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The percentage of Renewable Generation among all electrical generation has been increasing for the last two decades worldwide, because of increased awareness of environmental issues and desire of finding substitute energy source as fossil fuel reserves decreasing. The major obstacles of promoting renewable energy (RE) are high installation cost, low efficiency, the intermittency of energy flow and control complexity of feeding into the grid. (Nehrir el at. 2011)
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Wind energy harvesting technology had been developed for several decades. Initially, wind turbines directly connected with the grid using squirrel-cage induction generators, almost no dynamic control was applied except for some capacitor banks to ensure unity power factors. Investigation of power electronics for wind turbines system had become essential nowadays due to their increasing power capacity. (Blaabjerg et al. 2011)
This report will present academic literature review about trending technologies of wind turbine energy system, the goal of the project, summary of work done and time plan.
Overview of wind turbines systems design
Nehrir el at. (2011) reviewed on hybrid renewable energy storage (RES) systems and suggested that design of RES system should consider generation load, financial parameter, geographic factor, desire system reliability, cost requirement and others, minimizing the effect on environmental impact and cost.
Hahn et al. (2006) observed that although the downtime of power electronics failure is short compared to gearbox and generator, they are most likely to fail among all parts of wind turbines. Alewine and Chen (2012) mentioned two electrical winding failures occurred associated with power converter for wind generators, because voltage surge generates harmonic current and flashover and that leads to high bearing temperature and damage rotor leads. In addition, irregular voltage spikes generated increased the stress on rotor winding insulation. Proper design and protection are required to maintain good service life. Furthermore, the maintenance and repairment cost had increased significantly as new wind turbines location changes from on-shore to off-shore, also requires surviving under more severe environmental disturbance. Reliability is a key factor on establishing more efficient wind energy system (Blaabjerg et al. 2011)
Blaabjerg el at. (2011) reviewed 2 major types of power converters topology applied on MW scale variable speed wind turbines, which are partial scale power converters and full-scale power converters. Partial-scale converters are more compact and benefit in the economic point of view, they are most adopted with Doubly fed induction generators (DFIG). However, compare to permanent magnet synchronous generators (PMSG), DFIG has higher power losses and higher maintenance cost (typically slip speed is around 30% of synchronous speed). Slip rings and protection schemes are required for partial-scale converters if DFIG wind turbine is grid connected. Moreover, no reactive power required for PMSG as it operate at synchronous speed, unidirectional active power flow to the grid. (Liserre et al. 2011)
In terms of switching device, IGBT, IGBT press pack and IGCT press pack were reviewed. Busca et al. (2010) simulated high power IGBT life prediction model, which indicated that press pack IGBT achieved higher reliability on high power operation and better power cycling capability. Yet, press pack devices introduced new failure mechanism and still suffer thermo-mechanical issue, which is the most critical aspect on reliability. (Jakob et al. 2007)
For DFIG wind turbines, 2 level output voltage back-to-back (2L-BTB) converters were most commonly used for three phase power. Switching frequency generated by pulse width modulation (PWM), and it was dominating on WES due to simple construction, relatively low cost and mature technology. However, the performance of 2L-BTB solution is relatively poor as power level reaching above 10MW with higher losses and higher voltage change stress on generator. (Rodríguez et al. 2007)
Multilevel power converters are more suitable for PMSG high power wind turbine application, since they can achieve more output power levels, higher voltage amplitude and most important, larger power output. (Blaabjerg et al. 2011) Ma and Blaabjerg (2011) investigated the performance of 4 single cell converters, which can be category as natural-point diode clamped structure, flying capacitor clamped structure and cascaded converter structure. They conclude that 3-level and 5-level H-Bridge converter topologies perform better on thermal reduction and thermal distribution on switches than three-level neutral-point clamped converter. More advance design focused on multi-cell multilevel converters, such as cascaded H-bridge converter developed in European UNIFLEX-PM project. (Iov et al. 2009) Although higher output quality was achieved, but the technology is still lab-based, also reliability reduction and control complexity highly increased.
According to Liserre el at. (2011), three stages of control are needed in WES. The first stage is to ensure basic function as controlling voltage and current in the system. Second stage is to maximise power output. Extra functions for future power system such as improving power stability and grid stability in the third stage.
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Chinchilla el at. (2006) applied a fully controlled frequency converter consisting of PWM rectifier, intermediate dc circuit and PWM inverter (shown in figure 1 and 2). They suggested maximum efficiency of the system with variable wind speed and different load by maximum power tracking. Current and speed control loop was applied, also an addition outer dc voltage control for active power control.
Figure 1 Rectifier control scheme
Figure 2 Inverter control scheme
Future research directions on wind turbines system
Hossain and Ali (2015) proposed that as PMSG is a promising application for wind turbines, full conversion converter should be further investigated. As mention above, system fails mostly occur on power electronics failure among all wind turbine operation faults, the goal of designing should be focus on reliability, cost and efficiency. However, these criteria are contradicted in most cases.
Methodology and Design
The model would be consisting of PMSG, power electronics and grid. Standard IGBT switches would apply on the simulation for simplicity. For research purpose, cost of power electronics would not concern but focusing on the efficiency and power output of the whole system.
Simulation will be run on Simulink, where power converter and control scheme will be built. 10 MW power level wind turbine and UK grid will be set for the system.
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