Low-voltage Ride-through Capability Research Articles

Low-Voltage Ride through Capability Augmentation of DFIG-Based Wind Farms Using Series-Parallel Resonance-Type Fault Current Limiter

The introduction of doubly fed induction generators (DFIGs) has facilitated the utilization of wind energy to a great extent and constituted distributed generation (DG) systems in remote places. Therefore, long transmission lines are required to interconnect with the utility grid and, consequently, different short-circuit faults interrupt this transmission. Use of different fault current limiters (FCLs) minimizes the effect of faults and allows normal operation with minimum interruption in power flow. In this study, a series-parallel resonance-type fault current limiter (SPRFCL) is presented for enhancing the low-voltage ride-through (LVRT) capability of DFIG-based wind farms. The SPRFCL preserves the nominal voltage and power quality within the permissible limit during normal operation and during disturbances irrespective of the type of fault. The effectiveness of the proposed SPRFCL is validated by simulating both symmetrical and asymmetrical faults. Alongside the SPRFCL, two state-of-the-art FCLs—the parallel resonance-type fault current limiter (PRFCL) and the capacitive bridge-type fault current limiter (CBFCL)—are considered to investigate and compare the relative performances. Several graphical and numerical studies assure the efficacy of the proposed SPRFCL in wind farm application in multiple aspect. Moreover, the stunning total harmonic distortion (THD) values with the proposed technique signifies the excellency over its competitors. Additionally, the sub-synchronous resonance (SSR) analysis confirms the supremacy of SPRFCL for series compensated lines.

A SMES-Based Dynamic Current Limiter to Improve the LVRT Capability of DFIG-Based WECS

To improve the low voltage ride through (LVRT) capability of the doubly fed induction generator (DFIG), a series-connected current source converter (CSC) based superconducting magnetic energy storage (SMES) device is proposed. Based on the controllable current source characteristic of the SMES and the electromagnetic principle of the transformer, the device will behave as a dynamic impedance to suppress the overcurrent during the voltage sag period. Simulation results based on MATLAB further verify its good current limiting performance.

Transient Stability Enhancement of a Grid-Connected Large-Scale PV System Using Fuzzy Logic Controller

This paper presents a new intelligent control strategy to augment the low-voltage ride-through (LVRT) potential of photovoltaic (PV) plants, and the transient stability of a complete grid system. Modern grid codes demand that a PV plant should be connected to the main power system during network disturbance, providing voltage support. Therefore, in this paper, a novel fuzzy logic controller (FLC) using the controlled cascaded strategy is proposed for the grid side converter (GSC) of a PV plant to guarantee voltage recovery. The proposed FLC offers variable gains based upon the system requirements, which can inject a useful amount of reactive power after a severe network disturbance. Therefore, the terminal voltage dip will be low, restoring its pre-fault value and resuming its operation quickly. To make it realistic, the PV system is linked to the well-known IEEE nine bus system. Comparative analysis is shown—using power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC) software—between the conventional proportional–integral (PI) controller-based cascaded strategy and the proposed control strategy to authenticate the usefulness of the proposed strategy. The comparative simulation results indicate that the transient stability and the LVRT capability of a grid-tied PV system can be augmented against severe fault using the proposed FLC-based cascaded GSC controller.

An Anti-Fluctuation Compensator Design and Its Control Strategy for Wind Farm System

Large-scale wind farms in commercial operations have demonstrated growing influence on the stability of an electricity network and the power quality thereof. Variations in the output power of large-scale wind farms cause voltage fluctuations in the corresponding electrical networks. To achieve low-voltage ride-through capability in a doubly fed induction generator (DFIG) during a fault event, this study proposes a real-time reactive power control strategy for effective DFIG application and a static synchronous compensator (STATCOM) for reactive power compensation. Mathematic models were developed for the DFIG and STATCOM, followed by the development of an indirect control scheme for the STATCOM based on decoupling dual-loop current control. Moreover, a real-world case study on a commercial wind farm comprising 23 DFIGs was conducted. The voltage regulation performance of the proposed reactive power control scheme against a fault event was also simulated. The simulation results revealed that enhanced fault ride-through capability and prompt recovery of the output voltage provided by a wind turbine generator could be achieved using the DFIG along with the STATCOM in the event of a three-phase short-circuit fault.

Enhanced Low Voltage Ride Through Capability for Grid Connected Wind Energy Conversion System

It is obvious that the current era has received much attention in the fields of science and technology, besides the continuous endeavor to provide environmentally friendly and resource-saving alternatives for conventional energy conversion systems. The rapid development of Wind Energy Conversion Systems (WECS) has made Permanent Magnet Synchronous Generator (PMSG) a primer choice because of its advantages. The current trend on WECS necessitates wind turbines to maintain continuous operation during voltage drops, which is referred to as Low Voltage Ride Through (LVRT). The PMSG control technique is a widely used approach for improving conversion efficiency as well as LVRT capability. This paper provides LVRT and power enhancement for grid-connected PMSG based WECS using control techniques. The LVRT capability has been investigated by using PI and Residue controllers. The simulation results show improved active power delivery and better LVRT capability during voltage dips when the Residue controller is implemented.

Strategies to Increase the Transient Active Power of Photovoltaic Units during Low Voltage Ride Through

Due to a limitation in the magnitude of the three-phase output inverter currents, the output active power of the photovoltaic (PV) unit has been de-rated during low voltage ride through, which brings great instability risk to the power system. With the increase in the penetration rate of new energy, the impact of the power shortage on the system transient stability increases. It is of great significance to analyze the impact of this transient power shortage on system stability. This article explores methods to improve the active power output capability of photovoltaic units during low-breakthrough periods. A transient simulation model of a grid-connected PV generator with low-voltage ride-through (LVRT) capability is presented, under the condition of meeting the overcurrent capacity of the PV inverter and the requirement of dynamic reactive power support supplied by the PV generator specified in the China grid codes (GB/T 19964-2012) during grid fault. An example system with high PV penetration is built. The change principle and influencing factors of PV transient active power output are analyzed. The simulation model is designed in PowerFactory/DIgSILENT, and several types of three-phase voltage sags are performed in simulation to assess the impact of the active current reference calculation method and the maximum inverter output current (Imax) limit value on the PV active power output. According to the three indexes, namely the maximum active power of PV unit during the fault, the power improvement gradient and the power surge after the fault is cleared. Simulation results showed that using the orthogonal decomposition method to calculate the active current reference can make full use of the current capacity of the converter. Setting Imax to 1.1 rated current of photovoltaic inverter (IN) can reduce the cost-effectiveness ratio of the transient active power output of the PV unit. Therefore, we aim to improve the unit’s transient active power output capacity and realize the optimal effect of improving the transient active power shortage of the system.

Internal mode control based coordinated controller for brushless doubly fed induction generator in wind turbines during fault conditions

Brushless double fed induction generator (BDFIG) based machines have gained popularity in wind turbine applications because of their easily accessible design. Low voltage ride through (LVRT) is critical for the reliable integration of renewable energy with the power grid. The refore, LVRT capability of brushless DFIGs makes them an attractive choice for maintaining voltage stability in grid. The existing works on BDFIG control suffer from two major drawbacks. Firstly, the methodology does not consider LVRT as a design metric, and secondly, these techniques do not have any means for coordinating between a machine side inverter (MSI) and grid side inverter (GSI). This results in sub-optimal controller design and eventually result in the violation of grid code requirements. To solve these issues, this paper proposes the use of brushless DFIGs in wind turbines using a control technique based on analytical modeling. Moreover, employing internal model control (IMC), the proposed technique can effectively coordinate the control between the MSI and GSI resulting in reduced oscillations, overshoots and improved stability under fault conditions. Furthermore, the simulation results for wind turbine generators show that the proposed scheme significantly improves the stability and compliance of grid codes ascompared to the existing hardware techniques.

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.

Robust control scheme for the braking chopper of PMSG-based wind turbines–A comparative assessment

Despite the grid-integration of permanent magnet synchronous generator (PMSG) wind energy conversion systems (WECS), utilities are facing challenges that are hindering wind penetration growth. Grid-side voltage sags at the point of common coupling (PCC) are considered as one of such uncontested issues. Due to imbalance in back-to-back (BTB) converter’s dc-link input-output powers, its capacitor and power electronics are highly susceptible to PCC voltage sags. Hence, provision of wind farm grid-codes (GC) for supporting grid operations is imperative and development of strategies for realizing low-voltage ride-through capability in WECSs have emerged as an integral GC requirement. One strategy is to install a braking chopper (BC) across the dc-link. This study presents (a) an overview of conventional BC control methods, (b) design of a robust BC controller for PMSG-based WECSs, and (c) comparative performance assessment of (b) with (a) for PMSG-based WECS operations during normal and grid fault conditions. The robust BC controller takes advantage of the sliding mode method to control BC and to determine the grid-side converter active power reference set-point. This nonlinear control scheme ensures proper execution of GCs and offers reliable protection to the dc-link. Chattering phenomenon and reaching-time is reduced significantly, by further adoption of exponential reaching law. In addition to its robustness, a prominent feature of this BC controller is its applicability to present BTB control structures. The simulation results reflect on effectuality and robustness of the proposed BC controller in comparison to conventional BC control strategies.

Low-Voltage Ride-Through of the Novel Voltage Source-Controlled PMSG-Based Wind Turbine Based on Switching the Virtual Resistor

Voltage source (VS) control based on inertia synchronization is a novel phase lock loop (PLL)-less autonomous grid-synchronization control strategy suitable for the permanent magnet synchronous generator (PMSG)-based wind turbine. It can autonomously sense grid frequency fluctuations by adopting the dynamics of DC-link capacitor, and it has the advantage of stable operation in an extremely weak grid. This paper further studies the low-voltage ride-through (LVRT) of the PMSG-based wind turbine under the VS control, and presents a wind turbine structure with the additional energy storage battery on the DC side, which not only improves its LVRT capability but also enables the wind turbine to participate in the grid primary frequency regulation. The transient characteristics of VS-controlled wind turbines after the occurrence of the short-circuit fault are analyzed, and a current suppression strategy via switching the virtual resistor in the control loop of the grid-side converter (GCS) is presented. Through coordination with the energy storage battery, the LVRT of the PMSG-based wind turbine is realized, which has the advantage of withstanding a long-time short-circuit fault. Finally, based on the PSCAD/EMTDC simulation platform, the feasibility of the control strategy and the correctness of the theoretical analysis are verified.

LVRT Enhancement of a Grid-tied PMSG-based Wind Farm using Static VAR Compensator

This paper presents an efficient Low Voltage Ride Through (LVRT) control scheme for a 10.0MW grid-tied Permanent Magnet Synchronous Generator (PMSG)-based wind farm. The proposed control strategy plans to enhance the power quality and amount of injected power to satisfy the grid code requirements. The proposed approach utilizes a static Shunt Var Compensator (SVC) to enhance the LVRT capability and to improve power quality. It has been observed from the outcomes of the study that the proposed SVC controller ensures safe and reliable operation of the considered PMSG-based power system. The proposed system not only improves power quality but also it provides voltage stability of the Wind Energy Conversion System (WECS) under abnormal/fault conditions. The results show the superiority of the proposed control strategy.

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.

Multilevel converter integration for low voltage ride through controlling renewable wind energy conversion systems

In this study, a new control of grid-connected doubly-fed induction generators (DFIGs) is introduced to attain lowvoltage ride-through capability. The grid-side converter is proposed to be a multilevel converter (MLC) controlled by conventional DC-link voltage controller. The MLC is designed and integrated into the grid-connected DFIG. The rotor-side converter is considered a two-level conventional converter controlled by the proposed virtual voltage strategy. The virtual strategy is to use the designed input control signal rather than the actual measured one during system disturbance. The system disturbance in this study is the voltage dip. The proposed virtual controller is designed to use busbar voltage during normal operation, whereas it utilizes the designed virtual voltage during wide range conditions of low grid voltages. The energy conversion system currents are increased due to voltage dip disturbances. The use of designed virtual voltage values is proposed as inputs to the controller to limit these currents at the rating values. These virtual voltages are extracted using direct-quadrature machine representation and fixed machine currents at the rated values or lower. On the basis of the proposed control concerning the virtual voltage concept, the generator currents are limited to the rated values, thereby protecting the energy conversion system during low grid voltages. Simulation results prove the validity of the proposed control scheme during extreme low voltages.

Design and Performance Analysis of a PV Control Scheme to Improve LVRT of Hybrid Power System.

Solar power stations worldwide have been rising every year as well as the usage of sustainable power has increased. The addition into the traditional electricity grid networks of renewable sources such as the photovoltaic (PV) system is an essential challenge, given its erratic power generation. The major issue is to detach PV from the faulty grid, which leads to fluctuation in the system's interconnected PV system and system-wide energy interruptions. The low voltage ride through (LVRT) enables the PV scheme should stay linked to the grid even under fault situations in short periods and must assist the grid return to regular situations according to the modern grid codes. Hence, this paper develops and analyzes five aspects of control strategy to increase PV station's LVRT capability. The in-depth simulation of the independent units of the grid-connected large-scale PV farm is accomplished using PSCAD/EMTDC to assess the feasibility of the suggested control tactic for the modified IEEE nine-bus system.

LVRT and Stability Enhancement of Grid-Tied Wind Farm Using DFIG-Based Wind Turbine

According to the grid code specifications, low voltage ride-through (LVRT) is one of the key factors for grid-tied wind farms (WFs). Since fixed-speed wind turbines with squirrel cage induction generators (FSWT-SCIGs) require an adequate quantity of reactive power throughout the transient period, conventional WF consisting of SCIG do not typically have LVRT capabilities that may cause instability in the power system. However, variable-speed wind turbines with doubly fed induction generators (VSWT-DFIGs) have an adequate amount of LVRT enhancement competency, and the active and reactive power transmitted to the grid can also be controlled. Moreover, DFIG is quite expensive because of its partial rating (AC/DC/AC) converter than SCIG. Accordingly, combined installation of both WFs could be an effective solution. Hence, this paper illustrated a new rotor-side converter (RSC) control scheme, which played a significant role in ensuring the LVRT aptitude for a wide range of hybrid WF consisting of both FSWT-SCIGs and VSWT-DFIGs. What is more, the proposed RSC controller of DFIG was configured to deliver an ample quantity of reactive power to the SCIG during the fault state to make the overall system stable. Simulation analyses were performed for both proposed and traditional controllers of RSC of the DFIG in the PSCAD/EMTDC environment to observe the proposed controller response. Overall, the presented control scheme could guarantee the LVRT aptitude of large-scale SCIG.

A DC chopper‐based fast active power output reduction scheme for DFIG wind turbine generators

Increasing penetration level of renewable energy results in unprecedented operational challenges of AC grids due to generation uncertainty and reduced inertia. In grid emergencies, Fast active Power output Reduction (FPR) is demanded to drop wind turbine generator (WTG) power outputs faster than usual. However, the power mismatch between WTG mechanical power input and reduced electrical power output resulted from FPR can lead to rotor overspeed, which cannot be timely nullified by slow-responding pitch control. Therefore, this paper proposes a novel FPR scheme for doubly-fed induction generator (DFIG)-based WTG through coordinated control of DC chopper and pitch angle. Specifically, the FPR command is executed by DFIG rotor-side converter control, while rotor overspeed is restricted by triggering the DC chopper to dissipate the mismatched power. The pitch angle is actuated at a maximal rate to eliminate the mismatched power and switch off the DC chopper. The effectiveness of the proposed FPR scheme is verified through comparative simulation studies under FPR command execution and a grid fault. It shows the proposed scheme can achieve FPR in tens of milliseconds, avoid rotor overspeed and provide low-voltage ride-through capability in a fully-controlled manner.

Second‐order sliding mode control of wind turbines to enhance the fault‐ride through capability under unbalanced grid faults

AbstractThe integration of wind generation to the grid is growing rapidly across the world. As a result, grid operators have introduced the so‐called grid codes (GC), which nowadays include a range of technical conditions and requirements, which wind generators must fulfill. Among these, the low‐voltage ride through (LVRT) is a requirement for wind turbines to stay connected to the grid and continue to operate during the disturbance. In this study, a control structure, combining inertial kinetic energy storage with a crowbar circuit, is proposed to enhance the ride through capability of a wind turbine generator (WTG) based on a wound‐field synchronous generator (WFSG) under unsymmetrical voltage dips. For the grid‐side converter (GSC), a decoupled double synchronous reference frame (DDSRF) d‐q current controller is used. Furthermore, a second‐order sliding mode controller (SOSMC) with super‐twisting (ST) algorithm is proposed for the GSC and the machine‐side converter (MSC) to improve the response speed and achieve an accurate regulation of the dq‐axis current components simultaneously. The main objectives of the GSC are to achieve a balanced, sinusoidal current and smooth the real and reactive powers to reduce the influence of the negative‐sequence voltage. A series of simulations are presented to demonstrate the effectiveness of the proposed control scheme in improving the LVRT capability of the WFSG‐driven wind turbine and the power quality of the system under unbalanced grid voltage conditions.

Phaselet-Based Limited-Time $q$-Axis DC Offset Voltage Injection for Detection of Islanding of DG and Fault in a Meshed Microgrid

Rapid grid integration of inverter-interfaced distributed generators (DGs) and low-voltage ride-through capability of the smart inverter, as per IEEE 1547 standard, has made unintentional islanding detection (UID) cumbersome. Also, the feeder reconfiguration capability in meshed microgrids with multiple DGs has made the existing UID techniques ineffective. The methods have limitations in UID for small power mismatch conditions characterizing nondetection zone (NDZ). In hybrid methods for UID, a disturbance injection is continuous or for an extended period, which degrades the power quality (PQ) and creates stability issues. Hence, a decentralized closed-loop hybrid islanding detection method (HIDM) based on the indices extracted from the subcycle phaselet (passive) technique with limited-time (only for two cycles) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$q$</tex-math></inline-formula> -axis dc offset voltage injection (active) is proposed for UID of DGs. The proposed HIDM mitigates the NDZ due to the closed-loop scheme. Subcycle estimations from the phaselet technique result in quick UID and fault detection and classification process. Furthermore, the PQ issue is reduced due to the limited-time voltage injection (based only on the command from phaselet technique) in the proposed HIDM. Simulation results from the real-time digital simulator on a meshed microgrid and performance comparison with other existing methods prove the efficacy of the proposed HIDM.

A novel model predictive control with an integrated SOC and floating DC-link voltage balancing for 3-phase 7-level PUC converter-based MV BESS

Compared to conventional multilevel topologies, the Packed U-Cell multilevel topology utilizes the minimum number of components and provides both maximum voltage levels and high reliability. When used in a battery energy storage system, its dc-link consists of a top branch battery storage and bottom branch flying capacitors, whose state of charge and voltages must be balanced.This paper proposes a novel control scheme with two cascaded controllers: the inner loop regulates the output filter current and dc-link voltages using a novel finite-set model predictive control, the outer one is fully integrated with the model predictive controller and regulates the battery state of charge by injecting a zero-sequence voltage. The proposed control scheme was successfully implemented and tested through simulation and real-time testing. The obtained results highlight good dynamic performance and robustness of the proposed scheme during active power injection/absorption and low voltage ride-through capability.

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

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