Interleaving techniques are adopted to reduce input/output current or voltage ripples for converters with critical conduction mode. The paper studies and implements two types of phase-shift control schemes for critical interleaved boost PFC converters. Sub-harmonic oscillation phenomena occur at high line voltage when one of them is adopted. So this paper presents a quantitative analysis on the phenomena and proposes a novel phase-shift control scheme. Experimental results are presented in order to verify the feasibility and performance of the novel phase-shift control scheme.

Considering the FB-MMC, a generalized pre-charging strategy is proposed for the MMC-based systems. It contains two stages, no control charging and controllable charging. The circuit model of the converter is established in the no control charging stage, and the submodule capacitance can be charged to about 37% of rated voltage. During the controllable charging stage, we proposed grouping charging strategy, the blocked and bypassed strategy of each group sub-module is formulated. According to the circuit model, the capacitance voltage of each module can be charged to the rated value through this strategy. Finally, we set up a 9 level full bridge MMC-HVDC simulation system in PSCAD/EMTDC. The charging strategies are simulated, and the results show that the proposed control strategy is feasible.

In a flexible DC transmission system with a large-capacity overhead line, a hybrid modular multilevel converter (MMC) consisting of the half bridge submodule (HBSM) and the full bridge submodule(FBSM) has become a research hotspot due to its self-cleaning capability of DC short-circuit fault. Under the operating conditions of reducing the DC voltage, due to the different charging and discharging characteristics of the half bridge submodule and the full bridge submodule, a capacitor voltage imbalance may occur between the two submodules. This paper first studies the number configuration of half-bridge submodules and full-bridge sub-modules in modular multilevel converters. Then, the capacitor voltage imbalance mechanism of the hybrid MMC sub-module is analyzed in detail, and the optimal control strategy of the capacitor voltage balance is derived according to the configuration requirements of the two sub-modules. Finally, a hybrid MMC-based flexible DC transmission system is established in the PSCAD/EMTDC simulation environment, low DC voltage operation of the system are simulated to verify the correctness and effectiveness of the control strategy.

With the continuous increase of wind power permeability, the dynamic behavior of the power system also changes, which also has a great impact on the setting of the low-frequency load-shedding scheme. The original low-frequency load-shedding scheme has no good adaptability to the system of high-permeability wind power. So, this article analyzes the principles of wind turbines in power grid frequency control. This paper expounds on the principle of low-frequency load-shedding strategy in the power system configuration. Connecting with the actual operation of the grid simulation, simulation results show that wind turbines in power grid frequency modulation can significantly reduce the low-frequency load shedding process of loads and wind power in frequency modulation of low-frequency with a positive impact.

Voltage source converter multi-terminal DC system(VSC-MTDC) have been used in many applications due to their many advantages. With conventional droop control, the voltage and power of the stable point are on a straight line with a specific slope. When subjected to a power disturbance, the voltage quality may be deteriorated or the power distribution effect of the converter station may be deteriorated. In this paper, a new adaptive control strategy is proposed for VSC-MTDC. The control system focuses on voltage quality at low loads and power distribution at high loads. It not only ensures voltage quality but also meets power distribution requirements. Simulation results are provided to verify the validity and effectiveness of the proposed method.