Investigation of power converters for AC and DC power collection in wind turbine systems

• Main Author: Khan, Md Shafquat Ullah
• Other Authors: Ali Iftekhar Maswood
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Power electronics
• Online Access: https://hdl.handle.net/10356/137185
• Institution: Nanyang Technological University

Wind turbine systems (WTSs) have become one of the significant contributors to renewable energy and pledges the formidable reduction of global dependence on fossil fuel. With the overall increase in wind power production, individual wind turbines are also getting bigger producing power rating as high as 20 MW. WTSs are a complex electro-mechanical system, and as the wind turbines are getting bigger, vibration issues are becoming one of the concerning issues. Torsional vibration is one of the dominant contributors of failures pertinent to mechanical aspects which statistically causes the highest downtime to the WTS. Though the electrical system has lower downtime than the mechanical system, the power electronic components in the system add to the highest annual failure rate. As the installation of WTS is on a high in offshore scenarios, the reliability, and failure contingencies become one of the major concerns for the utilities. The research world is continuously striving for solutions in this regards for high power applications. Wind dynamics and grid side disturbances affecting the stability of the system required further analysis more than ever before. This thesis focuses on both control and topological aspects of WTS. Addressing the challenges in electro-mechanical scenario the active damping techniques of torsional vibrations in WTSs are investigated for a voltage source converter (VSC) based back to back (B2B) WTS. A novel control algorithm for the front end converter called speed difference damping controller (SDDC) is proposed, which reduces the effect of load dynamics and transient torsional oscillations caused due to wind dynamics. SDDC is designed based on segregated control of natural frequency and damping. The appropriateness of the converter control is verified in a comparative forum with other available algorithms. The system is emulated in simulation and experimentation for dynamic conditions from input wind speed, and output integrated grid. With the high power produced by the large wind turbine generators, the voltage and current rating for the power electronic converters are very high as well. Traditional semiconductor switches are failing to keep up with the ever-increasing voltage, and current ratings as their physical limits are reached. As a solution, newer elements are being explored and researched alongside advanced converter topologies. Multilevel converter topologies are becoming a popular solution as it accommodates off-the-shelf devices for high power applications. A three-level neutral-point-clamped (3L-NPC) converter is one of the most prominent in the research of multilevel converter topologies for B2B WTS applications. This topology is investigated in this dissertation addressing the dc-link aberrations related to the wind variations and grid-dynamics. The effect of wind variations and grid-side disturbances destabilize the dc-link voltage due to the varying active and reactive power flow demands. This problem is solved by the integration of an energy storage system (ESS) via a bidirectional dc-dc converter. An algorithm is analyzed for the operation of the dc-dc converter. The algorithm for control is implemented first via PI-controller. A simplistic model predictive deadbeat control based dynamic reference tracking method is also implemented for the dc-dc converter current control. The dynamic reference design tracks the dc-dc inductor current reference within one step of predictive horizon making it fast in response. The two control techniques are discussed based on the results of simulation analysis. Offshore wind farms showcase several advantages over onshore ventures. And going offshore dc-power collection configurations have superiority over ac power collection configurations in many regards. For offshore scenario, converters with simple configu- ration aiding to lower computational power requirement, lower switch count, simpler cooling system, and higher reliability are convenient and highly preferred. The aspect of simpler generator side converter in the offshore scenario is addressed and thus in this dissertation studies a topology with three-phase diode rectifier with auxiliary circuitry called unity power factor rectifier (UPFR). Considering application in high power scenarios as above, the parallel unity power factor rectifier (PUPFR) topology is proposed as a plausible front end converter for offshore WTS. The control objective of the PUPFR includes the input current control to ensure unity power factor operation which is done using hysteresis current control; current imbalance control between the parallel branches to ensure equal power-sharing, and dc-link voltage control assuring constant dc-link voltage with capacitor voltage balancing. The viability of the converter is investigated in simulation, and the simulation results are validated in scaled-down experimentation for wind dynamics and grid side variations. In this dissertation, VSC, parallel VSC (PVSC), 3L-NPC, UPFR, and PUPFR are investigated for front end converter topologies for WTS. Component failure based reliability estimation provides an outline or superficial idea of applicability of a power converter in a specific scenario. The component failure based reliability analysis of all the converter topologies discussed in the dissertation is carried out in a comparative platform. The reliability estimation analysis is done based on individual component failure data obtained from a tool book. For the topology of PUPFR and UPFR, an economic feasibility study is conducted, which includes cost-estimations, power loss economics, component failure dynamics, and capacity factor estimation. Thus it can be concluded saying that the SDDC for torsional vibration decouples the natural frequency control and the damping control providing the system with a higher degree of freedom to the control of the front end converter. Integration of an ESS via a dc-dc converter is analyzed for its control for application in 3L-NPC based grid-connected WTS. A simpler PUPFR is proposed as a plausible topology for high power offshore WTS. Reliability estimation investigation carried out to investigate the applicability and economic aspects of PUPFR application in offshore WTSs.