1- Introduction

2- Problem formulation

3- Case studies

4- Validation of the proposed algorithm in the laboratory

5- Conclusion

References

Abstract

Relay coordination ensures the appropriate isolation of healthy feeders from the faulty areas in a power system. The dynamic nature of renewable sources such as wind generators (WGs) can create variations in fault levels which affect the relay settings. This causes coordination problems, which can lead to the faulty operation of overcurrent relays (OCRs). The settings of the relay need to be modified appropriately in line with the variations of fault levels, which in turn change with the wind dynamics. As a solution, wind forecast based technique for the coordination of the OCRs in wind-integrated substations is suggested in this work. In the proposed algorithm, wind speed and its direction are predicted and the settings of the relay are predetermined using an optimization technique. This reduces the computation time required for the algorithms in the current period. The algorithm is implemented in the modified IEEE 9-bus system with wind farms and also in a typical wind-integrated substation. Further, it is validated through an experimental setup in the laboratory. The results were found to be promising and the algorithm can be applied to any substations integrated with wind farms for avoiding relay mal-operations.

Introduction

Wind power generation is emerging as the fastest developing technology providing the largest share of the distributed generations. The total installed capacity of power generation from the wind has been increasing annually at an average of about 20% across the world [1,2]. The integration of wind farms with power networks is intermittent, according to the operating conditions of the WGs [3,4]. Wind farms are usually connected to the distribution feeders and then integrated into the grid. The most commonly used protection scheme for the feeders is the inverse type of overcurrent protection. The OCRs in the distribution feeders should be coordinated with their backup relays in the main feeders which connects the distribution feeders to the grid [5,6]. The settings of the OCRs are fixed at the rated capacities of the connected load and the wind farm. The penetration of wind farms into the power grid changes the conventional distribution system’s short-circuit power causing malfunctions and coordination problems in OCRs, as reported in [7]. Therefore, the settings of the relays need to be modified according to the various operating condition of the WGs. The methods commonly adopted to determine the relay settings are the conventional approaches and optimization techniques [8]. The conventional method is to predetermine all fault currents during abnormal conditions and system contingencies.

1- Introduction

2- System modelling

3- Surge protective device

4- Stochastic analysis

5- Conclusion

References

Abstract

Vacuum circuit breaker operation causes transient overvoltages that may lead to severe damages to transformer insulation. Since the parameters affecting these overvoltages have stochastic nature, a statistical analysis may provide detailed insight into the overvoltages from the point of view of insulation coordination applications. In this paper, a statistical analysis of the overvoltage variations of the step-up transformer during vacuum circuit breaker operation is conducted. Some variables, including switching angle, current chopping, high-frequency current quenching capability, and wind turbine power, are assumed as random variables. Besides, some indicators representing the switching overvoltage characteristics such as the amplitude, the number of restrikes, and the rate of rise are extracted. By performing a probabilistic analysis, the destructiveness due to switching overvoltages on typical transformer insulation can be investigated. The sensitivity of different surge protective devices and their effectivities are also analyzed and statistically evaluated. The applied methodology is very useful because of the stochastic nature of the switching overvoltages. In this work, the impact of protective equipment on each of the overvoltage indicators is analyzed. It is also found that considering only the amplitudes of transient overvoltages is not enough to draw conclusions about the safety of the transformer when exposed to frequent switching surges.

Introduction

VCBs are commonly used in offshore wind farms due to their ability to eliminate high fault currents and their low maintenance costs. Due to wind speed variations, numerous switching actions (energization and de-energization) commonly occur in wind farms every day. VCB switching transients reach the step-up transformer through the short cable connected to the wind turbine. In case, the amplitude of a switching transient voltage rises above the BIL; it can lead to transformer winding failures. Previous statistical studies related to VCB opening were only focused on the amplitude of transient overvoltages regardless of their rise times [1–4] However, some transformer failures due to VCB operations have indicated that the leading cause of the failure was the high du/dt [5]. Recently performed detailed research showed that due to fast transients and during resonance conditions, the voltages with high amplitudes are non-linearly distributed along the windings [6], and the high du/dt accelerates failure mechanisms because of high interturn overvoltages in the transformer windings; causing coil-to-coil failures in the top or in the middle of the winding. Besides, the critical voltage of the transformer BIL remarkably decreases with the reduction of the rise time [7,8] and the amplitude [7] of the transient voltage applied. Moreover, the experimental studies confirm the effects of lightning impulses on oiled paper [9] and very fast transients on cast-resin insulation [10]. Accordingly, the rate of rise and the number of transient surges should be considered alongside their amplitude in a stochastic analysis in order to accurately assess the effects of the switching overvoltages on transformer insulation lifetime. The characteristics of the transient overvoltages occurring due to VCB operation depend on various parameters. The current chopping phenomenon in VCBs being one of the key parameters that contribute to the reignition overvoltages. Experimental tests conducted in Ref. [11] however, showed the low dependency of this parameter on the operation instant.

1. Introduction

2. Numerical model description

3. Aerodynamic and sea wave loadings

4. Fragility analysis of wind turbine

5. Conclusions

Acknowledgements

References

Abstract

To more effectively extract the vast wind energy in marine areas, offshore wind turbines have been constructed with slender tower and large rotor. External vibration sources such as aerodynamic, sea wave and seismic loadings can threaten the safety of these energy infrastructures. It is important to evaluate the reliability of offshore wind turbines subjected to external vibration sources. Previous research works on the wind turbine fragility analyses only considered the fragility of the tower by assuming the wind turbine was in the parked condition with the blade mass lumped at the top of the tower. The study of the fragility of the blade which is one of the most important components of a wind turbine has not been reported. In the present study, a detailed three-dimensional (3D) finite element (FE) model of the NREL 5 MW wind turbine is developed in ABAQUS, and the tower and blades are explicitly modelled to realistically estimate the aerodynamic loads and structural behaviours of the wind turbine. The uncertainties of the structural mass, stiffness and damping are taken into account to develop the probabilistic wind-induced demand models for the tower and blades. The dynamic behaviours of the wind turbine subjected to the simultaneous aerodynamic and sea wave loadings are investigated in a probabilistic frame and the fragility curves for both the tower and blades under the parked and operating conditions are derived and discussed.

Introduction

Wind energy as one of the renewable energies is becoming a main contributor to the new electricity generation. The growth and expansion of wind farms increased rapidly in the past decade. As reported by the Global Wind Energy Council (GWEC), the worldwide installed capacity of wind power at the end of 2018 reached 591 GW, with an increase of 9.6% compared to that at the end of 2017 [1]. Due to the fact that the power generated by the wind turbine is proportional to the rotor area and cube of wind speed, multi-megawatt wind turbines with slender tower and large rotor are widely constructed in the state-of-the-art designs. These wind turbines are very flexible and lightly damped 38 since they are normally manufactured by the light-weight high-strength materials. They are thus susceptible to external vibration sources such as aerodynamic and sea wave loadings, which are experienced constantly during the whole lifetimes by offshore wind turbines. Moreover, many wind farms are located in the regions of high seismic activities such as western of United States, Japan and China [2], seismic loading is another possible vibration source during their lifetimes in these regions. The excessive vibrations may slow down the conversion of wind energy to electricity, reduce the 44 fatigue life of the structural components or even lead to the structural collapse in extreme conditions.

In this paper, a distributed optimal reactive power control (DORPC) scheme is proposed for minimizing the total losses of doubly fed induction generator (DFIG)-based wind farms (WFs), including the losses of generators, converters, filters, and networks. The DORPC minimizes total WF losses by optimally coordinating reactive power outputs of the DFIG stator and the grid-side converter. The optimal control problem is solved in a distributed manner by using the consensus alternating direction method of multipliers (ADMM). With the consensus ADMM, the total WF loss optimization problem is transformed into a distributed optimal power flow problem considered with DFIGs’ optimal operation. The optimization problem with local constraints considers the reactive power limit of DFIG-based wind turbines (WTs) and the voltage limits at all WT terminal buses inside the WF. In the DORPC, the optimal control problem is solved by the collector bus station controller and WT controllers in parallel, only with the information exchange between immediate neighbors. It eliminates the need of a central controller and centralized communication, implying better robustness and plug-and-play capability. A WF with 20 DFIG-based WTs was used to validate the proposed DORPC scheme.

Introduction

Wind power has become a widely used renewable energy source (RES) with substantial potential and mature technology. With wind power generation expanding, the intermittency of wind power and the interaction between wind farms (WFs) and power systems introduce challenges [1]. The doubly fed induction generator (DFIG)-based wind turbine (WT) has been widely used in modern WFs due to high controllability and small converter rating [2]. With power electronic converters, DFIG-based WFs can regulate reactive power independently and provide reactive power support for power systems [3]. Voltage and reactive power control of WFs has motivated numerous studies. The WF is required to maintain the power factor within the limit at the point of connection (POC) [4,5] or provide reactive power support for power systems while tracking the dispatch command from the transmission system operator (TSO) [6]. Dynamic power electronic devices, such as static var compensators (SVC) and static synchronous compensators (STATCOM), are used in WFs for providing rapid reactive compensation and voltage control [7,8]. In DFIG-based WFs, each DFIG-based WT is equipped with power electronic converters. The DFIG-based WF can utilize the capabilities of the DFIG-based WTs for providing reactive power support to meet grid code requirements. The most widely used reactive power control scheme in WFs is the proportional dispatch (PD) scheme, which is simple, easy to implement, and considers the reactive power margin of each DFIG-based WT [9,10]. However, without the optimizing reactive power references for individual WTs, the WF controller cannot achieve WF optimal operation. In [11], the particle swarm optimization (PSO) was adopted to dispatch reactive power of WTs by minimizing total active power losses along the cables and the transformers of WTs.

Wind energy as one of the renewable energies is serving as an indispensable role in generating new electric power. The worldwide installation of wind farms has considerably increased recently. To extract more wind resources, multi-megawatt wind turbines are usually designed and constructed with large rotors and slender tower. These flexible structures are susceptible to external dynamic excitations such as wind, wave and seismic loads. The excessive vibrations can compromise the wind energy conversion, lead to the structural fatigue damage and even result in the catastrophic failure of wind turbines in harsh environmental conditions. Various control devices have been proposed and used to mitigate the unwanted vibrations of wind turbines to enhance their safety and serviceability. This paper aims to provide a state-of-the-art review of the current vibration control techniques and their applications to wind turbines. Firstly, the widely used control strategies in engineering structures are briefly introduced. Their applications to suppress the adverse vibrations of the structural components of wind turbines, mainly the tower and blades, are then reviewed and discussed in detail. It can be concluded that the vibration mitigation of wind turbines is very challenging due to the fact that the dynamic behaviours of wind turbines are very complicated, which are associated with the aerodynamics, rotation of the blades, interaction between the tower and rotating blades, and soil-structure interaction, etc. Moreover, it is a challenge to straightforwardly use many of the conventional control devices because of the limited spaces in the tower and blades.

Introduction

Wind energy as one of the renewable energies plays an attractive means to generate new electric power. Wind farms have experienced rapid growth and expansion recently especially in the last decade. As reported by the Global Wind Energy Council, the worldwide installations of wind turbines reached about 539 GW at the end of 2017, with an increase of 2155% compared to that in 2011 [1]. Moreover, numbers of wind turbines are erected far away from coastlines to more efficiently extract the huge wind resources. The offshore wind power reached a historical record in 2017 with 4334 MW new installations, and the cumulative capacity was 18,814 MW [1]. Fig. 1 shows the global annual and cumulative installations of wind turbines from 2001 to 2017, and the total installations of offshore wind farms in 2011–2017 are shown in Fig. 2. It can be seen that though the offshore wind capacity in 2017 was only about 3.5% of the total wind capacity, it is growing very quickly and with great prospects. Wind turbines can be broadly grouped into the horizontal and vertical axis categories depending on the orientation of the rotation axis of the blades. Fig. 3 shows the typical geometrical configurations of the horizontal and vertical axis wind turbines (HAWTs and VAWTs) [2]. As implied by the name, the blades of HAWTs rotate about the horizontal axis, and they are perpendicular to the direction of wind flow, while the blades of VAWTs rotate vertically and they are not required to face the wind. As the HAWTs dominate the utility-scale wind turbine market currently, only the research works on the vibration control of HAWTs are reviewed and discussed herein.

The energy interaction between smart homes can be a solution for developing renewable energy systems in residential sections and optimal energy consumption in homes. The main objectives of such energy interactions are to increase consumer participation in energy management‚ boost economic efficiency‚ increase the user’s satisfaction by choosing between electricity sellers and buyers‚ and reduce the electricity purchased from the grid especially at peak hours. Thus, the innovations of this study includes defining an energy exchange method between smart buildings in an off-grid mode considering renewable energy systems, considering both thermal and electrical equilibrium and studying the lightning loads. it is assumed, here, that smart homes are off-grid‚ and the critical loads are supplied by the energy transfer between the homes using mixed integer linear programming. A compromise between the cost and time interval for using home appliances is considered to provide consumer’s comfort. An objective function is introduced considering programmable and non-programmable loads‚ thermal and electrical storages and lighting loads aiming to optimize the cost of energy between different smart buildings. Based on the method, which is tested in two different cases not only does the total cost of the smart buildings decrease but also the cost is reduced significantly when lightning loads are managed.

Introduction

The applications of hybrid energy systems have been studied with the focus on smart homes (SHs) and their optimal management in various studies. In (Yin, et al., 2017)‚ a hybrid DC Microgrid (MG) (based on power supply from photovoltaic (PV) cells) was investigated. This research shows that if the smart grid (SG) is in the off-grid state‚ the diesel generator with the electrochemical storage can be a better choice as a backup source. On the other hand‚ diesel generator cannot supply a stable power at the start-up. In (Arabul, et al., 2017)‚ the smart home loads are supplied by fuel cells (FCs), wind and solar energy resources. The method used in this research which is balancing and optimization of energy production and consumption using the Energy Management Algorithm (EMA), has been investigated. This method enhances the efficiency of the whole system improves the penetration of renewable energy sources‚ and increases the reliability of the system compared to conventional load shifting methods. In (Pilloni, et al., 2018)‚ the energy management method based on renewable energies in MG is presented., The energy sharing between SHs considering renewable energy participation is analysed in (Celik, et al., 2018). (Wu, et al., 2018)‚ adopt a method for energy management in SHs which is discussed considering renewable energy systems.

1- Introduction

2- The model

3- Global boundedness under control limits

4- Hopf bifurcation and periodic attractors

5- Conclusion

References

Abstract

In this paper we investigate whether continuous oscillations are probable for the wind turbines dynamics when connected to the grid. This can have negative consequences on the mechanical parts of the system, the power grid and, possibly, on other power systems connected to the same grid. We consider the wind turbine when the pitch control is activated, exposing it to higher wind speeds. In order to find out if there are continuous oscillations in the state variables that may realistically occur, we investigate if there are periodic attractors for the dynamics that are still allowed within the control limits suggested by many in industry, such as, and not limited to General Electric. The paper provides rigorous mathematical proofs for boundedness of the systems state variables and their derivatives under the imposed control limits by industry. This establishes the existence of attractors in a bounded system under the controls. We then find that there is a Hopf bifurcation in which periodic attractors within the control limits exist. The results are supported by simulations. The nonlinear model used in the analysis is validated versus a real measured data to magnify the finding of this paper. The conclusion of this paper is significant for the nonlinear dynamics and control literature for wind turbines complex system, and the possible consequences on studying/questioning the control limits (limeters) suggested by industry.

Introduction

According to the US Department of Energy [1], wind is the fastest growing energy resource being used. This rapid expansion requires more scientific research and studies to comprehend the dynamics of Wind Turbine Generators (WTGs), if we are to gain the most from this valuable resource. Governments and corporations are working on understanding the challenges and consequences of integrating WTGs within large cities with/without other power systems. As a result of the complexities of the WTG mechanical and electrical systems, dynamic and control studies have increased recently. This has been observed by the comprehensive review article [2]. Additionally, the reader may consider [3,4] for studies that are looking into the challenges facing the future of power grid with large share of wind and renewable energies. According to [5], three-bladed (type-3) WTGs are more efficient in extracting power from the air streams, when compared to other types. Coefficients of Performance ( curves) of type-3 can go to up 0.4-0.5 efficiency (also see [6]). Most agree that Doubly Fed Asynchronous/Induction Generator (DFAG/DFIG) is mostly the technology used with WTGs systems. A detailed study describing this can be found in the literature review of [2] with citations to many sources in the literature that focus mainly on investigating the implementation and advantageous of DFAG/DFIG technologies.

1- Introduction

2- The multiscale modeling framework

3- Multi-scale modeling procedure

4- Model validation

5- Damage evolution of the wind turbine blade

6- Conclusions

References

Abstract

Composite structures have been widely used in wind turbine equipment for their high stiffness to mass ratio and high strength. A major concern in the use of composite materials is their susceptibility to various micro damage, such as fiber breakage and matrix crack, which will lead to macroscopic structural fracture. In this paper, a multi-scale modeling strategy is proposed to investigate failure mechanisms and damage evolution of composite blades with initial defects from microscopic damage (including fiber fractures and matrix cracks) to macroscopic fracture. At the microscopic scale, an isoparametric micromechanical model is developed to calculate microscopic stresses and simulate microscopic damage. At the laminar scale, the classic laminate theory is employed to evaluate the laminate stiffness. At the structural scale, a reverse modeling technology is proposed to accurately acquire structural dimensions of a wind turbine blade, and a macroscopic 3D model is implemented into ANSYS/LS-DYNA software. By comparing with the experimental data, it is demonstrated that the proposed multi-scale method is suitable to predict mechanical properties of complex composite structures effectively.

Introduction

As one of the most abundant renewable and green resources, wind energy is increasingly playing an important role in reducing CO2 emission for environmental protection and has viable commercial values [1-3]. It has been one of the most important new energies due to its availability and accessibility. To maximize the energy that can be harvested from wind, an efficient and safe design of a wind turbine, including its strength and damage tolerance, is essential. Complex dynamic loads may lead to a catastrophic failure of wind turbine blades [4-5]. According to the data from Caithness windfarm information forum [6], blade failure is the most common accident of wind turbines, estimated at around 3800 incidents per annum. Therefore, it is crucial for researchers to have a better understanding of the failure mechanisms, which will improve the safety and reliability of wind turbines. Wind turbine blades, which are normally manufactured by continuous glass fiber-reinforced resin matrix composites, accounts for nearly one-fifth of the cost of a wind turbine. Experimental [7-8] and theoretical methods [9-10] have been used to investigate the mechanical features and damage mechanisms of composite blades. However, for a heterogeneous material, it is difficult to reveal the intrinsic relations between its macroscopic and microscopic characteristics, such as fiber arrangements and fiber shape [11-14], by experimental methods. As a result, more and more researchers are using numerical methods to study their mechanical properties and optimally design composite wind turbine blades. Haselbach et al. [15] studied ultimate failure of a 34m long blade.

ABSTRACT

In this research work an introduction to the concept of an actively controlled horizontal axis wind turbine through varying blade tip sweep, is presented. The concept refers to variable tip swept rotor blades, that have the ability to pivot collectively aft, about an axis located at the blade tips. Quantities to be controlled are power production and blade loads. The investigation is carried out with a modified Blade Element Momentum (BEM) model that takes into account variable tip swept rotor blades and the modifications are based on results from a lifting line theory based model. The simulations refer to the 5MW NREL reference wind turbine that incorporates a suitable controller and preliminary results show beneficial behaviour in all of the investigated areas.

INTRODUCTION

Over the past few years there is a continuous effort for increasing energy production and reducing dynamic loads of wind turbines. In [1] there is an extensive review of the current status in smart rotor control that goes beyond the borders of conventional control methods like pitch or stall regulation. In particular, smart rotor control refers to an integrated system equipped with sensors, actuators and one or more microprocessors that operate in a feedback loop and control the blade aerodynamic loads. The latter is achieved either by enhancing the flow around the blade with the deployment of microtabs and use of boundary layer control methods (like vortex generators and active synthetic jets) or by altering the shape of the airfoil utilizing camber control, active twist or flaps. However, all of these features have to be carefully designed in order to compensate for their complexity and the fault cases that they may impose. In this research work an introduction to an innovative control method is presented through tip swept rotor blades that have the ability to pivot simultaneously aft (in-plane movement) about an axis located at the blade tips. The swept tip can be either part of the main blade with an internal mechanism or an added surface (add-on) to the blades, as it is shown in Figure 1. The purpose of this control feature is to actively adjust power at specific operating areas and reduce fatigue blade loads or extreme loads during a wind gust through small sweep angle variations in the tip area. Similar research efforts like [2], [3] and [4] have already concluded that aft sweeping of blades plays a beneficial role on reducing the loads while fore sweeping increases them. However, the aforementioned methods refer to passive load control methods, in which changes in wind speed are counteracted through the passively adapting aeroelastic response of the rotor blades (for example tension – torsion, bend – twist, sweep – twist coupling), whereas this research intends to fill in the gap of active load/power control through varying blade tip sweep for an otherwise nondeformable rotor.

1- Introduction

2- Wind Energy System

3- WECS Topologies and Generators

4- Power Converters

5- Conclusions and Recommendations

Reference

Abstract

Sources of renewable energy such as wind energy are indigenous and can help in decreasing the reliance on non-renewable energy sources. After introducing the history of wind energy production in Egypt and worldwide besides its techno-economic importance, this paper presents a comparative review on the wind energy conversion systems (WECS). The horizontal and vertical types of wind turbines with their mathematical dynamic models are discussed. Different types of electrical generators used in WECS in addition to their advantages and disadvantages are illustrated. The various AC-AC converters topologies for WECS are explained with a detailed discussion for their features.

Introduction

Worldwide energy demand is rapidly expanding due to the continuous economic progress and the power electronic technologies development. Conversely, traditional petroleum derivatives, for example, coal, oil and flammable gas, which have been the key energy source since the industrial revolution, are confronting rapid consumption. Therefore, the mission to create inexhaustible and clean sources of energy from wind, solar and hydrogen energy becomes more convenient. Among these numerous sustainable assets, wind energy encourages business prospects in substantial power in favor of its safe and naturally sustainable behavior, zero emission, competitive and inexhaustible nature [1, 2]. In 2018, wind energy provides around 5% of the total energy consumption of the world [3]. It is anticipated that energy from wind will be about (20 % and 35%) of the total world’s energy in (2030 and 2050) respectively [4]. The worldwide wind energy production has reached 690.8 GW in 2018 from around 19057 wind farms and Europe had the first place with 259 GW and 13290 wind farms [6]. Figure 1(a) depicts the global wind power production in the last two decades in 2018[5]. Production of wind power for the top five countries across the world in 2018 is illustrated in Figure 1(b). China has the highest wind production in the world with 123.805 GW [5]. The USA provides the highest wind power in Americas and the second in the world with 98.94 GW [7]. Germany is the highest in Europe and the third in the world with 52.828 GW. Africa has the lower wind energy production in the world continents with 5.7 GW with 87 wind farms as depicted in Figure 1(c) [5]. Wind power in South Africa is the highest in Africa with 2295 MW [5]. Egypt has the third place and produces around 810 MW of wind energy and the potential of this production is expected to reach (1500 MW and 2500 MW) in (2030 and 2050) respectively [5]. The electrical power generation from wind energy in Egypt started in 1997 by 6 MW and increased gradually as shown in Figure 1(d) to reach 810 MW in 2018 [5]. In onshore, the blowing wind from the sea to land is usually less than perfect. In addition to, the tendency of waves increases and short period swell is created [8]. Therefore, there is an increased interest for offshore wind, as the breeze is regularly more grounded and more uniform in surface of ocean than aground and the expected generated power from the planned offshore is 143.36 GW [9]. European nations are the pioneers in offshore wind. For around 20 years, Denmark has been using it to supply power. Offshore wind in the USA and similar nations can possibly turn into a noteworthy energy hotspot for household applications as waterfront wind sources are plenty [10].

Wind conditions in the built environment are complex in nature and characterized by lower wind speeds and higher turbulence due to the presence of obstructions. A growing body of literature and research/testing activities related to performance evaluation of small wind turbines (SWTs) in urban wind conditions have inferred that urban installed SWTs are subjected to higher level of turbulence and face dynamic loading that impedes their performance, and reduces fatigue life. This paper reviews the diverse studies conducted on the application of SWT technology in the built environment to understand the characteristics of inflowing wind, their performance and identify the gaps in the knowledge. This review paper also investigates the extent to which the international design standard for SWTs, IEC 41400-2, is valid for urban installations. The findings from this review show that the wind models incorporated in IEC 61400-2 is not suitable for installation of SWTs in the built environment. The authors recommend a thorough study through measured data and characterization of urban wind to make current standard inclusive of wind classes that characterize urban wind conditions. Thus, SWT design can be made more consistent with urban wind conditions and their performance and reliability can be assured.

Introduction

With rapid growth in population, the global energy consumption is projected to increase by 56% between 2010 and 2040 [1]. In 2015, fossil fuel (coal, petroleum and natural gas) accounted for 78.4% of global final energy consumption, with the share of renewables (modern and traditional) and nuclear power at 19.3% and 2.3% respectively[2]. Worldwide, the share of renewable energy will increase to address global climate change by 2030. Modern renewable energy is being used increasingly in four distinct markets: power generation, heating and cooling, transport fuels, and rural/off-grid energy services [3]. In recent years, progress has been made in increasing the renewable energy share in the power 103 sector particularly in the wind, solar photovoltaic (PV) and hydropower sectors [4]. In 2014, 104 the total renewable power capacity was 712 GW out of which 370 GW was from wind energy through utility scale wind turbines [5]. The total global capacity of wind energy reached 432 106 GW at the end of 2015, representing a cumulative growth of 17%. Although large-scale generation using wind energy has taken shape, there are some problems for their sustainable development. For example, the main barriers of large scale on-shore wind farm are available sites, impact of grid power quality, public acceptability and losses in transmission and distribution of electricity to the consumers [6]. One alternative that reduces some of these barriers is the application of small wind turbine (SWT) technology.

1- Introduction

2- Theories and methodology

3- Results and discussion

4- Conclusions

References

Abstract

In order to account for rigid-flexible coupling effects of floating offshore wind turbines, a nonlinear rigid-flexible coupled dynamic model is proposed in this paper. The proposed nonlinear coupled model takes the higher-order axial displacements into account, which are usually neglected in the conventional linear dynamic model. Subsequently, investigations on the dynamic differences between the proposed nonlinear dynamic model and the linear one are conducted. The results demonstrate that the stiffness of the turbine blades in the proposed nonlinear dynamic model increases with larger overall motions but that in the linear dynamic model declines with larger overall motions. Deformation of the blades in the nonlinear dynamic model is more reasonable than that in the linear model as well. Additionally, more distinct coupling effects are observed in the proposed nonlinear model than those in the linear model. Finally, it shows that the aerodynamic loads, the structural loads and global dynamic responses of floating offshore wind turbines using the nonlinear dynamic model are slightly smaller than those using the linear dynamic model. In summary, compared with the conventional linear dynamic model, the proposed nonlinear coupling dynamic model is a higher-order dynamic model in consideration of the rigid-flexible coupling effects of floating offshore wind turbines, and accord more perfectly with the engineering facts.

Introduction

In recent years, floating offshore wind turbines (FOWTs) have been receiving increasing attention due to their prominent advantages, such as steadier and stronger wind available resources, lower operational noise, reduced visual pollution and fewer space limitations (Karimirad et al., 2011; Bachynski and Moan, 2012; Pérez-Collazo et al., 2015; Ma et al., 2015). FOWTs are complex rigid-flexible coupled multi-body systems (Namik and Stol, 2010; Wang and Sweetman, 2013; Nejad et al., 2015). Moreover, because the slender blades of an FOWT system typically work at a high rotational speed and are influenced by the motions of the floating platform, rigid-flexible coupled dynamic responses of FOWT systems are more complicated than those of the fixed bottom wind turbines. Rigid-flexible coupled multi-body dynamics have received considerable attentions during the development of modern high-speed airplanes (Shabana, 1997; Bauchau, 2011). In the 1970s, Winfrey (1971) proposed the “kinetoelastodynamics” (KED) method to model the dynamic behaviour of rigid-flexible coupled multi-body systems. In this method, the system is first modelled as a rigid multi-body system to calculate the motion and inertia forces on the system.

I- Introduction

II- Wind Turbine Configuration

III- Proposed Control System With the Higher Order Super-Twisting Sliding Mode Control

IV- Conventional and Proposed Inertial Power Based HC Methods

V- Experimental Results and Discussion

VI- CONCLUSION

References

ABSTRACT

Finding and tracking maximum power point are two important dynamics in the control of variable speed wind turbines, since they determine the efficiency of wind turbines. The conventional hill climbing possesses the problems of wrong directionality and low performance, since it does not take the inertial effect into account. In this study, a novel hill climbing method is proposed by considering the inertial effect to solve these problems. Besides, employing the exact model knowledge of the generator in the maximum power tracking control deteriorates the efficiency considerably, therefore it is required to design a parameter independent and robust control system if possible. Thus, the third order super twisting sliding mode and continuous integral sliding mode controllers are designed for the control of generator and grid side converters to track the maximum power trajectory accurately, and they are compared to each other for the chattering in experimental results. A comparison is also performed between the conventional and proposed hill climbing methods based on the captured energy from the wind. Experimental results, with a wind turbine emulator, demonstrate that the proposed hill climbing method relaxes the wrong directionality and sluggish performance of the conventional one.

NTRODUCTION

wing to the intermittent characteristic of wind, maximum power tracking is an imperative requirement to increase the efficiency of the wind turbines. To this aim, Variable Speed Wind Turbines (VSWTs) have been developed to increase the efficiency in comparison with constant speed wind turbines [1]. Maximum Power Point Tracking (MPPT) is achieved by adapting the turbine speed optimally below rated wind speed. Two important dynamics arise when the MPPT based control of VSWTs is the subject, these are the instantaneous detection and tracking of the Maximum Power Point (MPP). In the literature, there are widely used MPPT methods based on Generator Signal Feedback (GSF), Tip Speed Ratio (TSR), Optimal Torque Control (OTC), Wind Turbine Power Curves (WTPC) and Hill Climbing (HC) [47].