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Differing from a two-level voltage source converter (TL-VSC), the internal dynamics of a modular multilevel converter (MMC) are much more complicated. Since the fluctuation of submodule capacitor voltage is unavoidable when transmitting power, the MMCs output voltages cannot track the references perfectly. The interaction between the time-varying capacitor dynamics and controllers may lead to self-instability of the MMCs. In order to address this self-instability issue, this paper first establishes the small-signal closed loop transfer function matrices (TFMs) of the external and internal control loop by the harmonic linearization method. Further, a novel design principle of MMC controllers is proposed. Then, the critical impact factors on the stability of circulating current controller and AC voltage controller are investigated based on the generalized Nyquist criterion(GNC). The proposed models and analysis methods are validated by time-domain simulations in PSCAD/EMTDC.

In this paper, a centralized locating strategy of fault current limiter (FCL) in high voltage direct current (HVDC) grid is proposed and analyzed. By employing FCLs at the output sides of MMCs, the proposed centralized locating strategy could reduce the number of FCLs in multi-terminal high voltage direct current (MTDC) grid. A typical application scenario based on Zhangbei-Beijing 4-terminal HVDC grid is introduced in this paper. Mathematical model of the proposed strategy is built and numerical solutions are calculated. Then, the simulation model is built in PSCAD/EMTDC and different locating strategies of FCL are studied. Simulation results verify the rationality of theoretical analysis. Also, calculation and simulation results demonstrate that the proposed centralized strategy has similar current limiting performance with the conventional distributed FCL locating strategy. When the proposed strategy is applied, the MMCs could be protected from over currents. What’s more, the reliability and reconnection of partial system after faults is ensured. The healthy part of MTDC grid could maintain continuous operation and transmit active power as normal. When applying different FCL locating strategies, the total investment of FCLs are calculated and compared. Calculation results prove that the proposed centralized locating strategy can effectively reduce the investment of FCLs compared with the conventional distributed strategy and is more suitable for MTDC grid.

The small-signal stability of wind farm connected MMC-HVdc systems has been widely studied by impedance analyses. However, it is generally difficult to calculate the poles of wind farm connected MMC-HVdc due to complex internal dynamics. Thus the existing impedance analysis methods usually assume that the studied subsystems are stable. In some special cases, the MMC-HVdc itself may be unstable but the wind farm connected MMC-HVdc may be still stable. If the poles of the subsystem are not calculated, these phenomenons can not be well revealed by impedance analysis. In order to address these special issues. This paper divides the wind farm connected MMC-HVdc into three subsystems and proposed a 3-order minor loop-gain matrix to investigate the unified stability of both ac and dc systems. Based on the proposed model, the impact of ac/dc current-based damping controls in wind farm side MMC on the system stability are investigated. As a result, the proposed model makes up for the deficiencies in ac/dc impedance analysis method and provides a new sight at evaluating the impact of active damping controls on system stability. The simulation results in PSCAD/EMTDC validate the proposed models and stability analyses.

High-frequency resonance (HFR) is a key issue of power electronics-based systems. Two solutions can be applied for addressing HFRs, including active damping method and passive damping method. For modular multilevel converters (MMCs), most of the existing harmonic resonance suppression methods are focused on active methods, including active filtering and virtual resistance. However, the active filtering may lead to negative damping in its attenuation band and the virtual resistance may become negative in high-frequency range when influenced by control delay. In order to avoid the negative effects of control delay and increase the damping of MMCs in high-frequency range, this article reconsiders the arm inductor in MMCs and proposes a passive damper design method for MMCs. With the coordinated design between active controller and passive damper, the negative damping of MMCs in high-frequency range can be effectively eliminated and the system stability can be improved. Also, the proposed passive damper has little impact on the steady-state operation and increases few losses of the MMC. The effectiveness of the proposed passive damper is validated by electromagnetic transient (EMT) simulations in PSCAD/EMTDC.

This article approaches to design an attack-aware detection and defense framework to resist adversarial attacks on the security-critical artificial intelligent systems. We first make efforts to test the performances of adversarial attacks and present classifying and grading rule (CGR) for the fine-grained grouping of adversarial example attacks. According to CGR, adversarial attacks can be divided into six groups. Then, we propose a feature squeezing and CGR-based detector to detect adversarial attacks, which can be aware of the detailed attack group and is evaluated to be effective by extensive experiments. We also test the defense performances of typical defense methods against these six groups of adversarial attacks, and finally give the defense recommendations for each type of adversarial attack.

In the past, most of the transient electromagnetic (EM) analyses focused on the scattering from the target. The transient scattering from rough surface was rarely studied, especially for the layered rough surface. When the target is above the rough surface, it needs to solve the composite transient scattering from the rough surface and the target above it. In this article, we mainly focus on the time-domain (TD) analysis for transient scattering from the target and layered rough surface based on time-domain ray tracing (TDRT) modeling. Applying geometrical optics (GO), the multiple bounces of the transient EM waves between layered rough surfaces and target are traced to obtain the incident field of the irradiated surfaces. Then considering the particularity of the layered rough surface, we modify TD Gordon integral and reduce the TD physical optics (TDPO) integral to closed-form expressions to improve the computing efficiency. To demonstrate the efficiency and accuracy of the proposed algorithm, the transient scattering simulations of the composite model involving target and layered rough surface are compared with frequency-domain ray tracing (FDRT) with inverse fast Fourier transform (IFFT), as well as the results obtained by the multilevel fast multipole algorithm (MLFMA) of FEKO in conjunction with IFFT (IFFT-MLFMA). The characteristics of transient scattering from the composite model are further studied to show the physical phenomenon of scattering mechanisms. Specifically, the transient scattering from the composite model for different parameters is simulated for sensitivity investigation of the transient scattering to the variations in the related parameters.

The utilization of demand response flexibility has become a significant method to cope with the intermittence of renewable energy sources in distributed systems. This paper proposed a new pricing method for demand response resources managed by a distribution system aggregator, which is deduced from analyzing the operating revenue within the timescale from hours to years. In the proposed model, the hourly decision-making of an aggregator is formulated as a newsvendor model and uncertainties in the long-term decisions are modelled by a backward valuation process. It maximizes the benefit of an aggregator by considering the price and quantity uncertainties of distributed load/generation in day-ahead and real-time wholesale electricity markets. Meanwhile, the coexistence of controllable and uncontrollable loads is also considered, where the former refers to electricity consumption from end-users who are equipped with smart devices for energy management, and the latter load demand of passive end-users who have no willingness or capability to participate in the demand response schemes. Finally, numerical studies are carried out to demonstrate the feasibility and effectiveness of the developed model and methods, and the impacts of active end-user percentage on the aggregator operation under the proposed pricing method are also compared and illustrated.

Replacing expensive IGBT with thyristor to design dc circuit breaker (dcCB) is attracting attention recently due to its advantages in terms of low investment and low conduction loss. However, due to the half-controlled characteristic of thyristor, the existing thyristor-based dcCBs (T-CBs) cannot be actively turned off . As a result, the opening process is not fully controllable and reliable. In this letter, a simple active T-CB (AT-CB) topology is proposed. The proposed AT-CB can interrupt both fault current and operation current actively, and the current breaking process is fast, reliable, and fully controllable. Moreover, the proposed AT-CB can be improved to be bidirectional without increasing the conduction loss, and the topology remains simple. Parameters design principle and operation process of AT-CB are presented. Scaled-down experiment verified the effectiveness of the proposed solution.

Sudden increases in the dc-link voltage, caused by ac grid side voltage drops, can lead to potential damage in the equipment. Therefore, dc choppers are widely used in offshore wind power links to absorb the excess energy and keep the voltage within safe margins. This article investigates the existing chopper topologies and summarizes their advantages and disadvantages. On this basis, a novel dc chopper topology with both centralized and distributed resistances is proposed. It is a combination of two mainstream chopper topologies and gives the flexibility to tradeoff among the pros and cons of them. In this way, the proposed topology achieves great electromagnetic interference performance, precise power control, and very small system disturbance. In addition, the maximum power dissipation of the distributed resistance is reduced by 75%. The switching frequency and the loss of insulated gate bipolar transistors (IGBTs) are also decreased. Finally, the claimed attributes of the proposed topology are verified by test and simulation.

SCR-based solid-state dc circuit breaker (SCR-DCCB) topologies have been widely studied for replacing the traditional expensive insulated-gate bipolar transistor (IGBT) based DCCB. Yet, the existing SCR-DCCB cannot be actively turned- off since SCR is a half-controlled device. In this letter, an active Z-source DCCB (AZ-DCCB) is proposed by the combination of IGBT and SCR. The proposed AZ-DCCB has advantages of simple and compact topology, economical design, low conduction losses, and active and bidirectional current breaking capacity. The operation process of the proposed AZ-DCCB is discussed in detail, and the designing principle of circuit parameters is also presented. Experimental results verified the effectiveness of the proposed AZ-DCCB.

Smart home scheduling, facilitated by advanced metering, monitoring, and manipulation technology, plays an important role in the energy transition in terms of accommodating intermittent renewable energy and improving energy consumption efficiency. The key functionalities of home energy scheduling are usually implemented by leveraging the flexibility of household appliances, such as thermostatically controlled loads (TCLs) and energy storage units, to improve the peak-to-average ratio for utilities and reduce energy bills for customers. However, the consumption patterns of appliances are greatly influenced by a variety of factors, including real-time tariffs, ambient temperature profiles, indoor activities, and solar irradiance. Hence, smart home energy scheduling is a challenging task because most of these impacting factors are stochastic and difficult to predict. To properly model and manage the uncertainty factors associated with smart home appliance scheduling, an economic model predictive control (MPC)-based bilevel smart scheduling scheme is proposed in this paper. The comprehensive modeling of distributed generation and household appliances is performed at the single-household level. The home energy scheduling problem is formulated on two levels, with the upper level emphasizing the economic impact and the lower level focusing on capturing TCL responses. The correlations among different TCLs and their performance under the influence of various uncertainty factors, such as environmental impacts and user behaviors, are considered. The efficiency of the proposed MPC-based bilevel optimization model and the effectiveness of the home energy scheduling strategy in managing uncertainties are validated and illustrated in numerical studies.

Memtransistor, a hybrid structure that integrates the function of memristor and transistor, is a promising device prototype for the realization of complex neuromorphic learning owing to its diverse functionality and additional flexibility in emulating synaptic behaviors. Memtransistor of two-dimensional (2D) chalcogenide materials have received many interests as it has distinctive memristive mechanism quite different from conventional oxide memristors. Here, we report a memtransistor based on the two-dimensional thin films (2DTFs) of non-layered β-In2S3. The In2S3 2DTFs grown by physical vapor deposition method have microscopically visible grain boundaries (GBs) formed by the stacking and interconnecting of 2D In2S3 flakes. The memtransistors of In2S3 2DTFs show tunable bipolar resistive states with resistance ratio up to 105, endurance over 200 cycles, and a retention time of 104 s. Illumination of laser light from visible and near-infrared are able to induce intermediate resistance states in memtransistors, enabling optical-modulated multilevel memory storage. Also, the memtransistors are able to emulate the synaptic function of long-term potentiation (LTP) and long-term depression (LTD) with tunable synaptic weight in response to presynaptic stimuli of drain/gate pulses. Interestingly, the plasticity of LTP and LTD behavior can be switched in a highly tunable manner by simply varying the gate voltages. The diverse optoelectronic properties and controllable functionality of memtransistors based on the emerging 2D In2S3 offer a useful guide to potential application in electronic memory and artificial synapses.

A dc–ac converter for voltage-source converter (VSC)-HVdc technology has a significant influence on the performance of the entire power transmission system. This paper introduces a new dc–ac converter composed of submodules and series insulated-gate bipolar transistor (IGBT) switches. The basic idea of this hybrid solution is to shape the ac voltage by submodules but, per half cycle, reconnect them to different electrical points by IGBT switches. This concept can help to reduce the quantities of submodules, thereby reducing the energy storage requirement significantly. Another advantage is that the IGBT switches can be soft switched by utilizing the high controllability of the submodules. This brings extra benefit of power losses reduction. In this paper, the operating principle of this hybrid converter is explained. Its performances are also presented in detail and compared with those popular VSC dc–ac converters. The feasibility of the new concept is also verified by simulation and experimental results.

Increasing deployment of distributed energy resources (DERs) is re-sculpturing the modern power systems in recent years. Future smart power distribution systems should be competent at accommodating extensive integration of DERs and managing the associated uncertainties at the distribution level. The electricity market has been proved to be an efficient way to employ market signals to direct behaviors of users and DERs with large capacity and homogeneous pattern. However, existing market frameworks cannot effectively handle a large number of small-scale DERs due to their diverse characteristics and arbitrary behavior patterns. In this context, an aggregated model which can represent and manage a diverse collection of DER, load, and storage is proposed. An additional trading platform, namely the energy sharing market, is established to reinforce the coordination and collaboration among various aggregators as well as operators. Energy sharing scheme is applied and a corresponding dynamic dispatch platform is designed to solve the crowdsource problem. The efficiency of the proposed model is validated by the numerical studies, and the market performance and impacts of energy sharing on the power systems are illustrated.

The determination and optimization of the rated capacitance of a capacitor voltage transformer (CVT) is very valuable for the optimal design of a CVT since it is a fundamental parameter for solving many problems such as minimizing the effect of stray capacitance (SC), ensuring a high accuracy of voltage measurement, and fast transient response. SC is known to seriously affect the capacitance of an ultra-high-voltage (UHV) CVT resulting in a larger additional measurement error and a higher false operation rate of relay protection; it is usually unknown or poorly quantified and cannot be measured directly. A new effective and reliable method for rated capacitance design of a new UHV equipotential shielding CVT under the interference of SCs is proposed and successfully demonstrated in this work. An equivalent mathematical model is established to derive the CVT rated capacitance and the SCs are extracted using a finite element method. The performance of this method is verified in the experiment to demonstrate the effectiveness of the method.

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results.

Reducing the capacitance of submodules (SM) and handling short-circuit faults in HVdc systems are crucial for economical and reliable operation of modular multilevel converters (MMC). This paper proposes an SM capacitance reduction method for full-bridge submodule (FBSM)-MMC, which combines circulating current injection and ac voltage boosting. An optimized circulating current control method considering the limitation of fixed semiconductor current rating is introduced. By adding second-harmonic voltage to the reference arm voltage, the capacitor voltage ripple can be controlled to be a targeted value under all operating conditions. To further decrease the SM capacitor voltage ripples, the negative voltage state of FBSM is fully utilized to boost the ac voltage under fixed dc voltage with the modulation index up to 1.414. Consequently, the capacitance requirement for ac voltage boosting FBSM-MMC can be significantly reduced as compared to the conventional FBSM-MMC. Experimental results confirm the feasibility and validity of the proposed SM capacitance reduction method.

Using voltage-source converter high-voltage direct current (VSC-HVDC) to interconnect weak ac power systems has drawn much attention. Under the traditional vector control for the VSC that the response of AC voltage loop cannot match that of the active power loop may limit power transmission capability of the VSCs that interconnect with weak power systems. To enhance the power transmission capability and stability, this paper proposes a power compensation control that is based on the analysis of characteristics of power–voltage (PV) curves. This method can improve the damping control performance of the active power and the ac voltage by adjusting the compensated values adaptively. A small-signal model of the integrated system is established to study the performance of the proposed control method. The impact of the compensation coefficients and steady-state operating points on the system stability is studied. Motivated by an on-going back-to-back project in China, corresponding electromagnetic transient simulation models are constructed in MATLAB/Simulink environment to verify the effectiveness of the proposed method on the performance of operating limits, damping performance, disturbance withstanding capability, and fault ride-through capability.

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables.

The hybrid multilevel voltage-source converter, combining semiconductor switches and power module cells, is a new family for VSC-HVDC applications. The hybrid multilevel converter in parallel arrangement (PHMC), in terms of converter investment, is very adaptable for cable transmission systems. However, the dc voltage of the converter, synthesized by the output voltage of three-phase cell stacks, is strongly coupled with ac voltage variations, which influences the stability of converter operation. This paper proposes a dc voltage compensation strategy for the parallel hybrid multilevel converter. This strategy, implemented by introducing an isolation period, can effectively cope with the dc voltage deviation, especially under ac network faults. Simulation results in PSCAD/EMTDC verify the effectiveness of the proposed strategy under various circumstances.

An unbalance voltage between a positive pole and negative pole will exist for a long time in an ungrounded modular multilevel converter (MMC) HVDC transmission system. A zig-zag transformer has been proposed to construct an artificial neutral point and keep voltage balance between the positive pole and the negative pole. An efficient grounding system consists of the zig-zag transformer and the neutral grounding resistor. In this paper, the different impedances, including positive-, negative-, zero-sequence impedance, and dc current impedance, have been analyzed, respectively. Then, the computer simulations have been performed to evaluate the characteristic of the zig-zag transformer under normal voltage, zero-sequence voltage and unbalanced dc voltage, respectively. The simulation results show that 1) the zig-zag transformer can effectively balance the unbalanced voltage in dc side of the MMC under different work conditions; 2) the zero-sequence impedance is low and zero-sequence current is convenient for the detection of a single-phase fault; 3) the power consumption of the neutral grounding resistor is of a light rating, and the cooling of the resistor is unnecessary. The method presented in this paper proves to be an efficient grounding method for the MMC-HVDC transmission system.

An equivalent circuit for a core snubber is established in this paper. The circuit can approximately express a parallel resistance and inductance. The resistance can be directly calculated according to the resistivity of the magnetic core and structure dimensions of the core snubber referring to Fink-Baker-Owren (FBO) method, and the eddy current factor is adjusted to one. In this paper, the shunt inductance neglected by FBO is first considered, and it can be obtained from critical characteristics of the magnetic core and structure dimensions of the core snubber. The two electrical parameters are varied with saturated zone of the inner turn of tape. Then, the simulated model of a core snubber is set up. Finally, a miniature core snubber is analyzed, and its parallel resistance and inductance are obtained from the experiment condition. The test data validate the analytic model. The method presented in this paper is proved to be useful in the design of core snubber for the Experimental Advanced Superconducting Tokamak and the International Tokamak Experimental Reactor.