Power Quality of Grid-Connected Wind Turbines with DFIG and Their Interaction with the Grid

Institutional and governmental support on wind energy sources, together with the wind energy potential and improvement of wind energy conversion technology, has led to a fast development of wind power generation in recent years. The continuous increase of the wind power penetration level brings a result that wind power generation gradually becomes an important component of power generation in the grid, which makes the study on the wind power quality issues and the interaction between the wind turbines and the grid necessary and imperative.

The research documented in this thesis examines power quality issues of grid-connected wind turbines and the interaction between wind turbines and the grid. The specific goal of the research has been to investigate flicker emission and mitigation of grid-connected wind turbines with doubly fed induction generators (DFIG) during continuous operation, and voltage recovery of such kind of grid-connected wind turbines after the clearance of a short circuit fault in the grid.

As a basis of the research, a model of grid-connected wind turbines with DFIG is developed in the dedicated power system analysis tool PSCAD/EMTDC, which simulates the dynamics of the system from the turbine rotor, where the kinetic wind energy is converted to mechanical energy, to the grid connection point where the electric power is fed into the grid. The complete grid-connected wind turbine model includes the wind speed model, the aerodynamic model of the wind turbine, the mechanical model of the transmission system, models of the electrical components, namely the DFIG, PWM voltage source converters, transformer, capacitor, and the control system. The grid model and the electrical components of the wind turbine are built with standard electrical component models from PSCAD/EMTDC library. The wind model, the aerodynamic model, the mechanical model and the control system are built with custom components developed in PSCAD/EMTDC.

Two control schemes are implemented in the developed grid-connected wind turbine model: speed control and pitch control. The speed control scheme is composed by two vectorcontrol schemes designed respectively for the rotor-side and grid-side PWM voltage source converters. Cascade control is used in the vector-control schemes. Two design methods, poleplacement and internal model control, are applied for designing the PI-controllers in the vector-control schemes. The pitch control scheme is employed to regulate the aerodynamic power from the turbine. The performances of the control schemes, respectively current control loops, power control loops, DC-link voltage control loop and pitch control loop, are illustrated, which meet the design requirements. Simulation results show that the wind turbine is capable of providing satisfactory steady state and dynamic performances, which makes it possible that the wind turbine model can be applied to study the power quality issues of such kind of grid-connected wind turbines and their interaction with the grid.

To evaluate the flicker levels produced by grid-connected wind turbines with DFIG, a flickermeter model is developed according to the IEC standard IEC 61000-4-15, which simulates the response of the lamp-eye-brain chain and provides on-line statistical analysis ofthe flicker signal and the final results. Based on the developed model of grid-connected wind turbines with DFIG and the flickermeter model, the flicker emission during continuous operation is studied. The influence factors that affect flicker emission of grid-connected DFIG wind turbines, such as wind characteristics (mean speed, turbulence intensity) and grid conditions (short circuit capacity, grid impedance angle) are analysed. The effects of the influence factors are compared with previous research results related to the fixed speed wind turbine. In particular, the effects of mean wind speed, turbulence intensity and grid impedance angle are different from that in the case of the fixed speed wind turbine.

It is possible to regulate the reactive power flow on the connection line so that the voltage fluctuation caused by the active power flow can be compensated by that caused by the reactive power flow. Based on this principle, two effective measures are proposed to mitigate the flicker levels produced by grid-connected wind turbines with DFIG, respectively by wind turbine output reactive power control and using STATCOM. Simulation results demonstrate that these two measures are effective for flicker mitigation regardless of mean wind speed, turbulence intensity and short circuit capacity ratio.

The voltage recovery study is started with grid-connected wind turbines with dynamic slip control, which are simple, cost-effective, partially variable speed wind turbines, for gaining a good understanding of transient responses of induction generators in an external short-circuit situation. The model of a variable speed wind turbine with dynamic slip control in the simulation tool of PSCAD/EMTDC is presented, and the control schemes, respectively dynamic slip control and pitch control, are described. The transient process of grid-connected wind turbines after an external short-circuit fault is analyzed in detail. It is concluded from the analysis that increasing the electromagnetic torque or decreasing the aerodynamic torque helps to recover the voltage after the clearance of an external short-circuit fault. For the wind turbine with dynamic slip control, after the clearance of an external short-circuit fault, the electromagnetic torque may be strengthened by adjusting the generator slip, and the aerodynamic torque may be reduced by regulating the pitch angle, which helped to slow the rotor speed down and re-established the voltage at the wind turbine terminal. Simulation results demonstrate that pitch control, dynamic slip control and combined control are effective measures for voltage recovery of grid-connected wind turbines with dynamic slip control. An emergency pitch regulation scheme is developed and applied in the case of pitch control.

Based on the acquired knowledge, the voltage recovery of grid-connected wind turbines with DFIG is studied. Two kinds of situations are studied which depend on whether the rotor protection devices in the DFIG are triggered or not.

When the situation after an external short-circuit fault is not serious enough to trigger the rotor protection devices, the control schemes of the DFIG operated as normal and are capable of forcing the rotor speed down and re-establishing the voltage at the wind turbine terminal after the clearance of the short-circuit fault, which are demonstrated by simulation results. The performances of the wind turbine as well as the control schemes are illustrated in detail.

If the situation after an external short-circuit fault is serious enough, the protection devices in the rotor circuit will be triggered which yields a result that the generator rotor is shortcircuited and the rotor-side converter is deactivated. In this situation, a control strategy is proposed to re-establish the voltage at the wind turbine terminal and restore the wind turbine's normal operation after the fault clearance, which is verified by simulations results. The control strategy, which takes advantage of the benefits of the rotor circuit protection device and the emergency pitch control scheme, are performed in three steps, respectively protection device activation, voltage recovery assisted by pitch control, and normal operation restoration of the wind turbine with DFIG.