Current-limiting Mode Research Articles

Criteria for Grid-Forming Inverters Transitioning Between Current Limiting Mode and Normal Operation

Criteria for Grid-Forming Inverters Transitioning Between Current Limiting Mode and Normal Operation

To the question of construction of welding inverter sources with a high power factor

Purpose. Design a power-factor-corrected, energy- and cost-efficient switchmode power supply for welding. Methodology. It is proposed to use the principle of direct conversion and to develop the "hybrid" power circuit consisting of a main power path using direct conversion and an auxiliary power path having a converter with an intermediate DC link with relatively low stored energy. Since a load to the welding power supply is the welding arc, its instantaneous power fluctuations during the mains voltage period essentially do not influence the welding process due to the thermal constant of the weld pool molten metal. Effective use of this features of the welding arc allows to optimize the power processing, reduce the amount of energy stored in the reactive elements of the circuit and improve the economical, mass and dimensional parameters of the supplies. Findings. A new power factor corrected switchmode power supply topology is proposed utilizing beforementioned principles of power processing. The algorithm for calculating the switching frequency of supply power switches is given. To confirm the adequacy of the developed provisions, an experimental sample of the power supply was created. Oscillograms of mains voltage and source current consumption and oscillograms in output current limitation mode are obtained. The results of experiments showed that the supply has a power factor > 0.94 in a wide range of powers. Originality. A distinctive feature of the supply is the absence of additional inductive components in the power path, a lower capacitance of the DC-link bulk capacitor and a simplified scheme for limiting its inrush current during initial charging. The supply has an increased open circuit voltage, independent of the mains voltage, which allows for easy arc ignition during manual arc welding. Practical value. Due to the increased power factor, the current drawn from the mains is 30 % to 45 % lower than that of widespread inverter supplies without a power factor corrector. These features allow to simultaneously use more power supply units with the same mains current load, and/or to provide stable welding in conditions of a "weak" mains.

Transient Stability Analysis of Grid-Forming Converter in Current Limiting Mode Based on Hamiltonian Theory

Transient Stability Analysis of Grid-Forming Converter in Current Limiting Mode Based on Hamiltonian Theory

Fault Recovery Analysis of Grid-Forming Inverters With Priority-Based Current Limiters

Grid-forming (GFM) inverters are required to operate robustly against grid faults. However, due to the limited over-current capability of inverters, current-limiting controls are usually applied to protect these semiconductor devices, which may prevent GFM inverters from a successful fault recovery. To understand this phenomenon, this study analyzes the fault recovery process of a GFM inverter with a priority-based current limiter. According to whether the GFM inverter can ensure transient stability and exit the current-limiting mode after fault clearance, three post-fault scenarios are identified, including <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">normal operation, current limitation, and oscillations</i> . Further, the impacts of the short-circuit ratio and control parameters on the post-fault behavior of GFM inverters are demonstrated. To illustrate the implications of these theoretical results, typical numerical examples are presented. Finally, the theoretical findings are validated through experimental tests.

Synchronization Stability of the MMC-Connected Wind Farm Under Severe Asymmetrical Faults

This paper analyzes the phase-locked loop (PLL) synchronization stability of an offshore wind farm connected via the modular multilevel converter (MMC)-based high-voltage direct-current (HVDC) transmission system. The investigated wind farm is composed of full-scale type-4 wind turbines, and severe asymmetrical faults between these turbines and the MMC are considered. The MMC has dual-sequence control requirements under such faults, which interacts with the control systems of the wind turbines and further complicates the transient stability problem for the system. Thus, we thoroughly investigate how the interaction caused by the MMC affects the synchronization stability of wind turbines. Under severe faults, the MMC would be switched into a current limitation mode and act as a current source. Therefore, the influences of the MMC’s positive- and negative-sequence currents as well as their assignments between dq axes, on transient synchronization stability of wind turbines, are studied according to the phase portrait method. Furthermore, the optimal assignment region (OAR) is proposed for the stability-oriented assignment of MMC’s positive- and negative-sequence currents. This theoretical analysis is validated through extensive simulation results obtained from MATLAB/Simulink.

Study of Electromagnetic and Thermal Transients in a High-temperature Superconducting Transformer during a Short Circuit

Today, high-temperature superconducting (HTS) current limiters and transformers allow to limit the surge short circuit current during failure without negatively affecting on the power grid complex at the normal operation mode. However, the transition of a superconductor to a resistive state at the moment of current limitation can cause significant heat generation, which can destroy the transformer windings. The research goal is to provide optimal technical characteristics of the HTS transformer to achieve effective short circuit current limitation and prevent thermal breakdown of its windings. To achieve this goal, a mathematical model of a HTS transformer was developed. The presented method considers the material type and geometry of the superconducting tape, the critical parameters of the superconductor (current and temperature), the parameters of the cryogenic liquid, dependence of the resistance and heat capacity of the HTS tape layers on temperature. The simulation model was created in the Matlab/Simulink software. The most important result is the possibility of obtaining optimal electrical and thermal parameters of the HTS transformer windings during the short circuit current limitation, as well as ensuring the thermal stability of the superconducting tape at the quench moment. The obtained results are significant in the design and operation of HTS transformers. For efficient and safe operation in the current-limiting mode, it is necessary to take into account heat generation on the transformer windings. It is important for the superconductor returning to the superconducting state without causing significant overheating of the windings.

LVDC SiC MOSFET Analog Electronic Fuse With Self-Adjusting Tripping Time Depending on Overcurrent Condition

This article deals with the design of an electronic fuse for 380 V dc distribution systems. It has been devised for applications that require current limitation up to 3 A, so approximately 1.2 kW. To protect the main semiconductor against excessive energy dissipation, the tripping time, defined as the time that circuit operates in current-limitation mode, varies with the current fault magnitude, and eventually for very hard fault conditions, i.e., short-circuit, the circuit acts as a circuit-breaker avoiding current-limitation operation. Furthermore, thermal foldback characteristic, defined as the variation of current limitation setpoint with temperature, and tripping time variation with temperature are also considered. Mathematical analysis, circuit design, simulations, and experimental validation have been carried out using a SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MOSFET</small> as power semiconductor. Experimental results working under different conditions show an excellent performance.

Current Limiting in Overload Conditions of an LLC-Converter-Based DC Transformer

The dc transformers based on an LLC converter operated in open loop near resonant frequency have several crucial advantages compared to other topologies. These advantages are the stiff voltage ratio with a natural power flow, the dc grid isolation via medium-frequency transformer, and the high efficiency due to the resonant converter nature. Despite these advantages, the open-loop operation does not provide any options to limit the transferred current during overload conditions in the dc grid. This is critical, since it prevents the grid from recognizing that either some local primary regulation or load shedding is required. This article proposes a closed-loop control method that utilizes variable duty cycles to limit the current once the overload situation is detected. An applicable control scheme is derived and implemented. Since the duty-cycle modulation increases the losses of the converter, an online thermal model is implemented to protect the dc transformer during the overload conditions. The proposed current-limiting mode with the thermal model are validated using an exemplary low-voltage dc transformer prototype, demonstrating the general feasibility and overall good performance.

Passivity-Based Dual-Loop Vector Voltage and Current Control for Grid-Forming VSCs

This letter proposes a passivity-based dual-loop vector voltage and current control method for grid-forming voltage-source converters (GFM-VSCs). A passive output impedance of GFM-VSC is guaranteed in both the voltage control mode and the current-limiting mode with a wide range of time delay. The frequency-domain analysis, simulation and experimental tests validate the effectiveness of the approach.

Analysis of Start-Up Response in a Digitally Controlled Boost Converter With Constant Power Load and Mitigation of Inrush Current Problems

In this paper, the start-up response of a digitally controlled dc-dc boost converter with a constant power load is studied. A mathematical expression of the system response during start-up is derived using a nonlinear averaged model. The time-domain response of the system during start-up when the converter is operating under current limitation mode is derived by analytically integrating the mathematical equations describing the dynamic behavior. Numerical simulations from a detailed switched model validate the theoretical results and a good agreement is obtained. A boost converter prototype is built to validate the theoretical finding and the numerical simulations. It is observed that even when the analysis and the simulation predict zero inrush current, the experimental circuit still exhibits an unsuitable initial current peak and an unacceptable transient response during start-up. This phenomenon is mainly attributed to the presence of an unavoidable delay due to the computation time required by the digital control platform. The resulting inrush current is more pronounced in the presence of nonlinear load such as a CPL and saturable inductor. Some solutions are proposed to mitigate the inrush current problems. Simulations results taking into account the delay and experimental measurements demonstrate the validity of the proposed solutions.

Dual-Functional Dynamic Voltage Restorer to Limit Fault Current

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.

A Novel Multi-Function Saturated-Core Fault Current Limiter

A novel multi-function saturated-core fault current limiter (MFCL) with two operating modes is proposed in this paper. MFCL can achieve fault current limitation and power flow management by switching operating mode: current-limiting mode and power-flow-controlling mode, which realizes technology convergence. The MFCL consists of an iron core with rotatable magnetic valve, ac coils, dc coils, and a fault-limiting reactor. The operating principle of the MFCL and the design of the rotatable magnetic valve are analyzed in this paper. The finite-element analysis (FEA) model based on Maxwell is used to analyze the MFCL. Simulation results depict that the MFCL can limit fault current effectively and provide variable impedance for power flow controlling.

Analysis of design features of inductive electromagnetic current limiter

The design circuits of an electromagnetic short-circuit current limiter of inductive type with movable armature and moving core are considered. A comparative analysis of the design features of the electromagnetic current limiter with copper or superconducting winding is carried out. The operation principle of the electromagnetic short-circuit current limiter of inductive type with copper or superconducting winding is considered. The peculiarities of operation modes of the electromagnetic short-circuit current limiter of inductive type with copper or superconducting winding are analyzed. Using results of the mathematical modeling of the magnetic system of the electromagnetic short-circuit current limiter by the Finite Element Method in the FEMM software environment, the character of the distribution of the magnetic flux density in the nominal mode and in the current limiting mode is determined, which allows to determine the parameters of the current limiter and design features. In accordance with the design features of the electromagnetic short-circuit current limiter of inductive type, the sequence of calculation of the basic parameters of the magnetic system, taking into account the voltage drop factor, is given. The basic equations of the electric equilibrium of the voltage drop of the electromagnetic short-circuit current limiter of inductive type for nominal mode and short-circuit conditions are given. The use of the design circuit of the electromagnetic short-circuit current limiter with superconducting winding can provide energy-saving in nominal mode. The use of high-temperature superconducting wire due to increased critical parameters will ensure preservation of the superconducting state of the winding in short-circuit mode.

Operation of offshore wind farms connected with DRU-HVDC transmission systems with special consideration of faults

The diode rectifier unit (DRU)-based high-voltage DC (DRU-HVDC) system is a promising solution for offshore wind energy transmission thanks to its compact design, high efficiency, and strong reliability. Herein we investigate the feasibility of the DRU-HVDC system considering onshore and offshore AC grid faults, DC cable faults, and internal DRU faults. To ensure safe operation during the faults, the wind turbine (WT) converters are designed to operate in either current-limiting or voltage-limiting mode to limit potential excessive overcurrent or overvoltage. Strategies for providing fault currents using WT converters during offshore AC faults to enable offshore overcurrent and differential fault protection are investigated. The DRU-HVDC system is robust against various faults, and it can automatically restore power transmission after fault isolation. Simulation results confirm the system performance under various fault conditions.

A New Distance Protection Method Considering TCSC-FCL Dynamic Impedance Characteristics

Ignoring the dynamic impedance characteristics of a thyristor-controlled series-capacitor-type fault current limiter (TCSC-FCL) may lead to maloperation/fail-to-trip or slow response of distance protection. To solve the above problem, the TCSC-FCL dynamic impedance analysis model was established. Taking the special operation mode of the TCSC-FCL into account, the time-varying characteristics of the TCSC-FCL during the whole process from the series compensation mode to the current-limiting mode were analyzed. Then, this paper proposed the function expression of dynamic impedance characteristics of the TCSC-FCL and put forward the distance protection setting strategy based on a dynamic offset impedance circle in order to eliminate the influence of TCSC-FCL dynamic impedance characteristics to the distance protection operation. By establishing a typical simulation model including the TCSC-FCL, the correctness of dynamic feature's analytic expression and the distance protection setting strategy is proved with extensive simulation results.

Fault Detection and Isolation in Medium-Voltage DC Microgrids: Coordination Between Supply Power Converters and Bus Contactors

A fast and effective method for detecting and isolating faults in medium-voltage dc microgrids relies on rapid coordination between power supply converters and bus segmentizing contactors to limit currents and isolate faults without any need for fast communication between these active elements. The power converters independently enter current-limiting mode as soon as they recognize a fault condition and the segmentizing contactors autonomously decide whether or not to open based on their local interpretation of time-to-trip curves as functions of apparent circuit resistance. This method allows converters and contactors to use only local measurements when discerning whether or not to trip in order to isolate the faulted section. Simulation and experimental results show that low-impedance short-circuit faults can be isolated within 10–20 ms and the system can be reenergized within 40–60 ms. The method is effective for a wide range of fault and system configurations, and that range can likely be expanded by applying additional discrimination methods.

Alternative model of space-charge-limited thermionic current flow through a plasma.

It is widely assumed that thermionic current flow through a plasma is limited by a "space-charge-limited" (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. Here, we formulate a fundamentally different current-limited mode. In the "inverse" mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting the circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modes should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. The inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.

Current-Limiting Fuses: New NFPA 70-2017 Section 240.67, Arc Modeling, and an Assessment Based on the IEEE 1584-2002

When operating in current-limiting mode, current-limiting fuses can effectively mitigate the heat and pressure hazards associated with an arc event because they limit the current in the first one-quarter cycle and interrupt current flow in less than 8.3 ms. The effectiveness of current-limiting fuses is reinforced by their inclusion in NFPA 70E-2015: Note to Table 130.7(C)(15)(A)(b), Annex D.4.6, and Annex O.2.4(4). The incident energy levels associated with current-limiting fuses can be determined from the IEEE Standard 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE 1584-2002 presents a method for directly determining incident energy without first determining arc current or arc duration. The fuse equations were derived from 600-V arc tests involving a range of current-limiting fuse sizes. These data are reviewed to assess the performance of current-limiting fuses in reducing arc energy in accordance with NFPA 70-2017 Section 240.67. The IEEE 1584-2002 equations are used to assess the performance of these fuses in comparison with an arc energy reduction maintenance switch and the combination of both technologies. The viability of predicting arc current and incident energy for installations involving current-limiting fuses and implementing NEC Section 240.67 using general arc models is discussed.

A superconducting direct-current limiter with a power of up to 8 MVA

A resistive switching superconducting fault current limiter (SFCL) for DC networks with a nominal voltage of 3.5 kV and a nominal current of 2 kA was developed, produced, and tested. The SFCL has two main units—an assembly of superconducting modules and a high-speed vacuum circuit breaker. The assembly of superconducting modules consists of nine (3 × 3) parallel–series connected modules. Each module contains four parallel-connected 2G high-temperature superconducting (HTS) tapes. The results of SFCL tests in the short-circuit emulation mode with a maximum current rise rate of 1300 A/ms are presented. The SFCL is capable of limiting the current at a level of 7 kA and break it 8 ms after the current-limiting mode begins. The average temperature of HTS tapes during the current-limiting mode increases to 210 K. After the current is interrupted, the superconductivity recovery time does not exceed 1 s.

Multilevel Cascaded-Type Dynamic Voltage Restorer With Fault Current-Limiting Function

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.