Ride-through Performance Research Articles

Parallel R-L configuration crowbar with series R-L circuit protection for LVRT strategy of DFIG under transient-state

This paper proposes an impedance protection circuit configuration to enhance the ride-through of doubly-fed induction generators (DFIGs) based wind turbines (WTs) during faults. The proposed impedance protection circuit configuration comprises of a parallel R-L impedance crowbar and a series R-L impedance circuit. Traditionally, resistive-crowbar (i.e., R-crowbar) circuit is applied as the means of protection for the DFIG WT which efficiently protects the rotor-side converter (RSC) and dc-link capacitor against the effects of severe grid faults. However, the conventional R-crowbar circuit disconnects the RSC leading to the loss of generator excitation control. During that time, the WT generator becomes a conventional squirrel-cage induction generator (SCIG) which obtains its magnetization current from the point of common coupling (PCC) via stator winding terminals. This phenomenon further deepens the voltage-dip at the PCC. Alternatively, integrating the R-crowbar with series R-L branch partially maintains the RSC connection to the rotor winding. Hence the generator excitation control is also partially maintained but oscillations of the rotor currents and dc-link voltage can still affect the ride-through performances of WT driven by the DFIG. With the proposed protection circuit configuration, it compensates the deficiency of the two conventional protection circuits configuration. In order to validate the performance of the proposed scheme, extensive simulation studies are carried out using MATLAB/Simulink software. From the comparative simulation results, improved performances of the DFIG under transient-state are achieved with the proposed protection circuit configuration than the conventional protection circuits.

Study of a Modified Flux-Coupling-Type SFCL for Efficient Fault Ride-Through in a PMSG Wind Turbine Under Different Types of Faults

Regarding the permanent-magnet synchronous generator (PMSG)-based wind turbine system, this paper proposes a modified flux-coupling-type superconducting fault current limiter (SFCL) to enhance its fault ride-through (FRT) performance. The modified SFCL's structural principle and theoretical influence on the PMSG ride through capability are conducted, and a comparison of the SFCL and a dynamic braking chopper (BC) is performed. Using MATLAB, a detailed model of a 1.5-MW PMSG-based wind turbine integrated with the SFCL/BC is built, and the simulations of symmetrical and asymmetrical faults are done. From the results, introducing the modified SFCL can limit the fault currents in the generator and grid sides. In addition, using the modified SFCL, we are able to compensate the generator voltage and alleviate the dc-link overvoltage. Thus, the wind turbine system's power balance is improved, and also the fault recovery process can be accelerated. On the whole, the modified SFCL is better than the BC for assisting the FRT operation of the PMSG.

Impact of series compensation on operation performance of large‐scale PV plants

Application of series compensation transmission lines to enhance the long-distance transmission capability of photovoltaic (PV) plants is the most economical technical measure. To investigate the impact of series compensation on operating performance of large-scale PV plants, the simulation research is performed in this study. The detailed model of large-scale PV plants connected to the grid through series compensated lines is developed using power systems computer aided design (PSCAD)/electromagnetic transients inccluding DC (EMTDC) software. Using the detailed model, the performances of active and reactive power outputs, voltage and current profiles of PV plants are investigated for different levels of series compensation and strengths of AC system during short-circuit faults. The simulation results show that series compensation and strong AC system can effectively improve the transient operation performance of the PV power plant. With the increase of the series compensation degree and the strength of AC system, the fault ride-through performance of PV plants is improved.

Advanced Fault Ride-Through Management Scheme for VSC-HVDC Connecting Offshore Wind Farms

This paper proposes a novel configuration and transient management scheme for Voltage Source Converter-High Voltage Direct Current (VSC-HVDC) connecting Large Scale Wind Farm (LSWF). The proposed configuration aims to fully utilize the HVDC converters with shunt and series reconfiguration during steady state and fault conditions respectively. Thus, it targets the employment of the available converters controls capabilities to enhance the Fault Ride-Through (FRT) performance of the VSC-HVDC and achieve smooth power evacuation to the electric grid. The control strategy is built in such a way to guarantee three factors: 1) enhancing the voltage profile of the electrical grid during symmetrical and asymmetrical grid faults, 2) providing smooth power evacuation from the LSWF, and 3) mitigating the DC link oscillations caused by unbalanced faults. Hence, the proposed scheme demonstrates superior performance of the VSC-HVDC system during steady state and transient conditions. Comprehensive simulation studies are carried out to demonstrate the gorgeous performance of the proposed VSC-HVDC configuration using PSCAD/EMTDC. In addition to that, a lab-scaled experimental set up is constructed to validate the functioning of the proposed VSC-HVDC configuration.

A Unified Current Loop Tuning Approach for Grid-Connected Photovoltaic Inverters

High level penetration of renewable energy sources has reshaped modern electrical grids. For the future grid, distributed renewable power generation plants can be integrated in a larger scale. Control of grid-connected converters is required to achieve fast power reference tracking and further to present grid-supporting and fault ride-through performance. Among all of the aspects for converter control, the inner current loop for grid-connected converters characterizes the system performance considerably. This paper proposes a unified current loop tuning approach for grid-connected converters that is generally applicable in different cases. A direct discrete-time domain tuning procedure is used, and particularly, the selection of the phase margin and crossover frequency is analyzed, which acts as the main difference compared with the existing studies. As a general method, the approximation in the modeling of the controller and grid filter is avoided. The effectiveness of the tuning approach is validated in both simulation and experimental results with respect to power reference tracking, frequency and voltage supporting.

Assessment and Enhancement of a Full-Scale PMSG-Based Wind Power Generator Performance Under Faults

A full-scale permanent-magnet synchronous generator (PMSG)-based wind turbine with dc-link voltage control via the machine-side converter has the potential to provide inherent low-voltage ride-through (LVRT) performance without additional hardware components. However, several important performance aspects related to this topology are not addressed in this literature. This paper investigates the impacts of the LVRT control on the stability and risk of resonance, successful operation, and fatigue in a full-scale PMSG-based wind power generation system. An analytical model, considering the double-mass nature of the turbine/generator and typical LVRT requirements, is developed, validated, and used to characterize the dynamic performance of the wind generation system under LVRT control and practical generator characteristics. To enhance the operation and reduce the fatigue under LVRT control, two solutions, based on active damping control and dc-link voltage bandwidth retuning, are proposed, analyzed, and compared. The detailed nonlinear time-domain simulation results validate the accuracy of the developed model and analytical results.

An improved low-voltage ride-through performance of DFIG based wind plant using stator dynamic composite fault current limiter

In this paper, an enhanced low-voltage ride-through (LVRT) performance of a grid connected doubly fed induction generator (DFIG) has been presented with the usage of stator dynamic composite fault current limiter (SDCFCL). This protection circuit comprises of a suitable series resistor–inductor combination and parallel bidirectional semiconductor switch. The SDCFCL facilitates double benefits such as reduction of rotor induced open circuit voltage due to increased value of stator total inductance and concurrent increase of rotor impedance. Both effects will limit rotor circuit over current and over voltage situation more secured way in comparison to the conventional scheme like the dynamic rotor current limiter (RCL) during any type of fault situation. The proposed concept is validated through the simulation study of the grid integrated 2.0MW DFIG.

Test and analysis on sensitivity of low‐voltage releases to voltage sags

This study discusses the sensitivity (i.e. ride-through performance) of low-voltage releases to voltage sags on the basis of extensive tests, and presents two sensitivity models to assess the tripping characteristics of low-voltage releases, which have not been studied before. First, working principle of low-voltage release and existing standards are reviewed. Second, a detailed test scheme is proposed and ten kinds of low-voltage releases have been tested. Test results show that magnitude, duration and point-on-wave angle of voltage sag jointly determine the sensitivity of low-voltage releases. After extremely processing test results and applying the curve fitting theory, the functions representing the relationship between magnitude and duration at different point-on-wave angles are formed, which symbolise a detailed sensitivity model of low-voltage release. Moreover, the other simpler and more practical sensitivity model is proposed by introducing rectangular envelope lines of voltage tolerance curves at different point-on-wave angle.

Improved Demagnetization Control of a Doubly-Fed Induction Generator Under Balanced Grid Fault

Doubly-fed induction generators are one of the most popular machines for variable-speed wind turbine. However, they are very sensitive to grid fault since the stator is directly connected to the grid. This paper analyzes the low-voltage ride-through performance of a doubly-fed induction generator-based wind turbine under balanced grid fault. An improved demagnetization control, immune to system parameter variation, is proposed to shorten the dynamic process. The investigation shows that the proposed method increases the probability of a successful ride-through at the recovery moment of balanced grid fault. Feasibility region for a representatively sized system is also investigated for balanced grid fault. In order to verify the proposed control method, laboratory experiments are carried out and the results demonstrate the analysis.

Implementation of Fault Ride-Through Techniques of Grid-Connected Inverter for Distributed Energy Resources With Adaptive Low-Pass Notch PLL

The amount of distributed energy resources (DERs) has constantly increased worldwide. As the power ratings of DERs have become considerably high, the grid code requirements are necessary to secure reliable power generation and transmission for the public electric network. In order to follow grid codes of the various countries and optimize the function of grid-connected inverters for DERs, a robust phase-locked loop (PLL) is essential for extracting the grid phase information accurately, when the grid voltage is polluted by harmonics and unbalanced. This paper proposes fault ride-through techniques based on a low-pass notch (LPN)-PLL. The LPN-PLL has not only fast and smooth transient responses to a sudden transition of the grid voltage but also has robustness to the distorted and unbalanced grid conditions. Therefore, the stable performance in the grid fault conditions is expected without the system trip. Furthermore, a universal voltage sag generator for the various grid codes with six parameters is proposed for the verification of low voltage ride-through (LVRT) performance. Experimental verifications are presented to show the LVRT performance based on the LPN-PLL with a three-phase grid-connected inverter (10 kV·A) and a prototype of the voltage sag generator (10 kV·A).

A novel control strategy for offshore DFIG-based wind farm integrated through diode-rectifier-based HVDC transmission

Summary This paper investigates the control strategy of the offshore DFIG-based wind farm integrated through a diode-rectifier-based HVDC link. The HVDC link is consisted of an uncontrolled diode rectifier, a voltage source inverter and a HVDC line. Based on an in-depth analysis of the diode-rectifier-based HVDC link and the DFIGs, the main influence factors of the offshore ac-grid voltage are revealed, which proves that the offshore ac-grid voltage can be kept in a proper range automatically as far as the onshore VSC is operated in constant DC voltage mode. A control strategy is proposed for the offshore ac-grid frequency control and the maximum power point tracking of the DFIGs. The study shows that the system can be started up with the help of several small power supplies and has a good fault ride-through performance during onshore PCC short-circuit faults. Moreover, the offshore wind farm is robust against wind speed variation, frequency reference value variation, capacitor bank switching and wind turbine disconnection. Finally, detailed PSCAD/EMTDC simulations are used to verify the theoretical analysis and the effectiveness of the control strategy. Copyright © 2015 John Wiley & Sons, Ltd.

Improving Low-Voltage Ride-Through Performance and Alleviating Power Fluctuation of DFIG Wind Turbine in DC Microgrid by Optimal SMES With Fault Current Limiting Function

The vital problems of doubly fed induction generator (DFIG) wind turbine are power fluctuation and low-voltage ride-through performance. To tackle both problems, the new circuit configuration and optimization technique of the superconducting magnetic energy storage with fault current limiting function (SMES-FCL) in a DC microgrid are presented. The SMES-FCL circuit mainly consists of two DC choppers with common superconducting coil (SC). During normal operation, the SMES-FCL acts as the SMES unit to suppress the power fluctuation of DFIG. When severe faults occur in the system, the SC is automatically connected to the system and used as the fault current limiter. Consequently, the fault current and the terminal voltage drop of DFIG can be alleviated. The energy function method is used to formulate the optimization problem of SC inductance, initial stored energy, and proportional-integral control parameters of choppers. Simulation study confirms the superior control effect of the SMES-FCL over the conventional SMES.

Equivalent Circuit Parameters for Large Brushless Doubly Fed Machines (BDFMs)

This paper presents analytical methods to calculate the equivalent circuit parameters for large-scale brushless doubly fed machines (BDFMs) with magnetic wedges utilized for closing stator open slots. The use of magnetic wedges reduces the magnetizing currents in the machine, reflected in the values of magnetizing inductances, but also increases leakage fluxes affecting the value of series inductances in the equivalent circuit. Though such effects can be modeled by numerical models, the proposed analytical methods are particularly helpful in optimizing machine design, inverter rating, reactive power management, and grid low-voltage ride-through performance. The conventional analytical methods cannot be readily applied to the BDFM due to its complex magnetic field distribution; this paper presents analytical methods to calculate the magnetizing and leakage inductances for the BDFM with magnetic wedges used in the stator slots. The proposed methods are assessed by experimentally verified finite-element models for a 250 kW BDFM.

Grid to Standalone Transition Motion-Sensorless Dual-Inverter Control of PMSG With Asymmetrical Grid Voltage Sags and Harmonics Filtering

This paper describes a variable-speed motion-sensorless control system for permanent-magnet synchronous generator (PMSG) connected to grid via back-to-back inverters for wind energy generation. The grid-side inverter control system employs proportional-integral (PI) current controllers with cross-coupling decoupling and line-voltage feedforward disturbance compensation. Also, a D-module filter is used to robustly extract the line voltage positive sequence followed by a phase-locked-loop (PLL) based observer to estimate the positive-sequence angle for control, including the case of asymmetric voltages and automatic seamless transfer method from grid connected to stand alone and vice versa. In stand-alone mode, a voltage control scheme with selective harmonic compensation is employed. The PMSG motion-sensorless control system uses an active power controller and a PLL-based observer to estimate the rotor position and speed without using the electromotive force (EMF) integration and initial rotor position. The paper investigates and validates the ride-through performance of this proposed system during asymmetric power grid-voltage sags, transition from grid connected to stand alone and vice versa and voltage harmonics compensation. While some of the aforementioned issues have been treated rather individually in previous conference publications of the authors, the present paper integrates them into a comprehensive control system of PMSG.

Low voltage ride-through of DFIG and brushless DFIG: Similarities and differences

The brushless doubly fed induction generator (BDFIG) has been proposed as a viable alternative in wind turbines to the commonly used doubly fed induction generator (DFIG). The BDFIG retains the benefits of the DFIG, i.e. variable speed operation with a partially rated converter, but without the use of brush gear and slip rings, thereby conferring enhanced reliability.As low voltage ride-through (LVRT) performance of the DFIG-based wind turbine is well understood, this paper aims to analyze LVRT behavior of the BDFIG-based wind turbine in a similar way. In order to achieve this goal, the equivalence between their two-axis model parameters is investigated. The variation of flux linkages, back-EMFs and currents of both types of generator are elaborated during three phase voltage dips. Moreover, the structural differences between the two generators, which lead to different equivalent parameters and hence different LVRT capabilities, are investigated.The analytical results are verified via time-domain simulations for medium size wind turbine generators as well as experimental results of a voltage dip on a prototype 250kVA BDFIG.

Second-order sliding mode control for DFIG-based wind turbines fault ride-through capability enhancement

This paper deals with the fault ride-through capability assessment of a doubly fed induction generator-based wind turbine using a high-order sliding mode control. Indeed, it has been recently suggested that sliding mode control is a solution of choice to the fault ride-through problem. In this context, this paper proposes a second-order sliding mode as an improved solution that handle the classical sliding mode chattering problem. Indeed, the main and attractive features of high-order sliding modes are robustness against external disturbances, the grids faults in particular, and chattering-free behavior (no extra mechanical stress on the wind turbine drive train).Simulations using the NREL FAST code on a 1.5-MW wind turbine are carried out to evaluate ride-through performance of the proposed high-order sliding mode control strategy in case of grid frequency variations and unbalanced voltage sags.

Application of Series Voltage Boosting Schemes for Enhanced Fault Ridethrough Performance of Fixed Speed Wind Turbines

This paper presents new series grid interface topologies for enhancing the low voltage ridethrough capability of self-excited induction generator-based wind turbines. Two proposed schemes: static synchronous series compensator (SSSC) and controllable series braking resistor (CSBR) have been designed and simulated. The test system simulated represents a wind turbine connected to an electric grid with alternatively employing SSSC and CSBR. The potential of the two schemes is evaluated and analyzed using positive and negative-sequence reference frames in response to balanced and unbalanced fault scenarios. The simulation results using PSCAD/EMTDC demonstrate the performance of the proposed schemes for improving the fault ridethrough capability and transient stability margin in response to severe symmetrical and asymmetrical grid faults. The schemes have shown successful and fast mitigation of voltage disturbance consequences on the mechanical system. The schemes helped the wind turbine to remain connected with considerable isolation from grid faults.

멀티레벨 인버터의 순간정전 보상알고리즘에 관한 연구

Cascaded H-Bridge multi-level inverters can be implemented through the series connection of single-phase modular power bridges. In recent years, multi-level inverters are becoming increasingly popular for high power applications due to its improved harmonic profile and increased power ratings. This paper presents a control method for balancing the dc-link voltage and ride-through enhancement, a modified pulse width-modulation Compensation algorithm of cascaded H-bridge multi-level inverters. During an under-voltage protection mechanism, causing the system to shut down within a few milliseconds after a power interruption in the main input sources. When a power interruption occurs finish, if the system is a large inertia restarting the load a long time is required. This paper suggests modifications in the control algorithm in order to improve the sag ride-through performance of ac inverter. The new proposed strategy recommends maintaining the DC-link voltage constant at the nominal value during a sag period, experimental results are presented.

Voltage Booster Schemes for Fault Ride-Through Enhancement of Variable Speed Wind Turbines

This paper presents series grid interface topologies for enhancing the fault ride-through (FRT) performance of doubly-fed induction generator (DFIG)-based wind turbines (WTs). Two voltage booster schemes, 1) dynamic voltage restorer (DVR) and 2) resistive type high temperature superconducting fault current limiter (HTS-FCL), are designed and implemented. The test system represents a WT connected to an electric grid with alternatively employing DVR and HTS-FCL. Both schemes provide fast mitigation of voltage dip that maintains the nominal operating conditions for DFIG-WT. To achieve a flexible control solution for balanced and unbalanced fault conditions, the DVR employs positive and negative sequence controllers while the HTS-FCL is designed to perform fast quenching for each phase individually. The potential of the two booster schemes is evaluated and analyzed in positive and negative sequence reference frames. Comprehensive simulation studies are presented to verify the capability of the series grid interface schemes for ensuring the normal operation and smooth wind power evacuation with effective isolation from grid faults. Furthermore, the grid code requirements of reactive current support are evaluated for both schemes at various fault scenarios.

Study of Low-Voltage Ride-Through Performance for Wind Power Generation with Doubly Fed Induction Generator

Summary The introduction of wind power generation is increasing rapidly. The ratio of wind power generation to the total generation capacity is becoming higher and higher. When a phase-to-phase fault occurs in the power system, the frequency of the power system is lower due to disconnection of wind power generation with doubly fed induction generators (DFIG). Therefore, the power system might become unstable. This paper describes an LVRT (low-voltage ride-through) performance improvement scheme for wind power generation with DFIG. The wind power generator is disconnected from the grid in case of a power system fault. It is made to operate in isolation from the grid by controlling the inverters installed with the generators. After clearance of the power system fault, wind power generation is immediately reconnected to the grid. As a result, instability in the power system disappears. The performance of LVRT is confirmed by using the simulation software PSCAD/EMTDC. The simulation results show excellent results for the three-phase short-circuit fault with a voltage dip of 100%. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 185(1): 17–26, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22423