The awareness of global warming has increased significantly in the last two decades, with international organizations producing technical assessment reports that highlights the causes and impacts of global warming [1], [2]. These reports show that global warming will continue due to increasing global energy demand [1]. The significant increase in global energy demand is due to economic and population growth, with overdependence on fossil fuel for electricity generation and transportation being a critical factor [1]-[3]. With a projected 48% increase in global energy demand from year 2020 to year 2050, sustainable and efficient energy sources must be developed and deployed [3]. Therefore, the most sustainable solution to minimize global warming is systems based on renewable energy sources [1]-[3].
Among the various emerging renewable energy sources deployed in recent times for electricity generation, wind energy and solar energy are the two most well-known renewable energy sources [1]-[6]. However, geographical availability, technological advancement, and cost have made wind energy very prominent in the last decade [3], [5]-[7]. The installed global wind power capacity is currently about 98 Gigawatt (GW), as shown in Figure 1.1 [6], [8]. In addition, the installed global wind power capacity is projected to increase by 17% from year 2021 to year 2026 [8]. Furthermore, the total installed global wind power capacity is expected to be about 300GW in about a decade, as shown in Figure 1.1 [8]. This global trend of increasing installed wind energy conversion systems (WECSs) has resulted in the development of high-power systems because the maximum power produced by a WECS increases linearly with air density and swept area of the rotor blades [6], [8], [9]. Furthermore, the power capacity of commercially available WECSs is projected to be about 15𝑀𝑊 to 20 𝑀𝑊 in the next decade [6]. However, operating a WECS at such a high-power range with a low voltage (LV) level will result in the transmission of excessive electric current in the system [6], [10]-[11].
The power conversion stage of the system will consist of several power semiconductor devices connected in parallel to handle the high current value within its topologies [10]-[13]. Also, the associated cable losses and the connecting cables' cost are increased significantly [6].
Therefore, the electric current transmitted in a high-power WECS operated at a much higher voltage level results in reduced cable losses and cost [6].
2 Figure 1.1: Global installed wind power capacity from year 2019 to year 2030 [8].
The efficiency of a high-power WECS can be enhanced by increasing its voltage level to the medium voltage (MV) range and simultaneously reduce its electric current rating. However, it requires more complex subsystems, such as a medium voltage electric generator and multilevel power converter topology utilized in the system [6], [10]-[12]. The medium voltage electric generators are available within 3𝑘𝑉 to 6.6 𝑘𝑉 and 2𝑀𝑊 to 10 𝑀𝑊 voltage and power ratings, respectively [6]. A multilevel power converter topology is preferred because it is possible to produce higher output voltage amplitude irrespective of the voltage ratings of the power semiconductor devices deployed in the topology [12]-[15]. Also, the multilevel converter topology produces output voltage and current waveform with less harmonic distortion [13]- [18].
Therefore, this research explores the possibility of eliminating the need for a wind turbine transformer by developing a grid-side multilevel converter configuration that can operate at a medium voltage (MV) level [6], [14]. This thesis presents the analysis, design and development of a novel single-stage converter topology deployed at the grid-side of the transformer-less WECS. Furthermore, the theoretical analysis, control algorithm, real-time implementation, and prototyping of the proposed converter topology have been investigated extensively.
Therefore, the organization of Chapter 1 is as follows: a brief overview of reliability studies on existing WECSs is discussed in Section 1.1, and a detailed explanation of the motivation for carrying out this research is presented in Section 1.2. The objectives of this research work have been outlined in Section 1.3. Finally, the outline and organizational structure of this thesis are summarized in Section 1.4.
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Number of Installations in Gigawatts (GW)
Year
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1.1. Overview of Reliability Studies on WECS
The increased penetration of WECSs has resulted in the development of large-scale onshore and offshore WPP, as discussed earlier [6]-[19]. However, most of the large-scale WPP development will be situated offshore in the coming decades [8]. The wind energy resources available in offshore regions far exceeds onshore wind resources [19]. However, the cost of installation and operation of an offshore WPP is estimated to be about eight times more than an onshore WPP because the components of an offshore WECS are more prone to failure due to the harsh environmental conditions and system complexity [19]-[21].
Various reliability studies on installed WECSs have been presented in the literature [20]-[25].
A detailed survey on the failure statistics of WECSs in WPPs situated in Finland, Germany, and Sweden was carried out in [24]. This survey was based on operational statistics collected on the WPPs, to identify the most critical subsystem for both failure rate and downtime [24].
The electrical subsystem showed the highest failure rate in both onshore and offshore WPPs, as illustrated in Figure 1.2 [24]. Simultaneously, the multistage gearbox subsystem was identified as the component with the highest downtime [24]. Furthermore, this survey showed that WECS rated above 1𝑀𝑊 tends to have higher failure frequency, and the control subsystem was identified to have the second-highest failure rate, as shown in Figure 1.2 [24]. Another reliability study was carried out based on the electric generator technology deployed in WECS [25]. This study investigated about 2,000 WECSs consisting mainly of doubly fed induction generator (DFIG)-based WECSs and permanent magnet synchronous generator (PMSG)-based WECSs [25]. The DFIG-based WECS was more susceptible to failure than its PMSG counterpart due to gearbox and auxiliary system-related issues [25].
Furthermore, a comprehensive reliability analysis on the subsystem of 6,000 WECSs in Denmark and Germany was carried out based on the bathtub curve concept and analysed data collected over 11 years from the WIND STATS survey [23]. In this study, the subsystems with the highest failure rate in descending order are electrical subsystems, rotor, converter, electric generator, hydraulics, and gearbox [23]. From the results and discussion presented, high-power WECSs are prone to more failure due to their high complexity and direct-drive (gearless) WECS is more reliable than the geared WECS [23]. The power converter configuration of direct-drive and geared WECS exhibits a high failure rate due to environmental conditions [23]. Also, the electrical subsystem is the least reliable subcomponent of the WECS, as stated in [23]. The electrical subsystem consists of the transformer, power feeder cable, grid-side
4 filter, switchgear, power protection unit, circuit breaker, and surge arrestor [23]. Finally, the failure rate of high-power WECS increases with the system's operational years [20]-[25], as indicated in Figure 1.3. Therefore, the subsequent sections focus on the reliability-related issues associated with the electric generators, power converter topologies and electrical subsystem deployed in direct-drive WECSs of a WPP.
Figure 1.2: Breakdown of the number of failures in various components of a WECS connected in an onshore Swedish WPPs [24].
Figure 1.3: Failure rate with the respective rated power capacity of WECS showing their operational years [24].
Entire Unit; 3
Hub; 1 Blades/Pitch; 12 Generator; 5
Electric System;
18
Control System; 13 Drive Train; 1
Sensors; 14 Gears; 10 Mechanical Brake; 1
Hydraulics; 13
Yaw Systems; 7 Structure; 2
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