Load converter operating in the weak grid potentially suffers harmonic oscillation problem due to the impedance interaction. In this paper, considering DC-link voltage dynamics and frequency-coupling effects, the precise sequence impedance models are derived for the traditional load converter (TLC) and the load virtual synchronous machine (LVSM). Based on the derived models, their sequence impedance characteristics are analyzed and compared. The analysis shows that the positive-sequence impedance of the LVSM is generally inductive in the middle- and low-frequency bands, which is basically consistent with the grid impedance characteristics. In contrast, the positive-sequence impedance of the TLC is negative-resistive-capacitive (i.e., phase angle is between -180° and -90°) in the middle- and low-frequency bands, and its amplitude-frequency curve is easy to intersect with the amplitude-frequency curve of the grid impedance, which may lead to harmonic oscillation in the weak grid. Furthermore, based on the derived model and Nyquist stability criterion, the effects of the grid impedance and load power on the stability are analyzed for TLC and LVSM, which indicates that the LVSM has better stability in the weak grid than the TLC. Finally, the experimental results validate the correctness of the theoretical analyses.

A damping-controllable-commutated converter (DCCC) is proposed to enhance the commutation failure (CF) mitigation capability of line-commutated converter-based high-voltage direct current (LCC-HVDC) systems. The DCCC comprises a main branch and an auxiliary branch, with the main branch consisting of thyristors and a few high-current capacity integrated gate-commutated thyristors ( i -IGCTs) and the auxiliary branch composed of damping modules and a few high-voltage tolerance IGCTs ( v -IGCTs). In normal operating conditions, the DCCC operates like the LCC for commutation. However, under fault conditions, the commutation process in the DCCC is decomposed into a current-limiting process and a forced commutation process. During the current-limiting process, the damping module transfers the valve current to the auxiliary branch, and the i -IGCTs actively block the main branch in the zero-current state. When the current reaches the turn-off value of the v -IGCTs, the forced commutation process is initiated, and the v -IGCTs actively turn off the current to complete the commutation. Operation strategy for DCCC and parameters for damping and snubber circuits are devised based on the preceding work modes. Simulations show that the DCCC can effectively enhance the CF mitigation capacity of the system, with the switching device maintaining reasonable electrical stress.

In battery charging applications, the charger changes its output voltage in a wide range during the charging process. This makes the design of LLC converters difficult to be optimized between the efficiency and the gain range. In this paper, an improved resonant transformer is presented for LLC resonant converter charger to improve the gain adjustment and charger efficiency. The resonant inductance and magnetizing inductance are integrated in the designed LLC transformer, and the magnetizing inductance can be adjusted dynamically with the change of output voltage and load, which is realized by a switch-controlled inductor (SCI) parallel to the secondary winding of transformer. The proposed transformer has 22.4% reduction in losses under full load conditions compared to conventional solutions. Moreover, the conduction loss and switching loss of LLC resonant tank are reduced by dynamically adjusting the magnetizing inductance, which improves the comprehensive efficiency of the whole charging process. The proposed transformer design is verified on a 720 W prototype.

With the emergence of the hybrid multi-infeed high voltage direct current (HMIDC) system, there is complex dynamic interaction between the AC/DC system and converter stations, while the interaction mechanism of the HMIDC system is unclear. For this purpose, this paper establishes the HMIDC linearized model based on the motion equation concept. Then, after decoupling and reconstructing the interaction path of the HMIDC system, the damping coefficients are derived. The AC system strength and voltage source converter (VSC) terminal voltage controller parameters are selected to analyze the impacts on damping coefficients, and the interaction mechanism of the HMIDC system is analyzed. In order to solve the potential instability issue, an additional damping controller in the VSC terminal voltage loop is proposed to enhance the voltage stability of HMIDC system, which is realized by adjusting the reactive power/amplitude coupling damping coefficient. Finally, the RT-LAB-based hardware-in-the-loop experiments verify the accuracy of the interaction mechanism analysis of the HMIDC system and the effectiveness of the proposed controller.

This paper studies the impact of the transmission line frequency response on commutation failure (CF) and points out that the different frequency response of transmission line leads to different CF mitigation capability. Based on the commutating voltage time area theory, the double-tuned damped filter of 2nd and 3rd characteristic harmonics is added to the system to increase the CF mitigation capability. Simulation results based on the PSCAD/EMTDC show that the CF mitigation capability with different transmission line frequency response and the improvement of CF mitigation capability with the double-tuned damped filter.