Operational Year
BUS 1 PCC)
D. Cost-Effectiveness
One of the major advantages of a transformer-less WECS is the overall reduced cost from the system-level perspective [14]. However, the cost associated with the power conversion stage of a transformer-less WECS is increased when compared with the power conversion stage of a conventional WECS [6], [14]. Based on the additional functionality of the power conversion stage in a transformer-less WECS, the higher multilevel converter topology and additional control algorithm extend the cost of the power conversion stage.
In section 1.1.2, the different conventional multilevel converter topologies deployed in the back-to-back power conversion stage of a conventional WECS have been highlighted.
However, the component counts of these topologies will increase significantly in a transformer- less WECS [14]. Therefore, the associated cost of the power stage of a transformer-less WECS should be less than the cost of a wind turbine transformer. Furthermore, the cost-benefit analysis of deploying transformer-less WECSs in a WPP must be studied.
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1.2.2. Research Questions
According to the highlighted issues from the previous section, this research work seeks to address the following questions:
• What are the existing power converter configurations of a transformer-less WECS connected via a grid-side filter to the collection point of a WPP?
• What is the cost-benefit advantage of adopting a transformer-less WECS as compared to a conventional WECS?
• What type of power converter configuration is more suitable for a transformer- less WECS, which can effectively mitigate the drawbacks identified in the existing structure, such as excessive clamping devices, and multiple series-connected power semiconductor devices?
• What is the most appropriate technique for minimizing the injected dc component in a three-phase transformer-less WECS?
1.3. Research Objectives
Based on the highlighted research questions in section 1.2.2, the objectives of the research work are summarized as follows:
• Review existing power converter configurations and discuss their features and drawbacks for transformer-less WECS applications.
• Carry out a cost-benefit analysis of the proposed power converter configuration and existing power converter configuration of a transformer-less WECS.
• Design and develop a novel single-stage grid-side power converter with minimal series- connected power semiconductor devices, minimal clamping devices, and voltage boosting capability.
• Develop an experimental testing rig for verification based on a power hardware-in-loop (PHIL) configuration.
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1.4. Thesis Outline
The presented research work is organized into seven chapters. The work carried out in each chapter is summarized as follows:
Chapter-1: An overview of reliability studies on WECS is presented with additional discussion on DD-PMSG, power converter topologies, and electrical subsystem of high-power WECS.
The design challenges for deploying a multilevel converter topology at the grid-side of the power converter configuration of the transformer-less WECS.
Chapter-2: This Chapter presents a comprehensive review of existing power converter configurations for transformer-less WECS. These configurations are grouped into the following categories, namely: generator-converter configuration and three-stage power converter configuration. The operating principles, advantages, and drawbacks of these power converter configurations are presented in this chapter. Furthermore, the cost-benefit analysis of a WPP with transformer-less WECS is studied using the life-cycle cost analysis of the existing power converter configurations and collection systems of a WPP.
Research Outputs from Chapter 2:
Journal Paper Published:
Akinola A. Ajayi-Obe, M. A. Khan, and P. Barendse, "Techno-Economic Evaluation of Five- Level Nested Neutral Point Clamped (NNPC) Converter Topology for Transformer-less Connection of High-Power Wind Energy Conversion Systems" Journal of Energy in Southern Africa (JESA), Vol. 3, No. 3, pp.33-43, August 2019.
Conference Papers Presented:
Akinola A. Ajayi-Obe, M. A. Khan, and P. Barendse, "Techno-Economic Evaluation of Five- Level Nested Neutral Point Clamped (NNPC) Converter Topology for Transformer-less Connection of High-Power Wind Energy Conversion System" WindAC 2018, Cape Town, South Africa, 5th – 6th November 2018.
Akinola A. Ajayi-Obe, M. A. Khan, and P. Barendse, "Comparative Evaluation of Transformer- less Configurations for Wind Energy Conversion Systems" WindAC 2017, Cape Town, South Africa.
20 Chapter-3: In this chapter, the generalized multilevel converter topology method for deriving a four-level Nested Neutral-Point-Clamped (4L-NNPC) and five-level NNPC (5L-NNPC) converter deployed as the grid-side converter topology of a transformer-less WECS is presented. Furthermore, combining the generalized multilevel topology method and the parallel-series cell method to derive a seven-level Modified Nested Neutral-Point-Clamped (7L-MNNPC) converter topology is discussed extensively. Simulations are used to verify the presented theoretical analysis and design for the 4L-NNPC, 5L-NNPC and 7L-MNNPC topologies. Furthermore, the grid-connection scenarios of deploying a transformer-less WECSs in a WPP are studied and verified through simulations.
Chapter-4: This chapter presents the proposed tapped inductor quasi-Z-source (qZS)-NNPC converter topology. The theoretical derivation, steady-state analysis, design of the tapped inductor is discussed. The proposed modulation technique and voltage balancing control technique of the proposed converter topology are discussed. The implementation of these techniques on a field-programmable gate array (FPGA)-based platform is presented. The feasibility of the proposed converter topology and the FPGA realization for the modulation and voltage balancing control techniques are verified through the developed experimental test rig.
Research Outputs from Chapter 4:
Akinola A. Ajayi-Obe, and M. A. Khan, "Analysis and Design of a Quasi-Proportional- Resonant Based Voltage Balancing Control for Grid-Connected Nested Neutral Point Clamped Converter" in IEEE-Energy Conversion Congress Exposition (ECCE) 2018 Proceeding, pp.
2982-2987, December 2018.
Akinola A. Ajayi-Obe and M. A. Khan, "Analysis of a Five-Level Dual Tapped Inductor Quasi Impedance Source-Nested Neutral Point Clamped Converter" in IEEE-Energy Conversion Congress Exposition (ECCE) 2017 Proceeding, pp. 2150-2155, November 2017.
Chapter-5: The proposed modified voltage filtering technique reduces the dc component produced by the proposed converter topology. Analysis, design, and experimental results are presented to verify the method.
Chapter-6: In this chapter, the development of power hardware-in-loop (PHIL) configuration for the experimental verification of the proposed converter topology is discussed. Furthermore, the prototyping process and grid connection of the proposed converter topology is presented.
21 The efficiency and cost-benefit analysis of the proposed converter topology are discussed in detail compared to a three-stage power converter configuration.
Chapter-7: The main contributions of the conducted research are summarized in this chapter.
Therefore, the main contributions of this presented work are stated as follows:
• A comprehensive overview of the existing power converter configurations for transformer-less WECS was carried out. Furthermore, a comparative benchmark factor was developed to evaluate the topologies deployed in these configurations; MMC- based topologies are less efficient and reliable because of their high number of power semiconductor devices, while higher voltage-level DCC topology combined with simple voltage boosting topology is more efficient and reliable for a transformer-less WECS.
• Theoretical derivations and simulation results of a three-phase four-level NNPC, five- level NNPC and seven-level MNNPC converter topologies deployed as the grid-side converter topology of a transformer-less WECS have been presented. Furthermore, the severity of the grid voltage unbalances (i.e., voltage sag) on the transformer-less WECS have been studied and investigated.
• Analysis, design, and development of a three-phase five-level tapped inductor quasi-Z- source-NNPC converter topology. The proposed converter topology is more efficient and more cost-effective than the conventional converter topology used in a transformer- less WECS.
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