure of single-star multilevel topology poses challenges for centralized ES installation. As a result, double-star multilevel topology is commonly implemented, which leads to an enlarged footprint and increased cost. To overcome these limitations, this paper proposes an E-STATCOM topology that employs a zigzag transformer to decouple the ES from the single-star multilevel converter. Furthermore, this paper presents a control strategy to ensure the stable and reliable operation of the proposed E-STATCOM topology. The feasibility and effectiveness of the proposed topology and control strategy are validated through simulation and experimental results.

This paper proposes a novel method to emulate virtual impedance (VI) for the current-limiting control (CLC) of grid-forming (GFM) converters. By leveraging the voltage at the point of common coupling (PCC), the proposed method enables the virtual amplification of the filter inductance without requiring a digital differentiator, thereby achieving a genuine emulation of physical inductance. The proposed VI method incorporating the virtual inductance can be adaptively increased based on the current magnitude for CLC. In contrast to conventional adaptive VI control based on algebraic impedance, which is prone to instability and requires a low-pass filter for stabilization, experimental results confirm that the proposed approach exhibits stable dynamic performance and effectively limits current during transients, while significantly reducing control complexity and design burden.

In a power system, when a fault occurs, a Static Synchronous Compensator (STATCOM) is often required to inject negative-sequence current to compensate for unbalanced conditions. In high-voltage applications, single-star multilevel converters are commonly used in STATCOM systems to efficiently interface with high-voltage levels. However, during unbalanced condition compensation, these circuits experience cluster energy imbalance, which must be addressed through the balancing control. The conventional cluster energy balancing method utilizes zero-sequence voltage to redistribute active power for system balance. Under severe unbalance, a large negative-sequence current must be injected to compensate grid or load asymmetry. In conventional single-star multilevel converters, the zero-sequence component of the cluster output voltage is then actively regulated to eliminate the resulting cluster-energy imbalance; however, the required increase in this component can drive the converter into over-modulation. This limitation reduces the effectiveness of negative-sequence current injection, making it difficult to effectively compensate for unbalanced loads and grid conditions. To overcome this limitation, this paper proposes a modified power converter structure that retains the single-star multilevel converter topology while incorporating a zigzag transformer, along with a new cluster energy balancing control method. The proposed approach significantly enhances the range and capability of negative-sequence current compensation. The effectiveness of the proposed control strategy and converter design is validated through mathematical analysis, simulations, and experimental results.

This paper presents a control technique to emulate a high-power synchronous motor using a modular multilevel converter without adding extra physical filter inductors. Motor emulation can be achieved through either fast current control or by adding auxiliary inductors to replicate the current response of the stator inductance in the virtual motor. The first approach requires high switching frequency, which is unsuitable for high-power converters. The second approach demands adjustments to the auxiliary inductance when motor parameters change, limiting the test flexibility of the emulator. To overcome these challenges, this paper proposes an output voltage control that allows the emulator to realize the stator inductance of the virtual motor without hardware modifications or fast current control. The validity of the proposed method is verified through simulation, demonstrating its effectiveness in flexible motor emulation.

This paper proposes a fast internal voltage source (IVS) controller for grid-forming (GFM) voltage source converter (VSC) to enhance the transient stability. While conventional methods frequently suffer from degraded synchronization performance due to current limitations, the proposed approach significantly reduces the post-fault transient period during which the current limiter remains active. In particular, when a phase jump fault occurs in a weak grid, a GFM-VSC staying in the current-limited state for an extended time exacerbates the recovery of the point of common coupling (PCC) voltage. Therefore, applying the proposed controller to the GFM-VSC can notably improve the PCC voltage recovery speed after a phase jump fault in a weak grid. The effectiveness of the proposed control is demonstrated through down-scaled experiments.