To address the issue of power quality, shunt active power filter (APF) has been installed to regulate the grid current in previous research. However, the resonance risk exists due to the interactions among grid impedance, APF and compensated load. Though existing methods offer solutions, their reliance on fixed models compromises the performance in dynamic scenarios. The effectiveness of active damper remains unrevealed, which is typically installed externally for dynamic suppression. Therefore, this paper analyzes the system characteristics of APF system and reveals the impedance modification effect of APF on the compensated loads. Further, the oscillation mechanism of APF system is analyzed. Ob this basis, stabilization effect of active dampers in different locations is investigated. When the active damper is installed in the scope of APF’s harmonic compensation, the negative resistor emulating strategy is proposed to replace traditional positive emulating strategy. The proposed strategy could deal with the modification effect of APF properly and stabilize the system with less power loss. Besides, the harmonic emission of active damper can be compensated by the APF, guaranteeing the power quality. Simulations and experiments are performed to verify the resonance analysis and proposed strategy.

Gallium nitride (GaN) power devices, with their high-frequency operation, low power dissipation, and high efficiency, contribute significantly to the design of electric vehicle power supply by enhancing performance and reducing weight. Nevertheless, the cascode configuration presents a limitation for GaN-based power converters: interactions with parasitic elements in the gate driver and power loop can cause significant switching overshoot and oscillations, which may leads to false turn-on and potential device breakdown of low-voltage Si MOSFET. An comprehensive analytical model of switching process considering gate driverparameters, coupling of parasitic inductance, nonlinear junction capacitance and transconductance characteristics is proposed in this paper. the influence of power loop parameters and gate drive loop parameters on transient switching behavior is investigated from perspective of switching speed, voltage and current overshoot, etc. Guidelines to make the best of cascode GaN devices are given.

With widespread proliferation of power electronic devices, harmonic pollution has become increasingly severe in power distribution network. Owing to its simple structure and high compensation accuracy, source-current-detected active power filter (APF) has been employed for harmonic cancellation. This type of APF generates the current reference through closed-loop control scheme without sensing the load current. However, this operational principle renders conventional current-limiting strategies, such as reference truncated limiting, impractical. Because they will destroy the inner closed-loop relationship. Effective residual capacity utilization can not be achieved while maintaining fast response. Therefore, this article proposes an improved fast current-limiting strategy for source-current-detected APF. By output current feedforward, closed-loop relationship is modified to achieve partial compensation. Further, this article optimizes multifrequency feedforward coefficients for different cases. Fast-limiting control and total harmonic distortion optimization control are integrated in a cascade to form a hybrid strategy, which exhibits a more balanced performance. By instantaneous current triggering, the proposed strategy substantially reduces the response time and overcurrent risk. On this basis, available capacity of APF is better utilized. Finally, the proposed strategy is verified by simulations and experiments in which a 2.2-kW load is compensated.

The conventional overcurrent protection fails to isolate the permanent short-circuit fault with the power electronic converters interface, hindering the utilization of various converters for power supply restoration. A novel protection scheme based on the harmonic undervoltage is proposed in this paper to realize accurate and rapid fault isolation. From the system-level perspective, a three-stage harmonic undervoltage protection method is proposed, comprising instantaneous, time-delayed, and inverse-time harmonic undervoltage protection, enabling coordinated operation between primary and backup protection. The operation principle, parameter design, and extension scheme for complex networks of this novel protection method are introduced, enhancing the computational efficiency and adaptability. From the converter-level perspective, the transient overcurrent limited strategy based on voltage feedforward dynamic gain adjustment is presented for avoiding undesired device shutdown during faults. Besides, the active harmonic injection loop is added to the control system, assisting the operation of the proposed protection method. The simulation and the experimental results indicate that the above method can rapidly locate various short-circuit faults in networks with converters, facilitating flexible control of the distribution networks.

The dc-link capacitors in back-to-back (BTB) converters perform as a necessary dc-link voltage supporter and power buffer for both sides. However, the dc-link capacitor is normally over-designed due to performance considerations, causing a larger volume and higher cost. This article proposes a more comprehensive and accurate design scheme of the dc-link capacitance for the BTB three-level neutral-point-clamped (NPC) converter, achieving the dc bus lightweight design. Multidimensional aspects are considered when quantifying the capacitance constraints, including small-signal interactive stability, dc-link voltage fluctuation facing a given power step change from both ac sides, and switching ripple filtering performances. Corresponding to the above multidimensional aspects, the stability constraint, response speed constraint, and filtering performance constraint of the dc-link capacitance and the control parameter value are analyzed. In addition, the active capacitor is adopted to suppress the NP voltage low-frequency oscillation and to avoid the unfavorable boundary reduction of the dc-link capacitance selection. The effectiveness of the proposed lightweight dc-link capacitance design scheme is validated through simulations and experimental results. The experimental results indicate that the dc-link capacitance of each converter can be reduced to 10 & micro;F in the 2-kW experimental BTB converter, realizing over 60% volume reduction of the dc bus compared with conventional designs

In a multi-parallel inverter system, wideband resonance is frequently induced by the interactions among inverters. On this basis, varying grid impedance will shift original resonance frequency, rendering pre-designed damping methods ineffective. To dynamically suppress the harmonic amplification, active damper has been proposed, which emulates a frequency-dependent virtual resistor. However, its damping range is restricted by control delay. Complicated compensation unit is typically required. Besides, resistance value fails to accommodate different resonance states, either. In some cases, harmonic distortion is aggravated conversely. To address these issues, this article proposes an adaptive damping strategy for active damper based on interharmonic power. In view of power flow, resonance characteristics are investigated. Direct interharmonic active power absorption is verified to be effective in both resonance suppression and harmonic elimination. On this basis, an adaptive power output mechanism is developed, which strikes the balance between damping effect and harmonic anti-interference. Besides, available damping frequency region is expanded by interharmonic reactive power closed-loop control. They collectively enhance the dynamic adaptability of existing active damper, making it more suitable for industrial applications. The detailed parameter tuning and stability analysis are presented. Experiment results validate proposed strategy under various operating conditions.

This paper presents a transient stator current filtering (TSCF) method by a static synchronous compensator (STATCOM) to suppress subsynchronous torsional oscillations. Using the modal series method, the shaft torques can be derived. The analysis shows that TSCF can enable the subsychrnous modes to oscillate with minimal, constant amplitude during the transient period. Finally, a time-domain simulation was conducted to validate the effectiveness of TSCF in the IEEE First Benchmark Model (FBM).

Neutral-point (NP) voltage regulation is essential for ensuring the safe and reliable operation of three-level neutral point-clamped (NPC) converters. When the NP voltage becomes unbalanced, the converter may suffer from distorted modulation signals and increased even-order harmonic emissions. Meanwhile, the intrinsic self-balancing capability weakens as the output power increases, and existing active balancing methods often exhibit limited power-factor adaptability or introduce additional harmonic currents. This paper establishes a unified impedance based model that reveals the NP self-balancing mechanism, characterizes its associated harmonic emission behavior, and explains the loss of balancing capability under high-power conditions. Based on this model, the harmonic behaviors and power-factor limitations of conventional balancing methods are also systematically analyzed. To address these issues, a minimum harmonic emission (MHE) balancing method is proposed by adaptively injecting corrected absolute modulation-wave components through coordinated DC and even-order harmonic channels. The method maintains NP voltage balance over the entire power-factor range while minimizing even-order harmonic emissions across mainstream CBPWM schemes without introducing additional switching losses. Experimental results verify the accuracy of the balancing model and the effectiveness of the proposed method across the full power-factor range through comparative evaluation.

In contemporary distributed power systems, the proliferation of power electronics and dispersed non-linear loads causes increasingly severe harmonic issues. The traditional point-to-point compensation method requires the deployment of numerous control devices, leading to high costs and suboptimal system-wide mitigation outcomes. To overcome these limitations, this paper proposes a dual time-scale coordinated harmonic mitigation strategy employing a few active power filters (APFs). On a short time-scale, a multi-bus harmonic mitigation model is developed and solved using nonlinear programming and graphical auxiliary analysis. This yields the current references for the APFs, ensuring that the harmonic distortion of each bus remains below predefined limits. On a long time-scale, a multi-objective genetic algorithm is employed to dynamically determine specific, system-wide limits for each bus, optimizing harmonic voltage distortion, harmonic power loss, and total APF capacity. Finally, simulation results based on an 18-bus system verify that the strategy achieves well-balanced multi-objective mitigation performance and dynamic response capability.

The software-based overcurrent capability enhancement for the grid-connected converter is essential in actual engineering applications. An optimal operation parameter determination methodology for most efficiently enhancing overcurrent capability is proposed in this article, utilizing the DC-link voltage, switching frequency, and the modulation strategy as variables. Firstly, the mechanism of the non-monotonic relationship between overcurrent capability and switching frequency/DC-link voltage is elucidated, from the perspective of the output current switching ripple and the thermal distribution. Secondly, the parallel-computable loss distribution calculation model is designed, considering the output current switching ripple, ensuring both accuracy and speed in computation. Thirdly, the optimal operation determination methodology is introduced based on the above loss distribution calculation model. Combined with the actual analysis case, the optimal DC-link voltage and the switching frequency are presented by visualizing the variation behavior of the maximum loss concerning the operation parameters. Finally, the simulation and the experimental results verity the precision of the trend of the maximum loss variation and the optimality of the selected operation parameters. Based on the verification case, an improvement of 50% in overcurrent capacity can be achieved by adopting the proposed method.

The integration of the distributed generations (DGs) stations distorts the conventional single-phase grounding (SPG) fault localization results. Considering the flexibility of DGs, a novel SPG fault localization methodology for the neutral noneffectively grounded distribution networks (NNGDNs) is proposed in this article. In the proposed method, DGs are controlled to inject the harmonic currents to actively participate in the localization process. First, the SPG fault characteristic at the harmonic frequency is analyzed, and the measured harmonic impedance at the feeder junction (FJ) is shown to correlate with the fault distance. Subsequently, typical control strategies of the grid-following (GFL) and grid-forming (GFM) DGs under the SPG fault are modified to assist the harmonic impedance detection. Then, the coupling effect between DGs at various feeders is studied, and this coupling effect is eliminated by modifying the output impedance of the control-heterogeneous DGs in the special frequency band. Finally, the SPG fault localization can be realized through the coordination operation of the multiple control-heterogeneous DGs, and the simulation model and the hardware-in-loop (HIL) experimental results verify the effectiveness under various testing conditions.

Grid-forming (GFM) converter can emulate the characteristics of synchronous generators to control voltage and frequency. However, their overcurrent capability is limited by current constraints of power electronic devices. During faults, GFM converters may switch to current-limiting mode, which degrades stability and even lead to fault propagation. The aforementioned stability issues are directly linked to the overcurrent capability of the inverter. To overcome this, a coordinated strategy is proposed that combines current limiting with operational adjustments that include reducing DC-link voltage, switching frequency, and adapting modulation. This approach enhances transient overcurrent capability, ensures device safety, and improves power angle stability, allowing smooth recovery to normal operation after low-voltage ride-through events. The effectiveness of the proposed coordinated control method has been verified through simulations.

With the increased penetration of renewable energy sources, the grid-forming (GFM) energy storage (ES) has been considered to engage in primary frequency regulation (PFR), often necessitating the use of a frequency deadband (FDB) to prevent excessive battery charging cycling and miti-gate frequency oscillations. Implementing the FDB is relatively straightforward in grid-following (GFL) control. However, implementing the FDB in GFM control presents a significant challenge since the inverter must abstain from providing active power at any frequency within the FDB. Therefore, in this paper, the performance of PFR control in the GFM-ES inverter is analyzed in detail first. Then, the FDB is implemented for GFM inverters with various types of synchronization methods, and the need for inertia response is also considered. Moreover, given the risk of oscillations near the FDB boundary, different FDB setting methods are proposed and examined, where an improved triangular hysteresis method is proposed to realize the fast response and enhanced stability. Finally, the simulation and experiment results are provided to verify the effectiveness of the above methods.

The DC bus neutral point of the three-level neutral point clamped (3L-NPC) converter is always connected to the neutral wire of the power system due to the requirement of zero-sequence output current, based on which the additional common resonance loop via the neutral wire will be introduced. The analysis of this introduced common-mode resonance is presented in this article, and the neutral point voltage fluctuation model is established considering the coupling of multi-frequency. The proposed neutral point voltage fluctuation model reveals that the output current of the 3L-NPC causes the fluctuation excitation sources of different frequencies, and the resonance will occur when the DC capacitors and the line inductance form a resonance loop at exactly this frequency. An additional extra active filter circuit is proposed in the article to suppress the fluctuation excitation sources, and the common-mode resonance won't be motivated as a matter of course. The control strategy and the stability are analyzed based on the structure of the proposed active filter circuit. Finally, simulation results verify the accuracy of the derived neutral point voltage fluctuation model and the effectiveness of the proposed active filter circuit.

To ensure good power quality, frequency secondary restoration is one of the most significant parts of a microgrid. Its key technology is to realize synchronous control among all the converters. Under the event-trigger-based method, the specific event serves as a uniform signal to activate secondary control, providing an effective solution for converter synchronization. However, the lack of a global synchronous clock and the event detection delay may still result in an out-of-sync state during the secondary restoration process. As a solution, this paper proposes a synchronization secondary control strategy based on voltage zero-crossing detection, which can effectively realize synchronous frequency restoration. Under the proposed strategy, the zerocrossing point of the voltage is used as the global synchronous clock, and the communication line is not required. Besides, an automatic adjustment algorithm is designed to eliminate the compensation error caused by the detection delay. Simulation and experimental results have verified the effectiveness of the proposed strategy.

This paper proposes a novel AC-AC dual active bridge (DAB) converter topology specifically designed for solid-state transformer (SST) applications, which has garnered extensive attention due to its lightweight design. This paper focuses on the harmonic issues arising from grid voltage distortion and switching dead-time effects in this topology. Firstly, a variable-frequency linearization and phase-shift control strategy is employed, enabling the proposed AC-AC DAB converter to operate effectively under severely distorted grid voltage conditions. Additionally, an analytical dead-time correction approach is introduced, implementing a sensorless open-loop compensation method to improve system performance under large inductive load scenarios. Finally, simulation results validate the effectiveness of the proposed strategy, showing that the total harmonic distortion (THD) of the output voltage is reduced from 18.4% to 2.49% under severe low-order grid voltage harmonics, while the proposed dead-time compensation method reduces the output voltage THD from 31.3% to 1.53% under large inductive load conditions without requiring additional current sensing hardware.

The dc-link voltage synchronization (DCVS) can be applied in the grid-forming static synchronous compensator (STATCOM) to realize dc-link voltage regulation and synchronization with the grid simultaneously. However, DCVS may lead to low frequency oscillation (LFO) of grid-forming STATCOM. To analyze and resolve the LFO issues of grid-forming STATCOM, this article built the small signal model of grid-forming STATCOM with consideration of DCVS, ac voltage and current dual close loop control (VCDC) and reactive power control (RPC). It is revealed that the LFO is mainly caused by the -180 degrees phase characteristic of DCVS and phase lag introduced by VCDC. Besides, RPC may bring right half plan poles to the close loop and lead to subsynchronous oscillation. Based on the analysis, a lead-lag compensator is utilized and designed in this article to compensate phase lag in the close loop, which is simple but can effectively damp the LFO of grid-forming STATCOM. Besides, the proposed strategy is compared with the existing methods and shows the advantage of inertia provision to decrease the rate of change of frequency in the synchronization. Simulation and Experimental results validate the stability analysis and effectiveness of the proposed method.

Along with the progressive advancement of the "dual-carbon" goal, the penetration rate of new energy power generation is gradually increasing. And the performance of grid-connected inverters, as the core interface equipment, is related to the safety of grid-connection. Traditional grid-following inverters rely on phase-locked loops, which are prone to oscillate in weak grids or high proportion of new energy scenarios. In contrast, Grid-forming inverters mostly adopt virtual synchronous generator (VSG) control, which enables the enhancement of dynamic response. However, the actual control parameters of inverters often do not match with the values provided by the manufacturers. Therefore, it is essential to precisely identify the parameters of the VSG control loop. In this paper, a first-order inertia link is added to the damping link in the original VSG control to strengthen the frequency support capability of the transient process of the inverter. According to the improved control equations, a parameter identification method based on dynamic response data is introduced. It realizes the identification of virtual inertia, damping coefficient and sag coefficient. Finally, the validity of the parameter identification method is verified by simulation under different working conditions and parameters.

The dynamic interaction between voltage-source converters (VSC) and the grid may lead to high-frequency destabilization (HFDS). It has been shown that HFDS can be effectively avoided if the VSC input admittance (VIA) is compensated to be passive at all the frequencies below Nyquist frequency (NF). The effective approaches, auxiliary current feedback and advanced control sequence (i.e., oversampling or delayed sampling), are limited for cost-sensitive applications. Without the above approaches, full-range passive VIA is hard to achieve, even if existing self-tuning active damping methods are used. Therefore, this article proposes an adaptive parameter switching voltage-feedback active damping control (AFVC) method. Both positive and negative feedback are used to design preset AFVC parameters, making VIA passive in different ranges, which overlap and cover the frequencies below NF. Parameter switching is triggered by fast Fourier transformation (FFT). The data sequence length of FFT is designed to balance the speed and frequency spectrum resolution. Sampling frequency aliasing and PWM sideband frequency coupling challenge the admittance/impedance based stability analysis around NF, so they are modeled and proved to be negligible for high-frequency stability. The effectiveness of the proposed method is verified by both simulation and experiment.

Storage energies (SEs) have the flexibility of bi-directional power regulation, so they have been integrated into the power system via power electronic converters to reduce the power fluctuations introduced by converter-based renewables (CBRs), such as solar and wind. However, converter-based SEs may interact with CBRs through the power network, which increases the complexity and difficulty of the phase-lock-loop (PLL)-induced small-signal stability analysis in multi-CBR systems, especially in weak grids. In this context, it remains unclear how the placement of SEs influences the PLL-induced small-signal stability, particularly when the SEs absorb active power from grids. To fill this gap, this paper analyzes the impact of SEs on the PLL-induced small-signal stability in multi-converter systems from the perspective of grid strength and proposes a method for optimally placing SEs to enhance the small-signal stability. First, the analytic results reveal that when the SEs absorb active power from the power grid, they can enhance grid strength and thus the PLL-induced small-signal stability; moreover, the degree of system stability improvement depends on the location of SE placement. On this foundation, a grid-strength-based method is proposed for the PLL-induced small-signal stability improvement via SE placements. The proposed method is validated based on three test systems. This paper provides an effective way of better understanding the interaction among SEs and CBRs through power network, and coordinating the placements of SEs in future converter-dominated power systems.