Converter fault diagnosis and post-fault operation of a doubly-fed induction generator for a wind turbine

Wind energy has become one of the most important alternative energy resources because of the global warming crisis. Wind turbines are often erected off-shore because of favourable wind conditions, requiring lower towers than on-shore. The doubly-fed induction generator is one of the most widely used generators with wind turbines. In such a wind turbine the power converters are less robust than the generator and other mechanical parts. If any switch failure occurs in the converters, the wind turbine may be seriously damaged and have to stop. Therefore, converter health monitoring and fault diagnosis are important to improve system reliability. Moreover, to avoid shutting down the wind turbine, converter fault diagnosis may permit a change in control strategy and/or reconfigure the power converters to permit post-fault operation. This research focuses on switch fault diagnosis and post-fault operation for the converters of the doubly-fed induction generator. The effects of an open-switch fault and a short-circuit switch fault are analysed. Several existing open-switch fault diagnosis methods are examined but are found to be unsuitable for the doubly-fed induction generator. The causes of false alarms with these methods are investigated. A proposed diagnosis method, with false alarm suppression, has the fault detection capability equivalent to the best of the existing methods, but improves system reliability. After any open-switch fault is detected, reconfiguration to a four-switch topology is activated to avoid shutting down the system. Short-circuit switch faults are also investigated. Possible methods to deal with this fault are discussed and demonstrated in simulation. Operating the doubly-fed induction generator as a squirrel cage generator with aerodynamic power control of turbine blades is suggested if this fault occurs in the machine-side converter, while constant dc voltage control is suitable for a short-circuit switch fault in the grid-side converter.Wind energy has become one of the most important alternative energy resources because of the global warming crisis. Wind turbines are often erected off-shore because of favourable wind conditions, requiring lower towers than on-shore. The doubly-fed induction generator is one of the most widely used generators with wind turbines. In such a wind turbine the power converters are less robust than the generator and other mechanical parts. If any switch failure occurs in the converters, the wind turbine may be seriously damaged and have to stop. Therefore, converter health monitoring and fault diagnosis are important to improve system reliability. Moreover, to avoid shutting down the wind turbine, converter fault diagnosis may permit a change in control strategy and/or reconfigure the power converters to permit post-fault operation. This research focuses on switch fault diagnosis and post-fault operation for the converters of the doubly-fed induction generator. The effects of an open-switch fault and a short-circuit switch fault are analysed. Several existing open-switch fault diagnosis methods are examined but are found to be unsuitable for the doubly-fed induction generator. The causes of false alarms with these methods are investigated. A proposed diagnosis method, with false alarm suppression, has the fault detection capability equivalent to the best of the existing methods, but improves system reliability. After any open-switch fault is detected, reconfiguration to a four-switch topology is activated to avoid shutting down the system. Short-circuit switch faults are also investigated. Possible methods to deal with this fault are discussed and demonstrated in simulation. Operating the doubly-fed induction generator as a squirrel cage generator with aerodynamic power control of turbine blades is suggested if this fault occurs in the machine-side converter, while constant dc voltage control is suitable for a short-circuit switch fault in the grid-side converter

Power Quality Improvement and Low Voltage Ride through Capability in Hybrid Wind-PV Farms Grid-Connected Using Dynamic Voltage Restorer

© 2018 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission.This paper proposes the application of a dynamic voltage restorer (DVR) to enhance the power quality and improve the low voltage ride through (LVRT) capability of a three-phase medium-voltage network connected to a hybrid distribution generation system. In this system, the photovoltaic (PV) plant and the wind turbine generator (WTG) are connected to the same point of common coupling (PCC) with a sensitive load. The WTG consists of a DFIG generator connected to the network via a step-up transformer. The PV system is connected to the PCC via a two-stage energy conversion (dc-dc converter and dc-ac inverter). This topology allows, first, the extraction of maximum power based on the incremental inductance technique. Second, it allows the connection of the PV system to the public grid through a step-up transformer. In addition, the DVR based on fuzzy logic controller is connected to the same PCC. Different fault condition scenarios are tested for improving the efficiency and the quality of the power supply and compliance with the requirements of the LVRT grid code. The results of the LVRT capability, voltage stability, active power, reactive power, injected current, and dc link voltage, speed of turbine, and power factor at the PCC are presented with and without the contribution of the DVR system.Peer reviewe

Rotor fault analysis in a doubly-fed induction generator using impedance matrix technique

© 2017 IEEE. Condition monitoring is a standard method for scheduling maintenance and ensuring that catastrophic failures do not occur in industrial motors

Design and implementation of variable speed wind energy induction generator systems for fault studies

Includes bibliographical references (leaves [136]-139).Due to the economical and environmental benefits, Wind Energy Conversion Systems (WECS) have received tremendous growth in the past decade. The increased interest in wind energy has made it necessary to model and experimentally evaluate entire WECS, so as to attain a better understanding and to assess the performance of various systems. As a direct consequence of the increase in wind generation systems, comes the need for the reduction of operational and maintenance costs of these wind generators. The most efficient way of reducing these costs is by the early detection of the degeneration of these generators health, thus facilitating a proactive response, minimizing downtime, and maximizing productivity. The more common induction machine failures are caused by the deterioration of the stator insulation and by the breaking of rotor bars. The thesis describes the design, modeling and implementation of two different variable speed induction generator systems for studying faults in wind energy applications. This project served as a platform for further research into the development and evaluation of a non-stationary fault detection technique suitable for wind energy induction generator purposes. Some common faults are implemented on the wind generators in an attempt to identify them from measurements and by using a steady state fault analysis technique (Motor Current Signature Analysis). For variable speed wind generation, there are two systems using induction generators. The first consist of a squirrel cage induction generator, which uses back-to-back converters in the stator circuit, as shown in Fig. 0.1. The second consists of a wound rotor induction generator, whereby the stator is directly connected to the grid and the rotor circuit consists of back-to-back converters, as shown in Fig. 0.2. When both the rotor and stator are capable of delivering power as with the wound rotor induction generator, they are known as doubly-fed induction generators (DFIG)

Fault Analysis Of An Unbalanced Distribution System With Distributed Generation

In recent years there has been a lot of emphasis on renewable power integration due to environmental issues and to lower the dependence on fossil fuels. The presence of renewable sources in the distribution systems adds complexity to the calculation of the power flows and hence has a direct impact on the short circuit calculations, protection and control. The presence of unbalance in distribution systems worsens the situation since the three phase voltages and currents are no longer equal in magnitude and 120 degrees phase shifted. This thesis involves a fault study in a 14-bus distribution system with integrated wind and solar power generation and shows the impact of unbalance in the system on short circuit calculations. The effect of unbalance on the behavior of traditional synchronous sources is already known and has been shown to cause errors in fault current magnitudes in the system. This thesis aims at observing and comparing the behavior of distributed generators in a balanced and an unbalanced distribution system. Detailed modeling of the DFIG and a grid connected PV array has been carried out in PSCAD. A 14 bus distribution system has been built and the distributed sources have been integrated into it. Unbalance has been introduced into an originally built balanced system by applying unbalanced loads at the buses and using untransposed feeders. Therefore, two systems, balanced and unbalanced, have been simulated and the behavior of the integrated distributed sources during faults has been compared for both the cases

A New Converter Station Topology to Improve the Overall Performance of a Doubly Fed Induction Generator-Based Wind Energy Conversion System

This thesis presents a reliable and cost effective technique that calls for reconfiguration of the existing converters of a typical Doubly Fed Induction Generator to include a coil of low internal resistance. A coil within the DC link is the only hardware component required to implement this technique. With a proper control scheme, activated during fault conditions, this coil can provide the same degree of performance as a superconducting magnetic energy storage unit during fault conditions

Grid fault ride through for wind turbine doubly-fed induction generators

EngD ThesisWind farms must contribute to the stability and reliability of the transmission grid, if they are to form a robust component of the electrical network. This includes providing grid support during grid faults, or voltage dips. Transmission system grid codes require wind farms to remain connected during specified voltage dips, and to supply active and reactive power into the network. Doubly-fed induction generator (DFIG) technology is presently dominant in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially-rated power converter. However, the DFIG is sensitive to dips in supply voltage. Without specific protection to 'ride through' grid faults a DFIG risks damage to its power converter due to over-current and/or overvoltage. Conventional converter protection via a sustained period of rotor-crowbar closed-circuit leads to poor power output and sustained suppression of the stator voltages. This thesis presents a detailed understanding of wind turbine DFIG grid fault response, including flux linkage behaviour and magnetic drag effects. A flexible 7.5kW test facility is used to validate the description of fault response and evaluate techniques for improving fault ride-through performance. A minimum threshold rotor crowbar method is presented, successfully diverting transient over-currents and restoring good power control within 45ms of both fault initiation and clearance. Crowbar application periods were reduced to 11-16ms. A study of the maximum crowbar resistance suggests that this method can be used with high-power DFIG turbines. Alternatively, a DC-link brake method is shown to protect the power converter and quench the transient rotor currents, allowing control to be resumed; albeit requiring 100ms to restore good control. A VAr-support control scheme reveals a 14% stator voltage increase in fault tests: reducing the step-voltage impact at fault clearance and potentially assisting the fault response of other local equipment.EPSR

Detection of faults in a scaled down doubly-fed induction generator using advanced signal processing techniques.

The study ventures into the development of a micro-based doubly fed induction generator (DFIG) test rig for fault studies. The 5kW wound rotor induction machine (WRIM) that was used in the test rig was based on a scaled-down version of a 2.5MW doubly fed induction generator (DFIG). The micromachine has been customized to make provision for implementing stator inter-turn short-circuit faults (ITSCF), rotor ITSCF and static eccentricity (SE) faults in the laboratory environment. The micromachine has been assessed under the healthy and faulty states, both before and after incorporating a converter into the rotor circuit of the machine. In each scenario, the fault signatures have been characterised by analyzing the stator current, rotor current, and the DFIG controller signals using the motor current signature analysis (MCSA) and discrete wavelet transform (DWT) analysis techniques to detect the dominant frequency components which are indicative of these faults. The purpose of the study is to evaluate and identify the most suitable combination of signals and techniques for the detection of each fault under steady-state and transient operating conditions. The analyses of the results presented in this study have indicated that characterizing the fault indicators independent of the converter system ensured clarity in the fault diagnosis process and enabled the development of a systematic fault diagnosis approach that can be applied to a controlled DFIG. It has been demonstrated that the occurrence of the ITSCFs and the SE fault in the micro-WRIM intensifies specific frequency components in the spectral plots of the stator current, rotor current, and the DFIG controller signals, which may then serve as the dominant fault indicators. These dominant components may be used as fault markers for classification and have been used for pattern recognition under the transient condition. In this case, the DWT and spectrogram plots effectively illustrated characteristic patterns of the dominant fault indicators, which were observed to evolve uniquely and more distinguishable in the rotor current signal compared to the stator current signal, before incorporating the converter in the rotor circuit. Therefore, by observing the trends portrayed in the decomposition bands and the spectrogram plots, it is deemed a reliable method of diagnosing and possibly quantifying the intensity of the faults in the machine. Once the power electronic converter was incorporated into the rotor circuit, the DFIG controller signals have been observed to be best suited for diagnosing faults in the micro-DFIG under the steady-state operating condition, as opposed to using the terminal stator or rotor current signals. The study also assessed the impact of undervoltage conditions at the point of common coupling (PCC) on the behaviour of the micro-DFIG. In this investigation, a significant rise in the faulted currents was observed for the undervoltage condition in comparison to the faulty cases under the rated grid voltage conditions. In this regard, it could be detrimental to the operation of the micro-DFIG, particularly the faulted phase windings, and the power electronic converter, should the currents exceed the rated values for extended periods

Impact of hybrid renewable energy systems on short circuit levels in distribution networks

The effects of the distributed generation can be classified as environmental, technical and economical effects. It is playing a very vital role for improving the voltage profiles in electrical power systems. However, it could have some negative impacts such as operating conflicts for fault clearing and interference with relaying. Distribution system is the link between the utility system and the consumer. It is divided into three categories radial, Loop, and network. Distribution networks are the most commonly used to cover huge number of loads. The power system reliability mainly depends on the smooth operation and continuity of supply of the distribution network. However, this may not always be guaranteed especially with the introduction of distributed generation to the distribution network. This paper will examine the impact of hybrid renewable energy systems (using photovoltaic and doubly fed induction generators) on short circuit level of IEEE 13-bus distribution test system using ETAP software