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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]

Transient stability and frequency stability of virtual synchronous generator (VSG)-controlled converters during grid faults are critical for the stable operation of high-share renewable energy power systems. However, the existing transient stability methods often overlook the consideration of frequency stability. This paper presents a method to improve both frequency stability and transient stability of VSGs during grid faults. The influence mechanism of inertia and damping on transient stability and frequency stability is comprehensively analyzed under two types of faults, with or without stable equilibrium points. To address the conflict between frequency stability and transient stability of VSGs with large inertia under the first type of fault, an improved VSG control with angular frequency deviation feedback is proposed. The transient response of this control method is analyzed using small-signal and large-signal models. Furthermore, to achieve optimal dynamic performance, this improved VSG control introduces the optimal damping ratio and presents a parameter design method. Finally, the correctness of the proposed method and its theoretical analysis is verified through experimental results. [ABSTRACT FROM AUTHOR]

Photovoltaic-storage integrated systems, which combine distributed photovoltaics with energy storage, play a crucial role in distributed energy systems. Evaluating the health status of photovoltaic-storage integrated energy stations in a reasonable manner is essential for enhancing their safety and stability. To achieve an accurate and continuous assessment of the health status of photovoltaic-storage integrated energy stations, a dynamic evaluation method is proposed in this study. This method integrates both subjective and objective characteristics. Initially, considering the evaluation needs of low-carbon operation and health status for photovoltaic-storage integrated energy stations, a comprehensive health status evaluation system is developed. The significance of each indicator is subjectively analyzed, while also considering objective characteristics and sensitivity of indicators. The integration of subjective and objective characteristics is achieved using principles from game theory. Subsequently, through the establishment of the Grey-TOPSIS evaluation model, both positive and negative correlations of the health status of photovoltaic-storage integrated energy stations are determined, resulting in the derivation of a health status vector. Furthermore, the introduction of time-weight vectors and the incorporation of a time dimension enable dynamic evaluation and the comprehensive observation of health status. Finally, the scientific validity and effectiveness of the proposed evaluation method are demonstrated through practical examples, with comparisons made to traditional evaluation methods. The results clearly indicate that this method offers higher sensitivity when evaluating the health of photovoltaic-storage integrated energy stations. [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]

The large-scale development of distributed generators poses challenges to the operation of distribution networks in remote areas. The power grid structure of the regional distributed networks in remote areas is relatively weak. When a fault occurs in the connecting channel, the regional distributed network may be separated from the main grid and become an island. Those distributed generators not only cannot maintain stable operation of the island but may lead to reclosing failure. This may lead to power interruption of important loads. Aiming at the demand for continuous power supply of important loads in regional distributed networks, a self-healing control method based on the cooperation of source-load-storage is proposed in this paper. The characteristics of the regional distributed network are analyzed first. A real-time island stability control method based on the operating conditions combined with regulation with tripping is proposed. This method comprehensively evaluates the self-healing ability of the regional distributed network after the island. The regulation strategy is adopted preferentially to realize the stable operation of the island without losing any load. When the regulation ability is insufficient, the strategy combines regulation and tripping. Considering the importance of load and source-load ratio, and according to the real-time power of each feeder, the optimal feeders are cut off. The purpose of island stability control of the regional distributed network was achieved. In this way, the load loss is minimized, and the speed of island re-connection to the grid is also accelerated. The seamless power supply of important loads in a regional distributed network is guaranteed. A complex Hardware-in-Loop simulation test platform of RTDS is built to verify the correctness of the proposed method. [ABSTRACT FROM AUTHOR]

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]

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]

A soft open point (SOP) can improve the flexibility of power control and thus further enhance the reliability and economy of distribution networks. However, existing SOP regulation strategies have mainly focused on feeder imbalance regulation and line loss optimization in medium-voltage distribution networks, and the loss of SOPs and distribution transformers are often ignored, which has poor applicability in low-voltage distribution network (LDN). Therefore, a novel two-layer control strategy is proposed for SOPs working in LDN, which can improve economy and reliability of the system simultaneously. In terms of the system layer, the proposed load regulation strategy considering the economical operation area of the transformers can improve the system economy while realizing load imbalance regulation. In terms of the equipment layer, the proposed peer-to-peer control of each port in the SOP can effectively suppress the voltage fluctuation on the DC side. Thus, the system layer commands can be quickly tracked and well conducted. Finally, the feasibility and effectiveness of the proposed two-layer control strategy are verified by simulation and experimental results. [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]

This paper deals with a hybrid active power filter with injection circuit (IHAPF). It shows great promise in reducing harmonics and improving the power factor with a relatively low capacity active power filter. This paper concluded that the stability of the IHAPF based on detection supply current is superior to that of others. To minimize the capacity of IHAPF, an adaptive fuzzy dividing frequency-control method is proposed by analyzing the bode diagram, which consists of two control units: a generalized integrator control unit and fuzzy adjustor unit. The generalized integrator is used for dividing frequency integral control, while fuzzy arithmetic is used for adjusting proportional-integral coefficients timely. And the control method is generally useful and applicable to any other active filters. Compared to other IHAPF control methods, the adaptive fuzzy dividing frequency control shows the advantages of shorter response time and higher control precision. It is implemented in an IHAPF with a 100-kVA APF installed in a copper mill in Northern China. The simulation and experimental results show that the new control method is not only easy to be calculated and implemented, but also very effective in reducing harmonics. Index Terms--Dividing frequency control, fuzzy adjustor, generalized integrator, hybrid active power filter (HAPF).

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]

To solve the problem of low efficiency of the dual active bridge (DAB) converter under the traditional single‐phase‐shift control, a natural soft‐switching current stress optimisation (NSS‐CSO) method is proposed by considering the current stress optimisation and soft‐switching operation under dual‐phase‐shift (DPS) control. This study establishes the current stress and the soft‐switching analytical models of the DPS control global mode. Based on these models, the Karush–Kuhn–Tucker (KKT) condition is used to analyse the influence of the current stress optimisation methods on the efficiency when all switches achieve controlled soft‐switching or some switches achieve natural soft‐switching. It is found that the NSS‐CSO method is more efficient. Furthermore, the optimal phase‐shift ratio combinations and the corresponding current stress extremums of the NSS‐CSO method under various ranges of transmission power and voltage conversion ratio are deduced completely. The results of this study show that the NSS‐CSO method can effectively improve the operation efficiency of the DAB converter over the entire power range when the voltage conversion ratio does not match. Finally, experimental results confirm the accuracy of the theoretical analysis and the effectiveness of the proposed optimisation method. [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]

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]

Three‐port isolated dc–dc converter, as an active exploration and trial of photovoltaic access to dc distribution system, has the advantages of superior control flexibility and high reliability. Due to the changes in the phase‐shift (PS) angle, the conventional PS control method cause transient dc bias in inductor current and magnetisation current, further increasing the current stress of switches and threatening the safe operation of the converter. Based on superposition theorem analysis, this study discusses the formation mechanism of transient dc bias, constructs a mathematical model of drive signals, and proposes two typical combination strategies of drive signals. The proposed method can simultaneously suppress the dc bias of port inductor and transformer magnetisation inductor in one switching cycle by changing the duty ratio of square wave voltage or adding the zero‐voltage phase related to the PS angle. By simulating the PS angle change and output power fluctuation, the dynamic characteristics under conventional PS control and the proposed strategy are compared and discussed. The simulation and experimental waveforms show that the proposed method is not only suitable for the multi‐step step change of PS angle, but also sufficient for the continuous evolution of the PS angle. [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]

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]

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]

DC droop control strategy is usually used to improve load sharing in increasingly popular DC distribution networks. However, the conventional droop control strategy suffers from considerable terminal voltage drop and influences by line impedance. In this study, the authors propose a new robust droop control strategy to overcome these drawbacks. They first develop a mathematical model of the proposed robust controller and use the load terminal voltage as a feedback signal. They further treat the line impedance as part of the equivalent output impedance of individual power converters and minimise the inaccuracy of load sharing by regulating robust coefficients. They analyse the influence of the robust coefficients on the system stability. They have verified the robust control strategy with both power system computer‐aided design/electromagnetic transients including DC simulation and control hardware in the loop semi‐experimental method. It is shown that the robust concept improves the load sharing accuracy and suppressing the load voltage fluctuation. [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 paper proposes a new fault current limiting dynamic voltage restorer (FCL-DVR) concept. The new topology uses a crowbar bidirectional thyristor switch across the output terminals of a conventional back-to-back DVR. In the event of a load short, the DVR controller will deactivate the faulty phase of the DVR and activate its crowbar thyristor to insert the DVR filter reactor into the grid to limit the fault current. A fault condition is detected by sensing the load current and its rate of change. The FCL-DVR will operate with different protection strategies under different fault conditions. Design of the FCL-DVR involves selecting important parameters, such as DVR power rating, dc link voltage of the DVR, output filter reactors and capacitors, and grid-tied transformers is proposed. The design methodology of the proposed FCL-DVR is fully discussed based on power systems computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) simulation. A scaled-down experimental verification is also carried out. Both modeling and experimental results confirm the effectiveness of the new FCL-DVR concept for performing both voltage compensation and fault current limiting functions. [ABSTRACT FROM PUBLISHER]

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]