Grid-Connected Voltage Source Converter - Control Principles and Wind Energy Applications
The thesis focuses on a forced-commutated voltage source converter (VSC) connected to a grid in a wind energy application. The work consists of four parts. The first part addresses the type of electrical system which should be used in a wind turbine. The conclusion is to use variable-speed wind turbines, due to higher efficiency, lower noise and lower fatigue. If high power quality is demanded, a grid-connected VSC should be used instead of a grid-commutated thyristor inverter. By utilizing the high current control bandwidth of the VSC in a hybrid wind farm, consisting of wind turbines having different electrical systems, a cost-efficient solution is obtained. The VSC is used for reactive power compensation and active filtering, in addition to converting wind power. These additional features cause only a moderate increase in the VSC rating compared with only converting wind power.
In part two, an electrical system in a variable-speed wind turbine, in which the VSC uses the voltage angle control to track the reference voltage of the dc-link, is investigated. The proposed control method is based on a steady-state model of the system, which results in a low bandwidth but which is high enough to operate a wind turbine. To increase the bandwidth, the linear quadratic (LQ) control method has been introduced. Due to sensitivity to current harmonics, an extended Kalman filter has been added to the LQ-controller. Simulations show that the controller operates as expected.
A grid-connected VSC using a discrete vector current controller is investigated in the third part of the thesis. The influences of an incorrect controller tuning and grid voltage harmonics on current frequency responses at an operating point are investigated. The attenuation of low-frequency voltage harmonics decreases when their frequency increases. Furthermore, it is shown that the current controller handles parameter errors satisfactorily. It has been shown that frequency dependent losses in the grid filter affect current frequency responses at high frequencies. A compensation function has been introduced to compensate for non-ideal valves and non-ideal pulse width modulation. The function improves the small-signal current frequency response around an operating point. Also four different synchronization methods, which are adapted to digital controllers, have been investigated. A novel transformation angle detector based on a space vector filter has been introduced. The detector manages phase steps in the grid voltage, and the extended version of the detector also manages frequency changes in the grid better than the extended Kalman filtered detector, in spite of the smaller number of calculations.
The last part of the thesis deals with the current harmonics of a grid-connected VSC. By introducing a third-order LCL-filter as an alternative to an L-filter, current harmonics are decreased. Furthermore, reflections caused by high voltage derivatives are adressed.
voltage source converter
voltage angle control
vector current control