Fault Behavior of Wind Turbines

Synchronous generators have always been the dominant generation type in the grid. This fact affected both planning and operation of power systems. With the fast increase of wind power share in the grid in the last decade, the situation is changing. In some countries wind power represents already a consistent amount of the total generation. Wind turbines can be classified as non-synchronous generation and they behave differently than synchronous generation under many circumstances. Fault behavior is an important example. This thesis deals with the behavior of wind turbines during faults in the grid. The first part focuses on the fault currents delivered by wind turbines with Doubly-Fed Induction Generators (DFIG). The second part investigates the impact of faults below the transmission level on wind turbine grid fault ride-through and the voltage support that wind turbines can provide in weak grids during faults. A wide theoretical analysis of the fault current contribution of DFIG wind turbines with crowbar protection is carried out. A general analytical method for fault current calculation during symmetrical and unsymmetrical faults in the grid is proposed. The analytical method can be used to find the maximum fault current and its AC or DC components without the need to actually perform detailed simulations, which is the method used today. DFIG wind turbines may also be protected using a chopper resistance on the DC-link. A method to model the DC-link with chopper as an equivalent resistance connected to the generator rotor during symmetrical grid faults is presented. This allows to calculate the short-circuit currents of a DFIG with chopper protection as an equivalent DFIG with crowbar protection. This is useful since fault current calculation methods for DFIG with crowbar are available in the literature. Moreover, power system simulation tools include standard models of DFIG wind turbines with crowbar protection, but often not with chopper protection. The use of an aggregate model to represent the fault current contribution of a wind farm has been analyzed through simulations. It has been found that the aggregate model is able to reproduce accurately the total fault current of the wind farm for symmetrical and unsymmetrical faults. The use of aggregate models simplifies simulation models and saves simulation time. The Swedish grid code requires wind turbines at all voltage levels to ride through faults at the transmission network. For faults at voltage levels below transmission level fault clearing times are often longer and this could impact on fault ride-through of wind turbines. Simulation of study cases with faults at sub-transmission level, performed using the standard Nordic 32 test system, show that wind turbines should still be able to ride through such faults. Only in case of high dynamic load scenarios and failure of the protection system, wind turbines could disconnect from the grid. Load modelling is important when carrying out this analysis. Faults on adjacent MV feeders seriously endanger grid fault ride-through (GFRT) of wind turbines. Finally, an investigation on the voltage support of wind turbines in weak networks during faults has been carried out. A simplified model of the power system of the Danish island of Bornholm has been used as a test system. It has been found that the minimum requirements for voltage support set by grid codes do not result in satisfactory voltage recovery in weak grids after fault clearing. However, if properly controlled, wind turbines are able to provide a voltage support comparable to that supplied by power plants with synchronous generation

Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

Dynamic Characteristic Analysis of Doubly-fed Induction Generator Low Voltage Ride-through

AbstractFor studying the impacts of wind turbines integrated into grid, the relation between the protection of doubly-fed induction generator (DFIG) during low voltage fault and the dynamic characteristic of grid is established from the points of generator operation constrains and district grid voltage stability. Then the resistance value and switching strategy of crowbar are discussed. Based on analyzing the electric characteristic of the voltage or current during the short-circuit fault in wind turbines with crowbar switching, the equation to estimate peak current of stator and rotor of DFIG with crowbar switching and the value range of crowbar resistance are derived. The numeric test analyzes the impacts of crowbar switching on district grid voltage stability with different fault types, crowbar switching time and crowbar resistance values. Also the interaction impact of crowbar switching on multi-wind farms is analyzed. The results show that reasonable crowbar resistance value and switching strategy can improve low voltage ride through (LVRT) ability of wind turbines and reduce bad impacts on district grid voltage stability with large-scale crowbar switching of wind farms

The transient stability analysis of wind turbines interconected to grid under fault

Wind farm has been growing in recent years due to its very competitive electricity production cost. Wind generators have gone from a few kilowatts to megawatts. However, the participation of the wind turbine in the stability of the electricity grid is a critical point to check, knowing that the electricity grid is meshed, any change in active and reactive flux at the network level affects its stability. With a rate of 50% wind turbine penetration into the electricity grid, the stability of the rotor angle is a dynamic phenomenon which is only visible by the variation of the active energy. The purpose of this journal is to verify the impact of wind turbine integration on an electrical grid, by exploiting the relationship between the reactive energy produced by the Doubly Fed Induction Generator equipping most wind energy systems, and the stability of the rotor angle of the synchronous generators equipping the conventional power plants in the electrical system

Dynamic Phasor Modeling of Type 3 Wind Farm including Multi-mass and LVRT Effects

The proportion of power attributable to wind generation has grown significantly in the last two decades. System impact studies such as load flow studies and short circuit studies, are important for planning before integration of any new wind generation into the existing power grid. Short circuit modelling is central in these planning studies to determine protective relay settings, protection coordination, and equipment ratings. Numerous factors, such as low voltage situations, power electronic switching, control actions, sub-synchronous oscillations, etc., influence the response of wind farms to short circuit conditions, and that makes short circuit modelling of wind farms an interesting, complex, and challenging task. Power electronics-based converters are very common in wind power plants, enabling the plant to operate at a wide range of wind speeds and provide reactive power support without disconnection from the grid during low voltage scenarios. This has led to the growth of Type 3 (with rotor side converter) and Type 4 (with stator side full converter) wind generators, in which power electronics-based converters and controls are an integral part. The power electronics in these generators are proprietary in nature, which makes it difficult to obtain the necessary information from the manufacturer to model them accurately in planning studies for conditions such as those found during faults or low voltage ride through (LVRT) periods. The use of power electronic controllers also has led to phenomena such as sub-synchronous control interactions in series compensated Type 3 wind farms, which are characterized by non-fundamental frequency oscillations. The above factors have led to the need to develop generic models for wind farms that can be used in studies by planners and protection engineers. The current practice for short circuit modelling of wind farms in the power industry is to utilize transient stability programs based on either simplified electromechanical fundamental frequency models or detailed electromagnetic time domain models. The fundamental frequency models are incapable of representing the majority of critical wind generator fault characteristics, such as during power electronic switching conditions and sub-synchronous interactions. The detailed time domain models, though accurate, demand high levels of computation and modelling expertise. A simple yet accurate modelling methodology for wind generators that does not require resorting to fundamental frequency based simplifications or time domain type simulations is the basis for this research work. This research work develops an average value model and a dynamic phasor model of a Type 3 DFIG wind farm. The average value model replaces the switches and associated phenomena by equivalent current and voltage sources. The dynamic phasor model is based on generalized averaging theory, where the system variables are represented as time varying Fourier coefficients known as dynamic phasors. The two types models provide a generic type model and achieve a middle ground between conventional electromechanical models and the cumbersome electromagnetic time domain models. The dynamic phasor model enables the user to consider each harmonic component individually; this selective view of the components of the system response is not achievable in conventional electromagnetic transient simulations. Only the appropriate dynamic phasors are selected for the required fault behaviour to be represented, providing greater computational efficiency than detailed time domain simulations. A detailed electromagnetic transient (EMT) simulation model is also developed in this thesis using a real-time digital simulator (RTDS). The results obtained with the average value model and the dynamic phasor model are validated with an accurate electromagnetic simulation model and some state-of-the-art industrial schemes: a voltage behind transient reactance model, an analytical expression model, and a voltage dependent current source model. The proposed RTDS models include the effect of change of flux during faulted conditions in the wind generator during abnormal system conditions instead of incorrectly assuming it is a constant. This was not investigated in previous studies carried out in the real-time simulations laboratory at the University of Saskatchewan or in various publications reported in the literature. The most commonly used LVRT topologies, such as rotor side crowbar circuit, DC-link protection scheme, and series dynamic braking resistance (SDBR) in rotor and stator circuits, are investigated in the short circuit studies. The RTDS model developed uses a multi-mass (three-mass) model of the mechanical drive train instead of a simple single-mass model to represent torsional dynamics. The single mass model considers the blade inertia, the turbine hub, and the generator as a single lumped mass and so cannot reproduce the torsional behaviour. The root cause of sub-synchronous frequencies in Type 3 wind generators is not well understood by system planners and protection engineers. Some literature reports it is self excitation while others report it is due to sub-synchronous control interactions. One publication in the stability literature reports on a small signal analysis study aimed at finding the root cause of the problem, and a similar type of analysis was performed in this thesis. A linearized model was developed, which includes the generator model, a three mass drive train, rotor side converter, and the grid side converter represented as a constant voltage source. The linear model analysis showed that the sub-synchronous oscillations are due to control interactions between the rotor side controller of the Type 3 wind power plant and the series capacitor in the transmission line. The rotor side controls were tuned to obtain a stable response at higher levels of compensation. A real-time simulation model of a 450 MW Type 3 wind farm consisting of 150 units transmitting power via 345 kV transmission line was developed on the RTDS. The dynamic phasor method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for different sub- synchronous oscillation frequencies

Transient analysis of grid-connected wind-driven PMSG, DFIG and SCIG at fixed and variable speeds

This paper is aimed at presenting transient analysis of a grid-connected wind-driven permanent magnet synchronous generator (PMSG) at fixed and variable speeds by solving the system describing differential equations. For comparison purpose, wind-driven squirrel cage (SCIG) and doubly fed (DFIG) induction generators at fixed and variable speeds are studied using MATLAB/SIMULINK software package. Different from PMSG and DFIG the grid should feed the reactive losses of transmission lines and transformers at any value of short circuit ratio for SCIG. This increases the dynamic stability of DFIG and PMSG when compared with SCIG. Under three-phase-to-ground fault and single-phase-to-ground fault conditions, the time required to recover stability of PMSG wind farm is less than those  DFIG and SCIG wind farms for fixed and variable speeds indicating that PMSG is more stable than of DFIG and of SCIG wind farms

Large Grid-Connected Wind Turbines

This book covers the technological progress and developments of a large-scale wind energy conversion system along with its future trends, with each chapter constituting a contribution by a different leader in the wind energy arena. Recent developments in wind energy conversion systems, system optimization, stability augmentation, power smoothing, and many other fascinating topics are included in this book. Chapters are supported through modeling, control, and simulation analysis. This book contains both technical and review articles