Xiaoping Zhou

In medium-voltage transformer-less battery energy storage systems (MVT-BESSs), the single-phase AC/DC converter in each submodule induces secondary ripple current on the battery side, which shortens battery lifetime, degrades state-estimation accuracy, and increases system losses. To mitigate this issue, this paper proposes an active suppression strategy based on third-harmonic current injection. The mechanism of secondary ripple generation is first clarified by analyzing the AC/DC coupling behavior of the submodule. Building on the concept of suppressing DC-side ripple through AC-side third-harmonic injection, a dynamic method for computing the optimal third-harmonic current is formulated for both unity and non-unity power factor conditions. A 380-V/100-kW experimental prototype is developed, and the results show that the proposed method reduces the battery-side secondary ripple to 52.8% of its original level under both operating conditions, confirming its effectiveness.

Grid-forming energy-type STATCOM (GFM ET-STATCOM) presents a promising alternative to synchronous condenser in mitigating commutation failures. In the paper, the fundamental mechanism of GFM ET-STATCOM in mitigating commutation failure considering the advanced firing control is firstly analyzed, revealing that GFM ET-STATCOM should fully inject the reactive current into the receiving-end grid to maximize its commutation failure mitigation capability. Then, following the fundamental mechanism, considering that GFM ET-STATCOM with different inner loops exhibits significant difference in the fault transient characteristics, the performance of GFM ET-STATCOM with different inner loops in mitigating commutation failure is compared, thereby providing the guideline for the practical design of GFM ET-STATCOM. Finally, based on PSCAD/EMTDC, the correctness of theoretical analysis is validated by the simulation results.

Modifying the inverter of line-commutated converter-based high-voltage direct current (LCC-HVDC) to hybrid commutated converter (HCC) is currently a highly effective solution for mitigating the risk of commutation failure (CF). This paper investigates the transient performance of HCC from five perspectives while retaining the LCC-HVDC control system. Since constant extinction angle (CEA) controller leads to significant reduction in power transmission, constant voltage (CV) controller is considered a more appropriate choice as the main controller during AC faults. In addition, the extreme case of insufficient forced commutation capacity leading to CF is considered. It is revealed for the first time that the voltage clamping effect of the arrester will cause a new CF phenomenon. There will be no short circuit on the DC side and the fault current is greatly reduced. This leads to insufficient regulation capability of the CV controller, resulting in continuous CFs. Given the limited fault ride-through capability of the selected CV controller, this paper proposes an adaptive order regulation control (AORC) based on expected reactive power. It can adjust the DC current and voltage in a targeted manner according to AC faults, and determine the maximum allowable upper limit of the firing angle. This improves the reliability of HCC in resisting CF. Under extreme cases, it also effectively prevents the continuous CFs. The simulation results in PSCAD/EMTDC verify the the effectiveness of theoretical analyses and the proposed control method.

The hybrid commutation converter (HCC) based high voltage direct current (HVDC) system features controllable turn-off capability, eliminating commutation failure (CF) caused by single-phase grounding faults in the AC grid. However, under severe asymmetric grid fault conditions, significant voltage and current secondary ripples occur in the DC lines and lead to sending end renewable energy systems destabilizing or even off-grid, posing a serious threat to the stable operation of HVDC systems. To address this issue, this paper first investigates the DC secondary ripple generation mechanism and its transmission process between the sending and receiving ends. It finds that negative-sequence voltage at the receiving end grid is converted into DC voltage ripple by converter modulation, propagating through the DC line and controller to induce fluctuation in the sending end converter bus voltage. Then, a method for suppressing DC secondary ripple for HCC-HVDC is proposed, leveraging the wide-range firing angle adjustment capability of HCC. The method involves actively injecting a double-frequency disturbance signal into the inverter firing angle command. This enables the converter to trigger asymmetrically and introduces a negative-sequence component into the fundamental frequency-switching function. The negative-sequence component couples with the positive-sequence voltage of the AC grid to offset the DC voltage secondary ripple. Finally, a DC secondary ripple suppression controller (DCSRSC) is designed based on the proposed method and embedded in the HVDC control system. The controller dynamically adjusts the inverter's firing angle based on real-time detection of AC negative-sequence voltage, effectively suppressing DC secondary ripple and enhancing the stability of DC power transmission. Hardware-in-the-loop experiments validate the effectiveness of the proposed method.

During grid faults, the grid-forming inverter (GFMI) needs to suppress overcurrent and provide grid support. However, the grid-supporting capability is commonly overlooked while designing the current-limiting method. Therefore, a two-stage adaptive virtual impedance-based current limitation for improved grid-supporting capability is proposed. Theoretical analysis shows that the better response performance of reactive power can be achieved by setting a larger impedance ratio (n = ωL_v/R_v), but this will increase the peak magnitude of fault current. Hence, during the fault transient stage, the proposed method uses the fault voltage drop to quickly calculate the large magnitude of virtual impedance to suppress the peak magnitude of fault current. However, during the fault steady stage, the large magnitude of virtual impedance limits the capacity utilization, thus the amplitude of the fault current is used to build a state machine to further optimize the magnitude of virtual impedance, to improve the capacity utilization. Then, the fault current can be suppressed under various fault conditions and the grid-support capability of GFMI can be improved from two aspects: the response performance of reactive power and the capacity utilization. Finally, simulations and experimental results are used to verify the effectiveness of the proposed method.

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 actual line-commutated converter based high- voltage direct-current (LCC-HVDC) projects, multiple HVDC transmission lines are usually installed on the same tower to save transmission corridors and reduce construction costs. However, electromagnetic coupling effect between adjacent lines can cause commutation failure (CF) in the non-fault poles when a line or pole fails. Hence, this paper deeply investigates the mechanism and the prevention strategy of CFs caused by line coupling effect in single- and double-circuit HVDC systems. The influencing factors of line coupling effect and the coupling characteristics under fault conditions are analyzed first based on the modeling of parallel transmission lines and the traveling wave theory. Then the mechanism of CF caused by line coupling effect is investigated with the consideration of different fault conditions, different line structures, and different system operation modes. Based on the above analyses, prevention strategies for CFs of non-fault poles in single- and double-circuit systems are proposed, respectively. Finally, the simulation results verify the correctness of the theoretical analyses and the effectiveness of the proposed control strategies.