Control of Wind Turbines during Symmetrical and Asymmetrical Grid Faults

As installed capacity of the wind power plants (WPPs) in power system of certain countries increases, stability of the power system becomes more critical. In order to sustain stable power system operation with high share of wind power, system operators of some countries are enforcing more stringent grid code requirements, which are targeting to make the WPPs operate in a closer manner to the conventional power plants. Common to most of the grid codes, WPPs are required to stay connected during short-circuit grid faults, and also inject reactive current in order to support the grid voltage regulation. The uninterrupted operation of WPPs is required even when the grid voltage drops down to zero; and the current injection requirement is defined as positive sequence reactive current irrespective of the fault type; symmetrical or asymmetrical.
In this project, the response of full-scale converter type wind turbines (WTs), in an AC connected WPP, is investigated and control algorithms are designed for minimum disrupted operation and improved grid support, for both symmetrical and asymmetrical grid faults. WTs’ response with conventional control algorithms is studied regarding the impact on the WTs and the grid. Alternative control methods are proposed, which are basically active and reactive current reference generation algorithms in positive sequence and also in negative sequence.
It is observed that when WTs inject pure positive sequence reactive current in case of asymmetrical faults in accordance with the grid code requirement, positive sequence grid voltage is boosted, but negative sequence grid voltage also is also boosted due to the coupling. As a result higher overvoltages at the non-faulty phases occur. In this thesis an alternative injection method, where WTs are injecting both positive and negative sequence currents, is given and compared with the conventional method in the sense of grid support performance. Additionally, effect of the coupling between positive, negative and zero sequences during asymmetrical faults, is investigated, which was not considered in the wind power studies before.
It is shown that when reactive current injection is performed during severe symmetrical faults, where the grid voltage is dropping down close to zero, the wind turbines can lose the synchronism with the grid fundamental frequency, which potentially creates risk of instability for the control. This Loss of Synchronism (LOS) situation is investigated based on active and reactive current transfer limits which are derived first time in this thesis, together with stability analysis of the grid synchronization of WTs.
Novel control algorithms are developed to generate necessary active and reactive current references, which provide stable operation of the WTs and result in improved grid support.
In summary, the response of the WTs to symmetrical and asymmetrical faults is
improved by means of the proposed control solutions, which provide compliance with stringent grid codes, improved grid support with minimum disrupted operation, and a WPP behavior in a closer manner to conventional power plants.