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A grid-forming (GFM) inverter can effectively support active power and reactive power, and the stability problem induced by the low inertia can be thereby alleviated in a power electronics-dominated power system. Yet, the voltage source characteristic presented by the grid-forming inverter induces an overcurrent problem during a short-circuit fault. Furthermore, the time delay induces an inrush current in traditional digital control, triggered by a predefined timing sequence. To address the overcurrent problem of the GFM inverter controlled by the digital controller, the operation characteristics of GFM inverters under grid-voltage drops are investigated, and a mathematical model of the instantaneous fault current is established, which depicts the relationship between the instantaneous fault current's magnitude, grid-voltage drop severity, equivalent output impedance, and current inner-loop response speed. Then, a Set–Reset (SR) latch-based dynamic current limitation with event-triggered control is proposed for the low-voltage ride-through of the GFM inverter. In the proposed method, the current limitation is enabled during grid fault, and the active and reactive powers can be recovered rapidly after fault clearance. Meanwhile, the active and reactive power references are designed to enhance synchronization stability during the grid fault. The proposed method addresses the issue of the repeated switching of virtual impedance during grid fault and achieves rapid power recovery after fault clearance. In addition, the proposed method uses the logic of event triggers to respond to the overcurrent event in real time and realize overcurrent protection. The simulation and experimental results demonstrate the effectiveness of the proposed method in current limitation and active-power recovery after fault clearance. [ABSTRACT FROM AUTHOR]

Grid-forming (GFM) converters are regarded as the most promising solution for grid-connected converters of renewable energy due to their robustness against weak grids. However, attributable to their voltage source characteristics, GFM converters may experience overcurrent issues during large disturbances. The virtual impedance (VI) method is an effective method to solve this problem. Nevertheless, there exists a contradiction between the demands for VI posed by current limitations and transient stability. Firstly, the analytical equations of virtual impedance for limiting the fault steady-state current of a GFM converter with different depths of grid voltage sag are solved. On that basis, an adaptive method based on virtual impedance is proposed to limit the fault current. Moreover, the analytical equation of the output active power of the converter when using adaptive virtual impedance to limit the fault current is solved. On this basis, the effect of the virtual impedance ratio on the transient stability is investigated. Finally, an adaptive virtual-impedance-based current-limiting method with the functionality of transient stability enhancement for a grid-forming converter is innovatively proposed. The enhancement effect of the method is verified by the equal area criterion method and phase portrait method. Finally, the efficacy of the proposed method in limiting fault currents and enhancing transient stability is validated through hardware-in-the-loop (HIL) experiments. [ABSTRACT FROM AUTHOR]

The adoption of distributed energy resources in the electricity system is rapidly spreading worldwide, where power converters are the preferred method for better integration. However, the fault behaviour of these converter-integrated distributed energy resources is still uncertain and varies according to the considered model representation. Furthermore, the fault response of the converter-integrated resources is influenced by the thought control strategy, which adds complexity to the analysis. Aimed at a better understanding of the converter-integrated resources fault behaviour, this document presents an analysis of the fault response considering active-reactive power control as the main control strategy of the converter-integrated resources; different current limitation strategies are tested and linked to physical constraints of the power electronic converters, also aligning with the reactive power support during fault. The fault behaviour is studied in a microgrid operating in grid-islanded and grid-connected modes, where the obtained results validate the adequacy of the fault current limitation strategies by compelling the converter-integrated resource to have a more realistic behaviour. Additionally, this study investigates the impact of prioritising direct or quadrature current injection (id or iq) during fault. Lastly, this study demonstrates that the converter-integrated resource can adequately support the microgrid voltage while complying with the reactive power generation requirements, which are usual in national grid codes and not often considered in the revised fault current limiting strategies. Overall, this study provides several insights into the performance of converter-integrated resources under fault conditions while emphasising the importance of fault-current limitation strategies to assure a reliable and realistic microgrid operation. These findings affect modern power systems designs, analysis, and implementation of power control strategies.

This paper focuses on enhancing the resilience of Power Park Modules (PPMs), connected to the grid via power electronic units, under asymmetrical voltage sag conditions. We particularly address three interconnected constraints crucial for Low-Voltage Ride-Through (LVRT) capability of PPMs: grid voltage support, overcurrent protection, and the DC-link voltage quality. Existing literature often overlooks the holistic consideration of these factors, prompting this study to introduce and compare three flexible LVRT strategies compliant with recent German grid codes. The proposed strategies use distinct approaches to reinforce the LVRT capability of the PPMs. Strategy A employs static gain factors to control positive- and negative-sequence reactive current (PSRC and NSRC) and the negative sequence of the active current (NSAC). Strategy B aims to enhance the DC-link voltage quality by eliminating real power oscillations through the flexible choice of NSAC and gain factors controlling PSRC and NSRC. Strategy C enhances DC-bus voltage quality by eliminating NSAC and employing flexible equal gain factors for PSRC and NSRC, mitigating both real and imaginary power oscillations. Testing on an inverter-based PPM, involving 400 kW PV module and 200 A, storage system, reveals the superior performance of strategy C. Additional tests evaluate Strategy C’s performance with var/voltage regulation equipment (On-Load-Tap-Changing transformer), demonstrating stable and promising performance. Through the introduced approaches, the authors believe that this research work is useful for developing future grid codes and improving the safety and reliability of grid-connected power systems.

Active power filter (APF) control is a natural area of application for vibrating reference frame (VRF) transformation due to the intentional occurrence of higher harmonics in the active filter current compensating load current harmonics. Due to the vibrating frame transformation, the APF current can be represented by the DC values, and thus proportional-integral (PI) controllers are sufficient to control the converter current. However, in the typical approach, it may be impossible to combine harmonic filtration with reactive power compensation features, due to the transformation constraints. The solution to this issue is decoupling of the fundamental harmonic and high harmonic components and a separate control for each of them. This paper presents a decoupled control system of an APF, which uses VRF transformation for accurate control of high-current harmonics. Decoupling is a groundbreaking improvement of the VRF method. Moreover, different current limitation scenarios are proposed, considering both harmonics compensation and fundamental frequency reactive current compensation. Theoretical considerations are supported by simulation and experimental tests.

Grid-forming (GFM) converters are a promising solution to enable large scale integration of renewable energy sources into the power system. However, due to the intrinsic voltage-source behaviour of GFM converters, current limitation during large grid disturbances is challenging. This article presents a novel limitation strategy that preserves the GFM properties of the converter and at the same time effectively limits the converter current to the desired value. Through the limitation of the converter's internal voltage, stable operation even during faults and in case of large frequency disturbances in the grid is achieved. Experimental results show the effectiveness of the proposed current limitation strategy in case of various grid disturbances.

The droop-controlled voltage source converter (VSC) can support the voltage at the point of common coupling (PCC) and realize frequency adjustment by simulating the swing equation of the synchronous generator (SG) and has a higher stability margin in the extremely weak grid. Under large-signal disturbances such as short-circuit faults in the power grid, the active power output of the VSC decreases, and the continuous increase of the power angle causes the VSC to have a power angle instability similar to that of the SG. In addition, in order to ensure the safety of the equipment, the VSC control system contains a current-limiting strategy, which will cause a large change in the transient external characteristics of the VSC under large-signal disturbances. This paper firstly studies the transient synchronization mechanism between VSC and the power grid under grid faults and expounds on the reasons for the transient instability of VSC with the current limiting strategy. Based on the above analysis, a transient enhancement strategy is proposed to reduce the imbalance of active power and reduce the risk of VSC transient instability. It is worth mentioning that the proposed transient control strategy will not affect the steady-state output characteristics of the VSC. Finally, simulation and experiment verify the effectiveness of the proposed control strategy.

Grid-connected inverters are commonly used in systems of renewable energy to convert this energy source into AC power with parameters suitable for connection to the grid. In the normal operating conditions, the grid-connected inverters mainly generate active power to the grid. However, when a voltage sag or voltage imbalance occurs, the grid voltage imbalance in the conventional control methods causes negative sequence components and increases the output current magnitude of inverters. The increase of current can damage power semiconductor devices. This paper presents a strategy to limit the current magnitude of inverters under unbalanced grid voltage conditions. In this strategy, a multiple-complex-coefficient filter is used to eliminate the negative sequence voltage components. This method does not require any additional hardware. A three-phase gridconnected photovoltaic inverter system using a solar array of 20 kWp is also used for the survey. The effectiveness has been validated when comparing the simulation results on Matlab/Simulink of the proposed method with those of the conventional method.

There are some disadvantages in existing inverter-interfaced distributed generators (IIDG) low voltage ride-through control methods,such as unsatisfactory voltage support effects,single control objectives,and the risk of over-limiting the output current. In this paper,a low voltage ride-through control considering multiple control objectives is proposed. Firstly,the preset phase voltage is used as the constraint condition,and the voltage support equation is constructed to realize the voltage support control. Secondly,by restricting the magnitude of the negative sequence reactive current injected by the IIDG into the grid,the active output oscillation suppression control is realized. At the same time,to make full use of the remaining capacity of IIDG,the maximum active power output and current-limiting strategy are used to calculate the positive sequence active current reference value of the IIDG to ensure that the maximum active power output control is achieved under the premise that the output current of IIDG does not exceed the limit. The simulation results show that compared with the existing positive-sequence reactive current injection control method,the proposed low-voltage ride-through control method in this paper has a better fault control effect.

Unbalanced grid faults are very harmful to the network operation. Distributed static synchronous compensator (D-STATCOM) can help to support the grid voltage by injecting the reactive power into the inductive power grid. However, this approach turns to be ineffective for a low-voltage grid, where the network impedance is mainly resistive. In this article, a dynamic voltage equalization control (DVEC) scheme is proposed to suppress the negative sequence voltage, and therefore, reduce the voltage unbalance during the faults in the resistive network. This effect is achieved through transferring proper active power from the negative sequence channel to the positive sequence channel. And the dual-sequence active currents are controlled by DVEC without exchanging extra active power with the grid except the little circuit loss compensation. By flexibly adjusting the control parameter of DVEC, current peak and active power oscillation can be limited within a preset range, respectively, during the process of maximizing the voltage control. As a result, the DVEC scheme can ensure the safe operation of D-STATCOM and the stability of the power grid. Moreover, experimental results are reported to validate the correctness and effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

The article discusses and systematizes the causes of high values of short-circuit currents in electrical networks up to 1 kV and indicates their negative consequences. A brief analysis of some current limiting methods (both with and without additional investment) is given, which makes it possible to reduce the negative effects when significant short-circuit currents occur. The variants of the implementation of these current limiting methods recommended for study at the design stage or when replacing equipment that has exhausted its resource, are considered. The advantages and disadvantages of the methods of limiting short-circuit currents described in the article are indicated. Options for connecting equipment with low power consumption to nodes of the electrical network up to 1 kV with high values of short-circuit currents are also considered. Special attention is paid to current-limiting protection devices, a detailed description of the design of some of them is given, the advantages of their use in comparison with other current-limiting methods are indicated, the corresponding graphs of the limitation of the periodic component of the short-circuit current, shock short-circuit current and thermal pulse are presented. Based on the analysis of graphic materials, the issue of ensuring the selectivity of protections is considered. The degree of prevalence of equipment with the property of current limitation is determined. A method for determining selective current-limiting protection devices is given, viz. the method of energy selectivity, the corresponding graphs for determining energy selectivity are presented. The physics allowing to determine the degree of efficiency of limiting the short-circuit current is described. Recommendations on the use of current-limiting equipment are proposed.

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In this paper, a systematic short- and long-term voltage stability analysis of grid-forming converters with current limitations is conducted. For this, sampling studies are performed in an aggregated version of the Nordic test system. Here, the grid-forming control approach, the simulation type (EMT/phasor), current prioritisation and load composition are varied. Also, a stability-enhancing control method is developed to stabilise the converter during current limitation. The proposed method does not require any parameter tuning and can be applied to almost all grid-forming approaches. Its performance is demonstrated by comparing it to similar methods. The results show that the proposed method can greatly improve short- and long-term voltage stability for all investigated grid-forming approaches. In addition, accurate load models are crucial, as small modelling differences can have a greater impact on stability than the choice of simulation type. Here, both simulation types show the same voltage dynamics but have small differences in the stability limits. • Voltage stability analysis of various grid-forming converters with current limitation. • Variations in control approach, model type (phasor/EMT) and current prioritisation. • Derivation of a computationally efficient power system model for sampling studies. • Short and long-term voltage instability can occur due to current limitation. • A proposed stability-enhancing control method can stabilise the converter. [ABSTRACT FROM AUTHOR]

Grid-forming (GFM) inverters are promising solutions for future power systems with high penetration of renewables. The successful design of GFM inverters is crucial for achieving current limiting, enabling them to withstand disturbances such as voltage sags and phase jumps in grid voltages. However, when the parameters are not properly tuned, a cascaded voltage and current controller can compromise grid stabilization performance due to issues like controller windup, post-fault recovery failure, and harmonic instability. To address this challenge, this paper proposes a dual control scheme consisting of GFM and current control that work in parallel. The dual control scheme incorporates emulation signal generators integrated with the dual controllers. This integration allows for adjusting their internal states during standby mode to track to references, enabling a seamless transition between the running and standby modes. Numerical simulations and experiments demonstrate that the proposed controller exhibits promising performance under both normal and faulted conditions. • The proposed controller eliminates the issues caused by the cascaded controller. • The proposed controller avoids transient instability during the current limitation. • The proposed emulation signal generator enables smooth switching between GFM and current control. [ABSTRACT FROM AUTHOR]

The adoption of distributed energy resources in the electricity system is rapidly spreading worldwide, where power converters are the preferred method for better integration. However, the fault behaviour of these converter-integrated distributed energy resources is still uncertain and varies according to the considered model representation. Furthermore, the fault response of the converter-integrated resources is influenced by the thought control strategy, which adds complexity to the analysis. Aimed at a better understanding of the converter-integrated resources fault behaviour, this document presents an analysis of the fault response considering active-reactive power control as the main control strategy of the converter-integrated resources; different current limitation strategies are tested and linked to physical constraints of the power electronic converters, also aligning with the reactive power support during fault. The fault behaviour is studied in a microgrid operating in grid-islanded and grid-connected modes, where the obtained results validate the adequacy of the fault current limitation strategies by compelling the converter-integrated resource to have a more realistic behaviour. Additionally, this study investigates the impact of prioritising direct or quadrature current injection (i d or i q) during fault. Lastly, this study demonstrates that the converter-integrated resource can adequately support the microgrid voltage while complying with the reactive power generation requirements, which are usual in national grid codes and not often considered in the revised fault current limiting strategies. Overall, this study provides several insights into the performance of converter-integrated resources under fault conditions while emphasising the importance of fault-current limitation strategies to assure a reliable and realistic microgrid operation. These findings affect modern power systems designs, analysis, and implementation of power control strategies. [ABSTRACT FROM AUTHOR]

The increase in inverter-based resources associated with the increased installation of PV sources is a concern because it reduces the inertia of the power system during peak PV generation periods. As a countermeasure to reduce grid inertia, the addition of pseudo-inertia using virtual synchronous machines can be selected, and PV generation can cooperatively contribute to the stable operation of the power system by using the suppressed output as reserve power. However, few studies have analyzed VSMs that do not use batteries and use PV as a resource (PV-VSM) in simulations, including grid interconnection and solar radiation fluctuations, and it is necessary to clarify the issues and discuss countermeasures. In this study, electromagnetic transient response analysis was applied to a VSM connected to a two-generator system, simulations were performed, and the following findings were reported and countermeasure methods for the problem were proposed. When the PV capacity is insufficient for the output required by the VSM inverter, the PV-VSM control system may become unstable. This is caused by a drop in the capacitor voltage of the DC/DC converter due to insufficient PV output. The limiter control system is designed to address this problem by combining the headroom estimation system with the current limiting algorithm. The proposed limiter control system is validated on solar radiation ramp fluctuations as a test case and found that the system was effective in supressing PV-VSM instability. In our simulation case, the PV-VSM with our limiter control can continue to operate stably even if the PV available power is 0.03 [p.u.] short of the inverter's reference power by the solar power ramp fluctuation, as long as the inverter installation rate is less than 50%. [ABSTRACT FROM AUTHOR]

Traditional power systems usually rely on synchronous generators to provide inertia and maintain grid stability. However, with the continuous development of new energy power generation technologies, more and more converter power supplies provide voltage and frequency support for the power grid. The synchronizing instability issue of a converter-dominated power system becomes increasingly relevant, which exists in both grid-following (GFL) and grid-forming (GFM) converters. In this paper, the transient synchronizing stability of grid-forming (GFM) converters with droop-control during a grid fault is studied, with the influence of current saturation limitation considered especially. The current saturation limitation applied to limit overcurrent during grid faults brings transient voltage-source/current-source switched mode of the converter, which significantly influences the transient behavior. Considering the influence, an equivalent motion model similar to the rotor motion of a synchronous generator is developed first to depict the transient synchronizing dynamics. It has been identified that the current saturation limitation reconstructs the power angle characteristics, which significantly influence the transient stability. Then based on the developed model, transient synchronizing stability during a grid fault is studied based on the accelerating–decelerating area method. With the influence of grid impedance and grid voltage sag considered, several kinds of transient synchronizing stability issues are revealed clearly. Finally, simulated results are presented to verify the theoretical analysis. The analysis of this article paves the way for the follow-up research on the synchronizing stability of multi-converters and the corresponding control methods to improve synchronizing stability. [ABSTRACT FROM AUTHOR]

The adaptive virtual impedance (VI) control emerges as an attractive solution to limit the current of grid-forming voltage-source converters (GFM-VSCs) during grid faults. Yet, the adaptive VI-based current limitation relies on the detection of the current magnitude, which introduces nonlinear dynamics that challenges the controller parameters tuning of GFM-VSCs. This article develops the small-signal model of GFM-VSCs with adaptive VI-based current limitation, from which, the dynamic impact of the adaptive VI is explicitly revealed. Considering such impact, a holistic controller parameters tuning method for GFM-VSC is proposed to guarantee the small-signal stability of the system. Simulation and experimental tests are performed to verify the effectiveness of the theoretical analysis and the proposed parameters tuning method. [ABSTRACT FROM AUTHOR]

Due to the high penetration of grid-connected photovoltaic (GCPV) systems, the network operators are regularly updating the grid codes to ensure that the operation of GCPV systems will assist in maintaining grid stability. Among these, low-voltage-ride-through (LVRT) is an essential attribute of PV inverters that allows them to remain connected with the grid during short-term disturbances in the grid voltage. Hence, PV inverters are equipped with control strategies that secure their smooth operation through this ride-through period as per the specified grid code. However, during the injection of reactive power under LVRT condition, various challenges have been observed, such as inverter overcurrent, unbalance phase voltages at the point of common coupling (PCC), overvoltage in healthy phases, oscillations in active, reactive power and dc-link voltage, distortion in injected currents and poor dynamic response of the system. Several strategies are found in the literature to overcome these challenges associated with LVRT. This paper critically reviews the recent challenges and the associated strategies under LVRT conditions in GCPV inverters. The drawbacks associated with the conventional current control strategies are investigated in MATLAB/Simulink environment. The advanced LVRT control strategies are categorized and analyzed under different types of grid faults. The work categorizes the state-of-the-art LVRT techniques on the basis of the synchronization methods, current injection techniques and dc-link voltage control strategies. It is found that state-of-the-art control strategies like OVSS/OCCIDGS provides improved voltage support and current limitation, which results in smooth LVRT operation by injecting currents of enhanced power quality.

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed. [ABSTRACT FROM AUTHOR]

This paper deals with the transient stability of a grid-forming converter while embedding a current reference saturation strategy. The novelty of this work consists in investigating the impact of the current reference angle on the transient stability. In case of a balanced voltage sag, analytical formulas to estimate the critical clearing angle (CCA) and critical clearing time (CCT) while considering different values of the current reference angle are derived. It is demonstrated that the choice of this angle is constrained by the ability of the power converter to switch back to the voltage control mode. Based on that, its optimal value that enhances the transient stability and allows a switching from the saturated current control mode to the voltage control mode is calculated. Thereafter, the effectiveness of this optimal choice to guarantee the stability in case of a phase shift caused by a line re-closing event is verified. Time-domain simulations and experimental tests validate the correctness of the presented theoretical approaches. [ABSTRACT FROM AUTHOR]

Automatic battery equalizers require no sensing circuits, which reduces their cost, size, and complexity. However, among existing methods, additional resistors or diodes are used in the balancing paths to ensure safe operation, which degrades the conversion efficiency and balancing speed. To overcome this problem, this article proposes an equalizer architecture based on the half-bridge LLC converter. The inherent current limitation characteristic of the proposed equalizer improves the balancing speed. High conversion efficiency is achieved due to the soft-switching operations of the switches. The circuit topology and working modes are presented first, then the features of voltage equalization, inherent current limitation, and soft-switching conditions are analyzed in detail. Finally, the proposed equalizer is validated by simulation and experiment. [ABSTRACT FROM AUTHOR]

This article presents a current limitation technique for a multiphase bearingless machine featuring a combined winding system. This winding structure allows the machine to produce both suspension force and motoring torque. The main challenges with the combined winding configuration consist of decoupling the suspension force and torque generation and designing a proper current limitation algorithm. The former topic has been already tackled and presented in previous publications, while the latter will be addressed in this article. In particular, the proposed suspension force control technique will allow to prioritize either suspension force or torque generation. In this article, the priority is given to the rotor levitation, hence the suspension force rather than the torque is essential. Finally, simulation results and experiment validation on a $\text{1.5}\,\text{kW}/3000\,\text{r/min}$ prototype machine are provided. [ABSTRACT FROM AUTHOR]

This paper presents a grid-forming control (GFC) scheme for two-stage photovoltaic (PV) systems that maintains power reserves by operating below the maximum power point (MPP). The PV plant in GFC mode behaves like a voltage source that supports the grid during disturbances in full or limited grid-forming mode as per the reserve availability. This is a model-free method that avoids the estimation of MPP power in real-time commonly done in the literature, which makes it simpler and more reliable. The proposed control also features an enhanced current limitation scheme that guarantees containment of the current overshoots during faults, which is not trivial in voltage-sourced GFC inverters. A thorough investigation is done, exploring various generation mixtures of synchronous machines (SM), GFC and grid-following (GFL) inverters, and all common disturbances, e.g., load change, faults and irradiance transients. The results show very favorable dynamic performance by the GFC inverters, far superior to GFL inverters and directly comparable to SMs. It is found that replacing SMs with GFC inverters may improve the frequency profile and terminal voltage during disturbances, despite losing out in the mechanical inertia and the strict inverter overcurrent limits. [ABSTRACT FROM AUTHOR]

Grid forming (GFM) converter mimics the operational characteristics of the synchronous generator, serves as a voltage source that provides voltage and frequency support to the power system. However, due to its voltage source nature, the current of the GFM converter is highly determined by the external system, increasing the risk of overcurrent under large disturbances and potential damage to the converter. The conventional current reference limitation suffers from synchronization instability, which causes the converter to lose synchronization. To address this issue, this paper proposes a novel power matching based current limitation method, which maintains the synchronization of the GFM converter and also avoids overcurrent issues. Specifically, when a large disturbance occurs, considering the maximum output power of the GFM converter, the reference power of the converter is corrected using the frequency to match the output power in real time. Therefore, the output power of the converter is always its maximum value, with the current constrained to the maximum level allowed, avoiding overcurrent and instability. Finally, the superiority of the proposed method is verified by experiment and simulation. • To avoid overcurrent issues, a power matching based current limitation method for GFM converter is proposed. • The proposed method can avoid instability while limiting GFM converter current. • The proposed method can smoothly exit the power matching mode without causing power fluctuation. [ABSTRACT FROM AUTHOR]

This paper proposes a grid-forming control strategy with a generic implementation approach to meet the transmission system requirements asking for the massive integration of power electronic devices into the power systems. In this context, several grid-forming controls have been proposed in the literature either with or without a Phase-Locked Loop (PLL). The PLL-based techniques allow decoupling the different control functionalities (i.e. inertia emulation, frequency support, active power setpoint tracking in steady-state) while the PLL-free schemes, which aim to avoid the PLL drawbacks, create a compulsory coupling between the control functionalities. The proposed grid-forming control in this paper is able to decouple the control functionalities without any dedicated PLL, which makes it more advantageous compared to what have been already proposed in the literature. Since the power converters are exposed to the small and large grid events, the presented control has been tested in both situations. For the small grid events, a simplified small-signal model is developed to assess the active power and frequency dynamics. In case of large grid events, a current limitation algorithm is included to the control in order to protect the power converter. To deal with the transient stability issues linked to the current limitation and enhance the converter performance during the post-fault, a method based on adaptive inertia constant is proposed. To validate the overall approach, time-domain simulations (in Matlab-Simulink) and experimentations are performed.

The paper presents a control method for the three-phase power converter operating under unbalanced grid voltage conditions. The method uses a new transformation to the non-Cartesian frame, which makes the controlled current vector components balanced in this frame even if originally the three-phase current is referenced as unbalanced. Furthermore, Park’s transformation makes the controlled variables constant, which allows to apply proportional–integral terms as current controllers independent of the required control target. Several control targets known from literature have been analyzed with regard to the required new transformation parameters, and the transformation parameters for all targets have been found. Simulation results are shown to prove the theoretical analysis, and the experimental test results are presented as practical validation of the proposed use of the non-Cartesian frame in control.

This paper presents a new multi-objective control strategy for inverter-interfaced distributed generation (IIDG) to ensure its safe and continuous operation under unbalanced voltage sags. The proposed control strategy can effectively improve the low voltage ride through (LVRT) capability, reduce active power oscillations, and limit overcurrent simultaneously, which are marked as the most important control objectives of IIDG during unbalanced voltage sags. The advanced voltage support scheme, which utilizes positive sequence component, is firstly proposed to maximize the LVRT capability of IIDG during unbalanced voltage sags. Then, to ensure the safety of IIDG, the active power oscillation suppression and current limitation algorithm are designed individually. Based on the control algorithms of such objectives, the multi-objective control method, including scenario classification and reference current determination, is then presented to achieve such three objectives under various system conditions simultaneously. Finally, case studies and evaluations based on MATLAB/Simulink are carried out to illustrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this study, a DC micro-grid consisting of multiple paralleled energy resources interfaced by both bidirectional AC/DC and DC/DC boost converters and loaded by a constant power load (CPL) is investigated. By considering the generic dq transformation of the AC/DC converters' dynamics and the accurate nonlinear model of the DC/DC converters, two novel control schemes are presented for each converter-interfaced unit to guarantee load voltage regulation, power sharing and closed-loop system stability. This novel framework incorporates the widely adopted droop control and using input-to-state stability theory, it is proven that each converter guarantees a desired current limitation without the need for cascaded control and saturation blocks. Sufficient conditions to ensure closed-loop system stability are analytically obtained and tested for different operation scenarios. The system stability is further analysed from a graphical perspective, providing valuable insights of the CPL's influence onto the system performance and stability. The proposed control performance and the theoretical analysis are first validated by simulating a three-phase AC/DC converter in parallel with a bidirectional DC/DC boost converter feeding a CPL in comparison with the cascaded PI control technique. Finally, experimental results are also provided to demonstrate the effectiveness of the proposed control approach on a real testbed.

This work has aimed to design a system that can eliminate the surge current. Surge current occurs in circuits where very high capacitances are charged. The proposed system solves this problem in a new way. Using a Mosfet transistor, it limits the amount of current that flows into the circuit. This solution has many advantages over conventional methods of limiting surge current during capacitor charging. The exact amount of charging current in the system can be set. Furthermore, there is no surge current. According to the conditions of the application, the parameters can be changed, and effectively adapted to the application. For this reason, it is possible to use the system for various applications where it is necessary to limit the surge current, which has adverse effects on the device. The advantage of this is also the reduction of power loss. The proposed system has been validated with a simulation. The system has worked properly by simulation. While charging high capacities and high input DC voltage, surge currents have been reduced. [ABSTRACT FROM AUTHOR]

This paper presents a method of limiting the panel current to a safe level to improve the reliability of automotive OLEDs. The proposed circuit consisting of the buck converter and the inverting buck‐boost converter supplies the positive and negative voltage (ELVSS) to the OLED panel, respectively. The load current is limited by adjusting the ELVSS voltage when abnormal situations such as panel damage or partial short circuit occur while driving. Thus, OLED displays can keep providing safety‐related information to the driver without being turned off. A prototype of the proposed circuit demonstrates limiting the load current to 1A despite the sudden change in load resistance. [ABSTRACT FROM AUTHOR]

Virtual inertia control is a methodology to make inverter-based resources (IBR) behave like a synchronous machine. However, an IBR cannot fully emulate the response of synchronous machine because of its low-current capacity. When the inertial response of an IBR is affected by the current limitation, the synchronization of the synchronous machine simulated virtually inside the IBR controller with the other synchronous generators in the grid is affected, which may cause step-out of the simulated generator. We propose a methodology which can keep the synchronization by modifying internal induced voltage of the simulated generator to follow the system voltage change. The proposal is validated by the simulation using a nine-bus transmission system model including two synchronous generators and a large-scale IBR. The result of the generator trip simulation shows that the proposed method suppresses the phase angle variation while the current is limited, and avoids the instability regarding the synchronism. Furthermore, the impact of the current limitation on frequency stability is also evaluated through the simulation study and it is found that as the amount of output suppression increases, the frequency nadir falls, but the rate-of-change of frequency is hardly affected.

Computer assisted calculations consists in applying software for the simu-lation of models of certain devices and later analysing their behavior under given conditions corresponding to real working conditions in a specific environment.The paper proposes a circuit model of 2G YBCO super-conducting tape created in the PSpice program. The model consists of passive blocks and active user blocks of analogue behavioural modelling (ABM). ABM blocks calculate the conductances, the currents of the indi-vidual layers of the superconducting tape, its thermal capacity, the heating power, the cooling power and resulting temperature of the tape. The model uses table of thermal power density passed to the liquid nitrogen vs. temperature. Smooth transition of the YBCO superconductor layer into the resistive state is described by Rhyner’s power law. The developed model was used for generating waveforms of thermal and electrical quantities.

Power quality and current limitation are the most important aspects of the grid-connected power converters under fault. Since the distributed energy resources are widely used, fault management strategy is important for micro-grids applications. This paper presents a new control strategy for low-voltage ride-through for 3-phase grid-connected photovoltaic systems. The proposed method, which is designed in a synchronous frame using positive and negative sequence components, can protect the inverter from overcurrent failure under both symmetrical and unsymmetrical faults and provides reactive power support. The method does not require a hard switch to switch from MPPT to a non-MPPT algorithm, which ensures a smooth transition.

Grid-forming inverters usually use inner cascaded controllers to regulate output AC voltage and converter output current. However, at the power transmission system level where the power inverter bandwidth is limited, i.e., low switching frequency, it is difficult to tune controller parameters to achieve the desired performances because of control loop interactions. In this paper, a direct AC voltage control-based state-feedback control is applied. Its control gains are tuned using a linear quadratic regulator. In addition, a sensitivity analysis is proposed to choose the right cost factors that allow the system to achieve the imposed specifications. Conventionally, a system based on direct AC voltage control has no restriction on the inverter current. Hence, in this paper, a threshold virtual impedance has been added to the state-feedback control in order to protect the inverter against overcurrent. The robustness of the proposed control is assessed for different short-circuit ratios using small-signal stability analysis. Then, it is checked in different grid topologies using time domain simulations. An experimental test bench is developed in order to validate the proposed control. [ABSTRACT FROM AUTHOR]

In the transient process of the grid-connected converter (GCC), the existing research mainly focuses on the impact of the control loops. Little attention is paid to the transient stability issues driven by the imbalance between the input power and output power of GCC. This paper shows that the transient stability issues will still exist even if ignoring the dynamics of phase-locked loop (PLL) and current loop. In this paper, the models of the AC grid and the GCC are built under the assumption that the dynamics of the PLL and current loop are ignored. Then, by analyzing the transient process of GCC under non-severe remote voltage sag, the effects of the imbalance power on the transient stability of GCC are discussed. Moreover, for the GCC to operate stably after the transient process, there should be a maximum input power limit (MIPL) for GCC, and the imbalance power equation is applied in this paper to determine the transient stability of GCC. Furthermore, the effects of the current limitation on the transient stability of the GCC are also discussed. Finally, the theoretical analysis has been verified by means of simulations.

This letter proposes a grid-forming control method for Modular Multilevel Converter (MMC) with AC-side current limitation capabilities under fault ride-through scenarios (balanced and unbalanced faults). The novelty of the proposed controller is that no physical capacitor bank at the AC-side connection is considered as it is usually done. This is achieved with two cascaded controllers (for voltage and current) and elliptic limitations in double synchronous rotating reference frame. In normal mode, the currents are imposed solely by the AC-side load and the current controller is in by-pass mode. In case of AC-side faults, the current controller is automatically enabled to effectively limit the currents if needed. This is achieved without switching logic between normal and limitation mode. Simulation results prove the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Conductive atomic force microscopy (CAFM) has become the preferred tool of many companies and academics to analyze the electronic properties of materials and devices at the nanoscale. This technique scans the surface of a sample using an ultrasharp conductive nanoprobe so that the contact area between them is very small (<100 nm 2 ) and it can measure the properties of the sample with a very high lateral resolution. However, measuring relatively low currents (∼1 nA) in such small areas produces high current densities (∼1000 A/cm 2 ), which almost always results in fast nanoprobe degradation. That is not only expensive but also endangers the reliability of the data collected because detecting which data sets are affected by tip degradation can be complex. Here, we show an inexpensive long-sought solution for this problem by using a current limitation system. We test its performance by measuring the tunneling current across a reference ultrathin dielectric when applying ramped voltage stresses at hundreds of randomly selected locations of its surface, and we conclude that the use of a current limitation system increases the lifetime of the tips by a factor of ∼50. Our work contributes to significantly enhance the reliability of one of the most important characterization techniques in the field of nanoelectronics.

In the modern power grid, distributed generators are widely connected to the grid via voltage source converters (VSCs). Analyzing the transient stability of VSCs under large disturbances is useful for maintaining the security of the grid. However, this topic is very little studied. In this paper, the transient stability behavior of the droop-controlled VSC is theoretically explained. In particular, it is shown that transient instability can occur to the droop-controlled VSC when its current is saturated under large disturbances. In addition, the dynamics of the VSC under voltage sags that could incur instability problem is elaborately studied to consider special non-fault disturbances. To deal with this instability problem, a stability enhanced P-f droop control is proposed. Simulations and hardware-in-the-loop experiments verify the validity of the transient stability analysis and the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]