Low-voltage Ride-through Capability Research Articles

A Hybrid Compensator for Unbalanced AC Distribution System With Renewable Power

A hybrid compensator with a new coordinated control is proposed for mitigating power quality aspects and integration of renewable power in an unbalanced three-phase four-wire distribution system. The proposed compensator comprises of thyristor switched capacitor (TSC) banks, delta connected thyristor controlled reactor (TCR), a 3rd and a 5th harmonic tuned passive LC filter & a low rating three-phase four-wire D-STATCOM. The TSC-TCR operates asymmetrically for the base var compensation and also eliminates negative sequence current during unbalanced loading at the point of common coupling (PCC). The D-STATCOM works in coordination with the TSC-TCR and the passive filters to compensate fundamental var, zero sequence current, voltage flicker & 7th harmonic currents at the PCC. The D-STATCOM thus serves as a supportive var compensator as well as an active filter in selective harmonic compensation mode, thereby improving the var compensation and harmonic current filtering at the PCC. A new coordination control system is proposed to control the operation of the D-STATCOM which is also employed to integrate a PV source & a doubly fed induction generator (DFIG) based wind energy conversion system (WECS) to the PCC. This scheme with grid connected and islanded mode of operation using D-STATCOM exhibits low-voltage ride-through capabilities. Further, the chance of resonance of the passive filters used with source inductance is eliminated in the proposed scheme. Simulation results show that the proposed hybrid system effectively compensates var, negative & zero sequence current, reduces voltage flicker and harmonic currents at PCC.

A Case Study on the LVRT Capability of an Egyptian Electrical Grid Linked to the Al-Zafarana Wind Park using Series Resistor

In wind farms, a doubly fed induction generator (DFIG) has proven to be the most successful wind power generator in recent years. DFIG features various speeds of operation and autonomous management of reactive and active powers. Nevertheless, the extreme DFIG sensibility to voltage dips brings a lot of defiance in terms of complying with grid code regulations set by the electrical utility operators. Now, these grid regulations impose tougher constraints, particularly for low-voltage ride-through (LVRT). Indeed, this disruption brought on by the stator circuit affects the generating system. Originally, when a failure occurred, generators had to be disconnected. In time, a disconnect could cause the entire system to fail. Recent grid codes require that in addition to meeting operating irregularities like the dc-link voltage and stator and rotor currents, the generators also remain connected. To increase the LVRT capacity for the Egyptian Electrical Grid linked to the Al-Zafarana Wind Park. This article introduces the design and modelling of a protection scheme employing a series resistor. The protection scheme ensures the safety of energy converters, rotor circuits, and dc-link capacitors while limiting rotor currents, dc-link voltages, and torque changes. The effectiveness of the protective scheme was also assessed for asymmetrical and symmetrical fault situations. Matlab/Simulink is used to produce the simulation findings. According to the simulation findings, the protection scheme demonstrated an effective solution for improving the LVRT feature, of the Egyptian Electrical Grid linked to the Al-Zafarana Wind Park.

Integrated control algorithm for fast and accurate detection of the voltage sag with low voltage ride-through (LVRT) enhancement for doubly-fed induction generator (DFIG) based wind turbines

Low voltage ride-through (LVRT) is one of the essential aspects of grid codes for integrating doubly-fed induction generators (DFIG) to achieve reliable and uninterrupted electrical power generation. The detection time of the voltage sag is one of the crucial aspects for the LVRT improvement of the DFIG. This paper introduces a second-order generalized integrator (SOGI), quadrature signal generator (QSG) with a frequency locked loop (FLL) based algorithm to detect the symmetrical voltage sag fast and accurately. The proposed SOGI-QSG-FLL-based voltage sag detection algorithm (VSDA) works under a second-order band-pass filter with an alfa-beta stationary reference frame. The proposed VSDA is integrated with the novel feed-forward transient current compensation (FFTCC) scheme with an advanced interval type-2 fuzzy logic-proportional integral (IT2-FLC-PI) controller to enhance the LVRT capability of the DFIG. This FFTCC scheme is activated after the detection of voltage sag by using the proposed VSDA. The proposed FFTCC feeds the uninterrupted active and reactive power to the grid and reduces the torque ripple of the DFIG. The detection time of voltage sag by using the proposed VSDA is compared with existing algorithms. The experimental results are also presented to validate the proposed algorithm.

An Optimized LVRT Control Strategy of Cascaded Modular Medium-Voltage Inverter for Large-Scale PV Power Plant

Active power backflow is an inherent problem of three-phase cascaded H-bridge (CHB) photovoltaic (PV) grid-tied inverters during low-voltage ride through (LVRT), probably resulting in no balanced operating point of the system, and the inverter will be shut down and off-grid due to overvoltage fault. Aiming at this issue, this article first reviews the existing control methods and analyzes their limitations in application, that is, they cannot effectively suppress active power backflow under the scenario of deeper voltage drop and lower output power. On this basis, an optimized LVRT control strategy for cascaded modular medium-voltage PV power generation system is proposed, which adopts different control methods to suppress active power backflow under three types of asymmetric grid drop conditions: single-phase short-circuit fault with ground, two-phase short-circuit fault with ground, and two-phase short-circuit fault without ground, enhancing the adaptability of CHB PV grid-connected inverters to different output powers and different drop depths during grid voltage failure and then improving LVRT capability of the system. Finally, the effectiveness and feasibility of the proposed control strategy are verified by experimental results.

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.

Extended State-Based OSG Configurations for SOGI PLL With an Enhanced Disturbance Rejection Capability

Changes in the orthogonal signal generator (OSG) configuration of the standard second-order generalized integrator-based phase-locked loop (S-SOGI PLL) have shown to offer an improved low-voltage ride-through capability. However, the resulting PLLs: highpass SOGI (H-SOGI) and lowpass SOGI (L-SOGI) PLLs have no dc-offset estimation capability and lack robustness to disturbances. Therefore, this article focuses on maximizing the merits of H-SOGI and L-SOGI PLLs for grid and industrial applications by incorporating dc-offset estimation and enhancing their disturbance rejection capability. Accordingly, two varieties of SOGI PLLs, namely: extended state highpass SOGI (ES-H-SOGI) and extended state lowpass SOGI (ES-L-SOGI) PLLs are proposed. In the proposed PLLs, states of the OSGs are extended to achieve dc-offset estimation and rejection, while frequency-adaptive implementation of the moving average filter (MAF) is developed to achieve complete harmonic rejection at off-nominal frequencies of the input voltage. To alleviate the undesirable effect of the MAF in terms of the PLL's slow transient response, frequency-adaptive implementation of a phase-lead compensator is introduced. In this article, design guideline that achieves an optimal tradeoff between disturbance rejection and dynamic performance in the ES-H-SOGI and ES-L-SOGI PLLs is presented. Finally, experimental comparison between the proposed ES-H-SOGI and ES-L-SOGI PLLs and their ES-S-SOGI PLL counterpart is carried to show that the proposed PLLs outperform the ES-S-SOGI PLL in terms of dynamic performance while achieving enhanced disturbance rejection capability.

Transmission distribution coordination for DER fault ride through support

Rapid renewable energy sources (RESs) deployments in transmission and distribution networks have changed power systems’ operation and control mechanisms. This fundamental shift makes the interaction between transmission and distribution systems more sensitive and requires robust TSO–DSO coordination to ensure system stability and ancillary services for the entire network. This paper uses an electromagnetic transient (EMT) simulation tool using a real-time digital simulator (RTDS) to investigate the dynamic transmission–distribution coordination with a high share of RESs in fault ride-through (FRT) capability of distributed energy resources (DERs). Then, the paper implements fault current injection (FCI), incorporating inverter-based resources (IBRs) in the transmission system to support dynamic voltage responses for the distribution system. Different reactive current control schemes are examined to evaluate the FRT performance of DERs under different fault locations. Additionally, hardware in the loop simulation for the under-voltage protection relay is implemented to validate the low-voltage ride-through capability of DERs. The comparison results show that the transmission system can provide dynamic voltage support for the distribution system through FCI from IBRs to help DER ride through. The FCI from the transmission system also significantly enhances fault-induced delayed voltage recovery of voltage-sensitive load in the distribution system.

Investigation of LVRT capability of wind driven dual excited synchronous generator

This paper proposes an effective control technique for low voltage ride through (LVRT) capability in dual excited synchronous generator (DESG) wind turbines. The proposed control technique is dependent on controlling the field circuit parameters. Where the active power is controlled by the field-current space phasor magnitude and the reactive power is controlled by the field-voltage space phasor phase. With the proposed control strategy, the DESG can generate additional reactive power to support grid voltage recovery under grid faults. The DC-link voltage is kept within an acceptable limit since the excess power, due to the power mismatch between the mechanical and armature power is stored in the generator inertia. Using the proposed control strategy, the DESG can enhance the LVRT capability efficiently without using extra protection circuits or any additional control techniques during fault conditions. To test the proposed control method, simulation, and experimental results for a 1.1 kW DESG wind turbine system were obtained.

Decoupled power-based sliding mode control modeling enhancement for dynamic stability in doubly-fed induction generator-based wind turbines

Low-voltage ride through (LVRT) capability is the principal method used to counteract voltage dips and over-current, which are caused by various balanced and unbalanced faults. However, in providing LVRT capability during faults, oscillations that occur in parameter changes should be damped for a short time. In this study, a decoupled power-based sliding mode control (SMC) in doubly-fed induction generator (DFIG)-based wind turbines was developed in order to eliminate the oscillation problems that may occur during balanced and unbalanced faults. Besides, while the stator electromotive force model in DFIG is developed with the aim of improving simulation performance and enabling ease of calculation, the rotor electromotive force model is developed to dynamic stability in the system during various faults. The results obviously indicated that the proposed decoupled power-based SMC and stator-rotor electromotive force models enhanced the LVRT capability of the grid-code requirement and quickly restored dynamic stability to the system.

RETRACTION NOTICE: Low-voltage ride-through capability enhancement of wind energy conversion system using an ant-lion recurrent neural network controller

RETRACTION NOTICE: Low-voltage ride-through capability enhancement of wind energy conversion system using an ant-lion recurrent neural network controller

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.

A stored energy-based control strategy to improve LVRT capability of HRES using PSO & WCA optimized DVR

The article presented here utilizes the particle swarm optimization (PSO) and water cycle algorithm (WCA) based dynamic voltage restorer (DVR) to augment the low voltage ride through (LVRT) performance of a hybrid renewable energy system (HRES). Moreover, FACT equipment like the dynamic voltage restorer (DVR) is deployed to boost the quick responsiveness of compensation and good transient performance of HRES. Due to numerous disturbances brought on by WECS' nonlinearity, the best tuning of a PI controller for DVR control is investigated to match the LVRT requirement of the HRES. The design incorporates a battery energy storage system (BESS) that is connected over the DC terminals of the DVR to improve the compensating capabilities by regulating the D-Q axis voltage signal to control the voltage source converter gate signals. In comparison to DVR alone, the suggested technique increases the DC-link voltage and PCC voltage by 44% and 4.6 %, respectively. MATLAB simulation results show that the BESS-enabled PSO DVR performs better in all respect for the compensation of voltage sag and makes the system self-LVRT capable.

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

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

Improvement of Low-Voltage Ride-Through Capability for Wave Energy Conversion System

With the wave energy conversion system (WECS) integrated to grid, the lack of satisfactory ability to ride through transient voltage faults will reduce the stability and reliability of the grid. In this article, in order to enhance the low-voltage ride-through (LVRT) capability of the WECS, the structure and the mathematical model of a magnetic-gear permanent magnet synchronous generator (MG_PMSG) and a float-type wave energy converter are introduced first. The output power characteristic of the WECS is analyzed. Then, a novel coordinative LVRT control scheme based on supercapacitor energy storage system (SC_ESS) is proposed. The control scheme includes four modes, which are determined by the severity of voltage sags and active power deviation of the WECS. Machine-side converter control, grid-side converter, control and SC_ESS control are used coordinately in the four operation modes. The results of simulation and experiment have verified the effectiveness of the proposed control scheme in normal running conditions.

Complying the LVRT grid code requirement of a fixed speed wind energy system using unified voltage and pitch angle control strategy

This paper proposes a Unified Voltage and Pitch angle Control (UVPC) strategy for fixed speed wind farm. The focus put on guaranteeing the LVRT grid code requirement when the wind farm subjected to severe faults. The UVPC consist of static synchronous compensator (STATCOM) voltage control loops with adequate pitch angle control loops which are coordinated to enhance the low-voltage ride-through capability of the wind generators thereby, complying the low voltage ride through (LVRT) grid code requirement as desired by the Germany grid code. Different scenarios such as system without STATCOM and pitch-angle controller, with STATCOM only, and system with STATCOM and pitch-angle control (i.e. UVPC) are simulated. The simulation results show that the adoption of STATCOM and pitch angle controller fulfilling the LVRT grid code requirement to ensure the continuous operation of wind turbines. Furthermore, a concept of critical clearing time (CCT) is discussed and its utility has been emphasized. Calculations, simulations and measurements provide how the increased STATCOM rating can offer an enhanced stability margin.

Output power smoothing of wind power plants using unified inter-phase power controller equipped with super-capacitor

Due to the growing concerns regarding environmental and economic issues in power systems, nowadays the application of renewable energy sources (RES), especially wind energy has been increased. However, the increasing penetration of wind power plants (WPPs) into the grid has raised numerous challenges including output power fluctuations as well as the significant role these sources play in low-voltage ride-through (LVRT) in case of short circuit fault conditions. Energy storage systems (ESSs) are commonly employed to enhance WPP output power smoothing. On the other side, flexible AC transmission systems (FACTS) can be utilized to enhance dynamic transients of the WPPs under short circuit contingencies. This paper presents the utilization of unified inter-phase power controller (UIPC) and the super-capacitor to fulfill the aforementioned challenges in connection of WPPs to the grid. To achieve this, a novel power control scheme is proposed to compensate for output power variations. The WPP model studied here is of doubly-fed induction generator (DFIG) type wind turbine. Simulations and comparisons are performed in the MATLAB/Simulink® environment to demonstrate the effectiveness of the proposed scheme in smoothening WPP output power as well as enhancing their LVRT capability.

Enhancing LVRT capability of DFIG using cooperative control of BTFCL and RPC

ABSTRACT To integrate the high penetration of power from wind farms into a grid, one of the key requirements posed by new grid codes is Low Voltage Ride Through (LVRT) capability. This paper studies voltage compensation-based methods to improve fault ride-through capability of the grid-connected Doubly Fed Induction Generator (DFIG). Bridge Type Fault Current Limiter (BTFCL) and Reactive Power Control (RPC), which is acting as an active voltage compensator, are cooperatively controlled in this study to achieve better LVRT capability of DFIG. Additionally, the design considerations to select shunt impedance of BTFCL are described to get optimal performance of the system under consideration. The test system was modelled and simulated in MATLAB/SIMULINK environment. The effectiveness of the proposed method is tested by comparing it with the conventional Crowbar resistance method. Simulation results show that the cooperative control of self-acting BTFCL and an intelligent RPC can effectively reduce the fault current, improves the stator voltage of DFIG for continuous operation and provides reactive power support to the grid during fault conditions.

Enhancement of the LVRT Capability for DFIG-Based Wind Farms Based on Short-Circuit Capacity

In this article, an advanced transient voltage control (A-TVC) strategy is proposed to enhance the low-voltage ride-through (LVRT) capability of a doubly fed induction generator (DFIG)-based wind farm (WF). In severe voltage dip (VD) conditions, the DFIG-based WF should stay online to support the grid voltage. However, it may lead to transient voltage instability in the grid. This article proposes a novel A-TVC, which follows the VD conditions and, consequently, improves the LVRT capability of the DFIG-based WF. In this approach, a lookup table is presented to control the transient voltage based on the short-circuit capacity at the point of common coupling (PCC), VD index on the grid, and generated power limitations of the DFIG. By applying the proposed control strategy, the DFIG could not only smooth the dc-link voltage fluctuations and reduce the stator and rotor overcurrents in a faulty grid but also increase the PCC and stator voltages and, thus, enhance the transient voltage stability. Simulations and control-hardware-in-the-loop experimental results validate the effectiveness of the proposed control strategy for improving the LVRT capability of the DFIG in severe balanced and unbalanced VD conditions.

Low-voltage ride through of multi-port power electronic transformer

A low-voltage ride-through (LVRT) control strategy for the multi-port power electronic transformer (PET) based on power co-regulation is proposed. During the sag and recovery of the grid-side voltage of the medium-voltage ac (MVac) port, the grid-connected active power of the low-voltage ac (LVac) port, rather than the power from external renewable energy sources (e.g., photovoltaic (PV)), is adjusted quickly to rebalance the power flowing across all ports, thereby preventing overcurrent and overvoltage. Moreover, a power-coordinate-frame-based LVRT mode classification is designed, and a total of six LVRT modes are classified to meet the LVRT requirements in all power configuration scenarios of the PET. In this way, the PET is endowed with the LVRT capability in both power-generation and power-consumption states, which is significantly different from traditional power generation systems such as PV or wind power. Furthermore, by optimizing the active power regulation path during LVRT transition, the overcurrent problem caused by the grid-voltage sag-depth detection delay is overcome. Finally, the effectiveness of the proposed control scheme is verified by experiments on a hardware-in-the-loop platform.

Dynamic response and low voltage ride-through enhancement of brushless double-fed induction generator using Salp swarm optimization algorithm.

A brushless double-fed induction generator (BDFIG) has shown tremendous success in wind turbines due to its robust brushless design, smooth operation, and variable speed characteristics. However, the research regarding controlling of machine during low voltage ride through (LVRT) need greater attention as it may cause total disconnection of machine. In addition, the BDFIG based wind turbines must be capable of providing controlled amount of reactive power to the grid as per modern grid code requirements. Also, a suitable dynamic response of machine during both normal and fault conditions needs to be ensured. This paper, as such, attempts to provide reactive power to the grid by analytically calculating the decaying flux and developing a rotor side converter control scheme accordingly. Furthermore, the dynamic response and LVRT capability of the BDFIG is enhanced by using one of the very intelligent optimization algorithms called the Salp Swarm Algorithm (SSA). To prove the efficacy of the proposed control scheme, its performance is compared with that of the particle swan optimization (PSO) based controller in terms of limiting the fault current, regulating active and reactive power, and maintaining the stable operation of the power system under identical operating conditions. The simulation results show that the proposed control scheme significantly improves the dynamic response and LVRT capability of the developed BDFIG based wind energy conversion system; thus proves its essence and efficacy.