Ofshore Wind Park Control Assessment Methodologies to Assure Robustness

Abstract

The transition from fossil fuels to renewable energy is an expensive but necessary process to ensure a habitable world for future generations. Renewable energy sources such as hydro-, solar- and wind energy continues to increase their share of the total power production. With national goals set by the Renewable Energy Directive of the European Commission to decrease carbon dioxide emission, the demand for renewable energy is increasing. Wind energy has been harnessed since 1887 [1] and has seen a large growth since the first multi-megawatt turbine in 1978. Gradually the wind energy technology has matured to a point where turbines are reaching a production capability exceeding 6 megawatt and the turbines have moved offshore due to stronger wind, and to avoid proximity to populated areas. The placement of wind power plants (WPP) with a typical size of 60 large turbines in remote locations with a weak grid interconnection point, is a challenge with respect to power system stability. This dissertation considers the interaction between the offshore grid and the control of power electronic devices (PED), its effect on system stability and challenges with respect to unwanted interaction between controllers in the rather complex control hierarchy on an offshore WPP. The output waveform of modern turbines utilizing PEDs is distorted at high frequencies, and the stability of the control system is affected by resonances and harmonics present in the weak offshore grid. These phenomena pose a risk to drive the system to instability, as they exist within the bandwidth of the turbine controllers. The resonances and the number of turbines in operation are characteristics of the grid, which are partly unknown at the controller design stage. The uncertainty and the unwanted interaction in the grid are difficult challenges for control designers. This project deals with these challenges and provides insight in root causes to phenomena that have been issues during wind power plant commissioning in the past. This is done through development of design and validation methods for controllers, by analyzing turbine interaction with the grid and suggestion of design guidelines to ensure proper operation of stacked controllers. Two specific faults serve as basis for the analysis and development, a rotor blade deformation and an unwanted oscillation in the reactive power, both of which experienced at a WPP. The low frequency reactive power oscillations observed were suspected to be i caused by the voltage control at the point of common coupling. The fault was thought to involve the interaction between the static synchronous compensator (STATCOM), the wind turbine voltage control and the power plant control (PPC). By establishing bounds on the sets of possible parameters of all involved controllers, the thesis replicates the phenomena by simulation and a method is proposed that analytically finds the set of control parameters, which ensure stable operation. The method enables DONG Energy to calculate bounds on controller parameters based on network parameters and the thesis contributes by ensuring proper operation before energization. The analysis of the voltage control philosophy related to the reactive power oscillations showed the need for proper handling of the resonances introduced by the offshore grid in the turbine control structure. The dissertation contributes to this area with the development of a robust H∞ converter controller employing notch filters in the performance specification to suppress harmonics of the grid frequency. This method combines attenuation of selected resonance frequencies with system stability and performance within the defined envelope of uncertainty of the grid. The controller is tested in a model of the WPP, and is shown to improve performance, control effort and output disturbance rejection compared to standard PI control. The second fault was that a turbine rotor blade was observed to deform in a WPP. This severe fault was suspected to have contributory causes from both mechanicaland electrical systems. A preceding investigation was conducted which ruled out physical generator phenomena such as cogging torque, as well as network voltage disturbances and delays in the converter control system. The investigation indicated that the problem was an insufficient implementation of the rotor speed controller. The thesis addresses the problem by the development of control methods to limit the shaft stress, and thereby the rotor blade vibration. The contributions include a feedback linerization controller and an observer based backstepping controller for a wind turbine. The thesis consists of an introduction part that briefly describes the field, the investigations conducted in the study, the models developed and the controller designs suggested to deal with the challenges described above. The main results of the research are highlighted in the introduction and the detailed results are described in four papers, which are enclosed in the last part of the thesis.