Voltage Ride-through Capability Research Articles

Peak current control of grid-feeding inverter with low voltage ride-through capability

Peak current control of grid-feeding inverter with low voltage ride-through capability

Enhanced low voltage ride-through control of multilevel flying capacitor inverter based wind generation

This paper introduces a cost-effective control method to enhance the low voltage ride-through (LVRT) capability and smooth the output power of a three-phase multilevel flying capacitor inverter (FCI) in wind turbine-based permanent magnet synchronous generator (PMSG). The proposed approach utilizes the energy storage capability of flying capacitors to mitigate wind power fluctuations and address short-duration outages and deep voltage sags. Additionally, a nonlinear controller based Lyapunov theory is developed to regulate capacitor voltages, improve power factors, and balance DC-link voltage. Numerical simulations are conducted in MATLAB/SimPower systems environment to validate the effectiveness of this comprehensive control strategy across different grid operation scenarios.

A novel low voltage ride-through scheme for DFIG based on the cooperation of hybrid energy storage system and crowbar circuit

To improve the low voltage ride-through (LVRT) capability of DFIG, a novel LVRT scheme based on the cooperation of hybrid energy storage system (HESS) and crowbar circuit is proposed. The HESS composed of superconducting magnetic energy storage (SMES) and batteries is connected in the DC-link bus of DFIG. During grid faults, the HESS is switched to stabilize the DC-link bus voltage of DFIG, the grid-side converter (GSC) of DFIG is switched to inject reactive power to the grid for improving the grid fault voltage and the crowbar circuit is activated in real-time to avoid RSC from being damaging according to the rotor current. During normal operation, the HESS is controlled to deal with power difference between the turbine generation power and power dispatching command, and the GSC of DFIG is controlled to manage the output power of the DFIG with HESS. Finally, the simulations of the DFIG with HESS system is conducted in RTDS platform, and the simulation results show that the proposed scheme can effectively achieve the LVRT of DFIG during gird faults and flexibly manage the DFIG output power with HESS during normal operation.

Analysis of low voltage ride-through capability and optimal control strategy of doubly-fed wind farms under symmetrical fault

Given the “carbon neutralization and carbon peak” policy, enhancing the low voltage ride-through (LVRT) capability of wind farms has become a current demand to ensure the safe and stable operation of power systems in the context of a possible severe threat of large-scale disconnection caused by wind farms. Currently, research on the LVRT of wind farms mainly focuses on suppressing rotor current and providing reactive current support, while the impact of active current output on LVRT performance has not been thoroughly discussed. This paper studies and reveals the relationship between the limit of reactive current output and the depth of voltage drop during LVRT for doubly-fed induction generator (DFIG) based wind farms. Specifically, the reactive current output limit of the grid-side converter is independent of the depth of voltage drop, and its limit is the maximum current allowed by the converter, while the reactive current output limit of the DFIG stator is a linear function of the depth of voltage drop. An optimized scheme for allocating reactive current among the STATCOM, DFIG stator, and grid-side converter is proposed. The scheme maximizes the output of active current while satisfying the standard requirements for reactive current output. Compared to traditional schemes, the proposed LVRT optimization strategy can output more active power during the LVRT period, effectively suppressing the rate of rotor speed increase, and improving the LVRT performance and fault recovery capability of wind farms. Simulation results verify the effectiveness of the proposed scheme.

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.

A Current Saturation Strategy for Enhancing the Low Voltage Ride-Through Capability of Grid-Forming Inverters

This brief proposes a new current saturation strategy (CSS) for grid-forming (GFM) inverters to comply with the existing low-voltage ride-through (LVRT) capability requirements. The proposed control philosophy enables the GFM inverters to follow one of the standardized grid codes by limiting the output current during LVRT via a new control parameter, the power factor angle (PFA). First, a nonlinear mathematical model capturing the full-order averaged dynamics of a GFM inverter embedded with current limiting control has been developed. Following this, the proposed CSS has been analytically evaluated by developing a simplified equivalent circuit model. Finally, numerical and experimental results are presented to verify the accuracy of the CSS for assessing the large-signal stability of a GFM inverter subjected to severe symmetrical grid faults.

An adaptive fuzzy logic control technique for LVRT enhancement of a grid-integrated DFIG-based wind energy conversion system

The fast improvement of wind energy conversion technology with optimization algorithms has recently received a lot of attention. However, their slow convergence speed, huge computational load, and low efficiency are the main drawbacks and to improve these disadvantages, a new adaptive fuzzy logic controller strategy is proposed to enhance the low voltage ride-through of the grid-connected doubly fed induction generator during severe grid faults. The rotor side converter and the grid side converter are controlled using an adaptive fuzzy logic controller topology under cascaded structure to improve the performance. The novel application of the ‘generalized variable step-size diffusion continuous mixed p-norm’ adaptive filtering algorithm is proposed to modify the calibrating factors of the fuzzy logic controllers at a rapid convergence speed with low normalized misalignment error. The proposed adaptive algorithm-based fuzzy logic controller has a better low voltage ride-through improvement capability than that of using the particle swarm optimization-based proportional-integral controller during severe grid disturbances. The convergence speed of the proposed adaptive filtering algorithm is compared to that of existing algorithms such as least mean fourth, least mean square, and continue mixed p-norm algorithms. In addition, a comparison is also made with different common optimization methods. The field-programmable gate array-based real-time experimental results are presented to validate the proposed adaptive control topology.

Improving Voltage Ride-Through Capability of Grid-Tied Microgrid With Harmonics Mitigation

When the microgrid operates in grid-tied mode, the voltage ride-through (VRT) capability presents a challenge in its inverter controller design. This letter proposes an effective method to support the VRT capability of an inverter-based grid-tied microgrid during grid fault. In addition, the inverter control is implemented by a combined method to mitigate its output current harmonics under normal or faulty grid conditions, even if the grid voltage is distorted. Simulations are performed using Matlab/Simulink and implemented in a real-time simulator with a hardware setup to validate the usefulness of the proposed method.

Design and Analysis of DFIG-STATCOM Coordinated P2P Grid Connected System Using RMSProp

This research work establishes a relationship between STATCOM and DFIG wind turbines in a transmission network for coordinated operation during grid disturbances. A change in wind gust also produces a variable output nonlinearly, thereby making the system unstable. Assuring bus voltage and proper balance of load angle in all the connected generating stations becomes challenging. Therefore, increasing voltage security at all the lines and further enhancing system stability leads to the placement of STATCOM at the proper location in the transmission line. Appropriate coordinated control of STATCOM and DFIG can lead to adequate power flow in the system. This research includes a multi-objective optimization problem for properly tuning the PI- controller. The voltage at the controlled bus, low-frequency oscillating waveforms, and real power available at the bus under pre-fault and post-fault conditions are identified as objective function parameters. To avoid the overgrowth of error inside the search space due to lack of normalization, this method uses the RMSProp algorithm for proper convergence in the state vector. The coordinated control action has been investigated in the different shunt fault conditions. Again, to enhance the system stability, low voltage ride-through capability has been thoroughly verified using Matlab software.

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.

Coordinated control by ADRC strategy for a wind farm based on SCIG considering low voltage ride-through capability

Wind farms are integrated with the power grid system to provide active and reactive power. Because in a wind farm, wind turbines (WTs) are highly coupled to their operating conditions, a central wind farm supervisory unit must take into account these conditions when producing power control references for each WT. The aim of this paper is to manage and control the active and reactive power of wind farms based on squirrel cage induction generators and back-to-back converters. The proportional distribution algorithm is used for distributing wind farm power to individual WTs. In addition, we consider the development of a local power management and control units for WTs. This is in order to extract the maximum available power from the wind, and to provide the active and reactive power predetermined by the transmission system operator, or to satisfy the grid code requirements considering Low Voltage Ride-through capability. The power dispatch strategy is to be used on all WTs using the distribution algorithm while ensuring the control loops using the proposed Active Disturbance Rejection Control strategy. The results demonstrate that the proposed strategies are efficient and can guarantee the safe integration of wind farms into the grid while respecting grid code requirements and power system stability.

Research on Low Voltage Ride through Control of a Marine Photovoltaic Grid-Connected System Based on a Super Capacitor

With the increase of photovoltaic penetration rate, the fluctuation of photovoltaic power generation affects the reliability of ship power grids. Marine PV grid-connected systems with high penetration rates should generally have a low voltage ride-through capability. In the present paper, a strategy in which super capacitors are applied for energy storage in a marine photovoltaic grid-connected system is proposed, and an inverter adopts independent decoupling control of active and reactive currents to improve the LVRT capability of photovoltaic grid-connected systems. In addition, a comprehensive control strategy is also designed to control the supercapacitor, to regulate the active power through the control method of the voltage outer loop and the current inner loop, in order to maintain the DC bus voltage stability. At the same time, the inverter can increase the reactive power output to support the grid voltage. The advantage of this system is in smoothing the power imbalance in a short time, enhancing the low voltage ride-through capability of the photovoltaic grid-connected system, improving the power quality, and ensuring the safety and stability of the ship’s power grid. MATLAB/Simulink were employed to establish a ro-ro ship super capacitor–marine photovoltaic grid-connected power system model and to carry out simulation experiments by setting the grid voltage drop. The results show that when the grid voltage drops, the inverter adjusts the distribution of active and reactive power. The power factor drops from 1 to 0.77, and the effective value of the voltage drop increases from 150 V to 156 V, which proves that this strategy effectively reduces the depth of the grid voltage drop and improves the low voltage ride-through capability of the photovoltaic grid-connected system.

Enhanced Deadbeat Control Approach for Grid-Tied Multilevel Flying Capacitors Inverter

This paper proposes an enhanced Deadbeat Controller (DBC) for a grid-tied Flying Capacitors Inverter (FCI). The proposed DBC guarantees the balancing of the capacitors’ voltages while injecting current to the grid with lower Total Harmonics Distortion (THD). The proposed controller has the following advantages: 1) Improved current tracking quality even at zero crossing instants by using a weighted state-space model, 2) Superior steady-state performance (lower current THD) compared to other prediction-based control techniques such as Finite-Control-Set Model Predictive Control, 3) The generated duty cycles are normalized to the common base when the desired state is out of reach within the sampling time, 4) Voltage Ride-Through (VRT) capability, and 5) Robustness to parameters variation. Theoretical analysis, simulation, and experimental results are presented to show the effectiveness of the proposed control technique in ensuring uninterruptible and smooth transfer of energy to the grid during normal/abnormal operating conditions (severe voltage sags, parameters variation, etc.).

Requirement Analysis of Wind Turbine High Voltage Ride-through Capability Detection and Simulation

With the number of wind farms connected to the power system increasing rapidly, the impact of wind power generation system on power grid system stability is becoming increasingly prominent. Especially when the power grid has transient fault, there will be problems such as reverse power flow, large-area power failure of wind farm and so on. This will directly lead to the power system instability and seriously affect the power grid security and stability. Especially in the stage of voltage recovery after the fault is eliminated, the low-voltage fault of the power grid might lead to a sharp rise in the voltage of power grid and overvoltage on the high-voltage line connected to the power grid with wind farm, resulting in the disconnection of the wind power grid connected system. However, China’s current standards do not specify the high-voltage crossing of wind power grid connected system, especially the high-voltage crossing detection device, detection method and detection procedure. Therefore, it is urgent to study the detection technology of wind power high voltage crossing and put forward the detection method. There are many reasons for high-voltage power grid fault, and the interaction between power grid and inverter is more complex in high-voltage fault. If the type test method is used to simulate various high-voltage fault conditions in power system and carry out the test and study on the HVRT( high voltage ride through) performance of wind turbine, the human and material cost is huge and there are many potential safety hazards. At the same time, the grid connection performance of wind turbines are largely related to the control strategy of the controller. Therefore, the semi physical simulation method can be used to research the HVRT detection technology of wind turbine. This can effectively avoid the safety problem of large current and high voltage in the primary circuit, and realize the physical connection between the real controller and the flexible high voltage fault platform model through hardware in the loop simulation. It can simulate the wind turbine capability of high-voltage ride through under various high-voltage fault conditions. It not only has low test cost, but also can deeply study the interaction between power grid and power grid.

An internal parallel capacitor control strategy for DC-link voltage stabilization of PMSG-based wind turbine under various fault conditions

In recent years, wind energy conversion systems (WECSs) have been growing rapidly. Due to various advantages, a permanent magnet synchronous generator (PMSG) is an appealing solution among different types of wind generators. As wind power penetration level in the grid increases, wind power impacts the grid and vice versa. The most essential concerns in the system are voltage sag and swell, and grid code compliance, particularly for low voltage ride-through (LVRT) and high voltage ride-through (HVRT) capability, is a pressing necessity. This paper presents a parallel capacitor (PC) control strategy to enhance the LVRT and HVRT capability of PMSG. Furthermore, this study presents a method for the sizing of a PC system for the reduction of the overvoltage of the DC-link during voltage sags and swell. Fast Fourier transform analysis is used to determine the total harmonic distortion (THD) for the injected current into the grid. The obtained results illustrate the effectiveness of the proposed system in keeping the DC-link voltage below the limit, power quality improvement, and increasing the LVRT and HVRT capability. Models of wind turbine, PMSG, and PC control system are built using MATLAB/SIMULINK software.

Research on HIL-based HVRT and LVRT automated test system for photovoltaic inverters

As an important capability of the photovoltaic grid-connected inverter, high voltage ride-through (HVRT) and low voltage ride-through (LVRT) capability are related to the stability of the whole power grid system. Therefore, according to the requirements of HVRT and LVRT standards, an automatic test and analysis system of the photovoltaic inverter is designed based on a hardware-in-the-loop (HIL) simulation platform, which saves time and human resources. Firstly, this paper analyzes the specific requirements for HVRT and LVRT capabilities in Q/GDW 1617-2015. Secondly, a HIL simulation platform is built. Thirdly, an automatic test and analysis system is developed based on Python and m language using RT-LAB and Matlab software, which can realize the automation of HVRT and LVRT test, data collection and storage, data processing and standard index calculation, and test results report generation of the photovoltaic inverter under different steady-state conditions. Finally, the correctness of the system is verified on the HIL simulation platform.

LVRT Capability Improvement of DFIG-Based Wind Turbines With a Modified Bridge-Resistive-Type SFCL

The doubly fed induction generator (DFIG) has been required to operate continuously when confronted with faults. For improving its fault ride through capability (FRT), a novel modified bridge-resistive-type superconducting fault current limiter (MBR-SFCL) is put forward in this paper. Compared with the conventional SFCL, the steady-state operating loss is reduced, and the resistance value can be controlled. The model, operation principles, and control scheme of MBR-SFCL are described. The comparative analysis between the proposed MBR-SFCL and conventional SFCL in the DFIG system is studied. All the critical variables of the DFIG system can be kept in domain. The simulation results show that the proposed limiter can improve the voltage ride-through capability of DFIG during the fault and post-fault process.

AC Microgrids Protection: A Digital Coordinated Adaptive Scheme

A significant challenge for designing a coordinated and effective protection architecture of a microgrid (MG) is the aim of an efficient, reliable, and fast protection scheme for both the grid-connected and islanded modes of operation. To this end, bidirectional power flow, varying short-circuit power, low voltage ride-through (LVRT) capability, and the plug-and-play characteristics of distributed generation units (DGUs), which are key issues in a MG system must be considered; otherwise, a mal-operation of protection devices (PDs) may occur. In this sense, a conventional protection system with a single threshold/setting may not be able to fully protect an MG system. To tackle this challenge, this work presents a comprehensive coordinated adaptive protection scheme for AC MGs that can tune their protection setting according to the system states and the operation mode, and is able to switch the PDs’ setting. In the first step of the proposed adaptive algorithm, an offline setting will be adopted for selective and sensitive fault detection, isolation, and coordination among proposed protective modules. As any change in the system is detected by the proposed algorithm in the online step, a new set of setting for proposed modules will be performed to adapt the settings accordingly. In this way, a new set of settings are adapted to maintain a fast and reliable operation, which covers selective, sensitive, and adaptive requirements. The pickup current (Ip) and time multiple settings (TMS) of directional over-current relays (DOCR), as well as coordinated time delays for the proposed protection scheme for both of the grid-connected and islanded modes of operation, are calculated offline. Then, an online adaptive protection scheme is proposed to detect different fault types in different locations. The simulation results show that the proposed method provides a coordinated reliable solution, which can detect and isolate fault conditions in a fast, selective and coordinated adaptive pattern.

Stability Testing and Restoration of a DEIG-Based Wind Power Plant with Indirect Grid Control Strategies

ABSTRACT In the current scenario, because of government policies, environmental factors, and technological improvements, there is a rapid growth in renewable energy sector. The emphasis is to obtain better system performance by effective resource utilization and providing security and reliability. This paper discusses the design and implementation of indirect grid control of a wind power plant by controlling the parameters in both grid and rotor side converters. The proposed system consists of Doubly Excited Induction Generator (DEIG) with Wind turbine system (WTS) and Mechanical and Electrical Power Controlling Systems (MPCS-EPCS). Various transmission line faults (symmetrical and asymmetrical faults) incur power imbalances in power grid. The developed MPCS and EPCS are helpful to perform grid monitoring and controlling under different types of faulty conditions. The MPCS monitors the effective source utilization and EPCS helpful for matching the grid energy levels under normal and faulty conditions. Modification in the converter topologies to minimize the impact of adverse effects of faults on the DEIG-WTS and to improve resiliency in the power grid is also discussed. To improve the stability and enhancing resource utilization to improve the efficiency of the overall system with the enhancement of fault voltage ride-through capability in DEIG-WTS under fault conditions are also considered. The stability of the system is tested under steady-state and dynamic-state conditions by applying faulty conditions in MATLAB/Simulink environment.

An augmented state observer-based sensorless control of grid-connected inverters under grid faults

The increasing integration of grid-connected renewable energy systems impose major challenges of reliability and power quality within power grid system. Grid-connected systems must ensure grid stability, accommodate low voltage ride-through capability, and comply with grid codes under unbalanced voltage dips to avoid any grid-disconnection. In this context, this paper proposes a novel control strategy to improve grid-connected inverter control under unbalanced grid voltage conditions. Positive and negative sequence components of grid voltage are estimated using an augmented state and disturbance observer. The estimated values of grid voltage and phase-angle are used to protect the inverter from any over-current tripping even at unbalanced grid voltage dips. This can be achieved by reducing active power and suppressing its fluctuations with new current references, which are obtained in the synchronous reference frame according to grid codes. The effectiveness and flexibility of the proposed power control strategy have been demonstrated by numerical simulation and validated by experiments in both balanced and unbalanced grid fault conditions.