Low-voltage Ride-through Operation Research Articles

Low-Voltage Ride-Through Operation of the SDBC Based STATCOM Integrated With One Single-Phase Converter Through Medium-Frequency Transformer

Low-Voltage Ride-Through Operation of the SDBC Based STATCOM Integrated With One Single-Phase Converter Through Medium-Frequency Transformer

Over-current low voltage ride-through operation of grid-connected converters based on thermal analysis

The grid-connected converters are becoming an important role in modern power grid. The grid codes have been proposed for the operation of grid-connected converters. The low voltage ride-through, namely one of the grid codes, is the operation of grid-connected converters under grid fault. The reactive power is generated by gird-connected converters to support grid under low voltage ride-through operation. However, the maximum reactive power is restricted by the capacity of converter, which is designed for long-term operation. Since the low voltage ride-through is a short-term operation, the maximum output reactive power can be enhanced. An over-current low voltage ride-through operation is proposed, and it is designed based on the maximum short-term capacity. The maximum short-term capacity is designed to ensure the junction temperature of semiconductor devices in the converter under the reliable operation, which is calculated based on thermal analysis. Furthermore, the different approaches to enhancing the maximum short-term capacity is analyzed.

Fault Ride Through approach for Grid-Connected Photovoltaic System

This paper presents a fault ride-through approach for grid-connected photovoltaic (PV) systems, aimed at improving the system's response during voltage sags and limiting the maximum inverter current during symmetrical faults. A constant active current reactive power injection approach was developed for low-voltage ride-through (LVRT) operation of grid-connected solar PV inverters in low voltage grids. The method manages the active and reactive power references and satisfies grid code requirements while also addressing tripping problems caused by overcurrent. The approach improves the speed response of the power converters during voltage sags, resulting in better dynamic grid support. The authors evaluated the approach using a 2-kW single-phase grid-connected PV system and demonstrated its ability to inject the required reactive power during a fault to improve the voltage profile and achieve dynamic grid support requirements. The accuracy of the proposed methodology is identified to be 98.3%, and the detection time is reduced by a reasonable margin in accordance with the grid standards. Overall, the fault ride-through approach can enhance the performance and reliability of grid-connected PV systems, particularly in low voltage grids.

Sequence extraction-based low voltage ride-through control of grid-connected renewable energy systems

Various faults can cause voltage sag in the power grid at different voltage levels across the network. Balanced or unbalanced voltage sags lead to grid instability by tripping off a large number of wind or solar power plants from the electric power network. This is particularly problematic to maintain the stability of renewable energy-rich converter-dominated modern power systems. To mitigate the adverse effects of voltage sag, grid-connected converters (GCCs) need to be capable of operating in self-healing and fault-tolerant mode by embedding low voltage ride-through (LVRT) capability into the control system of GCCs. In order to facilitate the implementation of LVRT capabilities for unbalanced faults, fast and accurate frequency-adaptive sequence extraction of grid voltages and currents is essential. This motivated the present work of making a systematic comparison of adaptive observer-based sequence extraction techniques to provide LVRT capabilities into the control system of GCCs. In order to show the effectiveness of each observer, various comparative analyses were performed through Matlab-based numerical simulation. Different observers were benchmarked by the dynamic performance improvement during the low-voltage fault period. Experimental results using a laboratory-scale prototype GCC show that adaptive observers are a suitable choice of sequence extractors for LVRT operation of grid-connected converters in unbalanced and distorted grids. The results obtained in this work will contribute to enhancing the stability of modern power systems that are getting more and more converter-dominated.

Cooperative LVRT control for protecting PMSG-based WTGs using battery energy storage system

Low-voltage ride-through (LVRT) capability is crucial for wind power plants that are grid-connected. A grid code requires wind farms to remain on-grid and inject a specific reactive current when a grid fault occurs. The requirements are satisfied by a cooperative control scheme of a battery energy storage system (BESS) and grid-side converter (GSC) for improving the LVRT capacity of wind turbine generators using permanent magnet synchronous generators. The proposed approach uses the BESS to absorb a power difference that appears in the back-to-back converters during the LVRT operation. Hence, the DC-link voltage is regulated by the BESS. In addition, the GSC control allows reactive current to be injected into the grid according to the grid code requirements. Finally, the wind turbine generators, which use the proposed approach, are built in MATLAB/Simulink. The theoretical analysis of the proposed method is validated by simulation.

A Multiobjective Control Strategy for Harmonic Current Mitigation With Enhanced LVRT Operation of a Grid-Tied PV System Without PLL Under Abnormal Grid Conditions

This article proposes a novel multiobjective control strategy (MOCS) for improving the performance of grid-tied photovoltaic (GTPV) system without phase-locked loop (PLL) operating under abnormal grid voltage conditions such as harmonically distorted voltages, grid faults, and weak grid. The proposed control strategy effectively mitigates the harmonics in grid injected current, eliminates the oscillations in grid injected power, and enhances the low-voltage ride-through (LVRT) capability simultaneously during abnormal grid conditions. The LVRT capability of the GTPV system is enhanced by incorporating an active power regulator (APR) with dynamic reactive power support (DRPS) controller in the proposed control strategy. The proposed MOCS is designed in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> stationary reference frame. Furthermore, the proposed MOCS consists of an advanced phase compensated multiresonant (APCMR) current controller and a phase-shifted reference current generator (PSRCG) integrated with the APR with DRPS controller. The APCMR suppresses the grid current harmonics under weak and distorted grid voltages and the PSRCG eliminates active and reactive power oscillations at the grid side caused by grid faults. The synchronization mechanism of the GTPV system is included in the PSRCG, and therefore, the requirement of PLL is avoided. Consequently, the associated computational burden on the digital controller is reduced. The effectiveness of the proposed control strategy is validated using the OPAL-RT digital simulator and offers superior performance compared to the conventional control strategy (COCS).

A Review of State Estimation Techniques for Grid-Connected PMSG-Based Wind Turbine Systems

The power system network grows yearly with a large number of nonlinear power generation systems. In this scenario, accurate modeling, control, and monitoring of interface systems and energy conversion systems are critical to the reliability and performance of the overall power system. In this trend, the permanent magnet synchronous generator (PMSG)-based wind turbine systems (WTS) equipped with a full-rated converter significantly contribute to the development of new and renewable energy generation. The various components and control systems involved in operating these systems introduce higher complexity, uncertainty, and highly nonlinear control challenges. To deal with this, state estimation remains an ideal and reliable procedure in the relevant control of the entire WTS. In essence, state estimation can be useful in control procedures, such as low-voltage ride-through operation, active power regulation, stator fault diagnosis, maximum power point tracking, and sensor faults, as it reduces the effects of noise and reveals all hidden variables. However, many advanced studies on state estimation of PMSG-based WTS deal with real-time information of operating variables through filters and observers, analysis, and summary of these strategies are still lacking. Therefore, this article aims to present a review of state-of-the-art estimation methods that facilitate advances in wind energy technology, recent power generation trends, and challenges in nonlinear modeling. This review article enables readers to understand the current trends in state estimation methods and related issues of designing control, filtering, and state observers. Finally, the conclusion of the review demonstrates the direction of future research.

SWRDFT Controller for Single Phase Grid Tied PV system with Low Voltage Ride through Capability

Voltage control and power quality (PQ) management are crucial aspects for the control of grid connected voltage source converters (VSC). In this paper, a novel control technique for solar photovoltaic (PV) tied single-phase grid is described based on a Sliding Window Recursive Discrete Fourier Transform (SWRDFT). This article considers a single-phase, double-staged PV interconnection. The major goal of control is to operate the VSC as per the standard grid codes and ensure quality power is delivered to the grid even under abnormal grid conditions. The system is designed to work at unity power factor during typical operation. Reactive power injection is mandated as per a country's grid code and helps to regulate the voltage profile and low voltage ride through (LVRT) during grid voltage fluctuations. Furthermore, the paper focuses on two important aspects viz. load compensation and LVRT operation carried out in the event of grid voltage sags. During night time operation, in the absence of PV, the VSC and dc link capacitor operate as a shunt compensator. The proposed controller completes both the tasks as demonstrated by simulation and hardware results. Additionally, a comparison study using state of the art fundamental extraction techniques based on Second / Third order generalized Integrator (SOGI/TOGI) is presented.

An event-oriented reliability assessment for DC link capacitors in grid connected inverter with low voltage ride through operation

Low voltage ride-through (LVRT) capability has been standard into the operating specifications of grid-connected inverters in many countries. Due to the mismatch of the time scale and lack of stress description model under the LVRT operation, the traditional mission profile reliability analysis tools are difficult to be applied for capacitors under LVRT operation. To overcome the above issue, this paper analysis the electrical and thermal stress of the DC link capacitors under LVRT operation, and established a lifetime model for the LVRT operation by the accelerated aging test. An extended mission profile and a nonlinear damage accumulation model have been proposed to evaluate the impact of the LVRT event on the reliability of DC link capacitors. A reliability assessment has been conducted for a 7.5 kW inverter which experienced one LVRT incident as an analytical example.

Coordination of adaptive virtual resistance controller and flux-coupling-type fault current limiter for LVRT fulfillment of virtual synchronous generator

The virtual synchronous generator (VSG) technology equips power inverters with virtual inertia and damping to access generation and storage more friendly. However, the VSG's weak performance to withstand fault current inrush and fulfill low-voltage ride-through (LVRT) operation should be carefully addressed. This paper presents an innovative method to coordinate adaptive virtual resistance controller (AVRC) and flux-coupling-type fault current limiter (FC-FCL). First, the theoretical modeling and transient characteristics of the VSG are illustrated. Then, the implementation of the coordinated AVRC and FC-FCL is done, with an elaboration of the coordination philosophy and the proposed approach's parameter design rules. Further, simulation validations are performed in MATLAB/Simulink. Different severities and types of faults are imitated, and the comparisons of the proposed method against the traditional VSG strategy, the single AVRC or FC-FCL, the inner current control, and the virtual voltage control are conducted. The results prove that: i) the proposed approach offers superior abilities to repress the fault current, alleviate the terminal voltage fluctuation and keep the VSG power balance. ii) the proposed approach can handle different severities and types of faults in the VSG. A satisfactory LVRT is obtained for the VSG with favorable frequency stability and power angle stability.

계통 연계형 3레벨 TNPC 인버터의 LVRT &amp; HVRT

With the increase in renewable power generation facilities, strict regulations on grid connection standards are being presented. The standards of Low Voltage Ride Through(LVRT) and High Voltage Ride Through(HVRT), which are representative grid connection standards to improve grid stability by maintaining grid connection in low and high voltage situations when a grid failure occurs, are gradually being subdivided. In this paper, the LVRT and HVRT control algorithms suggested in the transmission and distribution system connection regulations of KEPCO(Korea Electric Power Corporation) are applied to the grid-connected 3-level TNPC(T-Type Neutral Point Clamped) inverter. And in the VRT situation, the active and reactive current injection strategies were applied to the grid connected inverter algorithm. In addition, various simulation cases were analyzed, such as LVRT & HVRT conditions of transmission and distribution system. In the distribution system LVRT simulation, the grid voltage operated at 150ms and 1500ms, respectively, within 0.5p.u. and 0.75p.u. In the transmission system simulation, the LVRT operation and the HVRT operation were also simulated. By analyzing various simulation cases, the characteristics of the VRT cases was analyzed in detail and VRT control performance of the designed TNPC inverter was verified.

A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control

Low-voltage ride-through (LVRT) and grid support capability are becoming a necessity for grid-tied renewable energy sources to guarantee utility availability, quality and reliability. In this paper, a swap control scheme is proposed for grid-tied permanent magnet synchronous generator (PMSG) MW-level wind turbines. This scheme shifts system operation from maximum power point tracking (MPPT) mode to LVRT mode, during utility voltage sags. In this mode, the rectifier-boost machine-side converter overtakes DC-link voltage regulation independently of the grid-side converter. The latter attains grid synchronization by controlling active power injection into the grid to agree with grid current limits while supporting reactive power injection according to the sag depth. Thus grid code requirements are met and power converters safety is guaranteed. Moreover, the proposed approach uses the turbine-generator rotor inertia to store surplus energy during grid voltage dips; thus, there is no need for extra hardware storage devices. This proposed solution is applied on a converter topology featuring a minimal number of active switches, compared to the popular back-to-back converter topology. This adds to system compatibility, reducing its size, cost and switching losses. Simulation and experimental results are presented to validate the proposed approach during normal and LVRT operation.

Improved Particle Swarm Optimization Based Selective Harmonic Elimination and Neutral Point Balance Control for Three-Level Inverter in Low-Voltage Ride-Through Operation

In high-power applications, selective harmonic elimination pulsewidth modulation (SHEPWM) method is widely used to eliminate low-order harmonics in three-level inverters with low power losses. However, it faces the challenges that the conventional optimization methods for solving harmonic elimination equations in SHEPWM are easy to be trapped in local optimum. Moreover, the neutral point (NP) voltage of three-level inverter cannot be balanced under low-voltage ride-through (LVRT) condition with conventional SHEPWM method. To address these problems, a SHEPWM scheme is proposed, which is on the basis of an improved particle swarm optimization (IPSO) algorithm to solve the harmonic elimination equations and an enhanced NP balance control method in LVRT operation. First, an IPSO algorithm is proposed to calculate the switching angles of the transcendental equations for harmonic elimination. The IPSO algorithm defines a nonlinear negative exponential inertia weight considering the current and optimal fitness value, which can dynamically adjust the local and global searching speed and step size according to the actual situation, greatly improving the convergence speed and solution accuracy and avoiding falling into the local optimum. Then, by increasing the degree of freedom of the output current direction and combining with the NP voltage deviation, an enhanced NP voltage balance control method of SHEPWM in LVRT operation is proposed. The validity of the proposed method is proved by simulation and experimental results.

Dynamic Voltage Support for Low-Voltage Ride-Through Operation in Single-Phase Grid-Connected Photovoltaic Systems

This article presents a dynamic voltage support (DVS) scheme for achieving low-voltage ride-through (LVRT) with a grid-connected photovoltaic (PV) inverter during the voltage sag fault. The DVS scheme is achieved by formulating an additional reactive active current control mode which is developed from a conventional reactive current control approach. This provides stable operation of the system and achieves higher effectiveness due to the lower <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ratio at the point of common coupling in low-voltage networks. Further, the performance of the proposed controller is assessed by simulating a 4 kW system for voltage sag faults. Further, the proposed controller tested with the real-time simulations and experimental setup for voltage sag conditions. The results presented demonstrate that the active and reactive power is regulated in concordance with grid code requirements. The controller achieves LVRT within the time limits of grid standard during symmetrical faults, which makes it appropriate for fast transient events. This operates the PV system under its nominal capacity, avoiding unwanted grid disconnection events.

A Comprehensive Modulation Strategy for the Neutral-Point Clamped Wind Power Converter Thermal Management During Low-Voltage Ride Through

The temperature fluctuations in the semiconductor devices within neutral-point clamped (NPC) wind power converter during low-voltage ride-through operation would reduce the lifetime cycle of the converter and jeopardize the reliability of the wind power system. In this article, a thermal management strategy based on carrier-based modulation techniques, i.e., zero-sequence and dipolar injections, is proposed to address this issue. The temperature fluctuations in semiconductor devices among three phases can be reduced and balanced. Besides, the neutral-point voltage balancing can be guaranteed. Simulation and experimental results are presented to verify the feasibility and effectiveness of the proposed technique. Its influences on the converter total power loss and output current harmonics are discussed as well.

Low-Voltage Ride-Through Coordinated Control for PMSG Wind Turbines Using De-Loaded Operation

Low-voltage ride-through (LVRT) is important for the grid integration of wind power systems (WPSs). Each grid operator defines a grid code, including the LVRT requirement, considering their power system characteristics. During grid faults, LVRT is needed for the protection of WPS because the over current can induce damage to the power converter system. An important issue in the LVRT method of WPS is the energy capacity because sufficient reserve energy capacities of wind turbine inertia and an energy storage system (ESS) are needed for a successful LVRT operation. Although ESS can store excess energy of the WPS during a grid fault, it is physically limited considering its energy capacity. Moreover, WPS has a low reserve energy capacity, especially when the WPS operates in high wind speed conditions. Thus, we propose the deloaded operation of WPS for the preparation of LVRT by reducing the power generation of WPS. The effectiveness of the proposed method is validated by modeling a WT and an ESS topologically and performing simulations using the MATLAB/Simulink SimPowerSystems toolbox.

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.

Neutral-Point Voltage Oscillation Mitigation Scheme for Transformerless Three-Level PV Inverter in LVRT Operation With Selective Space Vector Modulation

In transformerless photovoltaic (PV) generation system, the neutral-point (NP) voltage balancing and the ground current reduction of three-level inverter are carefully regulated using a space vector modulation (SVM) scheme. Nevertheless, in low-voltage ride-through (LVRT) operation, the simultaneous reduction of NP voltage oscillation and common-mode voltage (CMV) has not been researched yet. To overcome these issues, a selective SVM (SSVM) strategy is proposed in this article. First, the NP voltage oscillation of three-level inverter in LVRT operation is analyzed in detail, which reveals that the NP voltage is affected by phase currents and duty cycles. Based on the location of output voltage and the direction of grid currents, the correct small vectors are selected under different power factor angles. Then, the small vector force (SVF) including the grid currents and duty cycle on NP voltage is proposed for the first time. According to the value of SVF, the optimal small vector having the most effective impact on NP voltage among the correct vectors and switching sequence is determined. With the proposed method, the NP voltage oscillation and CMV are reduced simultaneously in LVRT operation. Finally, the effectiveness of the proposed method is verified with experimental results.

Investigation of a Modified Flux-Coupling-Type SFCL for Low-Voltage Ride-Through Fulfillment of a Virtual Synchronous Generator

To solve the low-voltage ride-through (LVRT) issue of a virtual synchronous generator (VSG) under severe grid fault, our research group proposes to use a modified flux-coupling-type superconducting fault current limiter (SFCL). In this study, the configuration structure of the VSG with the SFCL at the point of common coupling (PCC) is designed. The fault analytical circuit of the VSG is presented, and the functions of the SFCL are stated from the aspects of current restraint, voltage compensation, and energy dissipation. Using MATLAB/Simulink, symmetrical and asymmetrical faults are both simulated to assess the transient behaviors of the VSG, and the SFCL parameters are changed to clarify the quantitative effects. From the comparison results, the SFCL parameters are properly determined to provide favorable performance. The SFCL can visibly decrease the fault current of each phase within the permissible level, and facilitate the VSG to have a preferable voltage profile. Furthermore, the SFCL can positively stabilize the VSG by enhancing the power delivery, mitigating the frequency fluctuation and consuming the excess energy. A satisfactory LVRT operation is gained, and the efficacy of the SFCL on reinforcing the VSG robustness is validated.

Coordinated LVRT Control for a Permanent Magnet Synchronous Generator Wind Turbine with Energy Storage System

This study introduces a coordinated low-voltage ride through (LVRT) control method for permanent magnet synchronous generator (PMSG) wind turbines (WT) interconnected with an energy storage system (ESS). In the proposed method, both the WT pitch and power converters are controlled to enhance the LVRT response. Moreover, the ESS helps in regulating the dc link voltage during a grid fault. Previous LVRT methods can be categorized into strategies with or without an additional device for the LVRT. The latter scheme is advantageous from the perspective of no additional installation cost; in this case, pitch and converter controllers are used. Meanwhile, the former method uses an additional device for LVRT operation and hence, involves additional expense. However, it can effectively enhance the LVRT response by reducing the LVRT burden on the WT. Moreover, the additional device can be used for various WT power control applications and it is common that the ESS is interconnected to the WT for multiple objectives. Previous studies focused on these two aspects separately; hence, a method of coordinated control for an ESS and a WT is needed as more ESSs are required to connect to WTs for flexible wind power operation. The proposed method introduces a control method with different LVRT modes considering the ESS state of charge (SoC). When the WT does not have a sufficient inertial response operation range, the ESS reserve energy capacity is required for LVRT operation. This coordinated LVRT method employs both the WT and ESS controls when it is hard to handle the LVRT using the WT control alone at high wind speeds. In this case, power curve analysis is used to obtain the appropriate power reference during the fault period. In addition, a power reference is also used to ensure a safe operation. Using the proposed method, an ESS can be operated in a manner that is appropriate for WT operation, especially at high wind speeds. To validate the effectiveness of the proposed method, we considered two case studies. One study compares the LVRT response between the WT itself and the proposed method. The other research work compares the response of the conventional LVRT method that uses a WT and an ESS and with that of the proposed method. From these case studies, we concluded that the proposed method achieved a better performance while operating within the constraints of the WT rotor speed and ESS SoC limits.