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

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

Comparison of doubly-fed induction generator and brushless doubly-fed reluctance generator for wind energy applications

Phd ThesisThe Doubly-fed Induction Generator (DFIG) is the dominant technology for variable-speed wind power generation due in part to its cost-effective partially-rated power converter. However, the maintenance requirements and potential failure of brushes and slip rings is a significant disadvantage of DFIG. This has led to increased interest in brushless doubly-fed generators. In this thesis a Brushless Doubly-Fed Reluctance Generator (BDFRG) is compared with DFIG from a control performance point of view. To compare the performance of the two generators a flexible 7.5kW test facility has been constructed. Initially, a classical cascade vector controller is applied to both generators. This controller is based on the stator voltage field orientation method with an inner rotor (secondary stator) current control loop and an outer active and reactive power control loop. The dynamic and steady state performance of two generators are examined experimentally. The results confirm that the BDFRG has a slower dynamic response when compared to the DFIG due to the larger and variable inductance. Finally a sensorless Direct Power Control (DPC) scheme is applied to both the DFIG and BDFRG. The performance of this scheme is demonstrated with both simulation and experimental results.Engineering and Physical Sciences Research Council (EPSRC) and Overseas Researcher Scholarship (ORS

Low-voltage ride-through capability improvement of type-3 wind turbine through active disturbance rejection feedback control-based dynamic voltage restorer

Disconnections due to voltage drops in the grid cannot be permitted if wind turbines (WTs) contribute significantly to electricity production, as this increases the risk of production loss and destabilizes the grid. To mitigate the negative effects of these occurrences, WTs must be able to ride through the low-voltage conditions and inject reactive current to provide dynamic voltage support. This paper investigates the low-voltage ride-Through (LVRT) capability enhancement of a Type-3 WT utilizing a dynamic voltage restorer (DVR). During the grid voltage drop, the DVR quickly injects a compensating voltage to keep the stator voltage constant. This paper proposes an active disturbance rejection control (ADRC) scheme to control the rotor-side, grid-side and DVR-side converters in a wind-DVR integrated network. The performance of the Type-3 WT with DVR topology is evaluated under various test conditions using MATLAB®/Simulink®. These simulation results are also compared with the experimental results for the LVRT capability performed on a WT emulator equipped with a crowbar and direct current (DC) chopper. The simulation results demonstrate a favourable transient and steady-state response of the Type-3 wind turbine quantities defined by the LVRT codes, as well as improved reactive power support under balanced fault conditions. Under the most severe voltage drop of 95%, the stator currents, rotor currents and DC bus voltage are 1.25 pu, 1.40 pu and 1.09 UDC, respectively, conforming to the values of the LVRT codes. DVR controlled by the ADRC technique significantly increases the LVRT capabilities of a Type-3 doubly-fed induction generator-based WT under symmetrical voltage dip events. Although setting up ADRC controllers might be challenging, the proposed method has been shown to be extremely effective in reducing all kinds of internal and external disturbances