Grid Fault Conditions Research Articles

Transient Synchronization Stability Analysis and Enhancement Control for Power Self-Synchronization Control Converters

Conventional grid-forming control often destabilizes voltage source converters (VSCs) in stiff grids, and transient synchronization instability will occur in the grid fault condition. Therefore, power self-synchronization control (PSSC) is first introduced for enhancing the small-signal stability of grid-forming control in the case of a short circuit ratio ranging from 1 to infinity. Meanwhile, the transient synchronization instability for the grid-forming converter with PSSC in the grid fault condition is analyzed by the phase portrait, and a transient stability enhancement control (TSEC) scheme is combined with a PSSC-based VSC, which can efficiently eliminate the risk of losing synchronization in the arbitrary SCR. Finally, experimental results are provided to confirm the theoretical analysis.

Grid-Impedance-Based Transient Current Control for Offshore Wind Turbines under Low-Voltage Fault

In order to enhance the transient stability of offshore wind turbines (OWTs) in marine energy systems, the grid codes stipulate that OWTs should possess the low-voltage ride-through (LVRT) ability of being grid-tied and injecting reactive current during grid fault. However, the grid-side converter (GSC) of OWTs may lose stability under weak grid or severe fault conditions due to inaccurate current references. To address this issue, a novel transient current control method is proposed to improve the transient stability of permanent-magnet-synchronous-generator (PMSG)-based OWTs. The feature of DC-link overvoltage is investigated and is alleviated by utilizing the GSC’s overcurrent capacity and chopper. Additionally, the equivalent circuit of the PMSG-based OWT connected to the onshore grid is derived based on Thevenin’s theorem. The feasible current region (FCR) is then determined, taking into account the GSC capacity, pre-fault power ability, LVRT requirement, and synchronization stability. Furthermore, a grid-impedance-based transient current control method is designed to enhance the fault ride-through performance and mitigate power oscillation of the OWT under various transient grid impedance and fault conditions. Finally, a simulation model is conducted using PSCAD v4.6.3 software to validate the effectiveness of the proposed method.

Evaluation of LVRT capability and stability analysis of VSC based advanced control approach for grid connected PV system under grid fault conditions

Short circuit faults are a prevalent issue in power systems, causing disruptions to the grid's normal operation. Dynamic behaviours of the conventional power systems during short circuit faults have been extensively studied and understood. The bulk of ongoing research and development are focusing on the dynamic performance of grid-connected renewable energy systems under these fault conditions, due to changes in the grid code and a decrease in system inertia. The development of effective control strategies to enhance the system's reliability during fault conditions is of paramount importance. In this paper, a two-stages grid-connected photovoltaic system (GCPV) having a rated power of 2 MW was created in the MATLAB/Simulink environment. The dynamic behaviour of the presented system was evaluated in two scenarios: steady state conditions and short circuit faults. A line-to-ground short circuit fault was created at the grid side, and its effect on the PV system's operation was observed. An advanced control system was designed to maintain stability during fault conditions. The results demonstrated the efficiency of the designated control system in minimizing the effects of short circuit faults on the GCPV system's function, and restoring the system promptly after the fault was cleared. Furthermore, considering modifications in grid regulations, the low voltage ride through (LVRT) capability of the designed system was analysed and validated according to the UK standards. The Total Harmonic Distortion (THD) level at the common coupling point was also analysed for voltage and current, remaining below the acceptable level of 5% as specified in the IEEE Std. 519.

Large-Signal Stability Analysis Using Takagi-Sugeno Fuzzy Model Theory for Fractional Frequency Transmission System Under Grid Faults

The rapid development of renewable energy requires transmission technology innovation. Fractional frequency transmission system (FFTS) is a promising solution for large-scale renewable energy transmission, and the stability analysis is essential for engineering application. Especially for the grid faults, these large-signal disturbances are inevitable during operation. Thus, the system-level Takagi-Sugeno fuzzy model of FFTS is proposed considering the system parameters, control strategy of modular multilevel matrix converter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$M^{3}C$</tex-math></inline-formula> ) and phase locked loop in this study. Subsequently, the Lyapunov function and the large-signal stability domain of attraction (LS-DOA) are calculated expeditiously based on the proposed fuzzy model. Ultimately, the large-signal stability of FFTS is comprehensively analyzed combining with the different-frequency interaction features under grid fault conditions. Compared with the conventional method, the stability strength and affine angle indexes are proposed for quantitative analysis mathematically. The obtained influence law of the system parameters can guide the parameter optimization of FFTS effectively. The effectiveness of proposed methods is verified by the simulation-based study.

Operation and Hierarchical Coordination Control of Integrated Hybrid Distribution Transformer under Grid Fault Conditions

Operation and Hierarchical Coordination Control of Integrated Hybrid Distribution Transformer under Grid Fault Conditions

Fault Current Multi-Stages Calculation Method for DFIG-Based Wind Farms with Whole Fault Process Attributes under Asymmetrical Grid Fault Conditions

Fault Current Multi-Stages Calculation Method for DFIG-Based Wind Farms with Whole Fault Process Attributes under Asymmetrical Grid Fault Conditions

Enhanced DC-link voltage control of PMSG-based wind turbine generators by machine-side converter during asymmetrical grid faults

Under asymmetrical grid fault conditions, oscillating active power can occur on the grid-side converter (GSC) of permanent magnet synchronous generator (PMSG)-based wind turbine generators. The oscillating active power flows into the DC-link, causing voltage oscillations that can lead to a shortened lifetime of DC-link capacitors and additional harmonics injected into the grid. While conventional methods can mitigate the oscillating active power through negative sequence current injection by the GSC, this hinders the grid support of the GSC regarding negative sequence reactive current during grid fault. To address this issue, a novel control scheme is proposed that uses the machine-side converter (MSC) to transfer the oscillating power to the motor. Firstly, the active power capacity of the MSC is analyzed and compared with that of GSC, and the results show that MSC has enough capacity to accommodate the oscillating active power. On this basis, a novel MSC control scheme is proposed to transfer the oscillating active power on the DC-link to the motor, which has a much larger inertia to buffer the power oscillation. Compared with conventional methods, the proposed control scheme relieves the GSC from mitigating the power oscillation to support the grid. Simulation studies using DIgSILENT PowerFactory are presented to verify the effectiveness of the proposed control scheme for PMSG-based wind turbine generators.

Fault Ride-Through for PMVG-Based Wind Turbine System Using Coordinated Active and Reactive Power Control Strategy

This study investigates a fault ride-through capability for grid-connected permanent magnet vernier generator (PMVG)-based wind turbine system (WTS) using coordinated active and reactive power control strategy. To do this, the generator peak current and speed limitation schemes are proposed for the machine-side converter to satisfy grid code requirements during grid fault conditions. At the same time, the reactive power control is implemented on the grid-side converter to meet the reactive current requirement of the grid code. And then, to suppress the DC-link power unbalance during grid fault conditions, an efficient variable exponential reaching law-based sliding mode control is designed. Further, the coordinated control schemes are employed to ensure the stable DC-link voltage and maintain the converter&#x0027;s maximum peak current during grid faults. Finally, the proposed control methods demonstrate their superiority through simulations on 5-kW PMVG, 1.5 MW PMSG-based WTS, and its application potential is shown through the experimental prototype of a 5-kW PMVG-based WTS.

Stabilizing dc-link voltage and neutral point current fluctuation in grid-connected three-level NPC converter under generalized grid fault conditions

Stabilizing dc-link voltage and neutral point current fluctuation in grid-connected three-level NPC converter under generalized grid fault conditions

Analysis of Voltage Dip Impact on Doubly Fed Induction Generator under Dynamic Conditions

Doubly Fed Induction Generator (DFIG) is the most widely employed generator in the Wind Energy Conversion Systems (WECS) for the production of electricity. However, despite all of its various advantages, it is extremely vulnerable to grid faults such as voltage dip since its stator is directly coupled to the grid. A voltage dip problem is one the main issues among the power quality concerns. This fault causes the flow of excessive current across both the stator and the rotor terminals, which may lead to serious damage to the generator, power converters, and DC Link capacitor. On the other hand, the current Grid Codes (GC) requires the system to stay connected to the grid during this fault condition and support it in healing its nominal voltage. This capacity of the system is known as the Low Voltage Ride Through (LVRT) capacity. For the system to achieve such capacity, appropriate protection mechanisms or controlling strategies must be utilized. Therefore, in this paper, the crowbar protection technique, PI controller, and the Adaptive Neuro-Fuzzy Inference System (ANFIS) controller are employed. Furthermore, the performance of the system employing PI, crowbar, and ANFIS is analyzed and compared under grid fault conditions, i.e., a voltage dip with a magnitude of 0.1 pu (worst case) using MATLAB/Simulink software and based on actual data obtained from Adama II wind farm. The obtained results unveil that the settling time of ANFIS for controlling the rotor currents in d and q axes (idr and iqr) and DC link voltage is 3.6 s, 3.57 s, and 3.4 s, respectively. On the other hand, the settling times of the PI controller for controlling the rotor currents in d and q axes and the DC link voltage are found to be 4 s, 3.91 s, and 45.2 s, respectively, while the crowbar protection technique’s settling times are found to be 4 s, 6 s, and 4.9 s, respectively. It is evident from the aforesaid results that the ANFIS controller provides the best performance of the three strategies since it allows both the rotor currents and the DC link voltage to return to their steady state values faster than the other two techniques, employed in this investigation.

An improved four‐layer capacitor voltage control strategy combined with arm current control for transformerless modular multilevel converters

AbstractIn order to improve the performance of the transformerless modular multilevel converters (MMC) under AC and DC grid fault conditions, this paper proposes an improved four‐layer capacitor voltage control strategy combined with arm current control. The proposed control strategy consists of overall submodule capacitor voltage averaging control, leg‐balancing control with flexible weight factor W, arm‐balancing control with zero‐sequence suppression, and inner‐arm individual balancing control. The new concept of weight factor W for leg‐balancing control is also originated, considering negative‐sequence current and DC circulating current jointly from power point of view. The proposed approach effectively realizes the capacitor voltage balance, ac‐side current tracking, and unwanted circulating current elimination. The practical implementation of the proposed control strategy and the experimental results from a downscale three‐phase transformerless MMC prototype are presented to validate the performance of the proposed four‐layer capacitor voltage control strategy under grid faults.

An Adaptive Fault Ride-Through Scheme for Grid-Forming Inverters Under Asymmetrical Grid Faults

Three-phase four-wire grid-forming (GFM) inverters are promising to interface distributed energy resources into low-voltage networks. However, these inverters are prone to overcurrent under grid faults. Physically increasing the inverter current capacity is not cost-effective to cope with complicated fault conditions. In this article, an adaptive fault ride-through (FRT) scheme based on instantaneous saturators and virtual negative- and zero-sequence resistances is proposed. It features not only overcurrent limitation by modifying voltage references, but also seamless transition between normal and grid fault conditions. The proposed FRT scheme is first analyzed from different aspects, including the virtual sequence resistances, grid short-circuit ratio, fault types, and fault levels. The virtual sequence resistances are then designed to be adaptive to ensure high voltage quality at the healthy phase. The proposed FRT scheme is verified by MATLAB/Simulink simulations under asymmetrical faults. A laboratory platform with a grid-connected 3kW GFM inverter is further constructed to demonstrate its effectiveness (a video of the experimental results under three asymmetrical faults is attached).

R-SFCL and RRCOCS-TVC Based Cooperative Control for Enhancing LVRT Capability of DFIG System

For the extraction of wind energy through a doubly-fed induction generator (DFIG), low voltage ride through (LVRT) is an essential technical requirement specified by the transmission system operator (TSO). Under a grid fault condition, DFIG should remain in connection with the grid for a certain minimum period and offer reactive power support as required by the TSO. A cooperative control scheme consisting of hardware solution through a superconducting resistance type fault current limiter (R-SFCL) and software solution constructed on the rotor reference current orientation control strategy (RRCOCS) with transient voltage control (TVC), is proposed in this paper to address the LVRT requirement. In the proposed control strategy, RRCOCS will limit the rotor current directly during a fault condition. The reactive power needs to be generated during fault to maintain grid code which is achieved through TVC. At the same time, improvement of stator terminal voltage, as well as suppression of stator current, is achieved by R-SFCL. The suppression of stator current by R-SFCL is also transformed to the rotor side aiding the rotor current limiting. The proposed cooperative scheme’s performance is simulated and tested on a 9 MW grid-connected DFIG wind system. The results obtained by the proposed strategy are compared with RRCOCS and RRCOCS-TVC.&#x0D; HIGHLIGHTS&#x0D; &#x0D; Independent software and hardware LVRT methods fail under severe faults and strict grid codes&#x0D; Software methods are challenged by current suppression and voltage support limits while the hardware methods involve high costs and undergo high thermal stress&#x0D; A cooperative LVRT can overcome the challenges and limitations associated with these independently deployed methods. The proposed R-SFCL device, in cooperation with RRCOCS-TVC, successfully meets the grid codes, and the transient behavior of the system's electrical and mechanical variables gets improved&#x0D; The proposed scheme helps reduce R-SFCL resistance, reduces its thermal stress, brings cost reduction in R-SFCL design, and improves its recovery characteristics. The listed improvement in R-SFCL can prevent the system from overcompensation and the adverse effects of recurring faults&#x0D; &#x0D; GRAPHICAL ABSTRACT

A novel efficient adaptive-neuro fuzzy inference system control based smart grid to enhance power quality

&lt;span&gt;A novel adaptive-neuro fuzzy inference system (ANFIS) control algorithm-based smart grid to solve power quality issues is investigated in this paper. To improve the steady-state and transient response of the solar-wind and grid integrated system proposed ANFIS controller works very well. Fuzzy maximum power point tracking (MPPT) algorithm-based DC-DC converters are utilized to extract maximum power from solar. A permanent magnet synchronous generator (PMSG) is employed to get maximum power from wind. To maximize both power generations, back-to-back voltage source converters (VSC) are operated with an intelligent ANFIS controller. Optimal power converters are adopted this proposed methodology and improved the overall performance of the system to an acceptable limit. The simulation results are obtained for a different mode of smart grid and non-linear fault conditions and the proven proposed control algorithm works well.&lt;/span&gt;

Overcurrent Relays Coordination Considering Transient Stability Study of Induction Generator Using a Method Based on Terminal Voltage

Nowadays, with increasing penetration of Induction Generators (IG) in the electricity grid, it is of great importance that IG be connected to the grid in the fault conditions. In this paper, a terminal voltage based method for transient stability study of IG is proposed. For this purpose, an improved analytical method is used to determine Critical Clearing Time (CCT) of IG in the state of grid fault, including near and non-near faults. For the faults occurring far from the terminal of generator, electromagnetic torque is not equal to zero during fault. Hence, equivalent circuit model of IG and network is used to calculate the CCT. A new formulation based on terminal voltage of IG is presented to consider the stability constraint on Over Current Relay (OCR) coordination, to prevent disconnecting of IG in grid fault condition. Simulations are carried out on two samples network, and the results demonstrate the efficiency of proposed method.

Enhancement of Line-to-Line Voltage Support During Asymmetrical Microgrid Faults Using a Four-Leg Three-Level Inverter

Recent research is focused not only to keep a three-phase inverter connected to the grid but also to support the voltage at the point of common coupling (PCC) during grid faults. The line-to-line PCC voltage can be enhanced by increasing the amplitude of positive and negative sequence current supplied to the grid for a given inverter rating. 3-leg inverters can supply only positive and negative sequence current to the grid. However, 4-leg inverters have the ability to supply zero-sequence current along with positive and negative sequence currents. This extra degree of freedom of control remains un-utilized during low voltage ride through operation. This paper presents a novel method to select the amplitude and phase angle of the zero-sequence current in such a way that the peak current amplitude of the 4-leg inverter remains minimum. Hence, it is possible to supply more positive and negative sequence currents to the grid. The proposed strategy can be applied to any asymmetrical grid fault condition. Experimental verification for two specific fault conditions has been carried out and results are presented.

Transient stability improvement of wave energy conversion systems connected to power grid using anti-windup-coot optimization strategy

This paper introduces an enhancement to the transient stability of a wave energy conversion system (WECS) by using the coot optimization algorithm (COA) combined with an anti-windup method. This combination helps in elimination of the windup issue in the integral term of the proportional-integral (PI) controller during power system faults leading to a significant enhancement for the transient stability of the WECS connected to the grid. The COA is utilized to select the PI controller parameters and the anti-windup method back-calculation coefficients. The WECS converts the linear vertical motion into electrical energy by using an Archimedes wave swing device connected to a linear synchronous generator. Minimization of the generator's stator losses and maximization of the generator's real power is accomplished by the utilization of a generator-side converter (GSC). Also, both the point of common coupling voltage and the capacitor link voltage are set at their reference values by utilizing a grid-side inverter (GSI). An optimal design for the PI controllers in both converters is achieved by direct application of the COA to the MATLAB/Simulink model. A comparison is made among the results obtained by the COA and those obtained by other recent optimization algorithms under different grid fault conditions.

Coordinated Low Voltage Ride-Through of MMC-HVDC Transmission System and Wind Farm With Distributed Braking Resistors

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission is becoming a trend in offshore wind-farm integration. However, the large DC-side energy dissipation equipment, which is utilized to dissipate surplus wind power under grid fault conditions, will largely increase the cost of MMC-HVDC systems. To reduce the cost, a novel low voltage ride-through (LVRT) strategy is proposed in this paper. When a grid fault occurs and the DC voltage exceeds the limit, the sending end converter is controlled to reduce the AC voltage of the wind farm. The LVRT of the wind generators will be activated, and the output active power of the wind farm is reduced. With this coordination, the DC-side energy dissipation equipment only needs to dissipate surplus power in the early stage of the grid fault before the output active power of the wind farm drops. Therefore, the heat generated by braking resistors can be significantly reduced. On this basis, the braking resistors can be distributed into the submodules of the receiving end converter (REC) station. The LVRT problem can be solved without building an individual energy dissipation station. Using the proposed coordination strategy, the construction cost of the MMC-HVDC system with offshore wind farm integration can be significantly reduced.

Current model predictive fault-tolerant control for grid-connected photovoltaic system

&lt;abstract&gt;&lt;p&gt;This paper investigates the performance of the current model predictive control (CMPC) for controlling a two-stage transformerless grid-connected photovoltaic (PV) system under grid fault conditions. A maximum power point tracking (MPPT) controller was used to extract the maximum power of the PV panel. To stabilize the DC link and generate the reference current values, a proportional-integral (PI) controller was used. The CMPC strategy was implemented to control the output current of the inverter that connects the PV system to the utility grid. The system and control strategy were simulated via a MATLAB/Simulink environment. The performance of the proposed control strategy was investigated under fault conditions between the three-phase two-level inverter and the grid. Moreover, to validate the capability of the CMPC, comparative case studies were conducted between CMPC, PI, and sliding mode control (SMC) under grid fault. Case studies' results showed that under grid fault, CMPC did not introduce any overshoot or undershoot in the PV output DC current and power. However, PI and SMC produced undershoots of almost 15 kW for the output power and 45 A for the output current. Under the fault conditions, the active output power and three-phase current recovery time of the inverter was 50 ms using CMPC, compared to PI and SMC with recovery times of 80 ms and 60 ms, respectively. Moreover, a voltage dip of 75 V at the DC link voltage was recorded with CMPC under faulty conditions, while the voltage dips for PI and SMC were around 180 V.&lt;/p&gt;&lt;/abstract&gt;

Doubly Fed Induction Generator Low Voltage Ride Through Improvement Through Modular Multilevel Converter

This paper proposes a novel fault current suppression method for doubly fed induction generator (DFIG) low voltage ride through (LVRT) improvement employing modular multilevel converter (MMC). It is based on the inherent MMC arm impedance and the number of levels to provide transient damping of DFIG rotor and stator currents under severe grid faults, contributing effectively to maintaining the DFIG connection while ensuring controllability by avoiding the protection activation. Neither additional hardware nor control loops are necessary. DFIG dynamic equations under LVRT, including the contribution of the MMC, are presented, and a model is derived for control design. Performance assessment, including symmetrical and asymmetrical fault scenarios for MMC with 3, 21, 51, and 101 levels and comparison with existing and proposed controllers, highlighted that arm impedance and increased MMC level number contributed to DFIG stator and rotor fault current suppression, providing DFIG LVRT improvement under severe grid fault conditions.