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 transformer leakage reactance have to be taken into account. The point of common coupling (PCC) is a common point of weakness in the grid [1]–[2]. This may lead to stability and power quality problems of grid connected converter. Namely, the low-order harmonics generated by a non-linear load leads to the background voltage harmonics at the PCC due to the grid impedance, which further affects the connection between the grid and converter. In addition, the high-order switching harmonics generated by the operation of the equipment connected to the power electronic grid further degrade the grid power quality [3]–[4]. The LCL-type converters have been widely used in renewable energy grid-connected systems due to their excellent performance in high-frequency harmonic attenuation [5]. However, the phenomenon of resonance exists due to the circuit structure of the LCL-type converters. The active damping method can be employed to avoid these problems such as high hardware cost and reduced power transmission efficiency caused by passive damping method [6]–[15]. Besides, the performance of the control system should be studied considering the digital delay, which causes a phase lag in the control system and threatens system stability and anti-disturbance ability. For the converter-side current feedback (CCF) and grid-side current feedback (GCF), stability can be achieved without using a damping strategy if specified digital delay requirements are met [8],[16]. Many measures have been proposed by scholars to reduce the delay, including the double sampling method, the impedance reshaping method and the pole-zero delay compensation method [14]–[17]. An improved sampling technique is proposed in [14], which can reduce the delay time by sampling the signal twice in each carrier cycle. By adopting the state observer, the signal of the previous cycle can be used to predict the signal of the next cycle, so as to reduce the sampling delay caused by digital delay. However, this method requires additional computing resources, and the observer is extremely sensitive to the small range variation of parameters. For example, the parameter values of resistance and inductance will change with the length of current passing time, which may lead to estimation or prediction error [15]. The delay effect can also be effectively reduced by adding low-pass filter [5] and sampling reconstruction [14]. In addition, there are many researches on

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 harm 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) undertakes the high-frequency isolation function is also called the DC solid-state transformer (DC-SST) [1]. However, most reliability evaluations are based on AC converters [2]-[4], and researches about the reliability evaluation of DC-SST are relatively few. In [5], an optimized design method for DC-SST based on SiC MOSFET has been given. Besides, the lifetime of Si IGBT and SiC MOSFET have been compared under the daily load curve of the distribution network. However, the lifetime assessment for DC-SST only considers the thermal stress of the power semiconductor devices (PSDs), and does not include PSD aging and capacitor reliability. In addition, limited to the simulation speed of the simulation software, [5] did not perform the reliability assessment of DC-SST under the annual mission profile. Therefore, the system-level reliability assessment of DC-SST considering aging information of PSDs and capacitors under the long-term mission profile needs to be resolved urgently. Since the DC-SST is the interface connecting the medium and low voltage distribution network, there is a large voltage level difference between the primary and secondary sides. To meet the voltage and current requirements of PSDs, the DC-SST usually adopts a modular structure. Redundancy is one important measure to improve the reliability of the modular converter, including the DC-SST. The results of the reliability evaluation are usually used to guide the redundancy design of the system. For instance, in [6], the optimal cell number for the cascaded H-bridge (CHB) and the cascaded neutral-point clamped circuit (CNB) has been determined. Based on the optimal cascaded cell number, mean time to failures (MTTFs) under different redundant cell numbers have been analyzed. In [7], the reliability of IGBTs and capacitors in modular multilevel converters (MMC) have been evaluated separately, and the evaluation results are used to guide the redundancy design of MMC. In [8], the reliability of MMC has been evaluated considering the relevance of sub-modules, and the optimal module redundancy configuration scheme of MMC has been proposed based on the evaluation results. It is worth noting that when evaluating reliability, the above-mentioned research is only based on the stationary operation of the converter, and it is assumed that power devices have a constant failure rate. In fact, the long-term mission profile (ambient temperature, load fluctuation, wind speed, etc.) changes drastically, which will seriously affect the lifetime of power devices and converters. Moreover, the aging of critical components such as PSDs and capacitors needs to be considered.

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.