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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]

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]

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]

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 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]

• 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 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]