Chapter 1 Introduction
This introduction chapter provides information on the current state of the wind power industry, the motivation behind and the contributions of this thesis.
This chapter is organized such that some recent developments in the European market and key areas of wind power research are reported in Section-1.1 and Section-1.2 respectively. Section-1.3 provides an overview of the Wind Turbine Generator System (WTGS) faults. A review of the condition monitoring systems on offer is provided in Section-1.4. The motivation behind this thesis is the topic of Section-1.5 while Section-1.6 discusses its contributions. Section-1.7 lists the publications. Finally, Section-1.8 presents the structure of the rest of this report.
1.1 The Wind Power Business
A 30 % cut in greenhouse gas emissions by 2020 compared to the 1990 levels is a crucial first step to the 85-95 % emissions cut by 2050, agreed to by the heads of state. This is essential in industrialized countries to avoid a global temperature rise of 2 ˚C or more that would have catastrophic climate impacts [EWEA 2010].
The overall market for renewable power capacity including wind, solar, hydro and biomass reached record levels in 2010, increasing 31 % from 17.5 GW in 2009 to 22.6 GW in 2010 [EWEA 2011].
Wind power installations accounted for 17 % of the new electricity generating capacity in 2010. This is equivalent to 9.3 GW, bringing the total to 84 GW by the end of 2010. This capacity will in a normal wind year produce 181 TWh of electricity, meeting 5.3 % of overall EU electricity consumption [EWEA 2011].
Offshore wind power installations grew 51 % from 582 MW in 2009 to 883 MW in 2010 with 308 new turbines installed. This brings the total offshore capacity to 2964 MW from 1136 offshore wind turbines, which together would generate 11.5 TWh of electricity in a normal wind year. The United Kingdom (UK) is the European (and world) leader, with a total installed offshore wind capacity of 1341 MW. The UK is followed by Denmark 854 MW, The Netherlands 249 MW, Belgium 195 MW, Sweden 164 MW, Germany 92 MW, Ireland 25 MW, Finland 26 MW and Norway with 2.3 MW [EWEA 2011].
Between 1000 and 1500 MW of new offshore wind power capacity is expected to be fully grid connected in Europe during 2011. Ten European wind farms are currently under construction with a total of 3000 MW – these will more than double the installed capacity in the 45 already grid connected offshore wind farms. The European Wind Energy Association (EWEA) research shows that a total of 19 GW of offshore wind capacity is already fully consented. If constructed, it would generate 66.6 TWh of electricity in a normal wind year - enough to supply 14 of the largest capitals in Europe with electricity, including Paris, London and Berlin. Not included in this figure is large additional offshore
wind energy capacity planned but not yet fully consented in the UK [EWEA 2011].
The generation capacity of individual WTGS is increasing as well with manufacturers such as Vestas, General Electric (GE) and ENERCON offering systems rated at 3.0 MW (V112-3.0 MW permanent-magnet), 4.1 MW (4.1-113 direct-drive) and 7.5 MW (E-126/7.5 MW direct-drive) respectively [Vestas 2011] [GE 2011] [ENERCON 2011].
During 2010, 29 new offshore turbine models were announced by 21 manufacturers whereas 44 new turbine models have been announced by 33 manufacturers over the last two years [EWEA 2011].
1.2 Key Areas of Wind Energy Research
The key areas of wind energy research include [EWEA 2010]
• Improving the design and layout of wind farms.
• Increasing the reliability, accessibility and efficiency of wind turbines.
• Optimising the maintenance, assembly and installation of offshore turbines and their substructures.
• Demonstrating large wind turbine prototypes and large interconnected offshore wind farms.
• New methods of grid management to allow high level of wind power in the system.
• Expansion of education schemes and better training facilities.
1.3 Faults in Wind Turbine Generator Systems
The main components related to faults in a WTGS are shown in Figure-1.1 [Amirat et al. 2009].
Figure 1.1 – Wind turbine nacelle cross-section [Amirat et al. 2009].
A survey carried out for wind power plant faults between 2000 and 2004 in Sweden suggests that generator faults accounted for 5.5 % of the number of failures while their contribution to down time was about 9 %; see Figure-1.2 and Figure-1.3 [Ribrant & Bertling 2007]. Another study for German and Danish wind power plants for the years 1994-2004 indicates generator faults as the fifth most frequent cause of failure, Figure-1.4 [Tavner, Xiang & Spinato 2006].
Figure 1.2 – Percentage failures for Swedish wind power plants [Ribrant & Bertling 2007].
Figure 1.3 – Percentage of downtime per component in Sweden between 2000 and 2004 [Ribrant & Bertling 2007].
Figure 1.4 – Failure rates for Danish and German wind power plants between 1994 and 2004 [Tavner, Xiang & Spinato 2006].
1.4 Condition Monitoring Systems Available in Industry
The trend in wind power plant operations is to shift from time-based to condition-based maintenance. This is due to constraints such as the location of the energy conversion equipment in nacelles at the top of high towers and offshore installations where accessibility is subject to weather conditions. A continuous monitoring regime ensures that abnormal (icing on or penetration of water in the blades) and fault conditions (in generator windings, sensors, bearings, gearbox, etc.) are detected and proper steps taken to limit the damage.
Most of the condition monitoring is based on spectrum analysis of vibration signals from accelerometers to detect common mechanical faults such as unbalance, structural weakness, loose parts etc. Other signals such as shock- pulse measurement are used on roller bearings for bearing damage, lubrication condition and effects of alignment and load.
As much as 20-30 % of wind farm operation and maintenance costs are related to the gearbox. GE Energy’s Bently Nevada ADAPT.wind condition monitoring solution detects drive train issues and is the standard software on GE’s wind turbine units. The complete system incorporates accelerometers and speed sensors mounted on the gearbox, main bearing and the generator that provide input to a monitoring device for signal processing. The software then does the trending and diagnostic analysis [GE Energy 2010] [Hatch 2004].
The condition monitoring system WP4086 from Mita-Teknik provides long term surveillance of vibrations in order to predict the wear and tear of the gearbox and the generator. If the vibration parameters exceed the threshold of the
predetermined critical vibration levels, an alarm is generated. The system visualizes live vibration data and the historical data can be transferred to a central computer for trend analysis [Mita-Teknik 2011].
The SKF Multilog Online System IMx-W works with SKF @ptitute Observer software and is housed in the nacelle. It features a Digital Peak Enveloping technique. 16 analog and 2 digital channels are provided, which are simultaneously measured. The signal inputs are configurable for acceleration, velocity and displacement sensors. Each input can be configured for standard accelerometers and proximity probes. Adaptive alarm levels may be controlled by speed or load [SKF 2011].
The Shock Pulse Method (SPM) system from SPM Instrument monitors shock pulse parameters. It is used for condition monitoring of generator bearings and main rolling bearings on the low speed shaft. The SPM transducer is sensitive to shocks generated by every operational rolling bearing and its signal is a train of electric pulses of various magnitudes corresponding to the magnitude of the shocks. These sensors are installed on bearing housings. The system measures signal amplitudes, provides shock and vibration spectra and provides values on fault symptoms related to the state of the bearing inner ring, gear wheel etc.
[SPM n.d.].
Broadband vibration measurement made in horizontal, vertical and axial direction is another cost effective method for gearbox monitoring due to coupling and gear related problems on the high speed side of the gear box. It is converted to an RMS value for the vibration velocity, acceleration or displacement [Kazzaz & Singh 2003]. The Evaluated Vibration Analysis Method (EVAM) provides the possibility to present the velocity value of the fault symptom in relation to overall machine vibration [SPM n.d.].
1.5 Motivation
Most of the condition monitoring systems available in industry are focused on the failure of mechanical parts e.g. the gearbox and the bearings; see Section-1.4.
This may be due to the large downtime associated with the gearbox. However, the generator faults also account for a significant percentage of downtime as mentioned in Section-1.3. As compared to bearing faults that deteriorate the system performance over a larger period of time, the generator winding short- circuit faults can develop into failures relatively quickly due to excessive heating caused by high current in the shorted turns [Kliman et al. 1996] [Tallam et al.
2007] [Wu & Nandi 2010]. The principal motivation of this thesis, therefore, is to develop a method for rapid detection of winding short-circuits in a Doubly-Fed Induction Generator (DFIG) based WTGS after determining the best fault indicator (residual) from those available.
To be able to study any system in fault mode, it is first necessary to understand its operation in faultless mode. For this purpose, appropriate models are required that are detailed enough to take into account the phenomena of interest while allowing fast simulation speeds. To understand the operation of a DFIG
system. It is then required to validate the control system and the developed fault detection algorithms on a test bench. The test bench has to be properly dimensioned and steps needed to bring the system up to the point of operation, where the phenomenon of interest can be studied, need to be clearly defined.
The goal of the start-up procedure is to reduce the stress on the electrical and mechanical components of the wind turbine and for MW range WTGS, the effect on the grid as well. In case of prolonged grid faults that lead to stator disconnection, a reconnection has to be carried out soon after voltage recovery [Da Silva et al. 2008].
• Operation
Start-up of a DFIG based WTGS has been the topic of discussion in literature [Da Silva et al. 2008] [Zhang et al. 2006] [Abo-Khalil, Lee & Lee 2006] [Yuan, Chai &
Li 2004] [Bouaouiche & Machmoum 2006] while [El Aimani et al. 2003] [Iov 2003] have briefly mentioned the procedure. In [Da Silva et al. 2008] the steps taken during start-up have been listed. It is not shown how the phase difference between the stator voltage and the grid voltage prior to connection is corrected, which is an important aspect of the procedure. The controller gain calculation, performance of the current loops, the voltage controller required to ensure perfect synchronization and switching between different controllers has not been discussed either. DC-link capacitor charging has been explained in detail but an additional diode rectifier has been used, instead of the anti-parallel diodes of the converter, to charge the DC-link capacitor. [Zhang et al. 2006] explains that the initial rotor position error causes the phase difference mentioned above and requires a Proportional Integral (PI) controller to be designed for angle compensation, but does not show the PI controller gain calculation. The evolution of the quantities in the dq reference frame used to design the controllers, the moment of controller switching and the change in power reference have not been shown. DC-link capacitor charging has not been discussed either. [Abo-Khalil, Lee & Lee 2006] has used the same method as [Zhang et al. 2006] for position error correction but also does not show the controller gain calculation. Initial rotor position effect has not been explicitly mentioned either. Stator voltage control, the connection instant and controller switching have not been shown. [Bouaouiche & Machmoum 2006] and [Yuan, Chai & Li 2004] also mention the start-up procedure but do not show the results of the connection procedure. The initial rotor position problem has not been discussed either. Only [Yuan, Chai & Li 2004] has mentioned the need for voltage control to ensure perfect synchronization while [Bouaouiche & Machmoum 2006] has mentioned DC-link charging as an initial step. None of the reviewed papers dealing with the start-up procedure treat the entire procedure step by step. The important issue of phase difference correction has only been taken up by two of the authors. A non-ideal grid and therefore the necessity of a voltage controller have been overlooked by most. Controller gain calculation has not been shown.
In order to successfully carry out the required steps on the test bench, they have to be simulated first. For this purpose, a simulator needs to be developed that models the actual system on the test bench as closely as possible. Apart from the
simulator for the start-up sequence, another simulator that allows initial conditions, corresponding to a point of operation, to be imposed is required. The simulation can then be started in steady state and different control techniques can be compared quickly without the need to repeat the start-up sequence.
• Fault Detection
Some authors have studied faults in the windings of a DFIG based WTGS [Popa et al. 2003] [Lu, Cao & Ritchie 2004] [Douglas, Pillay & Barendse 2005] [Shah, Nandi & Neti 2007] [Djurovic et al. 2009]. In all but [Douglas, Pillay & Barendse 2005] the authors have used steady-state harmonic spectrum analysis on current and/or power to detect specific frequencies that exist in case of such a fault.
[Douglas, Pillay & Barendse 2005] have combined the Extended Park’s Vector Approach (EPVA) with Discrete Wavelet Transform and statistics to detect the frequencies of interest existing around twice the fundamental frequency.
Furthermore, most of the authors have connected a resistance or an inductance parallel to one of the phases, thereby reducing the impedance of that respective phase. Although it does create a winding unbalance, this does not represent the true condition inside the machine due to the short circuit. Only two of the authors have a machine simulation model that takes into account winding short- circuits [Lu, Cao & Ritchie 2004] [Shah, Nandi & Neti 2007]. In all of the papers consulted, it is not clear if an active control system is present, what effect it has on the harmonics and how fast can the fault be detected. The effect of fault location on harmonic magnitudes has not been considered either.
The review of existing literature on winding short-circuit fault detection in a DFIG based WTGS points, first of all, towards the need to have a simulation model of the machine that takes into account the changed internal condition of the machine and can deliver the indicators that appear, as a consequence of such a fault, in the measured quantities. It is of interest then to use this machine model in a WTGS, along with the complete control system implemented. This provides the possibility to study the effect of the control system and fault location on fault residuals, something that has not been studied before. A suitable residual can then be selected that allows the short-circuit fault to be detected rapidly and to generate an alarm.
• Test Bench
As mentioned earlier, the motivation behind the test bench is to have a platform to validate both the control and fault-detection algorithms. Trials on a test bench are an important intermediate step before the algorithms can be tested on a real WTGS. Some of the phenomena that do not appear in simulations, due to the trade-off between component modelling detail and the resulting simulation speeds, can be observed and analyzed on a test bench.
1.6 Contributions
The aim of this thesis was to develop a detection method for winding short- circuit faults in a DFIG based WTGS. For this purpose, simulators had to be developed and a test bench was installed. In total, three simulators for the electrical part of the WTGS were developed, as per requirements, at different stages of the project.
Firstly, in order to study the operation of a DFIG based WTGS at different operating points and to compare different control techniques, a simulator that models all the system components in dq domain was implemented. Very fast simulations can be done by modelling components this way. Matlab/Simulink was used for this purpose and the models can be easily modified as required.
This simulator can also be used, for example, to calculate machine currents in case of short-circuit faults on the grid.
The second simulator was required to clarify the start-up procedure of the DFIG based WTGS. The ‘physical modelling’ aspect of Matlab/SimPowerSystems toolbox provides a direct correspondence with the experimental setup. The sequence of steps was tested in simulation before they could be carried out in practice on the test bench. The three-phase component modelling environment provides the possibility to test the phase-locked loops and the pulse-width modulation blocks required in experiments but simulation speed is slower compared to the first simulator. Furthermore, the possibility to start simulations from steady state at any operating point is made difficult due to the limitations of the user interface utility in calculating the initial conditions for a wound-rotor induction machine.
The third simulator runs a three-phase model of the DFIG, developed to take into account short-circuit faults in the stator and rotor windings. It has been implemented in Matlab/Simulink along with the controllers for the electrical system. The decoupling provided by the DC-link of the back-to-back converter allows the grid-side converter part to be modelled in dq domain, thus increasing the simulation speed.
Proper dimensioning of components, their acquisition, installation and troubleshooting has been carried out for the test bench installed as a part of this thesis. Safety of the operator and that of the equipment are key features of the system. A detailed user’s manual has also been written to facilitate its use.
The contributions of this thesis are:
• Operation
The simplification of the method for stator voltage synchronization with that of the grid, which is a part of the start-up procedure of the DFIG based WTGS. As a consequence of the sequence of steps normally carried out during start-up, where the turbine is already rotating before the rotor-side converter control is activated, the initial rotor position may not be available. This will lead to a phase difference between the generated stator voltage and the grid voltage, a problem
that has been documented in literature. This phase difference has to be eliminated before the stator can be connected to the grid and so other authors have proposed a PI controller that can dynamically correct this error. In this thesis, the task of designing a PI controller has been replaced with a simpler method that uses a four-quadrant inverse tangent function on the stator voltage d and q components to calculate the phase difference. Then a sample-and-hold function simply updates the slip angle to minimize the phase difference. Perfect synchronization is ensured by the voltage controller that needs to be designed in any case. This method has been tested with arbitrary rotor positions, both in simulations and in experiments, and found to work satisfactorily. The compensation is done ‘on the fly’ as soon as the stator voltages have stabilized and saves the headache of designing as additional controller.
• Fault Detection
It has been demonstrated that the magnitude of the current harmonics, normally used as fault residuals, is different when the location of the short-circuit fault is changed within the windings of a phase. It has also been demonstrated that the control system based on Park’s transformation, that assumes perfect balance between the phases and a principal frequency that defines the angular velocity of the dq reference frame, greatly attenuates the harmonics in the currents. A simulation model that allows the fault to be created online and therefore observe the reaction of the control system has been developed. The use of the cumulative sum (CUSUM) algorithm allows fast detection of the change in the residual mean value. It is easy to implement and can be used for online fault detection.
As a result of an analysis carried out on current harmonic magnitudes and negative-sequence current components, the latter has been selected to be more suitable as a residual for short-circuit faults in the stator windings of the DFIG used in a WTGS. The dependence of this residual on asymmetry in the machine and in stator voltage has been included in the analysis by increasing the percentage of the shorted turns and imposing an unbalanced stator voltage respectively. It has been demonstrated that changing the number of shorted turns and the location of the fault has little effect on the residual while an unbalanced stator voltage within limits is not able to trigger a false alarm.
1.7 List of Publications
Some of the results of this thesis have been published in the following articles:
• Journal Papers
Zafar, J & Gyselinck, J 2010, ‘CUSUM based Fault Detection of Stator Winding Short Circuits in Doubly-Fed Induction Generator based Wind Energy Conversion Systems’, Renewable Energy & Power Quality Journal (RE&PQJ), issue 8, paper 398, viewed 01 January 2011,
http://www.icrepq.com/icrepq’10/398-Zafar.pdf
• Conference Papers
Zafar, J & Gyselinck, J 2010, ‘Effect of Winding Short-Circuit Fault Location and Control Action on Currents in a Doubly-Fed Induction Generator based Wind Energy Conversion System’, Proceedings of the European Wind Energy Conference (EWEC 2010), Warsaw, Poland, paper 289, viewed 01 January 2011,
http://www.ewec2010proceedings.info/allfiles2/289_EWEC2010presentation.p df
Zafar, J, Galvez-Carrillo, MR, Gyselinck, J & Kinnaert, M 2010, ‘Comparison of Classical and State-Space Control Techniques of Doubly-Fed Induction Generator based Wind Energy Conversion Systems’, Proceedings of the European Wind Energy Conference (EWEC 2010), Warsaw, Poland, paper 168, viewed 01 January 2011,
http://www.ewec2010proceedings.info/allfiles2/168_EWEC2010presentation.p df
1.8 Structure of the Thesis Report
The rest of this thesis report is organized as follows:
Chapter-2 is a general overview of WTGS. The principle of energy conversion in a generic WTGS is explained and the expression for energy extraction by the aerodynamic part of the system is derived. Different methods of power gain maximization and limitation are discussed. The existing types of WTGS are compared in terms of their performance and limitations. Finally, some recent improvements in WTGS components and their use are reported.
Chapter-3 is focused on the control of the DFIG based WTGS. Modelling requirements of the system are discussed and the necessary controllers have been designed. The operation of the grid-connected WTGS system in faultless mode has been simulated at different operating points. The start-up procedure, which is a prerequisite to any experiments on the test bench for validation purposes, has been clarified and tested in simulations.
Chapter-4 treats the subject of winding short-circuits faults and their detection in a DFIG based WTGS. A model of the DFIG that can take into account such faults is developed. It is then validated to ensure that it can give the desired frequencies in the currents that are slip dependent and have been used in literature extensively as fault residuals. This DFIG model is then used in a controlled WTGS and the complete electrical part has been simulated to study different fault scenarios. Finally, a fault-detection system has been developed that is robust to factors such a fault location, fault severity and stator voltage unbalance within limits.
Chapter-5 develops the test bench for a 10 kW DFIG based WTGS. The dimensioning of components is carried out and the components related to the DFIG operation have been described. Extensive details of the test bench are found in the user’s manual included as an appendix.
Chapter-6 presents the experimental results. The developed controllers have been validated in the context of the start-up of the system and stand-alone operation. The problem encountered in the operation of the rotor-side converter has been discussed and an investigation into the possible cause of the problem has been carried out. Harmonic frequency analysis has been done and the current harmonic suppression property of the control system has been demonstrated.
Chapter-7 is the last chapter and presents the overall conclusions drawn for this thesis work and future research directions.
Chapter 2
Wind Turbine Generator Systems
This chapter aims to provide a brief introduction to WTGS operation. It is organized such that Section-2.1 provides information on the wind turbine system’s aerodynamic part and derives the expression of power extraction.
Section-2.2 and Section-2.3 discuss the WTGS control objectives related to the required gain in electric power. Section-2.4 presents different WTGS available on the market, based on whether they are fixed or variable speed systems, and discusses their strengths and weaknesses. Finally, some research related to improvements in the electrical components of WTGS and their use is highlighted in Section-2.5. Section-2.6 concludes this chapter.
2.1 Wind Energy Extraction
A WTGS converts the energy of the wind to electrical energy. This process is illustrated through Figure-2.1 [Digital Mind 2008].
Figure 2.1 – How does a wind turbine work? [Digital Mind 2008]
A flow of wind over the blades of the turbine spins the rotor. This rotor drives a generator’s shaft, often using a gearbox, and thus the generator produces electricity. The gearbox is used to adjust the speed of the slowly rotating turbine to the high speed of the generator.
The energy contained in the wind is due to its movement. This kinetic energy Ew
can be given as
2 w w
w m v
12
E = (2.1)
where mw is the mass and vw the velocity of the wind. The mass can be given in terms of air density ρ and volume V of the cylinder that is formed by the wind passing the area covered by the blades, which thus gives
( )
w2w V v
12
E = ρ (2.2)
The volume of this cylinder is given as
( )
w2 blade w
bladel R l.
A
V = = π (2.3)
where Ablade is the circular area covered by the blades, Rblade the blade length and lw the length of the cylinder formed by the passing air through the blade area.
The length lw of the cylinder, that is formed, per unit time is the distance covered by the wind per unit time; its velocity. Thus the energy content of the wind per unit time, its power Pw, can be given as
3 w 2 blade
w R v
12
P = ρπ (2.4)
The energy in the wind that is extracted by the aerodynamic system is given by the actuator disc representation. Thus the fraction of the power in the wind extracted by the turbine and denoted by Pt is given as
) , ( C v 2 R
Pt = 1 ρπ blade2 3w P λTSR β (2.5)
The efficiency coefficient CP(λTSR,β), as indicated, is a function of the blade angle called the pitch angle, denoted by β, and the Tip Speed Ratio (TSR), denoted by λTSR. TSR is the ratio of the linear speed of the blade tip and the wind speed. It is given as
w t blade
TSR v
R ω
λ = (2.6)
ωt is the angular velocity of the turbine. For a certain blade design there is a fixed relationship between λTSR and CP for different β as, for example, given by Equation (2.7) and illustrated through the curves in Figure-2.2 [Galvez-Carillo 2011].
i 5 . 12
i TSR
P 116 0.4 5 e
2404 . 0 ) , (
C β λ
β λ λ
−
− −
= (2.7 a)
− +
= +
1 035 . 0 08
. 0 1
3 TSR
i λ β β
λ (2.7 b)
As these curves show the maximum efficiency exists for β = 0 thus, for the turbine operation when there is a possibility to control TSR, the pitch angle is kept at zero [Li & Chen 2006]. This aerodynamic model has been used to carry out the simulations presented in Section-3.6.
Figure 2.2 – Efficiency coefficient vs. TSR for different pitch angles [Galvez- Carillo 2011].
The maximum value of the efficiency coefficient can be 16/27 ≈ 0.593 which is called the Betz limit [Ackermann 2005] [Li & Chen 2006]. This means that the wind is ideally slowed down to a third of its original speed. Modern blade designs have a CP value of about 0.52 to 0.55 [Ackermann 2005]. If a turbine design were to achieve the Betz’s limit, then the turbine would be said to have 100 % efficiency.
2.2 Control Objectives for Wind Turbine Generator Systems
The control objectives for a WTGS can be described with respect to the Maximum Power Point Tracking (MPPT) objective or the defined power objective. MPPT refers to the control of the system to harvest maximum energy. There is also a possibility to simply define a specific, lower, power output without the aim of extracting the maximum possible power. This can be the case in deregulated market environment, for example, where the operator can request a lower generation to keep a spinning reserve [Camblong et al. 2006] [Morales 2006].
2.2.1 Maximum Power Point Tracking Control
The concept of CP, presented in Section-2.1, can be utilized to obtain another set of curves that relate the turbine rotational speed ωt to the power output for different wind speeds. This is given in Figure-2.3 [Camblong et al. 2006]. This
figure illustrates the requirement for maximum energy harvesting. The curve indicated as the Maximum Power Curve has to be tracked for maximum aerodynamic efficiency. This is then known as Maximum Power Point Tracking (MPPT). The turbine speed is limited to its rated value above the variable speed region of WTGS operation.
Figure 2.3 – Maximum Power Curve [Camblong et al. 2006].
For a modern WTGS, due to the variable nature of the wind speed, the variation in the input energy can be controlled to appear in the speed of the generator or the active power sent to the grid. Thus, the reference input to the control system can be a speed reference, a power reference or a torque reference [Zhao et al.
2006] [Yuan, Chai & Li 2004]. Each has its advantages and disadvantages. If it is desirable to maintain the maximum gain in energy then any form of reference to the control system has to be derived respecting the λTSR value that achieves the maximum CP value. The control strategies are described in the following.
2.2.1.1 Torque Control Strategy
The control of the electromagnetic torque in the generator has the advantage that the power delivered to the outer system has less variations. The system is allowed to change speed and find a new stable point when there is a variation in the mechanical driving torque [Hansen & Michalke 2007]. Consider Equation (3.1j) presented in Chapter-3. If the electromagnetic torque Tem is constant then a sudden variation in the mechanical driving torque Tm, due to wind speed variation, will either speed up the system or slow it down. This reduces stress on the drive train and the electric power output of the system is not affected. The disadvantage, of course, is that if the electromagnetic torque reference is not updated to reflect the energy content of the wind then maximum energy harvesting is not achieved.
The torque control strategy, also called the Indirect Speed Control strategy, is characterized by its slow speed response and lower aerodynamic efficiency
Moreover, the maximum power point tracking is not guaranteed since the speed is not directly controlled. The torque reference is derived by the following method.
Assuming that the wind speed is an available parameter, then rearranging Equation (2.6) for vw and substituting in Equation (2.5), the mathematical relations between the optimal turbine velocity ωopt and turbine power Pt,opt and also with turbine torque Tm,opt at the maximum achievable efficiency coefficient CP,max are obtained as [Li & Chen 2006] [El Aimani et al. 2003] [Abo-Khalil, Lee &
Lee 2006]
3 opt opt opt ,
t k
P = ω (2.8)
2 opt opt opt
opt , t opt ,
m P k
T ω
ω =
= (2.9)
with
3 opt , TSR
max , P 5 blade opt
R C 2 k 1
ρπ λ
=
λTSR,opt is the optimal TSR at which CP,max exists. The electromagnetic torque reference T*em can thus be given using the steady-state form of the drive-train equation, here Equation (3.1j), as
opt eq opt , m
em T B
T∗ = − ω (2.10)
where Beq represents the total viscous friction of the lumped-mass model. The electromagnetic torque in the machine is controlled by the currents, as shown in Section-3.5.4.5. The current control loops are fast and thus the commanded torque is achieved quickly but the speed variation of the system is defined by the mechanical inertia of the system, which is not included in the closed-loop control. Thus, the aerodynamic efficiency is lower since optimal TSR is not strictly followed at all times. In a reference, this efficiency is obtained as 95 % compared to 99 % achieved with direct speed control [Camblong et al. 2006].
2.2.1.2 Speed Control Strategy
From the knowledge of λTSR,opt, an optimal turbine rotation speed exists for a specific wind speed. It is however difficult to obtain an accurate measurement of the wind speed. An estimate of the turbine torque Tm,est by using an observer that has electromagnetic torque and the turbine speed as inputs can be used [Camblong et al. 2006] [Steurer et al. 2004]. Thus the speed reference ω*opt is given as in Equation (2.11).
opt est , m
opt k
= T
ω∗ (2.11)
A cascade control structure can be used with speed loop being the outer loop providing reference to the inner current loop and hence the electromagnetic
torque in the machine is dependent on the speed control [Zhao et al. 2006]
[Yuan, Chai & Li 2004]. This is unlike the previous case where speed was the consequence of the difference between the uncontrolled driving and the controlled electromagnetic torque.
The direct speed control method for MPPT gives better aerodynamic efficiency but the disadvantage is that it has more oscillations in the torque and therefore the power fed into the outer system [Camblong et al. 2006] [Zafar 2007] [Zhao et al. 2006]. The reason is that due to the variation in the driving torque, with the wind speed, the controller tries to match the electromagnetic torque of the generator to maintain speed at the optimal.
The speed control has the ability to control the overall dynamic response of the system, depending on the performance of the controllers. It is shown in literature that the higher bandwidth of the speed controller can give good speed reference tracking but more electromagnetic torque oscillation, due to better disturbance rejection capability [Camblong et al. 2006]. The driving torque is a disturbance for the speed loop.
2.2.2 Power Control Strategy
The power control strategy can be used to directly control the power sent to the grid. It can be required to produce a power lower than what can be generated.
The speed of the system then has to be controlled by an external system such as pitch angle control of the turbine blades to reduce the aerodynamic efficiency and so avoid the acceleration of the system [Hansen & Michalke 2007]. Usually the controller design is carried out considering the stator active and reactive power control [Morales 2006] [Zhi & Xu 2007] [Zhao et al. 2006] [Nho et al.
2007]. The power control strategy is explained in Section-3.5.4.4.
2.3 Aerodynamic Power Control
The methods discussed here are more concerned with power limitation through aerodynamic efficiency control. Power limitation is activated when the power produced by the WTGS exceeds its rating. The usual methods of power limitation are stall control, active stall control and pitch control.
2.3.1 Stall Control
The stall effect is shown in Figure-2.4. This control method was utilized in the older fixed-speed wind turbine designs. The blade is designed such that the laminar flow of air over the blade is disrupted above the rated wind speed. This reduces the aerodynamic efficiency of the blade naturally thereby limiting the power generated by the system. This is called Passive Stall where the blade orientation is always fixed. This is in contrast to Active Stall designs where the blade is turned around its own axis to induce the stall effect. The active-stall regulation results in a high sensitivity of power variation to angular change [Ackermann 2005].
Figure 2.4 – The stall effect.
The disadvantage of passive-stall regulation is that the decline in blade efficiency reduces the power above the rated wind speed instead of maintaining it at the rated value. This is illustrated in Figure-2.10 against the advantage of pitch control.
2.3.2 Pitch Control
Pitch control is the modern method of power limitation where the pitch angle β is controlled to keep the power at its rated value. β is defined as the angle between the chord of the blade and the plane of rotation; see Figure-2.5. vtip is the linear speed of the blade tip while the relative wind speed vrel is the vector sum of vw and vtip.
The pitch angle variation speed is usually 5° per second whereas it can be as high as 10° per second in emergencies [Ackermann 2005] [Hansen & Michalke 2007].
Since wind speed differs with height, it causes an unbalanced torque on the blades. To minimize this effect modern turbines can pitch each blade independently [Wind Plant Collector System Design Working Group 2009].
Figure 2.5 – Definition of pitch angleβ.
2.4 Types of Wind Turbine Generator Systems
The different types of WTGS can be classified broadly as being fixed-speed or variable-speed systems. Together with the power electronic devices and the control methods used they can be further analysed in terms of their capabilities and performance respectively. For example, the capability to fulfil their reactive power requirement, terminal voltage requirement, reactive power support to the
grid etc. and the control performance during grid short circuits, Low Voltage Ride Through (LVRT) and flicker emission [Wind Plant Collector System Design Working Group 2009].
2.4.1 Fixed-Speed WTGS
The fixed-speed wind turbine is what is called the Danish Concept and can be found up to a maximum class of 1.5 MW WTGS [Heier & Saiju 2006]. It represents earlier designs which use a Squirrel-Cage Induction Generator (SCIG); see Figure-2.6. Since the machine is directly connected to the grid it has to rotate at an almost constant speed. The generator does not have its own excitation and therefore it depends on the grid for the voltage level at its terminals. At no load (idling), the reactive power consumption is about 35-40 % of the rated active power and increases to around 60 % at rated power [Chen 2005]. In order to reduce its dependence on the grid for reactive power, compensation capacitors are installed at the stator terminals to improve the power factor as seen from the grid. During start-up, a soft-starter is used to limit the inrush current to less than twice the generator rated current and thereby the high torque spike in the drive train of the wind turbine and also the loading effect on the grid. This provides the possibility to slowly bring the turbine to near synchronous speed and therefore does not require a synchronization device [Chen, Guerrero & Blaabjerg 2009].
Figure 2.6 – Fixed-speed WTGS based on squirrel-cage induction generator.
Figure-2.7 presents power flow at the SCIG terminals, which shows large variability of power with a small variation of speed [Wind Plant Collector System Design Working Group 2009]. The machine works as a generator above synchronous speed. There is a possibility to change the speed for low and high wind speed conditions by changing the number of pole pairs of the machine [Chowdhury & Chellapilla 2006]. This means that for a pole-pair setting the speed is still constant.
This type of generator system although quite simple and robust is heavily dependent on the grid for its voltage and frequency reference. This has particular implications in cases when the grid is not strong enough, meaning that there are voltage and frequency fluctuations. The islanded operation is difficult since the machine loses its frequency reference and the terminal voltage can not be maintained. In case of short circuits, the machine is not able to transfer as much active power to the grid so the energy input from the wind increases the turbine speed until it is disconnected by the over-speed protection.
Figure 2.7 – Variation of active and reactive power for SCIG [Wind Plant Collector System Design Working Group 2009].
2.4.2 Limited Variable-Speed WTGS
This type of WTGS uses a wound-rotor induction generator. The manner of connection of the stator to the grid is the same as that for the fixed-speed WTGS, requiring a soft-starter and capacitors. However, in the rotor circuit variable resistances controlled through power electronics are added, either externally through slip rings or directly on the rotor to avoid the brushes and slip-ring associated problems. The latter design is called the Weier design [Wind Plant Collector System Design Working Group 2009]. Figure-2.8 shows the schematic of this WTGS.
Figure 2.8 – Limited variable speed WTGS based on wound-rotor induction generator.
The V47 series from Vestas uses the rotor resistance method with the market name OptiSlip. This method allows speed variations of up to 10 % to cope during violent gusts of wind thereby reducing the load on various parts of the turbine and better power quality [Tande, Marzio & Uhlen 2007]. It can also influence the
machine’s dynamic response during grid disturbances. The generator, however, still has to run above synchronous speed.
Figure 2.9 – Variation of active and reactive power with external rotor resistance [Wind Plant Collector System Design Working Group 2009].
Figure-2.9 presents the power curves for this method corresponding to different rotor resistances [Wind Plant Collector System Design Working Group 2009]. It is seen that some freedom to change the speed of the turbine, according to optimal TSR, is gained.
2.4.3 Variable-Speed WTGS
The variable-speed WTGS provide several advantages and owe their existence to power electronic converters. The advantage of variable-speed turbines, using pitch angle regulation, is that there are two degrees of freedom for the control of power. The pitch angle variation time constant can be longer thanks to the possibility of speed variation. Furthermore, the noise generated by the blade tip can be reduced by slowing down the turbine rotational speed [Muller, Deicke &
De Doncker 2000]. The advantage of pitch control and variable-speed operation is illustrated in Figure-2.10.
The generic designs for the variable-speed WTGS are based on the synchronous generator and the induction generator. Although many different topologies have been considered within these types, the two common ones are described here [Aimani et al. 2003] [Marques et al. 2003] [Petersson 2005] [Lindholm 2003].
One feature that is shared among these variable-speed wind turbines is that they utilize power converters, which convert the power generated at a variable frequency to the grid frequency.
Figure 2.10 – Advantage of pitch control and variable-speed operation for power gain [Muller, Deicke & De Doncker 2000].
2.4.3.1 Full-Scale Power Converter based WTGS
The design based on the synchronous or induction generator is presented in Figure-2.11. The system uses a converter that has to be about the same rating as the generator, which is one of its disadvantages. This means that substantial cost is involved and converter efficiency has a profound effect. The filters at the output of the converter have to be rated at full as well since all of the power generated has to follow this route to the grid.
Figure 2.11 – Variable-speed WTGS based on full-scale converter.
The advantage of the converter is that reactive power can be generated and thus an induction generator can also be used. For the synchronous generator since the excitation is separately provided to the machine, the winding current capacity can be fully utilized for active power.
The presence of the converter provides an opportunity to remove the gearbox altogether. This is important since the gearbox is prone to failures; see Section- 1.3. This has led to a gearbox free design being offered by one of the big wind turbine manufacturers ENERCON [ENERCON 2011]. The turbine is multi-pole which allows it to rotate at slow speeds according to the TSR. This also means that the generator is more bulky and has a large diameter to accommodate more poles.
With the reduction of cost for the permanent magnets, they are being utilized in higher rated designs of wind turbines whereas before only small rated turbines were using them economically [Vestas 2011].
2.4.3.2. Partial-Scale Power Converter based WTGS
The other design of a variable-speed WTGS now popular and the subject of investigation in this thesis is the doubly-fed induction generator. It has been used in one of the largest offshore wind farms, 160 MW, at Horns Rev [Sorensen et al. 2005] [Chen 2005]. The layout of the system is presented in Figure-2.12.
Figure 2.12 – Variable-speed WTGS based on partial-scale converter: The DFIG.
The main advantage of this design is the fraction rating of the converter used, compared to the full rating of the system. This has substantial cost savings and the converter losses are only a small part of the overall power since the main route for power flow is different. The converter is rated according to the variable speed range of the system. This range can be ±30 % around the synchronous speed. This requires a 30 % rated converter, of full rating of the generator, in the rotor circuit which is accessible through the slip rings [Wind Plant Collector System Design Working Group 2009]. The complete operational details of this system will become clear in the coming chapters.
2.5 Improvements for Wind Turbine Generator Systems
The trend for semiconductors used in power electronic devices is toward silicon carbide from silicon that will render greater power densities [Chen, Guerrero &
Blaabjerg 2009]. The breakdown voltage and the current carrying capacity of these devices are also increasing. A need to reduce the harmonic content of the generated voltage and to be able to connect the converter to higher voltage levels in the grid is driving research into multilevel converters topologies [Baroudi, Dinavahi & Knight 2007] [Ikonen, Laakkonen & Kettunen 2005] [Lindholm 2003]; see Figure-2.13. The disadvantage is that complex control techniques are required for such converters.
Some research has also been done to use the grid-interface converter as an active filter for harmonic compensation, at or beyond the Point of Common Coupling (PCC) [Tremblay, Chandra & Lagacé 2006]. In another study, the rotor connected converter of a DFIG based WTGS is used for harmonic compensation of a non- linear load connected to its stator terminals [Toufik, Machmoum & Poitiers
Figure 2.13 – Three-level neutral point clamped inverter leg (single phase) [Lindholm 2003].
Apart from converters as a part of the WTGS, the power electronics based FACTS technology such as the STATCOM is helping wind farms using grid-vulnerable WTGS achieve compliance with the grid codes. The good dynamic response of the STATCOM helps wind farms ride through grid faults and the voltage recovers faster after the fault. There are also gains in terms of smaller STATCOM power required as compared to other reactive power compensators [Bagnall et al.
2008]. In fact, it has been shown how the STATCOM can bring the fixed-speed and limited variable-speed induction generator based systems to comply with the grid codes [Moinas, Suul & Undeland 2007] [Chen, Guerrero & Blaabjerg 2009].
The LCL filter, Figure-2.14(a), is becoming the state-of-the-art for grid-interfaced converters compared to the L filter, shown in Figure-2.14(b) [Lindgren &
Svensson 1995] [Blaabjerg et al. 2003] [Teodorescu et al. 2003] [Liserre, Blaabjerg & Hansen 2005] [Lang et al. 2005] [Peltoniemi et al. 2006] [Roufi &
Lamchich 2004]. With an LCL filter, the same level of attenuation of switching harmonics in the current can be obtained with smaller component values, see Figure-2.14(c) [Roufi & Lamchich 2004]. Alternatively, if the same value of components is used, a reduction in switching frequency is possible [Lindgren &
Svensson 1995]. The LCL combination has an associated resonance peak below which the system acts as a first-order filter and above which it is a third-order filter, providing 60 dB/decade attenuation [Roufi & Lamchich 2004] [Lindholm 2003]. Thus placing the resonance peak at approximately 10 times the fundamental frequency and half the switching frequency provides effective attenuation [Peltoniemi et al. 2006] [Liserre, Blaabjerg & Hansen 2005]. The performance is evaluated, against an L filter, by analysing the current Total Harmonic Distortion (THD) below the resonance frequency and the attenuation of the current harmonic at the switching frequency, which lies beyond the resonance frequency [Liserre, Blaabjerg & Hansen 2005].
(a) (b)
(c)
Figure 2.14 – Comparison between LCL and L filter.
(a) LCL filter. (b) L filter.
(c) Frequency response from converter output voltage to line current [Roufi & Lamchich 2004].
The principal disadvantage associated with the LCL filter is the laborious design process although simplified procedures are being sought. The system has to be studied for stability since a non-ideal switching of the converter can lead to excitation of the inherent resonant modes [Lindgren & Svensson 1995] [Liserre, Blaabjerg & Hansen 2005]. The attenuation performance of the filter is also dependent on the grid stiffness and the parameters related to control, such as the analogue filter cut-off frequency [Blaabjerg et al. 2003]. The stability is affected by the placement of current sensors and the selection of switching frequency while sampling frequency, in relation to the resonance frequency, affects the reliability of the stability analysis [Teodorescu et al. 2003] [Liserre, Blaabjerg &
Hansen 2005]. To prevent oscillations at the resonance frequency, a resistor is used for passive damping which causes losses. Active damping can be done but then more current sensors or complex control techniques are required [Teodorescu et al. 2003].
2.6 Conclusions
This chapter provides a brief introduction to WTGS. The principle of wind energy conversion to electrical energy has been explained. Control objectives of the WTGS have been discussed. Different types of WTGS have been introduced and the improvements in the components and their use have been highlighted. The
aerodynamic model presented in Section-2.1 will be used in Section-3.6 to carry out simulations for a 2 MW DFIG based WTGS.
