Hua Lin

This paper focuses on the system stability of grid-forming (GFM) control through passivity criterion. An accuracy small-signal impedance model under dq-frame is established, which is used to analyze the influence of controller coefficients on system stability by passivity theory. And the inherent characterization for the risk of low-frequency resonance is revealed, indicating that its non-passivity region always exists, regardless of controller coefficients and specific control schemes. Finally, the theoretical analysis is verified by simulation results and the experiment test.

Grid-side current control based on capacitor-current-feedback (CCF) active damping is a common control method for LCL -type grid-tied inverters. The control system should not only ensure inverter stability under complex grid conditions, but also be simple in structure. Therefore, the multiobjective design for the CCF control is proposed to enhance the passivity, while improving the internal stability. First, the passive constraint of CCF control is derived, from which two general control schemes, namely, negative CCF-based phase-lead (N-Lead) control scheme and positive CCF-based phase-lag (P-Lag) control scheme, are proposed. Then, the two schemes are implemented by the design of a first-order phase-lead CCF controller and a first-order phase-lag compensator-based positive CCF controller. Besides, the parameters design is also given for the convenience of engineering application. Finally, the experimental results verify the theoretical analysis and the effectiveness of the proposed design methods.

To guarantee system stability, proportional capacitor-current-feedback (CCF) active damping control has been widely used in LCL -type grid-connected inverter. However, when digital control is adopted in a grid-side current-controlled inverter, proportional CCF active damping will show negative damping characteristic in frequency band higher than one-sixth of the sampling frequency ( f _s /6), causing poor robustness in weak grid. In this article, an unstable second-order phase lead compensator is proposed to insert into the CCF path for eliminating the negative effect resulting from the digital control delay. In doing so, the positive damping region is extended to almost Nyquist frequency ( f _s /2), which achieves high robustness against grid impedance variation. To ensure system stability, the discrete rule is investigated for the proposed compensator and system performance is analyzed when the LCL resonance frequency changes in the range of (0, f _s /2) in discrete domain, a particularly simple and practical design is further presented for the proposed compensator and the current regulator. Finally, the effectiveness of the proposed strategy and parameters design is verified by the experimental results.

Connecting low-voltage insulated gate bipolar transistor (IGBT) in series to withstand much higher dc -link voltages has some advantages over using a single high-voltage IGBT or the multilevel topologies. However, severe voltage imbalance during turn-off transient remains to be solved. In all existing methods, adjusting driving signal time delay (ADSTD) is attractive and has no penalty of sacrificing the switching performance of IGBT. Moreover, instead of using a snubber, the voltage imbalance caused by the difference in tail currents can be mitigated by using ADSTD as well. An active voltage balance method based on ADSTD is proposed and verified by experiments in this article. Besides, a more accurate equivalent capacitance for IGBT considering the stored carriers in the n-drift region is discussed. The factors affecting a proper driving signal time delay are also discussed in detail. These contents are well supported by experimental results as well.

Modular multilevel converter with integrated battery energy storage system has been verified as a better choice for large-scale battery energy storage system. However, battery power and its unbalanced distribution in submodules lead to significant increase of capacitor voltage ripple. Larger submodule capacitance is required to maintain the ripple within reasonable limits, increasing the cost and volume of the system. To address the problem, this article proposes a novel battery integration method. Each battery pack is connected with two submodules that belong to upper and lower arm, respectively, through an isolated three-port dc-dc converter. The proposed method can ensure the power balance between upper and lower arm to eliminate fundamental circulating current and reduce capacitor voltage ripple. Three-port converters provide power channels for the fundamental power fluctuation of arms; thus, capacitor voltage ripple is further reduced. Then, the small-signal model of the proposed structure is built, and the control strategy is designed to realize both capacitor voltage ripple reduction and battery power control. Finally, a downscaled prototype is built and detailed experiment results verify the effectiveness of the proposed battery integration method and control strategy.

Modular multilevel converter (MMC) is regarded as one of the most potential topologies for high‐voltage direct current transmission system. However, the multi‐frequency behaviour and complex control structure of MMC bright great challenges to the stability analysis, identification of factors affecting stability, and controller design. To address this problem, the harmonic state‐space (HSS) theory is applied to MMC in this study. First, the small‐signal HSS model is established to describe the multi‐frequency behaviour of MMC. Capacitor voltage control, circulating current control, and output current control are also taken into account in the modelling. On this basis, the stability analysis is performed by evaluating the eigenvalues of the HSS model, and participation factor is introduced to identify the factors affecting stability. In addition, the proposed approach reveals the influence of different controllers on the system stability, providing guidance for the tuning of multiple controller parameters. Finally, the effectiveness and accuracy of the proposed approach are verified by simulation and experiment results.

Limited by low availability, high price, and poor switching performance of high voltage power devices, connecting low voltage devices in series to block much higher voltages is always an option. However, severe voltage unbalance during turn-off transient remains to be solved. Most of existing methods designed for low speed Silicon (Si) IGBT cannot be directly transplanted to series-connected Silicon Carbide (SiC) MOSFETs with high switching speed. In order to maximum the switching performance of SiC MOSFETs, an elegant implementation of adjusting driving signals time delay method is proposed. In addition, a simplified model during drain-source voltage rising transient is discussed to basically reveal features and problems of series-connected SiC MOSFETs. Factors affecting the appropriate time delay are discussed as well, especially the influence of the load current. The simplified model and the implementation are both verified by experiments. Indeed, the proposed active voltage balancing control works well and has no penalty of sacrificing switching performance of SiC MOSFETs.

Modular multilevel converter with integrated battery energy storage system (MMC‐BESS) has been proposed for energy storage requirements in high‐voltage applications with large‐scale renewable energy resources. The MMC‐BESS is essentially a three‐port converter which can transfer energy between any two of the ac port, dc port, and BESS, constituting multimode operation of the system. Different from conventional MMCs, the state‐of‐charge (SOC) inconsistency among batteries would magnify the submodule capacitor voltage unbalance issue in the MMC‐BESS. In this study, an improved capacitor voltage balancing method applicable for multimode operation of MMC‐BESS is proposed by adjusting ac and dc modulation indexes simultaneously. After that, the ratio of ac and dc modulation indexes is optimised to enhance the tolerance of unbalanced power among SMs within phase arm, greatly improving the SOC equalisation rate. Based on the proposed capacitor voltage balancing method, the control structure is given and the dynamic model is conducted for the analytic design of the closed‐loop controller. Finally, the simulations and experimental results validate the effectiveness and feasibility of the proposed control strategy.

A modular multilevel converter with an integrated battery energy storage system (MMC-BESS) has been proposed for high-voltage applications for large-scale renewable energy resources. As capacitor voltage balance is key to the normal operation of the system, the conventional control strategy for the MMC can be significantly simplified by controlling the individual capacitor voltage through a battery side converter in the MMC-BESS. However, the control strategy of the MMC-BESS under rectifier mode operation has not yet been addressed, where the conventional control strategy cannot be directly employed due to the additional power flow of batteries. For this defect, the rectifier mode operation of the MMC-BESS based on a battery side capacitor voltage control was analyzed in this paper, proposing a control strategy for this application scenario according to the equivalent circuit of MMC-BESS, avoiding passive impact on the state-of-charge (SOC) equalization of batteries. Furthermore, the implementation of a battery side converter control is proposed by simplifying the capacitor voltage filter scheme within phase arm, which enhances its performance and facilitates the realization of control strategy. Finally, simulation and experimental results validate the feasibility and effectiveness of the proposed control strategy.