Hailiang Xu

The virtual-return-based DC traction power supply system (VRTPS) has been studied to solve the stray current and rail potential issues in urban rail transit. By utilizing the power electronic devices (PEDs) and return line, VRTPS constructs the zero-resistance virtual return path (VRP) by diverting traction current from the running rail into this newly established path. However, the mitigation performance of existing VRTPS control strategies is still suboptimal, leading to incomplete suppression of stray current and rail potential within the train-occupied section. Thus, the reverse running rail current control strategy of VRTPS is proposed to further reduce the stray current and rail potential. Firstly, the topology and modeling of VRTPS are investigated to analyze the inherent limitations of the ideal zero-resistance VRP. Secondly, the detailed analysis of the reverse running rail current control for VRTPS is presented, which optimizes the running rail current distribution with the variable-resistance VRP. Thirdly, the comparative researches on both the stray current and rail potential distribution are conducted between the conventional system and VRTPS with different control strategies. Finally, the correctness and effectiveness of VRTPS with proposed strategy on the stray current and rail potential mitigation are validated through simulation and experimental results.

Virtual synchronous generators (VSGs) mimic the output characteristics of traditional synchronous generators, which can effectively address the challenges of low inertia and weak damping in modern power systems. However, under fault conditions, VSGs exhibit prominent transient instability, and the unclear transient stability boundary significantly impair their voltage/frequency support capability. To this end, this paper proposes a voltage deviation adaptive feedforward method, aiming to optimize the transient stability operating boundary of VSG. First, coupling relationship between reactive power-voltage droop characteristics and transient power angle characteristics is analyzed. Based on this, the coupling characteristics are harnessed to propose a transient stability control architecture incorporating voltage deviation feedforward. Furthermore, with the objective of optimizing the transient stability boundary of VSG, the voltage deviation feedforward coefficient is adaptively designed to enhance its transient stability performance. Finally, the effectiveness of the proposed strategy in this paper is verified through simulations and experiments.

Addressing issues such as dc-link voltage unbalance and dead-time effect is essential for the wide-spread application of the four-level (4L) π-type inverter. Effective strategies have been proposed for dc-link voltage balancing. However, existing strategies for suppressing the dead-time effect cannot be directly combined with dc-link voltage balancing strategies. In this article, investigations are carried out to both address the dc-link voltage unbalance issue and the dead-time effect of the 4L π-type inverter. First, a quasi-virtual discontinuous pulsewidth modulation (DPWM) for dc-link voltage balancing and its dead-time effect are briefly presented. Then, the overmodulation issue of the conventional dead-time compensation is analyzed based on the quasi-virtual DPWM. Afterward, the dead-time elimination PWM is tried to combine with the quasi-virtual DPWM, which however has a high dependency on current polarities. To further reduce the dependency on current polarities, a new double-modulation-wave quasi-virtual DPWM is proposed, which combines the quasi-virtual DPWM and the double-modulation-wave PWM. Comparative experimental results shown that, the proposed PWM scheme can both achieve dc-link voltage balancing and dead-time effect suppressing for the 4L π-type inverter.

The balance of dc-link voltages is quite a challenge in the four level (4L) π-type inverter, which is the main factor limiting its wide application. In this article, an investigation is conducted to simultaneously balance dc-link voltages and reduce switching losses in 4L π-type inverters. First, the mechanism of the dc-link voltage balancing method based virtual vectors is analyzed, which however will inevitably increase switching losses. To reduce the switching losses, a discontinuous pulse width modulation (DPWM) implemented based on virtual vectors is presented. Then, the vector sequences are rearranged proposing a quasi-virtual DPWM for balancing dc-link voltages of the 4L π-type inverter. And a simplified carrier-based implementation method is introduced with split modulation waves. Auxiliary closed control loops are designed for controlling and balancing dc-link voltages. In addition, the advantage of low common-mode voltage in the quasi-virtual DPWM is analyzed and presented. A three-phase 4L π-type inverter experimental setup is built based on a DSP-FPGA control board. The proposed quasi-virtual DPWM is finally verified by comparative experiments, showing its controllability of dc-link voltages, high efficiency, high linear modulation index, and low common-mode voltage.

The virtual synchronous generators (VSGs) have been found to be effective in supporting the voltage and frequency of power systems. However, VSGs may suffer from transient instability and overcurrent under grid fault conditions. Nevertheless, VSGs are required to output necessary reactive power during grid faults in compliance with the latest grid codes. Unfortunately, the internal connections between the transient stability and the fault current limitation, the reactive power support constraint, still leave unknown. To address this issue, this paper proposes a transient stability enhancement method that considers all these constraints. Firstly, the droop coefficient of the reactive power control loop is redesigned to regulate the VSG's current. Secondly, an adaptive control is introduced in the active power control loop to improve the transient power angle stability. Additionally, the design guidelines for various fault scenarios and constraints are proposed. The effectiveness of the proposed parameters regulation method is finally validated through both simulation and experimental results.

In recent modular multilevel converter (MMC)-based projects, high-frequency resonance (HFR) incidents have posed severe threat to the security of the power system. Existing HFR suppression methods mainly focus on specific operation conditions, and cannot be so effective when the grid impedance changes. The grid impedance adaptive methods, however, still get limit to suppress single HFR, and could not deal with the multiple HFRs issue. To overcome these challenges, an adaptive virtual impedance (AVI) strategy is proposed to detect and mitigate the multiple HFRs. The proposed strategy consists of resonance detection and virtual impedance-based suppression method. Particularly, the parameter of the virtual impedance is designed to be adjusted adaptively, according to the detected resonance frequency in real time. The AVI control is configured in each HFR risk frequency band to achieve mitigation of multiple HFRs. Simulation and experimental results validate that the unpredictable HFRs can be effectively mitigated by the proposed method.

Droop-controlled voltage-source converters (VSCs) can provide frequency and voltage support to power grids. However, during a grid fault, VSCs may experience transient instability, which can be significantly affected by both the control parameters and fault conditions. This mechanism has not been fully elucidated in previous studies. In particular, grid-voltage faults are commonly accompanied by a grid voltage phase-angle jump (VPAJ), which is typically ignored in the evaluation of the transient stability of VSCs. To address this issue, this study comprehensively assesses the impact of the VPAJ and key control parameters on the transient characteristics of VSCs. Furthermore, the critical clearing angle and critical clearing time are quantitatively calculated to define the transient stability boundary. In addition, a transient stability-enhancement control method that considers the transient stability constraints is proposed. Finally, simulations and experimental tests are conducted to validate both the theoretical analysis and proposed method.