Modular Multilevel Converter (MMC) secured its place in applications where high reliability, efficiency and effortless voltage/power scalability are of paramount importance. Without compromising these benefits, this paper proposes a novel control and appropriate design approach for Hybrid-MMC topology, comprising a mix of Full-Bridge and Half-Bridge submodules in converter branches, as a grid-side stage of a back-to-back MMC, enabling operation at variable DC link voltage and arbitrary power factor. In the midst of transition to Renewable Energy Sources-dominated power systems, variable DC link voltage operation opens door to the use of Hybrid-MMC in retrofit of large Pumped Hydro Storage Plants to variable speed operation, where existing machines would not tolerate high common-mode voltage stress imposed by fixed-DC-link-voltage operated machine-side MMC stage. In this way, highly flexible grid-scale energy storage use of existing hydro capacities is enabled. Converter design approach and additional control layers have been developed and verified through a set of test scenarios. Sizing and operation at typical operating points are compared to equally-rated Full-Bridge-based MMC. Improvements over the existing Hybrid-MMC solutions include unity-power-factor operation at the entire attainable DC link voltage range and equal loading of upper and lower phase-leg branches regardless of the operating point.

Due to the existence of grid impedance, the grid voltage often contains low order background harmonics, which will deteriorate the current quality of grid connected converters. In view of this situation, considering the delay effect and weak grid, the robustness of two single loop control modes of LCL-type grid connected converter, namely grid-side current feedback (GCF) and converter-side current feedback(CCF), is evaluated to improve the current quality of the grid current under distorted grid. Through the analysis and demonstration of above impedance models, CCF has the advantages of excellent stability and excellent power quality. Then, a direct harmonic suppression strategy based on CCF is proposed. The strategy takes the converter side current which contains richer low harmonic content as harmonic information source, and the control effect is more effective than the traditional strategy which uses capacitor current as harmonic information source. Without additional control strategy and controller, the hardware cost is reduced and algorithm complexity is simplified. The simulation and experimental results verify the correctness and feasibility of the theoretical analysis.

There are twelve high-voltage direct current (HVDC) transmission projects in China Southern Power Grid. If the inverter alternating current (AC) system fault leads to HVDC protection system maloperation, it’s harmful to the safe and reliable operation of the system. To solve the above problem, firstly, this paper selects the possible maloperation DC protection functions on AC faults according to RTDS (Real Time Digital System) test. Then influence of AC fault on DC control protection is analyzed. Combined with the test and principle analysis, the improved idea of using AC system side protection to remove faults and avoid DC protection maloperation or blocking is put forward. It is suggested that the DC protection delay setting should be adjusted and matched with the AC protection setting within the main equipment tolerance, and the corresponding scheme of DC protection optimization is given. Finally, the simulation tests were carried out by the RTDS based on 7 HVDC current projects, and the improved protection schemes were verified. The improved protection schemes have been put into effect in China Southern Power Grid HVDC projects. The operation experience shows that these schemes can effectively avoid the shutdown risk of HVDC system caused by AC system fault.

For high-voltage direct current(HVDC) transmission systems that use overhead transmission lines, the lightning disturbance is the main reason for the line protection mal-operation. To avoid the impact of lightning disturbance on the safety and stability of DC line protection, the model of HVDC transmission system including lightning strokes is constructed. On this basis, the complementary ensemble empirical mode decomposition(CEEMD) and Hilbert transform are combined as a time-frequency analysis method to calculate the energy distribution of the signals. Then the low-to-high-frequency energy ratio of the 1-mode voltage signal in a 3 ms data window after the protection startup is used as the protection criterion for identifying lightning disturbance, and that of the current signal in 1 ms is used as the criterion to further distinguish lightning and non-lightning faults. Verified through simulation tests, the scheme appears good adaptability and accuracy in different situations.

Power distribution systems are vulnerable to natural disasters and malicious attacks. An efficient and accurate online risk assessment tool is very necessary to provide timely warning information for emergency dispatch of resilient distribution systems. However, conventional analytical risk assessment methods are subject to known network information, while emerging data-driven methods rarely incorporate resilient resources into the risk assessment procedures, limiting their accuracies when applied to extreme events. To solve the problems, this paper proposes an improved online data-driven risk assessment method adaptive for resilient distribution systems. Twenty-five basic operational indexes from practical experience are chosen to indirectly reflect the system risk, and the complicated relationship between the indexes and risk is characterized by entropy weights and gray correlation degrees. The proposed method is validated on a modified 33-node system, and the results show that it has better accuracy compared with similar approaches in online risk assessment during extreme events. The whole scheme can be helpful for the software design and hardware layout of future resilient distribution systems.

In this paper, a novel control strategy of current source converter(CSC) is proposed for doubly-fed induction generator(DFIG) wind energy conversion system(WECS). Most of the studies on wind forms are based on voltage source converter, nevertheless, with the development of semiconductor technology and high switching frequency reverse block insulated gate bipolar transistor(RB-IGBT) devices being used in CSCs, the shortcomings of conventional CSCs including low switching frequency, large passive devices and slow dynamic performance can be overcome. Furthermore, WECS based on CSCs has various advantages, such like robustness, inherent short-circuit protection, fault ride through capability and so on. To implement the CSCs in DFIG wind energy conversion system, this paper analyzes the system configuration, operation principles, modulation strategy and control strategy. In addition, a novel control strategy based on DC-Link current optimal control is proposed. The simulation and prototype experiments verify the validity of system configuration and control strategy.

The modular DC solid-state transformer(DC-SST) is promising but restricted by limited reliability, which can be solved by redundancy. However, current redundancy designs are only based on the reliability evaluation in stable operation condition and seldom efficiency. In this paper, a redundancy design method of DC-SST is proposed considering both reliability and efficiency. The long-term mission profile and aging of power semiconductor devices(PSDs) and capacitors are considered in the reliability evaluation. Based on the result that the capacitor reliability dominates the system reliability, a simplified system reliability model is proposed. Besides, IGBT power loss considering the long-term mission profile is obtained, which dominates the system power loss. Finally, a multi-objective optimization is solved to obtain the optimal redundant number. Feasibility of the proposed method is verified by a prototype.

The modular multilevel converter(MMC or M2C) technology becomes the mainstream solution in today’s voltage source converter-high voltage direct current(VSC-HVDC) transmission lines because of its unique performance. Recently press pack IGBT(PPI) has come into focus as a preferred realization for high power rating VSC-HVDC stations, it could offer the highest current rating IGBTs and advanced features in reliability. A new PPI is introduced in this paper, which is designed for several innovative improvements based on the application demands from VSC-HVDC systems. The chip is optimized for low on-state voltage with advanced trench technology, and well protected in a chip-stack by double side sinter process. The disc package is designed with hermetic sealed ceramic housing to avoid environmental erosion and could provide a robust feature of short circuit failure mode(SCFM), which is fully qualified under the worst-case scenario. In addition to the PPI device, system solutions had also been developed and investigated to support field developments, including electrical and mechanical design. An example of a power stack is analyzed in commutation loop, pressure distribution, and power losses.