Low-voltage Ride-through Capability Research Articles

Active Power Control of Hydraulic Wind Turbines during Low Voltage Ride-Through (LVRT) Based on Hierarchical Control

To improve the power grid adaptability and low voltage ride-through (LVRT) capability of hydraulic wind turbines (HWT), an LVRT control method based on hierarchical control is proposed for the energy regulation of HWT. The method includes a top-level machine-controlled paddle, mid-level control based on variable motor swash plate angle, and an underlying control based on throttle opening. To achieve multivariable coordinated control of the HWT via the control process, the minimum wind, maximum inertial energy storage, and minimum energy consumption of the throttle valve of the wind turbine are optimized. The multiobjective control law is computed by a quadratic programming algorithm, and the optimal control law is obtained. The multitarget control strategy is simulated and analyzed by AMESim14 and MATLAB/Simulink R2014a software, and the control law is verified by a semiphysical test platform of an HWT. The results show that the proposed control method can effectively reduce the residual energy of the HWT during LVRT, reduce the impact on the generator, and improve the adaptability of the HWT.

Fault Ride-Through Capability Enhancement for Microinverter Applications

Due to the fast growth of single-phase grid-connected photovoltaic (PV) systems, the existing grid codes are expected to be modified to guarantee the availability, quality, and reliability of the electrical system. Therefore, the future single-phase PV systems should become smarter and support low voltage ride-through (LVRT) capability, which are required for three-phase wind power systems. In this paper, the operation principle of a flyback inverter in a low-voltage ride-through operation is demonstrated in order to map future challenges. The steady state performance of the flyback inverter under voltage rise and drop conditions at boundary conduction mode (BCM) and discontinues conduction mode (DCM) is studied theoretically. The simulation results of the flyback inverter for various grid faults are presented to verify the theoretical analyses. The results indicate the fact that the flyback inverter at BCM condition can provide LVRT capability for photovoltaic microinverter applications in distributed generation (DG) systems, even though it does not need any auxiliary control branches and any limitations in components design.

DC-Link Voltage Control of a Grid-Connected Solar Photovoltaic System for Fault Ride-Through Capability Enhancement

The high penetration level of solar photovoltaic (SPV) generation systems imposes a major challenge to the secure operation of power systems. SPV generation systems are connected to the power grid via power converters. During a fault on the grid side; overvoltage can occur at the direct current link (DCL) due to the power imbalance between the SPV and the grid sides. Subsequently; the SPV inverter is disconnected; which reduces the grid reliability. DC-link voltage control is an important task during low voltage ride-through (LVRT) for SPV generation systems. By properly controlling the power converters; we can enhance the LVRT capability of a grid-connected SPV system according to the grid code (GC) requirements. This study proposes a novel DCL voltage control scheme for a DC–DC converter to enhance the LVRT capability of the two-stage grid-connected SPV system. The control scheme includes a “control without maximum power point tracking (MPPT)” controller; which is activated when the DCL voltage exceeds its nominal value; otherwise, the MPPT control is activated. Compared to the existing LVRT schemes the proposed method is economical as it is achieved by connecting the proposed controller to the existing MPPT controller without additional hardware or changes in the software. In this approach, although the SPV system will not operate at the maximum power point and the inverter will not face any over current challenge it can still provide reactive power support in response to a grid fault. A comprehensive simulation was carried out to verify the effectiveness of the proposed control scheme for enhancing the LVRT capability and stability margin of an interconnected SPV generation system under symmetrical and asymmetrical grid faults.

Control of Photovoltaic Systems for Enhanced Short-Term Voltage Stability and Recovery

This paper investigates the impact of: 1) the Low Voltage Ride-Through (LVRT) and Dynamic Voltage Support (DVS) capability; 2) the active current recovery rate; 3) the local voltage control; and 4) the plant-level voltage control of large-scale PhotoVoltaic (PV) systems on Short-Term (ST) voltage stability and Fault-Induced Delayed Voltage Recovery (FIDVR). Moreover, the influence on transient and frequency stability is studied briefly. To evaluate FIDVR, a novel metric, the so-called Voltage Recovery Index (VRI), is defined. The studies are performed with the WECC generic PV system model on an IEEE voltage stability test system, namely the Nordic test system. The results show that without LVRT capability the system is ST voltage and transient unstable. Only the LVRT and DVS capability help to avoid ST voltage and transient instability. Considering voltage and frequency dynamics, an active current recovery rate of 100%/s shows the best performance. To further enhance voltage dynamics, plant-level voltage control together with local coordinated reactive power/voltage control should be applied. Moreover, the VRI provides useful information about the FIDVR and helps to compare different ST voltage controls.

A Low Voltage Ride Through Scheme for DFIG-Based Wind Farm With SFCL and RSC Control

Taking into consideration the low voltage ride through (LVRT) demand proposed in grid codes, a resistive-type superconducting fault current limiter (R-SFCL) LVRT solution is presented in this paper for doubly fed induction generator (DFIG) based wind turbine generators. The installed position, equivalent modelling of R-SFCL, and technical principle for improving LVRT capability are described. Cooperative control between R-SFCL and two advanced control strategies-reactive power control (RPC) and inductance-emulating control (IEC)-are introduced in this paper to help the DFIG generate reactive power and protect key parameters during grid faults. Simulations and performance evaluation are also conducted compared with the single IEC scheme and single RPC-IEC scheme, which demonstrate the effectiveness and superiority of the proposed scheme in enhancing LVRT capability.

Cooperative Control of SFCL and SMES for Protecting PMSG-Based WTG<roman>s</roman> Under Grid Faults

To meet the requirement of grid codes about low voltage ride through (LVRT) of wind turbine generators (WTGs), a cooperative control scheme of resistor-type superconducting fault current limiter (SFCL) and superconducting magnetic energy storage (SMES) for improving the LVRT capability and transient stability of permanent magnetic synchronous generator (PMSG) based WTGs is investigated in this paper. In the proposed scheme, the SFCL is utilized to increase the output active power from grid side converter (GSC) to half of the pre-fault value, and the other half is absorbed by SMES so as to reduce its maximum energy storage capacity. In addition, a SFCL's resistance estimation method is introduced by using the grid-side voltage phasor relation that is established based on the grid voltage vector oriented control of GSC. Finally, a PMSG-based WTG integrated with the SMES and SFCL is built in PSCAD/EMTDC. Performance evaluations and comparisons demonstrate the advantages of the proposed scheme in improving the transient stability of PMSG and reducing the capital cost for superconducting devices.

Cooperative Rotor-Side SMES and Transient Control for Improving the LVRT Capability of Grid-Connected DFIG-Based Wind Farm

This paper presents a novel scheme to enhance low voltage ride through (LVRT) capacity of a 5 × 1.5 MW doubly-fed-induction-generator-based (DFIG-based) wind farm. One superconducting magnetic energy storage (SMES) unit is introduced to connect in parallel with the rotor side of each DFIG. Two LVRT strategies of inductance emulating control and transient voltage control are used in the SMES energy storage side converter and rotor side converter, respectively. Comprehensive simulations and performance evaluations are carried out in both the symmetrical and asymmetrical grid fault cases. The results show that the proposed scheme can not only protect the key parameters of each DFIG unit, but also enhance the grid voltage effectively under different fault initial angles.

An adaptive control strategy of crowbar for the low voltage ride-through capability enhancement of DFIG

Abstract Doubly-fed induction generator (DFIG) is sensitive to the disturbance of grid voltage, and its low voltage ride-through (LVRT) technology is still a hotspot issue. Rotor side crowbar is widely used to realize LVRT. However, traditional fixed crowbar can’t strike a balance between the limitation of rotor over current and DC-bus over voltage, and the crowbar is usually activated repeatedly due to the inappropriate deactivation strategy. Based on the analysis of rotor current considering crowbar operations, an improved LVRT method that dynamically adjusting crowbar resistance and adaptively deactivating the crowbar circuit is proposed. The corresponding adaptive control strategy and the resistance setting method are given. With the proposed scheme, both the rotor current and DC-bus voltage can be controlled within the restriction. Besides, the crowbar can be deactivated in the least time under the precondition of avoiding being reactivated, which will effectively reduce the absorbed reactive power from the system and benefit the service life of switching devices. The effectiveness of proposed LVRT scheme is verified by MATLAB/Simulink simulation.

Enhancing voltage stability and LVRT capability of a wind-integrated power system using a fuzzy-based SVC

The static var compensator (SVC) has widespread applications in voltage regulation and stability improvement of power systems. This paper studies the SVC performance in a single machine (doubly-fed induction generator (DFIG) employed in a wind farm) infinite bus (SMIB) system. The SVC is employed to improve the voltage stability of the system and the low voltage ride-through (LVRT) capability of the wind farm. To enhance the effectiveness of the SVC, a supplementary fuzzy-based controller (FC) is proposed and is integrated into the SVC controller. The use of the proposed FC leads to the increasing the accuracy of the SVC controller that results in improving its performance in both transient and steady state. By using the proposed controller, the steady state voltage is enhanced and the maximum voltage drop under fault situations is decreased. Less voltage drop after the fault occurrence using the proposed FC results in improving the LVRT capability and makes the wind farm able to meet the grid code requirements in a larger range of load variations in the system. Using extensive simulations in MATLAB/Simulink, the effectiveness of the proposed method in improving the voltage stability and LVRT capability is validated and its effects on the performance of the DFIG-based wind turbine are studied. Three different cases are investigated in the system simulation, in which by using the proposed FC, the steady state voltage error at the load bus and the maximum voltage drop after the fault occurrence at the generator (wind farm) bus are decreased by 74.44% and 25.16% on average, respectively.

Computationally Efficient Distributed Predictive Controller for Cascaded Multilevel Impedance Source Inverter With LVRT Capability

This paper presents a decoupled active and reactive power control scheme for grid-tied quasi-impedance source cascaded multilevel inverter (qZS-CMI). For photovoltaic (PV) applications, the proposed control scheme is based on an enhanced finite-set model predictive control (MPC) to harvest the desired active power from the PV modules with the ability to provide the ancillary services for the grid. The proposed control scheme has two modes of operation: normal grid mode and low voltage ride through (LVRT) mode. In normal grid mode, the controller commands the qZS-CMI to operate at the global maximum power point (MPP). The proposed technique regulates the impedance network’s current and voltage according to the MPP of PV strings and grid current/voltage requirements. In LVRT mode, the controller commands the qZS-CMI to provide the required reactive power to the grid during voltage sags as an ancillary service from the inverter as imposed by the grid codes. The main features of the proposed system include the global MPP operation during normal grid condition, LVRT capability during a grid voltage sag, mitigation of the PV modules mismatch effect on overall energy harvesting, seamless transition between a normal grid and LVRT modes of operation, and an efficient predictive controller that exploits the model redundancies in the control objectives. Several real-time experiments are conducted to verify the system performance with transients in both the solar irradiance and the grid voltage.

Comprehensive Overview of Low Voltage Ride Through Methods of Grid Integrated Wind Generator

The wind power generation is a rapidly growing grid integrated renewable energy (RE) technology with an installed capacity of 539.291 GW. The capability of the wind energy conversion system (WECS) to remain integrated into the utility network in the case of low voltage events is called low-voltage ride-through (LVRT) capability. This paper offers a comprehensive overview of improvement techniques of the LVRT capability in WECS to increase the wind energy penetration level in the utility grid. Exhibited portrait manifests a broad spectrum of 1) wind turbines, 2) electrical generators used for wind power applications, 3) international grid codes applicable for grid integration of WECS, 4) LVRT fundamentals in WECS, 5) wind turbines LVRT methods by doubly fed induction generator (DFIG), 6) wind turbines LVRT methods by permanent magnet synchronous generators (PMSG), and 7) LVRT methods of wind turbines using squirrel cage induction generator (SCIG). This ready-reckoner paper critically reviews and classifies more than 190 research papers on LVRT issues, practices, and available technologies for grid integration in wind energy systems, and it aims to be a quick reference for the researchers, designers, manufacturers, and engineers working in the same field.

A New Control Strategy based on Reference Values Changing for Enhancing LVRT capability of DFIG in Wind Farm

Low voltage ride through (LVRT) capability enables a doubly fed induction generator (DFIG) – based wind farm (WF) to remain online supporting the electric grid during fault condition. This paper proposes a control-based strategy for improving LVRT capability of DFIG, in which by changing active and reactive powers reference values following a voltage dip (VD) condition. Moreover, the direct power control (DPC) method is modified to adaptively change the reference values based on the severity of VD. In the proposed modified DPC (MDPC) method, the parameters of the PI controllers are tuned using the imperialist competitive algorithm (ICA). For activating/deactivating the proposed strategy, a voltage dip index (VDI) is proposed which is updated within a moving time window including samples measured by phasor measurement unit (PMU). In order to evaluate rout mean square (RMS) value of the voltage from the measured values by PMU, the DFT technique is used. For activating/deactivating the control strategy, two threshold values I± and I² are defined. In the active mode, the active and reactive power are changed to zero and one p.u, while in the deactivate mode theses are changed to one and zero p.u, respectively. Based on the proposed control strategy, during a VD condition, DFIG will be able to smooth the DC-link voltage fluctuations and significantly reduces the oscillations of the stator and rotor currents. The simulation results show the effectiveness of the proposed control strategy for improving LVRT capability of DFIG.

Low-voltage Ride-through Methods for Grid-connected Photovoltaic Systems in Microgrids: A Review and Future Prospect

<span lang="EN-US">Power quality is a concern for utility and grid operators due to a large penetration of intermittent and stochastic renewable power generation sources. One of the major concerns, when designing and controlling grid-feeding photovoltaic (PV) inverters is meeting the grid requirements. International grid requirements demand low-voltage ride-through (LVRT) capability and maintaining grid functionality during fault conditions. This paper presents a comprehensive review for several control techniques to assure the LVRT capability of grid-feeding converters as well as discussing their respective advantages and limitations in detail. Areas for further research are identified afterwards. Finally, the conclusion gives a brief summary and critique of the findings.</span>

Voltage stability improvement of wind power grid‐connected system using TCSC‐STATCOM control

Aiming at the voltage stability problem of wind power grid-connected systems, a cooperative control scheme of Thyristor Controlled Series Compensator (TCSC) and Static Synchronous Compensator (STATCOM) is proposed in this study. The proposed scheme is applied in the wind power grid-connected system, a combined control strategy based on STATCOM double closed-loop feedback control and TCSC device is used to ensure that the system has enough reactive power to maintain its normal work during operation. The corresponding simulation model of wind power grid-connected system is built in MATLAB/Simulink. By analysing and comparing the waveforms of the operating examples with only TCSC, only STATCOM and TCSC-STATCOM, it is shown that the cooperative control of TCSC and STATCOM can quickly and effectively recover the bus voltage at the point of grid-connected wind farms after fault, which has a good performance on improving the voltage stability of wind power grid-connected system and increasing the low-voltage ride-through capability, and the cooperative control effect of TCSC and STATCOM is better than only using the control of TCSC or STATCOM.

Operating performance analysis on wind turbines with the speed regulating differential mechanism

In this paper, a hybrid transmission system is presented for variable-speed constant-frequency wind turbines (WTs). This approach employs an electrically controlled speed regulating differential mechanism (SRDM) so that the synchronous generator is able to rotate at a stable speed and, thus, generate electricity in constant frequency regardless of the changing wind rotor speed. In order to find out the operating performance of the proposed SRDM-based WT, a detailed 1.5 MW simulation model is established by which the speed-regulating accuracy, the electrical harmonic pollutions, and the system low-voltage ride-through (LVRT) capability are all studied in depth and then compared with those of the existing direct-drive or double-fed wind power systems. Simulation results indicate that, under various wind conditions, SRDM can efficiently regulate the output power frequency with satisfactory deviations by using a low-power (less than 262 kW) permanent magnet synchronous motor. Compared to the existing WTs, the 5th, 7th, 11th, and 13th current harmonics of the proposed WT are all ideally reduced to 0.4% of the fundamental current. Moreover, around 0.45 p.u., reactive power can be provided for the rapid recovery of the grid during severe voltage sags. The output power quality, system transient stability, and LVRT capability are all improved. Theoretical analysis and case studies give good verifications of the feasibility and superiority of the proposed SRDM-based WT.

Low-Voltage Ride-Through Operation of Grid-Connected Microgrid Using Consensus-Based Distributed Control

Since the penetration of distributed energy resources (DERs) and energy storage systems (ESSs) into the microgrid (MG) system has increased significantly, the sudden disconnection of DERs and ESSs might affect the stability and reliability of the whole MG system. The low-voltage ride-through (LVRT) capability to maintain stable operation of the MG system should be considered. The main contribution of this study is to propose a distributed control, based on a dynamic consensus algorithm for LVRT operation of the MG system. The proposed control method is based on a hierarchical control that consists of primary and secondary layers. The primary layer is in charge of power regulation, while the secondary layer is responsible for the LVRT operation of the MG system. The droop controller is used in the primary layer to maintain power sharing among parallel-distributed generators in the MG system. The dynamic consensus algorithm is used in the secondary layer to control the accurate reactive power sharing and voltage restoration for LVRT operation. A comparison study on the proposed control method and centralized control method is presented in this study to show the effectiveness of the proposed controller. Different scenarios of communication failures are carried out to show the reliability of the proposed control method. The tested MG system and proposed controller are modeled in a MATLAB/Simulink environment to show the feasibility of the proposed control method.

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.

Research Issues in DFIG Based Wind Energy System

Among different renewable energy sources, Wind energy is the most imperative energy source in power system. The development of grid connected wind energy conversion system expands, its grid connectivity issues has additionally been expanded. A portion of the issues are incorporates, for example, the regulation of voltage at Point of Common Coupling (PCC), reactive power absorption and injection, voltage sag and swell and etc. The Doubly Fed Induction Generator (DFIG) is most broadly utilized as a part of wind energy conversion system. DFIGs are extremely sensitive to grid voltage disturbances. With expanded entrance of wind energy as a renewable energy source the wind turbine should be connected to the grid during transient conditions like grid faults. The Fault Ride-Through (FRT) or 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 transients and fault conditions. In this paper consists of various grid connectivity issues and research areas of wind energy conversion system and also the various control schemes for improving the LVRT system. These several techniques are used to improve the LVRT capability and limit the fault current to avoid the disconnection of DFIG from the grid under fault conditions for enhancement of stability and the performance of wind turbine.

ZSI for PV systems with LVRT capability

This study proposes a power electronics interface (PEI) for photovoltaic (PV) applications with a wide range of ancillary services. As the penetration of distributed generation systems is booming, the PEI for renewable energy sources should be capable of providing ancillary services such as reactive power compensation and low-voltage ride through (LVRT). This study proposes a robust model predictive-based control strategy for grid-tied Z-source inverters (ZSIs) for PV applications with LVRT capability. The proposed system has two operation modes: normal grid condition and grid fault condition modes. In normal grid condition mode, the maximum available power from the PV panels is injected into the grid. In this mode, the system can provide reactive power compensation as a power conditioning unit for ancillary services from DG systems to main ac grid. In case of grid faults, the proposed system changes the behaviour of reactive power injection into the grid for LVRT operation according to the grid requirements. Thus, the proposed controller for ZSI is taking into account both the power quality issues and reactive power injection under abnormal grid conditions. The proposed system operation is verified experimentally, the results demonstrate fast dynamic response, small tracking error in steady-state, and simple control scheme.

Perturbation compensation‐based non‐linear adaptive control of ESS‐DVR for the LVRT capability improvement of wind farms

This study presents a non-linear adaptive control (NAC) for the energy storage system (ESS) embedded dynamic voltage restorer (DVR) in enhancing the low-voltage ride through (LVRT) capability of wind farms. The proposed NAC features a perturbation observer to estimate and then compensate the real perturbation of the whole system, including parameter uncertainties, measurement noise, and external disturbances such as different grid faults and intermittent wind power. It can achieve an adaptive and robust control without requiring accurate system model and full-state measurements. This control is then applied to the ESS embedded DVR (ESS-DVR) system, in which the ESS can store the blocked wind power for potential power fluctuation suppression. Simulation studies have verified that the proposed control for ESS-DVR can effectively enhance the LVRT capability of wind farms installed with different types of WTGs under different operating conditions. Moreover, its superiority has also been demonstrated by comparing with fixed gains-based conventional vector control and accurate system model-based feedback linearising control.