Low-voltage Ride-through Conditions Research Articles

Performance Analysis of Photovoltaic and Wind Turbine Grid-Connected Systems under LVRT Conditions

The integration of grid-connected renewable energy systems has gained significant attention and introduces several challenges and considerations. One of these challenges is ensuring the reliable and stable operation of these systems under various grid conditions. For example, faults at the grid could lead problems such as DC-link over-voltage and AC over-current that may cause disconnection or damage to inverter. This paper presents a comprehensive analysis of the performance of photovoltaic (PV) and doubly-fed induction generator (DFIG) wind turbine grid-connected systems under low voltage ride-through (LVRT) conditions. The study aims to investigate their behavior, and stability during LVRT events and provide insights for enhancing their grid integration capabilities. The PV and DFIG systems are modelled and simulated using MATLAB Simulink under three difference conditions, with and without using reactive current injection and DC chopper circuit. Various performance parameters, including grid voltage, grid current, DC-link voltage, active power, and reactive power, are analyzed to assess the system's behavior and compare their responses. The principal results reveal distinct performance characteristics of the PV and DFIG systems. The PV system shows higher overshoot currents, over-voltage, and significant drops in active power during fault occurrences, while the DFIG system exhibits lower overshoot currents and better stability in the DC-link voltage. Reactive power responses differ between the systems, with the PV system demonstrating a higher capability for support. The implementation of DC chopper shows more effective in the reduction of DC-link voltage and overshoot grid current in the PV system compared to the DFIG system.

Signature POWIs for DFIG under LVRT conditions: Analysis, experiments and recommendations

As one of the important characteristics of voltage sags, the point on wave of initiation (POWI) has been widely studied in its influence on sag sensitive devices. However, it is less mentioned in the research and test standards related to the low voltage ride through (LVRT) of doubly fed inductor generators (DFIGs). To fill this gap, a comprehensive study for the influence of POWIs together with the phase angle jump (PAJ) is carried out in this paper. Taking rotor current as the evaluation index, mechanism analysis and experimental verifications are conducted under different asymmetrical sags. The results demonstrate that POWIs affect the LVRT response through the transient dc stator flux and rotor current. The two POWIs with the greatest and lightest impact within half a cycle are defined as the signature POWIs, which depends on the sag type, sag magnitude and PAJ. Analysis under the signature POWIs, especially the most severe POWI, can more accurately evaluate the LVRT response of DFIGs. Besides, the PAJ will have a significant detrimental impact together with the POWI, manifested as the increase of the negative ac and transient dc rotor current. Suggestions for improving the LVRT testing standard are further put forward, that performing tests within the range of ± 15° around the most severe POWI.

Short-circuit modeling of three-phase 4-wire unbalanced networks in presence of single-phase photovoltaic systems

Due to the growing diffusion of single-phase Photovoltaic Systems (PVSs), the analysis of active unbalanced three-phase 4-wire distribution networks in short-circuit operating conditions is nowadays of great interest. However, this analysis is not trivial due to various factors (e.g., environmental conditions, inverter control system, limits of the inverter current, location and type of the fault, PVS self-protection systems) influencing the contribution of PVSs to the short-circuit conditions. Among these factors, the greatest challenge is to model the complex behavior of the inverter control system that must comply with actual and future grid requirements, or with national and international standards, in both normal and abnormal operating conditions.An analytical model based on the phase-coordinates approach is proposed in this paper to evaluate the short-circuit contributions of single-phase PVSs connected to unbalanced three-phase 4-wire LV distribution networks. The model of the PVSs takes into account the main factors that influence the PVS contributions, with particular attention to the model of the control system which injects reactive power to support grid voltage in normal operating conditions and to perform low-voltage ride-through (LVRT) dynamic grid support during the fault.In this paper, control systems for next-generation single-phase PVSs able to satisfy grid code requirements are explored; they include constant average active power control, constant active current control and constant peak current control. Extensive numerical experiments are presented with reference to the Cigrè European LV Distribution Network Benchmark, considering different fault types, effects of various control strategies and the evaluation of the impact of the PVSs in LVRT conditions w.r.t. the passive network, i.e., the network under fault with no installed PVSs.

Dynamic Low Voltage Ride through Detection and Mitigation in Brushless Doubly Fed Induction Generators

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These machines are composed of two back-to-back voltage source converters: the grid side converter and the rotor side converter. The rotor side converter is typically used for reactive current control of the power winding using the control winding current. A low voltage ride through (LVRT) fault is detected using a hysterisis comparison of the power winding voltage. This approach leads to two problems, firstly, the use of only voltage to detect faults results in erroneous or slow response, and secondly, sub-optimal control of voltage drop because of static reference values for reactive current compensation. This paper solves these problems by using an analytical model of the voltage drop caused by a short circuit. Moreover, using a fuzzy logic controller, the proposed technique employs the voltage frequency in addition to the power winding voltage magnitude to detect LVRT conditions. The analytical model helps in reducing the power winding voltage drop while the fuzzy logic controller leads to better and faster detection of faults, leading to an overall faster response of the system. Simulations in Matlab/Simulink show that the proposed technique can reduce the voltage drop by up to 0.12 p.u. and result in significantly lower transients in the power winding voltage as compared to existing techniques.

Topology and Control Strategy of PV MVDC Grid-Connected Converter with LVRT Capability

This paper proposes an isolated buck-boost topology and control strategy for the photovoltaic (PV) medium-voltage DC (MVDC) converter with low-voltage ride through (LVRT) capability. The proposed isolated buck-boost topology operates on either boost or buck mode by only controlling the active semiconductors on the low-voltage side. Based on this topology, medium-voltage (MV) dc–dc module is able to be developed to reduce the number of modules and increase the power density in the converter, which corresponds to the first contribution. As another contribution, a LVRT method based on an LC filter for MVDC converter is proposed without additional circuit and a feedback capacitor current control method for the isolated buck-boost converter is proposed to solve the instability problem caused by the resonance spike of the LC filter. Five kV/50 kW SiC-based dc–dc modules and ±10 kV/200 kW PV MVDC converters were developed. Experiments of the converter for MVDC system in the normal and LVRT conditions are presented. The experimental results verify the effectiveness of the proposed topology and control strategy.

A Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems

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

Impact of Phase Angle Jump on DFIG Under LVRT Conditions: Challenges and Recommendations

The increased penetration of wind power has made its low voltage ride through (LVRT) capability more crucial than ever. Currently, the LVRT performance of wind generators is verified using experimental voltage sags with different magnitudes and durations. The actual fault, however, also causes the phase angle jump (PAJ) of the grid voltage. In this paper, the impact of PAJ on the LVRT capability of doubly fed inductor generator (DFIG) under three-phase fault is analyzed. To achieve this, detailed analytical calculations and experimental verifications are conducted. The results demonstrate that the PAJ imposes more challenges on the LVRT by intensifying the rotor overcurrent through the natural stator flux. As a result, DFIGs that have passed the existing LVRT test may still fail to comply with the grid code requirements in practice. To tackle this issue, an improved test procedure is recommended. The new procedure considers the impedance angle of the target system as an additional test condition, thus can reveal the LVRT performance of DFIG under the potential PAJ.

동적 무효전력 예비력 지수를 이용한 풍력 발전 단지의 전압 안정성 향상 방안

To increase the penetration of wind turbine (WT) generators in the power system, grid-integration standards have been needed for the stable integration of a large-scale system connected wind turbine generators. Especially, the low voltage ride through (LVRT) function has been emphasized, as it relates to the voltage-reactive power control of the wind turbine generator. In addition, it is important to accurately determine the voltage stability of the system for fast voltage recovery in the system contingency. Therefore, this paper proposes the improved method using dynamic reactive power reserves (DRPR) among wind turbine generators in order to determine the system stability and satisfy the LVRT condition in large-scale system connected wind turbine generators. To prove the effectiveness of the proposed method, the simulation results demonstrate dynamic voltage recovery according to the LVRT conditions.

Dynamic Performance Improving Sliding-Mode Control-Based Feedback Linearization for PV System Under LVRT Condition

The low-voltage ride through condition (LVRT) attracts wider attention with the increased power of central grid-connected photovoltaic (PV) systems. The conventional double-loop-based control strategy designed for the nominal voltage is slow and potentially unstable under the LVRT operation. In this article, a novel sliding-mode control-based feedback linearization is proposed to improve response speed of the PV system under LVRT condition. To increase the robustness of the control to parameter variations, sliding-mode control is combined with the feedback linearization in this article. Further, the complete linearization of the PV system can be achieved over the whole operating range. Moreover, the dc-link voltage operating point of the PV panel can be dynamically adjusted based on the drop depth of the grid voltage, which will reduce the load changes during the LVRT and subsequently ensure the ac current levels to stay within the design limits in any fault situation and enhance the LVRT capability of the central grid-connected PV system. Finally, a 100-kW PV inverter is built to validate the feasibility and effectiveness of the proposed strategy for LVRT.

Enhancement of the Dynamic Behavior of Grid-Connected DFIG Based Wind Turbines by using Fuzzy Logic Controller under LVRT Conditions

In recent years, due to the interconnection of large capacity wind turbines to the power grid lead, there are serious issues in the stability of Grid and generation of electrical power. Also, it is showing effect on the dynamic performance of the electrical power systems. To maintain stability during sudden changes in the grid, the LVRT (Low Voltage Ride Through) capability of the Wind Turbines is one of the prime requirements. Wind turbines attached to DFIG (Doubly Fed Induction Generators) are advantageous which have LVRT capability at limited extent. In this paper, the elaborated discussion of the LVRT of Wind turbines shafted to DFIG's in the Grid. It also presents the complete description of the sudden changes in the systems like transient characteristics and the Doubly Fed Induction Generators dynamic response at the time of grid voltage faults (Symmetrical and Asymmetrical). The latest rotor side control technology is displayed in this paper for DFIG and wind turbines with improved capacity of low voltage ride through at the time of severe grid voltage sags. A Fuzzy Logic controller-based control technology is introduced in this paper which performs the balancing the rotor-side voltage and short circuits during the disturbances in the Grid. The advantage in this proposed control scheme is that it reduces the additional cost and reliability issues. So, the DFIG is efficient and usability company norms are satisfied with the proposed Fuzzy logic controller compared to regular controller like PI controller. The performance of the proposed system is simulated and verified in the computer. The results are displayed and it conclude that the control strategy of LVRT capability for Grid connected DFIG based wind turbine systems with Fuzzy Logic Controller are more effective than the conventional control Methods.

Enhanced Active Power Balancing Capability of Grid-Connected Solar PV Fed Cascaded H-Bridge Converter

Cascaded H-bridge converter is a promising solution for the next generation large-scale photovoltaic (PV) system having both active and reactive power exchange capacity. However, one challenge with this converter is unbalanced power generation among the clusters or cells due to variation in radiation level, temperature, dusting on PV cells etc. In order to inject balanced power to the grid, in this paper, a controlled exchange of reactive power is proposed so that the converter can work with unbalanced power generation within the clusters. The magnitude of reactive power exchange is derived and necessary conditions to prevent overmodulation are discussed. In addition, the magnitude of zero voltage is derived taking into account both active and reactive power exchange with the grid. By exchanging reactive power, the converter can also be used during low-voltage ride-through condition. This paper introduces a novel representation of unbalanced power generation using the barycentric coordinate system which helps to identify regions in the power plane which require the exchange of reactive power. The proposed method is verified through computer simulation and in a laboratory prototype.

A Hybrid Passivity-Based Control Strategy for Three-Level T-Type Inverter in LVRT Operation

The fast dynamic performance of a photovoltaic (PV) inverter is one of the most important issues in low-voltage ride-through (LVRT) operation, as the voltage dip period is short. Nevertheless, it is difficult to control the output currents with conventional methods, especially during transients or under low-power factor operation. In order to satisfy the LVRT requirements and improve the performances of a three-level T-type inverter (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I) in a PV system, the hybrid passivity-based control (HPBC) strategy with the fast transient response and strong robustness to system parameter changes and external disturbances is proposed in this paper. First, the Euler- Lagrange model of the 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I under the LVRT condition is derived and analyzed. Then, the 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I system is theoretically proved to be strictly passive by using some ingenious arithmetic and differential calculation techniques. Furthermore, based on the error energy function and the Lyapunov stability criterion, a passivity-based controller including a negative sequence of grid voltage feedforward control terms is designed to suppress the negative-sequence currents effectively. The balanced grid currents are obtained, which improve the stability of the system in LVRT operation. Compared with the conventional methods, the proposed HPBC scheme has the superior characteristics of fast and non-overshoot response in a transient state and accurate current tracking performance in a steady state. The theoretical analysis and effectiveness of the proposed strategy have been validated by means of both simulations and experiments.

Neutral-Point Voltage Analysis and Suppression for NPC Three-Level Photovoltaic Converter in LVRT Operation Under Imbalanced Grid Faults With Selective Hybrid SVPWM Strategy

Balanced issues of neutral-point (NP) voltage in the neutral-point-clamped (NPC) three-level photovoltaic converter have been studied in depth. Numerous algorithms for NP voltage control have been proposed and proven to be effective in normal operations. However, most of the previous studies fail to consider the effect of imbalanced loads on NP voltage, especially under low-voltage-ride-through (LVRT) condition, which faces problems of imbalanced grid voltages and low power factor. In this paper, through derivation of NP current model for different loading conditions, the behavior of NP voltage under LVRT condition is comprehensively analyzed for the first time. Then, theoretical limitations of NP voltage control based on two common space-vector-pulsewidth modulations (SVPWMs) that three-nearest-vectors and symmetric SVPWM are investigated, and additional low-frequency oscillation in NP voltage is analyzed. This oscillation is dominated by fundamental frequency and has different generation mechanisms under different imbalanced grid faults. Then, a novel method of selective hybrid SVPWM, as selective combinations of SVPWM and virtual SVPWM according to different imbalanced grid faults, is proposed to effectively suppress NP voltage oscillations in LVRT operation without any complex improvements and increase of dc-link capacitor values. Finally, the analytical result and performance of the proposed method are confirmed in an existing 300 kW NPC converter.

Detailed Investigation and Performance Improvement of the Dynamic Behavior of Grid-Connected DFIG-Based Wind Turbines Under LVRT Conditions

Power generation and grid stability have become key issues in the last decade. The high penetration of large capacity wind generation into the electric power grid has led to serious concerns about its influence on the dynamic behavior of power systems. The low-voltage ride-through (LVRT) capability of wind turbines during grid faults is one of the core requirements to ensure stability in the power grid during transient conditions. The doubly-fed induction generators (DFIGs) offer several advantages when utilized in wind turbines, but discussions about their LVRT capabilities are limited. This paper presents a comprehensive study of the LVRT of grid-connected DFIG-based wind turbines. It provides a detailed investigation of the transient characteristics and the dynamic behavior of DFIGs during symmetrical and asymmetrical grid voltage sags. A detailed theoretical study supported by computer simulations is provided. This paper also provides a new rotor-side control scheme for DFIG-based wind turbines to enhance its LVRT capability during severe grid voltage sags. The proposed control strategy focuses on mitigating the rotor-side voltage and current transients during abnormal grid conditions, without any additional cost or reliability issues. As a result, the DFIG performance is improved and utility company standards are fulfilled. Computer simulations are used to verify the expanded ride-through capability of the novel strategy and its effective performance compared to the conventional control schemes.

Fuzzy grey wolf optimization for controlled low-voltage ride-through conditions in grid-connected wind turbine with doubly fed induction generator

With its enormous environmental and monetary benefits, the wind turbine has become an acceptable alternative to the generation of electricity by fossil fuel or nuclear power plants. Research remains focused on improving the performance of wind turbines with maximum flexibility and gains. The main objective of the paper is to simulate a low-voltage ride-through (LVRT) control system that is convenient for the development of a controller that should have the ability to rectify fault signals. This paper proposes a novel method called grey wolf optimization with fuzzified error (GWFE) model to simulate the optimized control system. Further, it compares the GWFE-based LVRT system with the standard LVRT system, systems with minimum and maximum gain, and conventional methods like genetic algorithm (GA), differential evolution (DE), particle swarm optimization (PSO), ant bee colony (ABC), and grey wolf optimization (GWO) algorithms. Accordingly, it analyses the simulation results regarding qualitative analysis like active power, [Formula: see text] comparison, gain, pitch degree, reactive power, rotor current, stator current, and [Formula: see text] and [Formula: see text] measurements; and quantitative analysis like RMSE computation of [Formula: see text] with varying speed. Hence, the proposed GWFE algorithm is beneficial for simulating the LVRT system compared to other conventional methods.

Control Strategy of Three-Phase Photovoltaic Inverter under Low-Voltage Ride-Through Condition

The new energy promoting community has recently witnessed a surge of developments in photovoltaic power generation technologies. To fulfill the grid code requirement of photovoltaic inverter under low-voltage ride-through (LVRT) condition, by utilizing the asymmetry feature of grid voltage, this paper aims to control both restraining negative sequence current and reactive power fluctuation on grid side to maintain balanced output of inverter. Two mathematical inverter models of grid-connected inverter containing LCL grid-side filter under both symmetrical and asymmetric grid are proposed. PR controller method is put forward based on inverter model under asymmetric grid. To ensure the stable operation of the inverter, grid voltage feedforward method is introduced to restrain current shock at the moment of voltage drop. Stable grid-connected operation and LVRT ability at grid drop have been achieved via a combination of rapid positive and negative sequence component extraction of accurate grid voltage synchronizing signals. Simulation and experimental results have verified the superior effectiveness of our proposed control strategy.

Thermal optimised modulation methods of three-level neutral-point-clamped inverter for 10 MW wind turbines under low-voltage ride through

The three-level neutral-point-clamped (3L-NPC) converter is a promising multilevel topology for use in the megawatts wind power generation system. However, the requirements for the grid side inverter under low-voltage ride through (LVRT) of power grid could impose extreme stress to the switching devices in this converter topology. The study investigates the loss and thermal performances of a 10 MW 3L-NPC wind power inverter undergoing LVRT condition. A new space vector modulation method is proposed to optimise the thermal distribution of this extreme situation. It is concluded that, with the proposed modulation method, the thermal distribution in the 3L-NPC wind power inverter under LVRT becomes more equal and the junction temperature of the most stressed power device can be significantly reduced, whereas the control ability of DC-bus neutral point potential is not compromised.