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

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

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

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

High frequency impedance based fault location in distribution system with DGs

Distributed Generations (DGs) with power electronic devices and their control loops will cause distortion to the fault currents and result in errors for power frequency measurement based fault locations. This might jeopardize the distribution system fault restoration and reduce the grid resilience. The proposed method uses high frequency (up to 3kHz) fault information and short window measurement to avoid the influence of DG control loops. Applying the DG high frequency impedance model, faults can be accurately located by measuring the system high frequency line reactance. Assisted with the DG side recorded unsynchronized data, this method can be employed to distribution systems with multiple branches and laterals

Integrated electromechanical wind turbine control for power system operation and load reduction

With the penetration level of wind power in electric power networks increasing rapidly all over the world, modern wind turbines are challenged to provide the same grid services as conventional synchronous power plants. The dynamic interaction between wind turbines and grid has to be assessed first before replacing large amount of conventional power plants by wind power. Over the last few years many power system operators have revised their grid codes and established more demanding requirements for wind power connection. In the past, when wind turbines were small, they were allowed to simply disconnect during a grid fault/disturbance. However, as wind turbine size has increased considerably, their fault ride-through capability has to be improved if the penetration of wind power is to be further increased. Wind turbine design and control need to be improved to optimize the compatibility of wind power and the grid. Among the various requirements that wind turbines have to meet, fault ride-through is of great importance and a very challenging one. Grid faults cause transients not only in the electrical system, but also in the wind turbine mechanical system. The dynamic performance of wind turbines is determined by both mechanical and electrical systems. From the mechanical point of view, the grid disturbance adds extra loads on wind turbine components. Severe grid faults may even lead to wind turbine emergency shut-down. From the electrical point of view, wind farms may lose power generation during a grid fault, which deteriorates the fault impact and slows down the fault recovery. Advanced control and active damping is required to improve wind turbine operation and assist it to remain connected during a grid fault. The novelty of this research is the study of the interaction between mechanical and electrical systems of the wind turbine. The detailed modelling of both the wind turbine mechanical and electrical dynamics not only helps to identify possible problems that wind turbines encounter during grid faults, but also allows adopting a combined approach to design the wind turbine controller. This thesis aims at improving the wind turbine fault ride-through capability and the ability of wind turbine to provide network support during grid disturbances. The main contents are as follows: The detailed model of wind turbine and grid including wind turbine mechanical model, wind turbine controller, synchronous and induction generator model, doubly fed induction generator (DFIG) controller and a generic network model are presented; A wind turbine fault ride-through strategy considering structural loads alleviation is proposed; A controller for asymmetrical fault ride-through of DFIG wind turbines is presented; The effect of having Power System Stabilizer (PSS) on wind turbine is investigated. A multi-band PSS controller for DFIG wind turbine is demonstrated.With the penetration level of wind power in electric power networks increasing rapidly all over the world, modern wind turbines are challenged to provide the same grid services as conventional synchronous power plants. The dynamic interaction between wind turbines and grid has to be assessed first before replacing large amount of conventional power plants by wind power. Over the last few years many power system operators have revised their grid codes and established more demanding requirements for wind power connection. In the past, when wind turbines were small, they were allowed to simply disconnect during a grid fault/disturbance. However, as wind turbine size has increased considerably, their fault ride-through capability has to be improved if the penetration of wind power is to be further increased. Wind turbine design and control need to be improved to optimize the compatibility of wind power and the grid. Among the various requirements that wind turbines have to meet, fault ride-through is of great importance and a very challenging one. Grid faults cause transients not only in the electrical system, but also in the wind turbine mechanical system. The dynamic performance of wind turbines is determined by both mechanical and electrical systems. From the mechanical point of view, the grid disturbance adds extra loads on wind turbine components. Severe grid faults may even lead to wind turbine emergency shut-down. From the electrical point of view, wind farms may lose power generation during a grid fault, which deteriorates the fault impact and slows down the fault recovery. Advanced control and active damping is required to improve wind turbine operation and assist it to remain connected during a grid fault. The novelty of this research is the study of the interaction between mechanical and electrical systems of the wind turbine. The detailed modelling of both the wind turbine mechanical and electrical dynamics not only helps to identify possible problems that wind turbines encounter during grid faults, but also allows adopting a combined approach to design the wind turbine controller. This thesis aims at improving the wind turbine fault ride-through capability and the ability of wind turbine to provide network support during grid disturbances. The main contents are as follows: The detailed model of wind turbine and grid including wind turbine mechanical model, wind turbine controller, synchronous and induction generator model, doubly fed induction generator (DFIG) controller and a generic network model are presented; A wind turbine fault ride-through strategy considering structural loads alleviation is proposed; A controller for asymmetrical fault ride-through of DFIG wind turbines is presented; The effect of having Power System Stabilizer (PSS) on wind turbine is investigated. A multi-band PSS controller for DFIG wind turbine is demonstrated

Short circuit modeling of wind turbine generators

Modeling of wind farms to determine their short circuit contribution in response to faults is a crucial part of system impact studies performed by power utilities. Short circuit calculations are necessary to determine protective relay settings, equipment ratings and to provide data for protection coordination. The plethora of different factors that influence the response of wind farms to short circuits makes short circuit modeling of wind farms an interesting, complex, and challenging task. Low voltage ride through (LVRT) requirements make it necessary for the latest generation of wind generators to be capable of providing reactive power support without disconnecting from the grid during and after voltage sags. If the wind generator must stay connected to the grid, a facility has to be provided to by-pass the high rotor current that occurs during voltage sags and prevent damage of the rotor side power electronic circuits. This is done through crowbar circuits which are of two types, namely active and passive crowbars, based on the power electronic device used in the crowbar triggering circuit. Power electronics-based converters and controls have become an integral part of wind generator systems like the Type 3 doubly fed induction generator based wind generators. The proprietary nature of the design of these power electronics makes it difficult to obtain the necessary information from the manufacturer to model them accurately. Also, the use of power electronic controllers has led to phenomena such as sub-synchronous control interactions (SSCI) in series compensated Type 3 wind farms which are characterized by non-fundamental frequency oscillations. SSCI affects fault current magnitude significantly and is a crucial factor that cannot be ignored while modeling series compensated Type 3 wind farms. These factors have led to disagreement and inconsistencies about which techniques are appropriate for short circuit modeling of wind farms. Fundamental frequency models like voltage behind transient reactance model are incapable of representing the majority of critical wind generator fault characteristics such as sub-synchronous interactions. The Detailed time domain models, though accurate, demand high levels of computation and modeling expertise. Voltage dependent current source modeling based on look up tables are not stand-alone models and provide only a black-box type of solution. The short circuit modeling methodology developed in this research work for representing a series compensated Type 3 wind farm is based on the generalized averaging theory, where the system variables are represented as time varying Fourier coefficients known as dynamic phasors. The modeling technique is also known as dynamic phasor modeling. The Type 3 wind generator has become the most popular type of wind generator, making it an ideal candidate for such a modeling method to be developed. The dynamic phasor model provides a generic model and achieves a middle ground between the conventional electromechanical models and the cumbersome electromagnetic time domain models. The essence of this scheme to model a periodically driven system, such as power converter circuits, is to retain only particular Fourier coefficients based on the behavior of interest of the system under study making it computationally efficient and inclusive of the required frequency components, even if non-fundamental in nature. The capability to model non-fundamental frequency components is critical for representing sub-synchronous interactions. A 450 MW Type 3 wind farm consisting of 150 generator units was modeled using the proposed approach. The method is shown to be highly 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 non-fundamental frequency components present in fault currents during sub-synchronous interactions. Further, the model is shown to be accurate also for different degrees of transmission line compensation and different transformer configurations used in the test system

Voltage security assessment with high penetration levels of utility-scale doubly fed induction generator wind plants

The interconnection requirements set forth by FERC in order 661-A mandate the operation of wind plants within a power factor range of 0.95 leading / lagging. This operation drastically underutilized the reactive output of doubly fed wind turbines (DFIG). This work discusses the impact of utilizing the capability curve of DFIG based wind plants on steady state and dynamic power system operation. A test system is used to demonstrate that committing the full reactive capability of a DFIG park can increase system penetration levels. This additional reactive support was also found to improve post fault voltage profiles by damping oscillations and preventing overshoots immediately after being subjected to a disturbance. The utilization of extended reactive limits in voltage control may prevent system collapse