Current Limit Control Research Articles

Design Considerations of Current-Limiting Control for Grid-Forming Capability Enhancement of VSCs Under Large Grid Disturbances

Design Considerations of Current-Limiting Control for Grid-Forming Capability Enhancement of VSCs Under Large Grid Disturbances

Protection and Fault Isolation Scheme for DC Distribution Network Based on Active Current-Limiting Control

Protection and Fault Isolation Scheme for DC Distribution Network Based on Active Current-Limiting Control

Transient Synchronous Stability Analysis of Grid-Following Converter Considering Outer-Loop Control with Current Limiting

With the evolution of modern power systems, inverter-based resources have become increasingly prevalent. As critical energy conversion interfaces, grid-following converters exhibit dynamic performances, presenting challenges for system security and stability. This paper focuses on the transient synchronization stability of converters after disturbances, highlighting differences in mechanisms compared to synchronous generators. Although previous studies on the transient synchronization stability of converters have been conducted, they primarily concentrate on the dynamics of the phase-locked loop, with limited consideration of the effects of outer-loop control. This has created a cognitive bottleneck in understanding the transient synchronization mechanisms of converters. To address these challenges, this paper models a grid-following voltage source converter system, incorporating detailed converter control strategies and current-limiting control. The stability regions of the stable equilibrium point under various fault severities are first analyzed. Then, the impacts of outer-loop control, including PI control and current-limiting control, on transient synchronization are examined. The study systematically elucidates the influence of outer-loop control on the transient synchronization stability of converters. Finally, the validity of the proposed theory is confirmed through simulations conducted in PSCAD/EMTDC.

DC fault current clearance coordinated control strategy for DC grid with hybrid MMC

For DC grid based on the hybrid modular multilevel converter (MMC), the traditional DC fault current clearance scheme takes a long time and greatly affects the active power transmission. A novel DC fault current clearance coordinated control strategy is proposed, which can quickly interrupt the fault current and reduce the impact of DC fault on the DC grid and AC system. For fault-line connected MMC (FLMMC), a negative DC voltage control is employed, which can improve the current attenuation speed and ensure reliable fault isolation. For non-fault-line connected MMC (NFLMMC), the active current-limiting control (ACLC) based on the virtual reactor is adopted, which can reduce the current flow to the fault location and further shorten fault isolation time. To reduce the impact on the AC system during the DC fault, a short-time active power support control is designed. Finally, a four-terminal DC grid simulation model is built based on the RT-LAB OP5600 real-time digital simulation platform. The simulation results show that the proposed coordinated control strategy for the DC grid can quickly clear DC fault current, shorten DC fault isolation time, and strengthen active power support capability.

Saturation-informed current-limiting control for grid-forming converters

In this paper, we investigate the transient stability of a state-of-the-art grid-forming complex-droop control (i.e., dispatchable virtual oscillator control, dVOC) under current saturation. We quantify the saturation level of a converter by introducing the concept of degree of saturation (DoS), and we propose a provably stable current-limiting control with saturation-informed feedback, which feeds the degree of saturation back to the inner voltage-control loop and the outer grid-forming loop. As a result, although the output current is saturated, the voltage phase angle can still be generated from an internal virtual voltage-source node that is governed by an equivalent complex-droop control. We prove that the proposed control achieves transient stability during current saturation under grid faults. We also provide parametric stability conditions for multi-converter systems under grid-connected and islanded scenarios. The stability performance of the current-limiting control is validated with various case studies.

Propagation Mechanism and Suppression Strategy of DC Faults in AC/DC Hybrid Microgrid

Due to their efficient renewable energy consumption performance, AC/DC hybrid microgrids have become an important development form for future power grids. However, the fault response will be more complex due to the interconnected structure of AC/DC hybrid microgrids, which may have a serious influence on the safe operation of the system. Based on an AC/DC hybrid microgrid with an integrated bidirectional power converter, research on the interaction impact of faults was carried out with the purpose of enhancing the safe operation capability of the microgrid. The typical fault types of the DC sub-grid were selected to analyze the transient processes of fault circuits. Then, AC current expressions under the consideration of system interconnection structure were derived and, on this basis, we obtained the response results of non-fault subnets under the fault process, in order to reveal the mechanism of DC fault propagation. Subsequently, a current limitation control strategy based on virtual impedance control is proposed to address the rapid increase in the DC fault current. On the basis of constant DC voltage control in AC/DC hybrid microgrids, a virtual impedance control link was added. The proposed control strategy only needs to activate the control based on the change rate of the DC current, without additional fault detection systems. During normal operations, virtual impedance has a relatively small impact on the steady-state characteristics of the system. In the case of a fault, the virtual impedance resistance value is automatically adjusted to limit the change rate and amplitude of the fault current. Finally, a DC fault model of the AC/DC hybrid microgrid was built on the RTDS platform. The simulation and experimental results show that the control strategy proposed in this paper can reduce the instantaneous change rate of the fault state current from 19.1 kA/s to 2.73 kA/s, and the error between the calculated results of equivalent modeling and simulation results was within 5%. The obtained results verify the accuracy of the mathematical equivalent model and the effectiveness of the proposed current limitation control strategy.

Segmental equal area criterion for grid forming converter with current saturation

Previous research indicates that post-fault grid-forming (GFM) converters only work in current limiting control (CLC) mode. However, converters integrating the current saturation algorithm (CSA) can operate in both constant voltage control (CVC) and CLC modes. This paper firstly derives the angle-switched model of the grid-connected system governed by CSA and virtual synchronous generator (VSG). Then, combining the fault and three types of post-fault trajectories, critical clearing angle is analytically formulated through the Segmental Equal Area Criterion (SEAC). Finally, a range of values for the saturated current angle is proposed to ensure that the system is stabilized in CVC mode. Notably, the optimal saturated current angle, which is the minimum value within the range, provides the maximum critical clearing angle.

Transient Stability of Power Synchronization Loop Based Grid Forming Converter

Previous studies regards that the grid-forming (GFM) converter switches from the constant voltage control (CVC) to the current limiting control (CLC) at the point of intersection of the unsaturated and saturated power-angle curves, which might not be held in most situations. This paper firstly derives the switching rule of GFM converter between two controls from the current limiting inequality. Accordingly, the conventional algebraic-variable switched model is transferred to the proposed state-variable switched model. Based on Lyapunov Theory, the stability of equilibrium points (EPs) is studied and the stable regions under two controls are further derived. In addition, one necessary stabilizing condition for the power synchronization loop based GFM (PSL-GFM) converter being locked in CLC is derived. As a result, one global stabilization on CVC is proposed by regulating the angle between the saturated current vector and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d-axis</i> . Finally, the maximum system boundary of the first swing can be obtained by placing stable equilibrium points (SEPs) under CLC on the right switching line.

Nonparametric three-vector model predictive current control for permanent magnet synchronous motor

Multi-vector model predictive control of permanent magnet synchronous motor can effectively overcome the shortcomings of conventional single-vector model predictive control such as large motor current ripples and limited control accuracy, but its control performance is more susceptible to the influence of motor parameter perturbation. To improve the robustness of the multi-vector predictive control algorithm under the motor parameter perturbation, a novel nonparametric three-vector model predictive current control method for permanent magnet synchronous motors is proposed in this paper. Through constructing an ultra-local predictive current model based on the input and output signals of the permanent magnet synchronous motor, the influence of motor parameter perturbation on current prediction is avoided, and a disturbance observer is designed to estimate the non-modeled and perturbed parts of the ultra-local model. In addition, the direct calculation method of vector duty cycles based on current error is introduced to suppress the influence of motor parameter uncertainty on the duty cycle calculation link, which further improves the system robustness. Finally, simulations and experiments of the proposed method are demonstrated to compare with the conventional parametric three-vector model predictive current control, and the results show that the proposed control strategy can effectively suppress the disturbances caused by the motor parameters and ensure the steady-state performance during motor operation.

Virtual Armature Resistance-Based Control for Fault Current Limiting in a High-Order VSG and the Impact on Its Transient Stability

This article proposes a fault ride through (FRT) technique for a high-order virtual synchronous generator (VSG) that adjusts its virtual armature resistance. When a fault is detected by a dedicated algorithm, the proposed control adjusts the resistance parameter accordingly. The main contribution of this article is to adjust the virtual resistance directly in the machine model to limit the current during faults, unlike other techniques proposed in the literature that add another control loop to produce the virtual impedance effects. To validate the effectiveness of the proposed control, a hardware-in-the-loop real-time simulation platform was adopted using a Typhoon HIL 402 device and a Texas Instruments F28379D digital controller. The results demonstrate that the control effectively limits the converter’s current while still contributing to raising the system’s critical clearing time (CCT) and improving transient stability. The proposed FRT strategy is validated in a three-phase fault scenario in which a 500 kVA–480 V converter’s peak fault current is reduced from 5 kA to 1.4 kA, depending on the resistance value adjusted. The transient stability is also analyzed in 30 different scenarios and the VSG support on the CCT is reduced by 23 ms on average. However, when compared to the baseline scenario without the VSG, the system still sees an increase in CCT with the current limiting control applied. Additionally, the control allows the VSG to smoothly transition to island mode in a scenario where the fault is cleared and the grid is disconnected by a protection system.

Interruption method for commutation failure caused cascading reaction of HVDC with wind farm integration under grid fault

Line-commuted converter based high voltage direct current (LCC-HVDC) is a typical mode for wind power transmission. The fault of the receiving-end grid of LCC-HVDC can easily lead to the commutation failure at the inverter station. Improper controls during the recovery process of first commutation failure may cause the subsequent commutation failure, and lead to the high voltage ride through of wind generators at the sending-end grid. The regional and inter-regional cascading reactions are produced by commutation failure. Existing researches severally focus on control of suppressing subsequent commutation failure or overvoltage. The demand for commutation failure recovery of the inverter and overvoltage suppression at the sending-end grid cannot be satisfied simultaneously, and may even aggravate the cascading process. The stability of the power system will be threatened. Therefore, the new idea of feasible power region and adaptive current control of inverter station is proposed to interrupt commutation failure caused cascading reaction. The feasible ranges of DC current to avoid subsequent commutation failure and overvoltage are derived. The feasible power region is modeled by the active power and exchanged reactive power of inverter station. The principle of interrupting commutation failure caused cascading reaction based on adaptive current control of inverter station is proposed. Then, the command value calculation and implementation method of adaptive current control are further proposed. Finally, the performance of the proposed control method and voltage-dependent current order limiter (VDCOL) control is compared based on the CIGRE LCC-HVDC standard system, and the effectiveness of theoretical research is verified by the simulation results.

A novel low-voltage current-limiting control strategy with improved voltage compensation

By optimizing the design of the conventional low-voltage current limiting controller (VDCOL), the continuous commutation failure on the receiving end of the traditional LCC-HVDC transmission system is effectively suppressed. In order to overcome low insensitivity of conventional VDCOL under mild and moderate faults, the dc current deviation value is superimposed into the starting voltage of the VDCOL in the form of a compensation voltage to enhance sensitivity of the VDCOL to the dynamic changes of the dc current during mild and moderate faults. In the face of severe faults, the conventional VDCOL adopts a linear adjustment method with a fixed slope, which is bad for suppressing continuous commutation failure. Aiming at this problem, a nonlinear VDCOL strategy is adopted, which the dc current command and the dc voltage are a nonlinear function relationship with a variable slope with the increasing or decreasing of the dc voltage. In the PSCAD/EMTDC simulation platform, the proposed control strategy is verified using the International Council on Large Electric Systems (CIGRE) standard model. Simulation results reveal that the proposed nonlinear VDCOL strategy with improved voltage compensation method can well suppress continuous commutation failures in the case of single-phase and three-phase short-circuit faults, and it is beneficial to restore dc voltage and transmission power and ensure the normal operation of the HVDC transmission system.

Fault current limitation control of multiple distributed renewable generations under unbalanced conditions

For multiple distributed generation units (DG unit) parallel system, excessive fault current has adverse effects on the safe and stable operation of the utility grid. As a result, a fault current limitation control of multiple DG units is developed in this paper to limit the fault current of related fault branch and ride through the associated fault conditions. The injection fault current amplitude and phase angle of each DG unit for grid-side converter are controlled through the point of common coupling (PCC) voltage support and fault current limitation to limit the total current amplitude at the corresponding fault location. The validity of the proposed control strategy has been verified by simulation and hardware-in-loop (HIL) experiment results.

Research on dual-mode fault suppression strategy of AC isolation and current limiting based on hybrid MMC

Abstract The DC side fault of the high voltage DC transmission system based on modular multilevel converters high voltage DC (MMC-HVDC) is easily affected by the double influence of AC transient electrical quantity infeed and overvoltage and overcurrent, seriously threatening the safe operation of the power grid. Aiming at the problem above, a fault suppression strategy in dual mode of isolation and current limiting is proposed. Based on the half-bridge-full-bridge hybrid MMC, the fault transient mathematical model considering AC feed and sub-module switching feed is analyzed. AC isolation module with double conduction self-resistance structure and current-limiting control module is constructed, which can be adaptively adjusted to the system’s direct current parameters. The ±500 kV double-ended hybrid MMC system model is established to verify the scheme’s effectiveness. In addition, the influence of high AC modulation parameters on the suppression effect is investigated. PSCAD/EMTDC simulation results show that AC active and reactive power feed-in peaks are reduced by 388.69 MW and 119.82 Mvar, respectively, under dual-mode control. The peak value of DC overvoltage and overcurrent is reduced by 1211.53 kV and 22.5 kA, respectively, the fault self-clearing time is shortened by 43 ms, and the fault suppression effect is remarkable. In the high AC modulation ratio operating state, the modulation parameters are positively correlated with the AC power feed and negatively correlated with the overvoltage and overcurrent of the DC system. The research results provide support for the fault protection scheme of the hybrid MMC flexible and direct power grid.

Digital Hysteresis Control Algorithm for Switched Inductor Quasi Z-Source Inverter with Constant Switching Frequency

In this paper, a digital hysteresis current limit controller is developed for Switched Inductor Quasi Z-Source Inverter (SLQZSI). Traditional methods like hysteresis current fixed limit and adjustable hysteresis current limit techniques changes the hysteresis bandwidth in accordance to modulating frequency and gradient of reference current. The operating shifting frequency of typical approaches oscillates and crosses the intended steady shifting frequency under noise. It leads to undesirable heavy interference between the phases and more power loss. In the planned digital hysteresis current limit technique, the hysteresis current limit is calculated by resolving the optimization problem. In the proposed approach the operating shifting frequency is kept same or inferior to the intended steady shifting frequency even under noise. Hence the planned digital hysteresis current limit algorithm maintains the output current steady and power loss is minimized which is not promised by the conservative techniques. To compare different control strategies in terms of nature of operating switching frequency and harmonic performance simulations are built on the MATLAB/SIMULINK

DC fault current-limiting method based on dual-loop active control for modular multilevel converters

PurposeIn the event of a DC short-circuit fault in a flexible DC power grid, the high peak value of the fault current puts forward more stringent requirements on the DC circuit breaker. The existing fault current cutoff mainly focuses on changing the topology structure. To suppress the development of fault current and reduce the investment cost of the DC grid, this paper aims to propose a dual-loop active current-limiting control based on energy difference.Design/methodology/approachFirstly, the equivalent circuit at fault is established, and the parameters related to the fault current are analyzed. Then, the relationship between the output voltage change of the bridge arm and the difference between the AC and DC energy is deduced. Finally, the experimental results are discussed on the real-time simulation platform Opal-RT.FindingsThe proposed current-limiting measures can greatly reduce the fault current, reduce the breaking current of the circuit breaker and increase the capacitor voltage during the fault period, which is beneficial to the stability of the AC system. It is verified that the proposed method is also applicable to a certain high-resistance fault.Originality/valueThis paper applies the method of AC fault to DC fault and deduces the relationship between energy difference and voltage variation corresponding to different step lengths based on digital simulation. In addition, two variables are used as control structure parameters to reduce the probability of system misoperation.

Enhancing LVRT capability of DFIG using cooperative control of BTFCL and RPC

ABSTRACT To integrate the high penetration of power from wind farms into a grid, one of the key requirements posed by new grid codes is Low Voltage Ride Through (LVRT) capability. This paper studies voltage compensation-based methods to improve fault ride-through capability of the grid-connected Doubly Fed Induction Generator (DFIG). Bridge Type Fault Current Limiter (BTFCL) and Reactive Power Control (RPC), which is acting as an active voltage compensator, are cooperatively controlled in this study to achieve better LVRT capability of DFIG. Additionally, the design considerations to select shunt impedance of BTFCL are described to get optimal performance of the system under consideration. The test system was modelled and simulated in MATLAB/SIMULINK environment. The effectiveness of the proposed method is tested by comparing it with the conventional Crowbar resistance method. Simulation results show that the cooperative control of self-acting BTFCL and an intelligent RPC can effectively reduce the fault current, improves the stator voltage of DFIG for continuous operation and provides reactive power support to the grid during fault conditions.

Overvoltage Suppression Under Commutation Failure Based on Improved Voltage-Dependent Current Order Limiter Control Strategy

Commutation failure (CF) in the line-commutated converter based high-voltage direct-current system will result in a large amount of reactive power surplus on the rectifier ac system. It will cause the transient overvoltage and lead to the massive disconnection of the renewable sources. In this article, the mechanism of the transient overvoltage caused by CF is analyzed and the suppression strategy based on improved voltage-dependent current order limiter (VDCOL) control is proposed. First, during the process of CF, the dynamic behavior of reactive power consumption of the rectifier is analyzed. It is proposed that when the inverter returns to normal commutation, the sudden increase of the inverter dc voltage will cause the decreasing of the dc current, which will result in the large decreasing of reactive power consumption of the rectifier. Then, the dynamic change process of the rectifier ac voltage is deduced. Second, the peak value of transient overvoltage as affected by the occurrence and removal time of the short-circuit fault is deduced and analyzed. Then, by improving the operating characteristic of the VDCOL control, the control strategy of overvoltage suppression is proposed. Based on the PSCAD platform's CIGRE-HVDC benchmark model, the effectiveness of the proposed strategy is verified by the simulation results.

Research on fault characteristic of modular multilevel converter

Along with China proposes to build a new energy power system, various DC transmission technologies have been extensively studied. As the most serious fault type of the DC transmission system, permanent DC bipolar short-circuited fault is easy to cause converter damage. Modular multilevel converter can be used in the DC power transmission and distribution systems because of its modular design and strong ability on handling fault. When permanent DC bipolar short-circuited fault occurs, the research on fault characteristics of modular multilevel converter can offer important support for current-limiting control of converters and relay protection of power system. By analogy with the fault evolution mechanism of two-level converter, the equivalent zero-crossing voltage of sub-module capacitance is defined, and the fault evolution mechanism of modular multilevel converters can be obtained. The expressions of the fault characteristic parameters including the equivalent zero-crossing voltage of the sub-module capacitance, the DC steady short-circuited current, the peak value of the DC short-circuited current and their difference, and the peak time of the DC short-circuited current were deduced. By the way of analysis and simulation, the influence of the main circuit parameters in MMC on the characteristic variables with short-circuited fault can get the regularity, it can provide the theory basis for the current-limiting control and relay protection.

Grid-Forming Hybrid Angle Control and Almost Global Stability of the DC-AC Power Converter

This paper introduces a new grid-forming control for a grid-connected dc-ac power converter, termed hybrid angle control (HAC) that combines the dc-based matching control with a novel nonlinear angle feedback reminiscent of (though not identical to) classic droop control. The synthesis of HAC is inspired by the complementary benefits of the dc-based matching and ac-based grid-forming controls as well as ideas from direct angle control and nonlinear damping assignment. The proposed HAC is applied to a nonlinear converter model that is connected to an infinite bus or a center-of-inertia dynamic grid models. We provide parametric sufficient existence, uniqueness, stability, and boundedness conditions that are met by appropriate choice of control parameters. Next, we take into account the safety constraints of power converter, and synthesize a new current-limiting control that is compatible with HAC. Last, we present details on the practical implementation of HAC that are followed by a robustness analysis (which showcases a theory-practice gap), uncover the intrinsic droop behavior of the HAC, derive a feedforward-like ac voltage and power control, and illustrate the behavior of the closed-loop system with simulation case studies.