Tao Zhao

The solid-state transformer (SST) based on a cascaded H-bridge (CHB) topology has gained widespread adoption in grid-to-vehicle (G2V) and vehicle-to-grid (V2G) systems, attributed to its high-efficiency power conversion, flexible bidirectional regulation capability, and compatibility with high-voltage applications. Nevertheless, the CHB-SST inherently suffers from double-line-frequency ripple in the H-bridge (HB) dc-bus voltage and envelope ripple in the isolated dc/dc converter. These issues lead to an increased requirement for HB dc-bus capacitance, exacerbate current stress on power devices, and diminish system efficiency. To mitigate these challenges, a collaborative control strategy is proposed to simultaneously suppress both types of ripples without introducing any additional hardware circuits. Specifically, on the one hand, adaptive compensation of the third harmonic zero-sequence voltage is implemented based on system parameters such as power factor and modulation index, thereby minimizing HB dc-bus voltage ripple. On the other hand, a notch-filter-based control strategy reshapes the output impedance of the isolated dc/dc converter, effectively eliminating both the original double-line-frequency envelope ripple and the quadruple-line-frequency envelope ripple coupled from the dc-side due to third-harmonic compensation. Ultimately, a low-voltage, low-power experimental prototype was developed to validate the proposed scheme, with experimental results affirming its effectiveness and optimality.

The Cascaded H-Bridge (CHB) converter features modularity and high capacity, with its stepped voltage waveform and output reactance emulating the internal potential and output reactance of a generator, making it an ideal solution for inner-loop-free medium-voltage direct grid-forming (GFM) applications. To address the challenges of slow response at low switching frequency and overload ratio limitation, this paper proposes a transient current limiting approach that integrates a current-clamped negative feedback control strategy (CCNFC) with a hierarchical module-level fast current reaction control strategy (HMCC). This approach ensures stable GFM operation under normal conditions while enhancing grid-support performance through optimized parameters under fault conditions. The rapid reaction of bottom controllers suppresses transient overcurrent, ensuring the CHB converter’s safety and reliable grid support performance. Simulations and experiments validate the proposed approach.

This article proposes a novel five-level hybrid NPC (neutral point clamped)-ANPC (active neutral point clamped) H-bridge single-phase inverter. This inverter integrates an NPC arm equipped with silicon ( Si ) devices and an ANPC arm utilizing silicon carbide ( SiC ) devices with the objective of controlling the cost of SiC devices. A fractional frequency half-cycle (FFHC) space vector pulse width modulation (SVPWM) strategy is proposed as the means of enhancing efficiency, power density, and thermal stability. The FFHC-SVPWM strategy facilitates the switching devices of the NPC arm to operate at the fundamental frequency, whereas the switching devices of the ANPC arm switch at a high frequency for only half a cycle. This paper explicates the operational principle of the proposed hybrid NPC-ANPC H-bridge inverter in detail. Subsequently, a loss model and thermal model are developed to analyze the loss distribution, temperature distribution and the maximum junction temperature of the power devices. A prototype of the hybrid NPC-ANPC H-bridge inverter is constructed to validate the increase in efficiency, power density, and thermal stability.

The inductance parameter is crucial to realize high-precision model predictive control (MPC) for grid-connected inverter (GCI), while the traditional inductance estimation method is sensitive to the grid frequency deviation and cannot work normally when the active power is zero. To solve these problems, a new inductance online estimation method based on a full order sliding mode voltage observer (FSMVO) is proposed. First, the drawbacks of the traditional FSMVO-based inductance estimation method are analyzed. Second, a new inductance estimation method is proposed, which uses the error between the observed grid voltage and the actual one passed through a filter, whose transfer function is the same as the one of FSMVO. As a result, the new inductance estimation method is immune to the grid frequency deviation. Moreover, a new Lyapunov function is also designed in this article to deduce the new inductance estimation method, which can work well, no matter the active power is zero or not. Finally, by substituting the estimated inductance parameter into the MPC algorithm, the control performance of GCI can be improved. The effectiveness and accuracy of the proposed inductance estimation method are verified by experimental researches.

During asymmetric faults in power grid, the negative-sequence modulation voltage decomposed by three-phase isolated cascaded H-bridge photovoltaic grid-connected inverter will redistribute three-phase active powers, which may cause active power backflow problem, that is, the output power of one phase in the inverter to be less than zero. The backflow power will cause H-bridge dc-bus voltages to pump up, triggering overvoltage protection and shutting down the inverter from the grid. Although existing methods effectively address single-phase and two-phase short-circuit faults, there is still larger room for improvement in inter-phase short-circuit faults. Therefore, this paper proposes an adaptive zero-sequence voltage square wave compensation strategy, which reduces the modulation voltage amplitude by compensating for all odd harmonics of the line-frequency adaptive zero-sequence voltage, thus reducing the risk of overmodulation. On this basis, an active power backflow suppression strategy for mode switching is proposed. According to the real-time operating status of the inverter, the most suitable control strategy is selected to minimize the active power backflow range during inter-phase short-circuit faults. The proposed method reduces the active power backflow area by 22.08% compared to the existing AZSVCS. Finally, experimental results validate the effectiveness and feasibility of the proposed control method.

Active power backflow is a unique problem of three-phase isolated cascaded H-bridge (CHB) PV inverter during asymmetric grid voltage fault, resulting in the continuous rise of H-bridge dc-bus voltages and that the inverter will be eventually shut down and off-grid due to voltage out of control. The existing methods are able to completely suppress the active power backflow during single-phase short-circuit fault and two-phase short-circuit fault, but fails in a large operating zone when interphase short-circuit fault occurs. For this issue, this article proposes a novel multiple specific harmonic voltages injection method based on the existing fundamental-frequency zero-sequence voltage compensation strategy, which calculates and injects the optimal third, fifth, seventh, and ninth harmonic voltage according to the fault type, drop depth, and the normalized power, for minimizing the amplitude of three-phase modulation voltages and then significantly shrinking active power backflow zone of three-phase isolated CHB PV inverter. Then, the area of the active power backflow zones for different control methods is quantitatively calculated and compared by adopting the polynomial fitting to show the advantages of the proposed method. Finally, experimental results are achieved by a low voltage and low power experimental prototype to verify its effectiveness and feasibility.

Currently, the grid-tied guidelines all require large and medium-sized PV power stations to stay connected to the grid during low voltage ride through (LVRT). However, three-phase cascaded PV solid-state transformer (SST) have the inherent active power backflow problem during asymmetric voltage sags, causing that H-bridge dc-bus voltages are out of control, and that the converter will be disconnected from the grid because of overvoltage fault. The existing control strategy is capable of avoiding active power backflow in theory by injecting appropriate zero-sequence voltage, but the overmodulation range is larger during inter-phase short-circuit fault, weakening active power backflow suppression effect. For this issue, this paper proposes a combined control method of positive-negative-sequence maximum-minimum harmonic zero-sequence voltage injection strategy and adaptive zero-sequence voltage compensation strategy, which can shrink effectively active power backflow range of three-phase cascaded PV SST, increasing its adaptability to different output power and different grid drop depth, and then improving LVRT capability of the PV power generation system. Finally, a low-voltage and low-power experimental prototype is made to verify the effectiveness and feasibility of the proposed control strategy.