Current Limitation Strategy Research Articles

High-Altitude Operation of a Commercial 100 kW PEM Fuel Cell System

A commercially available 100 kW PEM fuel cell system designed for efficient operation on ground-level was tested at low ambient pressures between 750 mbar and 940 mbar in a low-pressure chamber. The current–voltage characteristics at 940 mbar and 900 mbar showed only small differences, while the system performed worse at lower ambient pressures. To enable operation at these low pressures, an additional current-limiting strategy had to be implemented, as it was found that the compressor could not deliver sufficient mass flow at ambient pressures below 867 mbar to reach the maximum current allowed by the system (420 A). The results show that the fuel cell system, which was designed for ground-level applications, can be operated at lower pressures if the proposed current-limiting strategy is implemented, although at the cost of a lower maximum current output at low ambient pressures. Based on the results, suggestions for further hardware measures to optimise the system for flight conditions are made.

Efficient Peak Current Limit Strategy and Active Power Oscillation Reduction in a Three-Phase Grid-Interfaced PV-Battery System (GIPVBS) with Low Voltage Ride-Through Control

Efficient Peak Current Limit Strategy and Active Power Oscillation Reduction in a Three-Phase Grid-Interfaced PV-Battery System (GIPVBS) with Low Voltage Ride-Through Control

Dynamic Current-Limitation Strategy of Grid-Forming Inverters Based on SR Latches

Dynamic Current-Limitation Strategy of Grid-Forming Inverters Based on SR Latches

Automatic Current Limit Strategy for LLC DC-DC Converter for Overload Operation

Automatic Current Limit Strategy for <i>LLC</i> DC-DC Converter for Overload Operation

Development and Validation of the High-Voltage Direct-Current Modular Multilevel Converter (HVDC-MMC) Model for Converter Transformer Protection Studies.

The electrical protection of power networks with fault contribution from inverter-based power sources imposes new application challenges that have to be dealt with by protection engineers. This paper describes the development of a study case model of an HVDC-MMC link for testing the protection behaviour of connected converter transformers. The paper summarises the implementation and validation of the converter control as well as enhancements to provide Fault Ride-Through capability and fast fault current injection as required by the German Technical Connection Rules for HVDC. The grid code standard requires positive- and negative-sequence reactive current injection in the case of grid faults. A Doubled Decoupled Synchronous Reference Frame Phase Locked Loop (DDSRF-PLL) for Vector Current Control (VCC) is implemented. Additionally, a Fault Detection and Fault Ride-Through Reference Generator with a Current Limitation strategy is introduced. Though these techniques are well described in the literature, the DDSRF is improved for current control stability. The relationship between the parameters of the PLL and the control, as well as the behaviour of the protection system, are demonstrated. Grid faults with large voltage dips pose a significant challenge to the stability of the control system. Nevertheless, it is shown that with the developed model, it is possible to make general statements about the protection behaviour in an inverter-based environment.

Current-Limiting Strategy for Asymmetric Short-Circuit of Three-Phase Inverter for Continuous Power Supply of Nonfaulty Loads

Current-Limiting Strategy for Asymmetric Short-Circuit of Three-Phase Inverter for Continuous Power Supply of Nonfaulty Loads

Voltage-Based Current Limitation Strategy to Preserve Grid-Forming Properties Under Severe Grid Disturbances

Voltage-Based Current Limitation Strategy to Preserve Grid-Forming Properties Under Severe Grid Disturbances

Transient stability improvement of grid-forming voltage source converters considering current limitation

Transient instability issues of grid-forming voltage source converters (VSCs) considering current limitation strategies are addressed in this paper. Take the virtual synchronous generator (VSG) as the representative grid-forming control, qualitative analysis of transient instability mechanism of the grid-forming VSC is performed firstly. After that, a novel synchronization method named hybrid synchronization control combining the synchronous generator swing equation and the phase-locked loop (PLL) principle is designed, which not only makes the VSC retain its grid-forming characteristics, but also greatly improves its transient stability characteristics. Furthermore, an improved current limiting strategy (ICLS) that can reliably enable and disable current reference limiting loop is proposed, overcoming the drawback that the existing strategy fails to recover from current saturation mode effectively. Finally, simulation and experiments are built to verify the accuracy of the transient stability analysis and the effectiveness of the proposed transient stability improvement scheme.

Reconstruction of time-shifted hemodynamic response

Regression of voxel time course onto expected response is a standard procedure in functional magnetic resonance imaging that relies on exact onset time and shape of superimposed hemodynamic response functions. Elegant capture of time deviation by time derivative regressors appears complicated by shape distortion and limited to ±1 s, and is usually not exploited for reconstructing the true time-shifted response function together with its magnitude. This analysis of the time-derivative approach provides closed-form functional relations between time shift and regression coefficients that allow for hemodynamic shifts of ±5 s and can explain shape distortion and reconstruction behavior. Reliable absolute latencies were no smaller than 0.6 s in a best-case experiment. Confusions of latency are a previously undiscussed shortcoming where current limitation strategy may eliminate correct latencies and protect incorrect ones.

Transient stability enhancement control strategy for droop-controlled voltage source converter

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.

OC/OL Protection of Droop-Controlled and Directly Voltage-Controlled Microgrids Using TMF/ANN-Based Fault Detection and Discrimination

In this article, we propose a generalized overcurrent (OC) and overload (OL) protection scheme for the droop-controlled and directly voltage-controlled (DVC) inverter-interfaced distributed energy resources (IIDERs) to be used for the protection studies of the alternating-current (ac)-islanded (autonomous) microgrids, and provide a management strategy during and after short circuit (SC) and consequent OLs. The fault is detected, and its type is discriminated by a new method that takes the advantages of both artificial neural networks (ANNs) and transient monitoring function (TMF). By using the proposed strategy, the output current magnitudes of the distributed energy resource (DER) units are limited, and after fault clearance, the microgrid is restored to its normal operating conditions. The proposed OC/OL protection scheme limits the output power of the DERs. In this regard, a generalized fault model is presented, which considers the effect of the control system of the IIDERs. This model exploits a current-limiting strategy (CLS) that can be implemented in different reference frames to enhance the microgrid fault ride-through (FRT) capability. Finally, to demonstrate effectiveness, accuracy, authenticity, and feasibility of the proposed OC/OL protection scheme along with the TMF/ANN-based fault detection method and their desirable performance, offline digital time-domain simulations are done in MATLAB/Simulink environment, and the results are experimentally verified by using the OPAL-RT real-time digital simulator (RTDS).

Research on Comprehensive Active Current- Limiting Control Strategy Applied in Two-Port MMC-HVDC Transmission System

Aiming at the defect that the current-limiting strategy based on the modular multilevel converter (MMC)-high-voltage direct current (HVDC) system only focuses on the unilateral ac side or dc side, this article proposes a current-limiting strategy that is suitable for both ac-side ground fault and dc-side short-circuit fault. First, for the ground fault on the ac side, the outer loop controller with power state equation and the inner loop controller with passivity-based control (PBC) strategy are used to control it, and the genetic algorithm, which takes the absolute value of the difference between the reference current and the actual current as the fitness function, is used to fit the optimal injection damping parameters for the PBC method. Then, for the short-circuit fault on the dc side, the capacitor discharge duty ratio is introduced on the basis of the equivalent capacitance without considering the voltage drop, and its segmented control is combined with the current-limiting reactors to achieve current-limiting at different fault points. Finally, a two-port +500-kV HVDC transmission system was built on MATLAB/Simulink software, and the effectiveness of the current-limiting method proposed in this article was verified.

Resilient Power Converter: A Grid-Connected Converter With Disturbance/Attack Resiliency via Multi-Timescale Current Limiting Scheme

With the high penetration of renewable sources and flexible DC transmission systems, the power electronics converters are playing an important role in the modern power system. However, the grid-connected power converters may suffer from disturbances or cyberattacks from grid faults and communication networks. In this paper, we propose a control framework with multi-timescale constraints for the grid-connected converters. Firstly, the response sequence of different control layers is analyzed for a high power modular multi-level converter (MMC). Then, the operation boundaries of different control layers are derived considering the system stability and the protection requirement. Based on the operation boundaries at various timescales, a current limitation strategy is embedded in the controller to achieve the disturbance/attack resiliency for the grid-connected converter. Finally, simulations of a three-phase 40 MW 21-level MMC connected to a weak grid are performed to verify the resilient operation of the system.

Distributed‐hierarchical control strategy to coordinate peer‐to‐peer energy transactions and node voltages at low voltage distribution networks

Peer-to-peer (P2P) energy market has emerged as a promising way to absorb local generations. However, unregulated P2P transactions are likely to exacerbate voltage violations at distribution networks. The challenge is how to ensure P2P markets to flourish whilst maintaining system voltages within the stator limits. This study proposes a distributed-hierarchical control structure consisting of a central controller and peer controllers to address the challenge. The central controller computes the optimised P2P transaction levels and the nodal voltage references simultaneously and sends the outcomes to the peers as routing-update messages at regular intervals. The peer controllers are developed based on individual phase decupled P–Q theory where two individual channels follow the optimised transaction levels and nodal voltage references. The peers, also, apply a current limit strategy to release the network capacity and improve voltage profiles when confronting with short-term voltage magnitude variations between the two update intervals. A case-study-based investigation shows that reactive power contributions improve the power transaction levels up to 30% while the node voltage violations are reduced. The effectiveness of the proposed strategy is validated using simulation and compared with state-of-the-art voltage mitigation methodologies over the IEEE 19-bus system.

The Suppression of Voltage Overshoot and Oscillation During the Fast Recovery Process From Load Short-Circuit Fault for Three-Phase Stand-Alone Inverter

When a three-phase stand-alone inverter feeds several loads simultaneously, to ensure high reliability and continuity of power supply for the large load network, the inverter needs to operate continuously, even during faulty conditions. Moreover, the inverter should provide large enough current by switching from the voltage control mode (VCM) to the current control mode (CCM) to trigger the circuit breaker connecting the short-circuit fault branch to recover the power supply. Therefore, when a load short-circuit fault occurs, the inverter will switch from VCM to CCM. In CCM, the short-circuit currents can be controlled by the current-limiting strategy. Once the fault is cleared, to quickly supply the remaining loads, the output voltages should be restored to the rated voltage as soon as possible. However, the fast recovery process from the load short-circuit fault may cause significant voltage overshoot and oscillation. The voltage overshoot may lead to load damage, and the voltage oscillation may extend fault recovery time. To effectively solve the above problems, the voltage overshoot introduced by large fault current and postfault load impedance after fault clearance is first analyzed in this article, and then the suppression strategy based on the parallel virtual impedance is proposed. Second, the voltage oscillation introduced by the initial phase difference between the voltage references and the output voltages when the inverter switches from CCM to VCM is analyzed, and then the effective suppression strategy which can achieve seamless mode switching after fault clearance is proposed. The experimental results have been carried to verify the effectiveness of the proposed strategies.

Fault Current Hierarchical Limitation Strategy for Fault Ride-Through Scheme of Microgrid

With the growing penetration of grid-connected microgrids (MGs), utility grid (UG) faces the challenge of high-level fault current during fault ride-through (FRT) of MGs. To tackle this challenge, a fault current hierarchical limitation (FCHL) strategy is proposed, which consists of four levels: 1) fault current primary limitation control (FCPLC) is based on the proportional-resonant (PR) current controller; 2) fault current secondary limitation control (FCSLC) is formulated for current generation of grid-connected inverters (GCIs) with peak fault current limitation; 3) fault current tertiary limitation control (FCTLC) is formulated for the phase angle shifting of MG’s fault current to neutralize its effect on UG’s fault current; and 4) fault current quartus limitation control (FCQLC) is developed for the coordinated operation of parallel GCIs within grid code requirement. Extensive tests have validated the effectiveness of proposed strategy with a successful FRT while the MG’s output fault current has a minute effect on the UG’s fault current amplitude.

A Robust Optimal Coordinated Droop Control Method for Multiple VSCs in AC–DC Distribution Network

AC–DC hybrid distribution network is viewed as a featured system to efficiently connect to all kinds of power sources and loads. In order to improve the system robustness, adapting to random fluctuations of renewable energy sources, this paper proposed a robust optimal coordinated control method for multiple voltage source converters (VSCs) in AC–DC distribution network. A robust optimization model was established to co-optimize the slopes of active and reactive power droop control of VSCs with the aim to minimize the total network loss, while ensuring the system security. VSC capacity coupling and power flow constraints were both considered. Then, to enhance the computational tractability of the problem, based on the column-and-constraint generation algorithm, ${V^2} - P$ and ${V^2} - Q$ were implemented in the droop control schemes holding the linear feedback characteristic so that the non-convex max–min sub-problem can be transformed into a mixed integer second-order cone (SOC) programming problem. Furthermore, to increase the accuracy of SOC relaxation in the sub-problem, a current limit strategy was designed where the maximum possible network loss was pre-identified to restrain the feasible range of branch current. Numerical experiments suggest that the proposed coordinated control method can improve the system security with little extra economic cost, and the reliability of the results is enhanced by the proposed current limit strategy.

Characteristics and restraining strategy of fault current in VSG

With the increase of distributed generation (DG), DG is needed to provide inertia for the grid to maintain the security and stability of the grid. Therefore, the virtual synchronous generator (VSG) technology is proposed to compensate for the loss of inertia and reserve capacity. Similar to SGs, the large transient inrush fault current will flow into VSG at the moment of fault, which may lead to the damage of inverters and the instability of the power grid. Therefore, the characteristics of the inrush fault current of the VSG are studied. On the basis of this, a fault current limitation strategy based on adaptive virtual impedance is proposed, and its effectiveness is verified by simulation.

Cooperative Operation of R-SFCL and Online Current Limitation Strategy of GCPVS Under Asymmetrical Faults

Grid-connected photovoltaic systems (GCPVS) should remain grid-connected and inject specified reactive current in accordance with fault-ride-through requirements. To minimize voltage ripples on the dc link voltage and restrict the maximum phase currents, a modified online current limitation strategy (OCLS) is presented in this paper. Transient performance of GCPVS is enhanced as resistive-type superconducting fault current limiter (R-SFCL) is applied under severe asymmetrical faults. By introducing proper resistance selection process, the cooperative operation of OCLS and SFCL is proved to be valid in performance improvement.

A Modified DC Chopper for Limiting the Fault Current and Controlling the DC-Link Voltage to Enhance Fault Ride-Through Capability of Doubly-Fed Induction-Generator-Based Wind Turbine

A simple conventional dc chopper is employed to protect the doubly-fed induction generator (DFIG) from overvoltage; however, it is not capable to keep the transient overcurrent in an acceptable level in stator and rotor sides. Therefore, an effective current-limiting strategy should be incorporated with the dc chopper to improve the fault ride-through capability of the DFIG. In this paper, a modified DC chopper is proposed not only to keep the dc-link voltage in an acceptable range, but also to limit the high-current level in the stator and the rotor sides in a permissible level without incorporating any extra fault-current-limiting strategy. Unlike the conventional dc chopper, in the proposed dc chopper, it is not required to stop rotor-side converter (RSC) switching and employ high-rated-current antiparallel diodes. The proposed modified dc chopper is placed between the dc-link capacitor and the RSC. In the proposed switching strategy of the modified dc chopper, three extra semiconductor switches are included, which are triggered to insert dc chopper resistance either in parallel or series connections with the dc link regarding the dc-link voltage level and the dc-link current level, respectively. Calculation of the dc chopper resistance is discussed. To prove the effectiveness and robustness of the proposed modified dc chopper in terms of both limiting the fault current and controlling the dc-link voltage of the DFIG, symmetrical and asymmetrical grid faults are applied in a power system including the DFIG-based wind turbine modeled in PSCAD/EMTDC so.