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The fault of the tie line between the photovoltaic (PV) station and the grid is a serious fault for the PV station. It will cause the PV station to operate into an unintentional island. The uncontrolled Island voltage and frequency will inevitably lead to the disconnection of the inverter in the station. This situation will bring great losses to PV operators and impact to the power grid. In order to deal with the tie line fault, this paper analyzes the operation characteristics of PV stations in case of tie line fault firstly. Then a tie line fault ride‐through method based on cooperative strategy of small capacity energy storage (ES), relay protection and PV inverters is proposed. The islanding switching control strategies of PV and ES are designed respectively. The cooperative strategy of protection, PV controller and ES controller is formulated as well. The real‐time digital simulator (RTDS) closed‐loop test platform including line protection device, ES controller and PV controller is built to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Under fault conditions, virtual synchronous generators (VSGs) are prone to lose transient stability, similar to synchronous generators. However, the existing studies on VSG transient stability are not comprehensive. The transient characteristics of VSGs are highly affected by control parameters, but the transient models of VSGs in existing studies are overly simplified. In addition, the critical clearing angle (CCA) and critical clearing time (CCT) are two indices used to measure the transient stability of a system, but there are few studies that consider both CCA and CCT. In this article, we analyze the CCA and CCT quantitatively to describe the transient stability boundary of a VSG under different control parameters and fault conditions. Then, an equal proportional area criterion is proposed to provide guidance for the control of virtual inertia in the VSG. Based on a theoretical analysis, a transient control method for a VSG is proposed, which can improve the transient stability of the VSG in terms of both CCA and CCT. Finally, simulation and experimental tests are performed to validate the correctness of the theoretical analysis and the proposed method. [ABSTRACT FROM AUTHOR]

The stability of the grid-connected inverter (GCI) system in weak grids is deteriorated due to the asymmetric positive-feedback loops (APFLs) introduced by the phase-locked loop (PLL) structure. Under the same main circuit parameters and control loop parameters, the small-signal models of the GCI controlled in dq domain and αβ domain are constructed, respectively. Compared with the small-signal model of the GCI controlled in dq domain, the number of APFLs introduced by the PLL of GCI controlled in αβ domain is one less. Moreover, the mechanism why the stability of the GCI controlled in αβ domain is stronger than that controlled in dq domain is analyzed. Meanwhile, the influence degree and mechanism of the APFLs in different paths of the small-signal models for the stability of GCI are revealed for the first time. This lays a theoretical foundation for the analysis and design of asymmetric control strategies used to improve the stability of GCI in weak grids. Finally, simulations and experiments verify the correctness of the theoretical analysis. [ABSTRACT FROM AUTHOR]

Based on the actual packaging structure of insulated gate bipolar transistor (IGBT) module, this article establishes the power loss model, and analyzes the influence of the position of the lift-off bond wire on the module. Through the theoretical analysis, it is found that the position of the lift-off bond wire is an important factor affecting the aging of the modules. And then the electrothermal coupling model of IGBT is established for simulation, and an experimental platform is built for verification. The results show that the distribution of bond wire lift-off position is one of the characteristics of bond wire failure. When the bond wire lift-off failure is concentrated on a single IGBT chip, it will accelerate the aging process of the modules. Therefore, this article considers that the position of the lift-off bond wire is a significant factor in the aging process of the modules, which should be fully considered in the bond wire state evaluation. In addition, the research results of this article can also provide theoretical reference for the optimization of IGBT module packaging structure. [ABSTRACT FROM AUTHOR]

Fault current-limiting dynamic voltage restorers (FCL-DVRs), also named as dual-functional DVRs, involving both the FCL and the voltage compensation (VC) function, have attracted more attention. However, in case of the FCL-DVR operating in VC mode, either the high capacity of storage system in the dc-link or the oversizing of power converter is needed, and in case of the FCL-DVR operating in FCL mode both the high dc-link voltage and the oversizing of power converter are needed. Thus, in order to overcome these drawbacks, an improved FCL-DVR, which consists of an LC filter coupled with series coupling capacitor (LCC) at the ac side, is proposed in this article. The improved FCL-DVR using energy-optimized control strategy can not only perform the dual functions well, but also achieve the capacity optimization of FCL-DVR key components. The operating principle and performance of the proposed FCL-DVR under FCL and VC mode are elaborated, respectively. The optimal design of LCC and selection of FCL branch are studied in detail to ensure high stability and reliability for the proposed FCL-DVR operating in both modes. The simulation and experimental results clearly validate the effectiveness of the proposed topology and method. [ABSTRACT FROM AUTHOR]

• By the characteristic analysis of VSG, the causes and influencing factors of TPAI and FOC are revealed, and the inherent relationship between TPAI and FOC is found. • Based on the phase portrait theory, the stability analysis of different influencing factors is carried out. It is found that the proposed method can significantly enhance the TPAS of VSG under serious grid faults. • According to the verification results, the proposed method can simultaneously realize the TPAS and FCS of VSG during the fault, and the virtual inductance based on QSA can effectively suppress IIC. In addition, this method has the advantages of simple operation, no need to freeze the reactive power loop, and strong adaptability to engineering application problems such as considering the change of fault impedance and the difficulty of grid fault voltage measurement. When a serious grid fault occurs, a virtual synchronous generator (VSG) tends to lose transient power angle stability (TPAS) and cause the fault current to exceed the limit. Most existing research usually neglects the inherent relationship between transient power angle instability (TPAI) and fault over-current (FOC), which leads to the two problems being solved separately rather than simultaneously. In this paper, the transient power angle and fault current characteristics of VSG are first studied, and the causes and influencing factors of TPAI and FOC are explored. Then, the TPAS analysis is carried out based on the phase portrait theory. A TPAS control method for VSG considering fault current suppression (FCS) is proposed. This method is fulfilled by the combined regulation of active power reference and reactive voltage regulation coefficient, in which the reference and coefficient can be adjusted adaptively according to different fault degrees and application scenarios. In addition, a quasi-static approximate (QSA)virtual inductance is introduced to limit the instantaneous inrush current (IIC). The proposed method achieves both the TPAS and FCS at the same time during the fault. Finally, simulations and experiments verify the correctness of the theoretical analysis and the proposed method. [ABSTRACT FROM AUTHOR]

This study proposes an adaptive soft starter control for induction motors using a multifunctional series compensator (MFSC). The proposed MFSC can operate in two modes: (i) the compensation mode for voltage sags and (ii) the virtual impedance mode for soft starting. Various MFSC functions are obtained by controlling the output characteristics of the series inverter. In the voltage sag compensation mode, a voltage is injected in series with the fault phase; as a result, the terminal voltage of the motor remains constant. In the soft‐starter mode, the proposed converter injects adjustable virtual impedance in each phase; as a result, the starting current can be limited to the desired value. A novel control scheme and the optimal parameter selection of the MFSC are also presented. The design methodology of the proposed MFSC is verified using simulations conducted in MATLAB/Simulink software. Experimental results prove the capability and effectiveness of the proposed MFSC. [ABSTRACT FROM AUTHOR]

This paper proposes a dual-function dynamic voltage restorer (DFDVR) with fault current-limiting capability. The new topology can operate under different modes: the short-circuit fault current-limiting mode and the compensation mode for voltage fluctuation. Different functions of the DFDVR are achieved by controlling a brake chopper circuit and antiparallel thyristors. During fault conditions, the antiparallel thyristors are deactivated to protect the dc-side capacitor, and the duration of the on- and off-periods of the semiconductor switch in the brake chopper circuit is controlled to adjust fault current amplitudes. During voltage sags, the antiparallel thyristors are activated, and the brake chopper circuit is turned off by controlling the gate turn-off thyristor in the brake chopper circuit, thus making the DFDVR similar to a conventional DVR. The control scheme and optimal parameter selection of the DFDVR are provided. The design methodology of the DFDVR was verified through simulations performed using the PSCAD/EMTDC software. The experimental results prove the capability and effectiveness of the DFDVR topology. [ABSTRACT FROM AUTHOR]

State estimation is critical for railway power supply systems (RPSSs). Pseudo‐measurement is commonly used in state estimation. However, the fluctuations of renewable generations and railway traction loads in RPSS may introduce data noise, which will jeopardize the accuracy of the generated pseudo‐measurements and thus impact the state estimation. Additionally, when learning the historical measurement data sequences, the traditional pseudo‐measurement model is likely to have overfitting, which will further impact the accuracy of pseudo‐measurements, thereby affecting the accuracy of state estimation. To address these issues, this paper proposes a high‐accuracy pseudo‐measurement‐based state estimation approach for RPSSs. Firstly, a denoising autoencoder‐based method is used to mitigate the impact of data noise on the accuracy of pseudo measurements, and a gated recurrent unit‐based method is used to adaptively learn the historical measurement data sequence, thereby improving the accuracy of pseudo measurements. Next, the pseudo‐measurement weights are obtained by generating pseudo‐measurement variances using the Gaussian mixture model. Finally, the pseudo measurements and real‐time measurements are integrated by weighted least squares to realize the state estimation of RPSS. The effectiveness and accuracy of the proposed method are verified by simulation on a modified IEEE 33‐node system which includes a railway traction substation and renewable generations. [ABSTRACT FROM AUTHOR]

This paper proposes a novel bridge-type solid-state fault current limiter based on dynamic voltage restorer (BFCL-DVR). The new topology uses a DC-link in the back-to-back converter of DVR. The proposed BFCL-DVR is equivalent to the conventional DVR during normal operation mode. At fault inception, the controller deactivates the series converter IGBTs in faulty phase and activates the DC-link semiconductor switches, thus establishing an equivalent bridge-type current limiter to restrict the fault current. The magnitude of the fault current is adjustable by controlling the frequency and duty ratio of the semiconductor switch that is in parallel with the discharging resistance. Moreover, the device is able to quickly recover from the current limiting mode to the normal operation mode. In this study, the operation sequence, optimal value of discharging resistance, duty cycle, and frequency of semiconductor switches are obtained. The design methodology of the proposed BFCL-DVR is fully verified using PSCAD/EMTDC software. Experimental results validate the proposed topology and theoretical analysis. [ABSTRACT FROM AUTHOR]

There are both economic and environmental urges for transition from the current outdated power grid to a sensor-embedded smart grid that monitors system stability, integrates distributed energy and schedules energy consumption for household users. Especially with the proliferation of intelligent measurement devices, exponential growth of data empowers this transition and brings new tools for the development of different applications in power system. Under this context, this paper presents a holistically overview on the state-of-the-art of big data technology in smart grid integration. First, the features of smart grid and the multisource of energy data are discussed. Then, this paper comprehensive summarizes the applications leveraged by big data in smart grid, which also contains some brand new applications with the latest big data technologies. Furthermore, some mainstream platforms and knowledge extraction techniques are looked to promote the big data insights. Finally, challenges and opportunities are pointed out in this paper as well. [ABSTRACT FROM AUTHOR]

• A coordinated voltage controller based on the improved droop control strategy which is proposed improves the voltage of each node of the feeder. • In order to simplify the calculation method of each slope of BESS group, a simple calculation method of BESS slope considering line impedance and capacity is proposed; • frequency deviation controller based on double hysteresis loop which is proposed in this paper activates the frequency control loop to control the frequency and voltage of the island together with the coordinated voltage controller, so as to realize the self-healing of the island, and then realize the grid re-connection of the island through reclosing. • In addition, a real-time closed-loop simulation platform Composed of RTDS and line protection device is built. The correctness of the proposed method is proved by simulation results. In order to solve the problem of voltage fluctuation caused by distributed generation connected to feeder and the failure of reclosing caused by disordered operation after isolated island, a comprehensive control method including coordinated voltage controller and frequency deviation controller is proposed in this paper, which is applied to distributed energy storage group. When the feeder is connected normally, the frequency deviation controller is invalid, and the coordinated voltage controller based on the improved droop control strategy improves the voltage of each node of the feeder.The improved droop control strategy adjusts the voltage control behavior of BESS through the slope to ensure that multiple BESS jointly undertake the voltage control task, and effectively suppress the voltage fluctuation of feeders. In order to simplify the calculation method of each slope of BESS group, a simple calculation method of BESS slope considering line impedance and capacity is proposed; When the feeder is isolated due to fault, the frequency deviation controller based on double hysteresis loop activates the frequency control loop to control the frequency and voltage of the island together with the coordinated voltage controller, so as to realize the self-healing of the island, and then realize the grid re-connection of the island through reclosing. In addition, a real-time closed-loop simulation platform Composed of RTDS and line protection device is built.The voltage control effect is verified by simulating the power fluctuation of feeders, and the effect of islanding self-healing is verified by simulating the whole process from feeder fault, fault isolation, island self-healing to grid-re-connection. Index Terms: BESS group; Improved droop control; Slope setting; Coordinated voltage control; Frequency deviation control; Island self-healing. [ABSTRACT FROM AUTHOR]

This paper proposes a combinational voltage booster technique for fault ride-through (FRT) capability enhancement of squirrel-cage induction generators (SCIGs) by a multifunctional series compensator (MFSC). This technique uses a couple of antiparallel thyristors across the ac terminals of a traditional dynamic voltage restorer (DVR). Two control schemes are applied: a DVR mode and a multifunctional mode. The MFSC can operate in the DVR mode to compensate the slight voltage fluctuations/imbalances at the point of common coupling (PCC). Also, when serious voltage sags occur in the line, the MFSC controller deactivates the faulty phase inverter and activates its corresponding thyristors to insert the equivalent impedance to maintain the voltage level during the fault. Then, it switches to DVR mode to improve the recovery time after the fault. An operational algorithm is applied to deal with different voltage sag depths caused by various fault conditions (both symmetrical and asymmetrical faults). Comprehensive simulations were carried out by using PSCAD/EMTDC software, and results show that the proposed technology is a very effective device to improve the FRT capability of the SCIG. A scaled-down experimental verification was carried out. Experimental results confirm the effectiveness of the combinational voltage booster technique of the MFSC for both slight and serious voltage sags. [ABSTRACT FROM AUTHOR]

This paper presents a new multilevel cascaded-type dynamic voltage restorer (MCDVR) with a fault current-limiting function. This topology can operate in two operational modes: 1) a compensation mode for voltage fluctuations and unbalances and 2) short-circuit current-limiting mode. The current-limiting function of the MCDVR is performed by activating antiparallel thyristors during the short-circuit fault, and deactivating them during normal operation. The mathematical model of the MCDVR system is also established in this paper. The control scheme design and optimal parameter selection are outlined based on the detailed theoretical analysis of the converter. The transient states of the MCDVR in the compensation mode and current-limiting mode are also analyzed. Simulation results based on the PSCAD/EMTDC software, and experimental results on a laboratory setup help to validate the proposed topology and the theoretical analysis. [ABSTRACT FROM AUTHOR]

This article proposes an improved hybrid parallel compensator (IHPC) for enhancing the power transfer capability of a photovoltaic (PV) grid-connected inverter. A thyristor switched capacitor (TSC) module is used in series with a conventional inductive-coupling voltage source inverter to reduce the dc-link voltage. Under a low dc-link voltage, the voltage across the insulated gate bipolar transistor (IGBT) will also be low, which will reduce the voltage stress of the IGBT, thereby improving the reliability and reducing the loss of the voltage resource inverter. First, the mechanism of reactive compensation for improving the PV inverter power transmission capability is analyzed. Then, the circuit configuration and operating principle of the IHPC are introduced. After that, an optimal control strategy for the IHPC is proposed to address the resonance problem caused by the TSC connected in series with the L filter and to improve the compensation performance of the IHPC. Finally, the effectiveness of the proposed IHPC and the control method is validated by the simulation and experiments. The results show that the IHPC with optimal control strategy can achieve continuous reactive power compensation under a low dc-link voltage, thereby enhance PV power transfer capability. [ABSTRACT FROM AUTHOR]

Improving the operational reliability of distribution networks is an important goal in many countries and regions. The increase in the total length of feeders and the proportion of cable lines creates a larger single-phase-to-ground fault current, which makes the grounding arc difficult to self-extinguish, and then it may easily cause permanent fault events in the distribution network. The ground fault regulation device, also known as an arc-suppression device, is regarded as an important piece of equipment for the reliable and safe operation of ineffective grounding distribution networks. However, owing to the extensive access of cable lines and power electronic devices in the distribution network, the proportion of harmonic and active power contents in the ground fault current has increased, resulting in higher requirements for arc-suppression device performance. Based on the aforementioned, this study systematically summarizes the research work regarding arc-suppression devices from three aspects, namely, the topology of the arc-suppression device with its evolution trend, the control strategy of the arc-suppression device with its optimization method, and the functional integration method of the arc-suppression and electrical power regulation devices. Furthermore, the most representative topologies and control methods in each category are thoroughly described and briefly compared. The main challenges and critical problems of arc-suppression devices in the new distribution network are analyzed and summarized. [Display omitted] • The present situation of the topologies and control methods for the arc suppression devices is systematically studied. • The active and passive hybrid arc suppression devices can effectively integrate the high performance and low cost. • The functional integration of arc suppression and power control is the future development trend of arc suppression device. [ABSTRACT FROM AUTHOR]

The traditional fixed value assessment of the renewable distributed energy hosting capacity of a distribution network cannot accurately and comprehensively reflect the change in hosting capacity; therefore, we propose the interval assessment method for the renewable distributed energy hosting capacity of a distribution network. The renewable distributed energy hosting capacity interval consists of an optimistic upper boundary and a pessimistic lower boundary. First, the optimistic upper bound is described by a deterministic model that takes into account the constraints of safe system operation. Second, the pessimistic lower bound is portrayed by a two-layer robust assessment model that accounts for the DG temporal uncertainty, DG spatial uncertainty, and active distribution network flexible resource dispatch uncertainty. Each pessimistic sub-model was constructed in turn, and then the model was solved by linear simplification using pairwise transformation, as well as McCormick relaxation. Finally, simulations were carried out in the IEEE 135 system, and the results validated the effectiveness and practicality of the proposed method. [ABSTRACT FROM AUTHOR]

A three-phase four-wire inverter is highly suitable for low-voltage distributed grids owing to its zero sequence control; however, essential aspects of grid-following inverters during faults, such as the power oscillation control flexibility and power injection capability, are rarely discussed in detail. Thus, by introducing zero sequence instantaneous power into a four-wire system, this article proposes a flexible control strategy that enhances the power injection capability of the inverter in the photovoltaic generation system during asymmetrical grid faults. First, a generalized reference current generation with flexible control of positive, negative, zero sequence current (FPNZSC), and power oscillations is presented. Second, a maximum allowable power injection (MAPI) algorithm based on the actual value constraints of power oscillations and peak current is presented according to the requirement. The proposed MAPI-FPNZSC strategy further extends the existing fault ride through methods in three-wire inverter systems, which is also suitable for single line-to-ground and double line-to-ground faults and can be easily realized in the natural or stationary frame. The effectiveness of the proposed strategy is verified by comparing the provided three typical objectives with the traditional method, and tested in a real-time hardware-in-the loop experimental platform. [ABSTRACT FROM AUTHOR]

■ By establishing a complete mathematical model, the efficiency characteristics of MS-PEMFC, auxiliary equipment, and converter are analyzed to clarify the relationship between output power and operating efficiency. ■ By calculating the optimal allocation mapping set and embedding it into the control system, the real-time optimal power allocation in MS-PEMFC is realized. ■ The coordinated operation strategy between MS-PEMFC and battery is proposed under light load and overload conditions. ■ Based on the complementation of transient and steady-state energy between MS-PEMFC and battery, a coordinated control between MS-PEMFC and battery is proposed to suppress the DC bus voltage fluctuation. Then, the stable operation of the hybrid system can be well ensured. The traditional control strategy for hydrogen-battery hybrid system is challenging to achieve an efficient operation in the entire power area and suppress transient DC bus voltage fluctuation. Therefore, a two-layer control strategy for hydrogen-battery hybrid system considering the efficiency characteristics of MS-PEMFC (multi-stack proton exchange membrane fuel cell) is proposed, including system optimization allocation layer and equipment coordination control layer. In terms of the system layer, the proposed optimal operation strategy can optimize the power allocation and avoid the light load condition operation of MS-PEMFC to improve the system economy. In terms of the equipment layer, the proposed coordination control between MS-PEMFC and battery can realize the transient and steady-state energy complementarity, and suppress the DC bus voltage fluctuation to improve the system stability. Finally, the feasibility and effectiveness of the proposed two-layer control strategy for hybrid system are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

• A two-stage optimal strategy for flexible interconnection distribution network based on SOP. • Considering the loss characteristic of key equipment in distribution networks. • Economical operation area of distribution transformers. • Improve the economy and reliability of the flexible interconnection distribution network. The flexible interconnection distribution networks (FIDN) based on soft open points (SOP) provides an opportunity to improve the economy and reliability of distribution networks. However, the existing optimal strategies have mainly focused on line power loss reduction, and the loss characteristic of SOPs and distribution transformers is ignored. Therefore, the capability of reducing total power loss for the existing optimization strategy needs further improvement. Thus, based on SOP's locating, sizing and optimal operation, a two-stage optimal strategy for FIND is proposed considering the loss characteristic of key equipment including SOPs and distribution transformers. Firstly, considering the transformer equivalent loss model and different load states, an improved loss sensitivity model is proposed to determine the optimal access location of SOP. Further, the SOP's capacity is designed by the total cost optimization model. Secondly, based on the loss characteristic of key equipment, the optimal operation strategy is proposed to reduce the total power loss of FIND while realizing optimal economic area operation of transformers. Finally, the case studies were performed on a FIND test system with 22-bus and 15-bus to validate the proposed two-stage optimal strategy, and the results of the case studies show that the economy of FIND is greatly improved. [ABSTRACT FROM AUTHOR]

• MF-ASD can realize reactive power compensation and arc suppression. • The variation law of the DC-link voltage of the MF-ASD under different conditions is analyzed. • The proposed alternate arc suppression method does not need additional DC power supply devices. • The MF-ASD can also realize the DC-link voltage stability when the active component of the fault current is large. To fully compensate reactive and active components of the single line-to-ground (SLG) fault current, additional DC power supply devices are usually used for arc suppression devices. Therefore, the alternate arc suppression method is proposed for the non-neutral multi-function arc suppression device (MF-ASD) in this paper, which can achieve energy self-balance in the MF-ASD during the SLG fault without additional DC power supply devices. Firstly, the DC-link charge–discharge characteristic of the MF-ASD is analyzed under different SLG fault conditions. Then, the alternate arc suppression principle and energy flow law between two non-faulty phases of the MF-ASD are described in detail. Further, the relationship between the alternate period, switching time, and the DC-link voltage are deduced as well. Then, the control parameters are reasonably designed for the DC-link voltage stability of two non-faulty phases of the MF-ASD. Finally, the correctness and feasibility of the proposed method are verified by simulation and experiment results. [ABSTRACT FROM AUTHOR]

High-performance β-Ga 2 O 3 -based Schottky barrier diodes (SBDs) and metal-semiconductor field-effect transistors (MESFETs) are fabricated on a non-delta-doped high doping level (>1018 cm−3) epitaxial wafer. Their electrical properties and stabilities after annealing at 200–400 °C are investigated. The ON/OFF ratios for all the SBDs are over 108. The ideality factors and Schottky barrier heights for the Au/β-Ga 2 O 3 SBDs with different annealing temperatures range from 1.5 to 2.5 and from 0.8 eV to 1.0 eV, respectively. There are obvious pinch-off and saturation characteristics for the β-Ga 2 O 3 MESFETs. The maximum drain current (I D, max) for the as-fabricated β-Ga 2 O 3 MESFET is 46.8 mA/mm at anode voltage of 2.0 V. They are much higher than the previously reported values of non-delta-doped and nanobelt β-Ga 2 O 3 MESFETs. Threshold voltages for the β-Ga 2 O 3 MESFETs shift to the negative direction with the increase of annealing temperature. This study is meaningful to push forward the development of β-Ga 2 O 3 -based electronic devices for practical applications. • High-performance β-Ga 2 O 3 -based Schottky diodes and metal-semiconductor field-effect transistors (MESFETs) are fabricated on a non-delta-doped high doping level (>1018 cm−3) epitaxial wafer. • Maximum drain current (I D, max) for the as-fabricated β-Ga 2 O 3 MESFET is 46.8 mA/mm at gate voltage of 2.0 V. They are much higher than the previously reported values of non-delta-doped and nanobelt β-Ga 2 O 3 MESFETs. [ABSTRACT FROM AUTHOR]

Series-connected voltage sources converters (SVSCs), (i.e., dynamic voltage restorer, fault current limiter, electrical spring, etc.), have been widely used in the electricity grid to improve the electricity grid performances in terms of voltage fluctuation compensation, flexible power flow regulation, fault overcurrent limitation, flexible access of renewable energy, etc. However, SVSCs have a relatively short effective operating time and a low utilization rate, because they are generally operated during abnormal grid conditions. The new trend is to increase the utilization by extending the functions of SVSC, e.g., integrating the voltage compensation and overcurrent limiting in a single SVSC. On the other hand, the series connection architecture results in the SVSCs experiencing overvoltage and overcurrent from the main grid, and consequently the reliability of SVSC is challenged. Based on these details, this paper systematically studies the state of the art multifunctional techniques and the corresponding strategies of SVSCs, which can be mainly divided into three categories as follows: function integration between different SVSC; function coordination between SVSC and parallel-connected VSC(PVSC); function integration between SVSC and PVSC. The most representative topology and control optimization method in each category is elaborated, and a brief comparison of them is conducted as well. On this basis, the key problems and challenges in the research and engineering application of multifunctional SVSC system are summarized. • The state of the art multifunctional techniques of SVSCs is systematically studied. • The function coordination between SVSC and parallel-connected devices improves the output performance of the parallel part. • The multifunctional SVSC with fault current limiting capability can protect itself and the power grid actively. • The function integration between SVSC and PVSC open a new way to improve the utilization rate of power electronic devices. • The multifunctional SVSC with minimized additional physical switches and topology complexity needs further studied. [ABSTRACT FROM AUTHOR]

Gallium oxide ( Ga 2 O 3 ) with ultra-wide band gap and high radiation resistance have great potential in solar-blind ultraviolet (UV) photodetectors, but its development is limited by expensive Ga 2 O 3 substrate materials. Sapphire substrate, due to its high hardness, mature production process and low cost, has become one of the most favorable substrate materials for optoelectronic devices. So, Ga 2 O 3 -based optoelectronic devices on sapphire substrates hold significant commercial value. In this work, using first-principles density functional theory (DFT) calculations, we investigate the influence of interfacial oxygen concentration on the β - Ga 2 O 3 (001)/sapphire(11 2 ̄ 0) heterostructure. The results show that the bonding strength between O-terminated and Al-terminated interfaces is strong, indicating favorable thermodynamic stability of this heterostructure. Furthermore, we discover that the metal-O-metal bonding configuration at the interface effectively reduces the number of interfacial defects, increase the built-in electric field strength within the heterostructure, and improves the deposition quality of the β - Ga 2 O 3 film. This work holds significant implications for the research on β - Ga 2 O 3 solar-blind UV photodetectors based on sapphire substrates. • The β -Ga2O3(001)/sapphire(11 2 ̄ 0) interface models with four heterostructures were investigated. • The oxygen concentration at the interface affects the interface interactions and the structural stability. • Al model has fewer interface defects and the highest thermodynamic stability. • The β -Ga2O3(001)/sapphire(11 2 ̄ 0) heterostructure still shows strong optical absorption capacity in the deep ultraviolet region. • This theoretical research is valuable for the growth of high-quality β -Ga2O3 thin films on sapphire substrates experimentally. [ABSTRACT FROM AUTHOR]

A novel space vector modulation (SVM) strategy called maximum-boost discontinuous SVM (MBDSV) is proposed in this paper to reduce the thermal stress of three phase impedance source inverters (ISI). By exploiting the switch clamping mechanism and arranging the shoot-through (ST) states properly, the proposed strategy obtains the flowing merits: further optimize the thermal performance, improve the reliability, reduce the current stress and maximize the dc-link utilization of ISI. The proposed MBDSV was compared to the existing modulation strategies and the feasibility of the MBDSV had been benchmarked by simulation and experiments. • Thermal optimized space vector modulation strategy for impedance source inverter. • Discontinuous SVM is developed to reduce the thermal stress of inverters. • Suitable for medium or high output frequency applications • Simulation and experimental tests are carried out with three-phase qZSI. [ABSTRACT FROM AUTHOR]

The instability issues of the three-phase grid-tied inverters are major concerns in the distributed generated systems. However, due to the coupling effects in dq frame, the instability mechanisms of the three-phase inverters have not yet been clearly revealed. This paper presents a divided dq small-signal model of the grid-tied inverter in dq frame. The specific expressions of a small-signal output admittance of the inverter are investigated based on the proposed model, which can reveal the influencing mechanism of coupling effects and phase-locked loop (PLL) on the stability of the inverter. Then, a stability analysis method based on the divided dq small-signal model is proposed, by which the procedure of checking the stability of the grid-tied inverters with strong/weak grid is executed step by step, and it could be helpful for the system parameters design. Furthermore, the stability of the multiparalleled inverter system is also studied, and it indicates that as the number of the paralleled inverters increases, the parameters of PLL could be adjusted for a stable operation. The theoretical stability analysis results are verified by simulations and experiments. [ABSTRACT FROM AUTHOR]

In the traditional power grid, inverter-inter-faced distributed energy resources are widely used in microgrids such as conventional synchronous generators (SGs) in the traditional power grid. They are expected to decrease the rotational inertia and spinning reserve capacity that are specific to SGs, while synchronverters can compensate for the loss. Thus, an increasing number of interfaced inverters may act as synchronverters in a microgrid. The dynamic stability of the synchronverter-dominated microgrid must thus be carefully evaluated to ensure the reliable operation of this type of system. This paper presents a nonlinear model of a synchronverter-dominated microgrid. The eigenvalues of the system state matrix, along with the participation factors, are analyzed to determine the predominant parameters affecting the stability. Bifurcation theory is then used to predict and describe the unstable phenomenon as the system parameters fluctuate; the effect of these parameters on the system stability is then examined. A simulation on MATLAB/Simulink and experiments were performed and results validated the presented analysis. [ABSTRACT FROM PUBLISHER]

Microgrid is becoming an attractive concept to meet the increasing demands for energy and deal with air pollutions. Distributed energy sources (DERs) in Microgrid are usually interfaced with the utility grid by inverters, so the characteristics of Microgrid stability are much different from that of a traditional grid. However, the classifications, guidelines, and analysis method of Microgrid stability are well behind of the Microgrid development. In this paper, a Microgrid stability classification methodology is proposed on the basis of the of Microgrid characteristics investigation, which considers the Microgrid operation mode, types of disturbance and time frame. Then a comprehensive review of the body of research on Microgrid stability is presented in order to identify and advance the field. Finally, some challenges and suggestions of Microgrid stability for further researches are discussed. [ABSTRACT FROM AUTHOR]

In the single-phase photovoltaic (PV) grid-connected distributed generation (DG) system located at the end of the feeder, it is necessary to provide quickly active and reactive power to the local loads. In this paper, an improved reactive current detection and power control method are proposed to realize active power control and reactive power compensation. To detect quickly the loads reactive current, a fast reactive current detection method using the derivative and ip\!\!-\!\!iq algorithm is presented, and it can overcome the long delay in a conventional phase-shift method. In the inner current loop, the quasi-proportional-resonant (QPR) control method with grid voltage feed-forward is presented to control accurately the grid current with zero steady error, and to reduce the current distortion due to the frequency offset and distortion of the grid voltage. The inner loop stability is analyzed, and the proper parameters are selected. In the outer loop, proportional–integrator (PI) controller is applied to stabilize the dc-link voltage, and power feed-forward is introduced to speed up system response. Simulation and experimental results verified the validity of the proposed control method. [ABSTRACT FROM PUBLISHER]

Reliability of bond wires in power modules has been a main subject of interest in both industry and academia. Until now, analytical modeling of thermo-mechanical stress mainly focuses on the loop height variation of bond wires instead of plastic strain, which is the key factor causing the fatigue failure. In this paper, an analytical model for analyzing plastic strain of bond wires in IGBT modules is proposed. Using a laser displacement sensor, the dynamic loop height variations of bond wires under different heating currents are experimentally measured, based on which the finite element analysis (FEA) simulation is calibrated and verified to reveal the actual thermo-mechanical stress. In this way, the usefulness of our proposed method is verified by comparing its simulation results with those from the multi-physics fields FEA method. Finally, the relationship between the bonding angle and the plastic strain at the heel of bond wires is established. This study would provide new perspectives into further research on thermo-mechanical fatigue analysis and optimal design of bond wires in power modules. • An analytical model for analyzing plastic strain of bond wires in IGBT modules is proposed. • The FEA simulation is calibrated based on the experimental results of loop top displacement. • The relationship between the bonding angle and the plastic strain at the heel of bond wires is established. • The models have been compared with FEA simulation results with good agreements. [ABSTRACT FROM AUTHOR]

Physical lifetime prediction methods based on inelastic strain energy density (energy-based lifetime prediction) have been proven to be effective in producing reliable results on small solder joints (ball-grid arrays or chip-scale package) due to their capabilities to represent physical characteristics of soldering materials and loading history. However, in the existing energy-based lifetime models for solder layer failure the inelastic strain energy density is usually assumed to unchanged during aging, which is untrue particularly for large-area solder joints when crack propagation is considered. In this paper, an energy-based lifetime prediction method is proposed for failure of IGBT modules by explicit emulation of solder layer degradation. Inelastic strain energy density variances with respect to aging are meticulously modeled. The large-area solder layer is evenly divided into meshed elements, birth and death technique in finite element analysis (FEA) is used to successively emulate crack propagation. In this way, the increment of thermal resistance due to the solder layer degradation can be predicted. The lifetime prediction results by our proposed method show nice agreement with the experiment under different aging conditions. • An energy-based lifetime prediction method is proposed for failure of IGBT modules • Birth and death technique in FEA is used to successively emulate crack propagation • The increment of thermal resistance due to the solder layer degradation can be predicted • ΔW will change with different aging stage [ABSTRACT FROM AUTHOR]