Power Generation fromRenewable Energy Sources
Flashbacks of Last Lecture
•The power extracted by the wind turbine blades is equal to
•Define the ratio of downstream to upstream wind speed
and the rotor efficiency
then
•The maximum possible rotor efficiency is (Betz efficiency orBetz’ law)
Flashbacks of Last Lecture
•Example 6.7 on page 327 of the textbook: youshould master it!
Flashbacks of Last Lecture
•Grid-connected wind turbines use ac generators includingsynchronous generators and asynchronous inductiongenerators
•Synchronous generators spin at a precise rotational speeddetermined by the number of poles and the frequencyneeded for the power grid
–An exciter, slip rings and brushes are needed, which adds to themaintenance
•Most of current world’s wind turbines use inductiongenerators, which do not turn at a fixed speed
–The key advantage of induction generators is that their rotors do notrequire the exciter, slip rings and brushes, so they are less complicatedand less expensive
Wind Power Systems – WindTurbine Generators
•Classification of AC generators used in windturbines
–Synchronous generators
•Wound rotor synchronous generators (WRSG)
•Permanent magnet synchronous generators (PMSG)
–Induction generators
•Squirrel cage induction generators (SCIG)
•Wound rotor induction generators (WRIG)
•Could you make a brief survey of the use ofPMSG and WRIG for wind power?
Wind Power Systems – Wind TurbineGenerators
•Squirrel cage induction generators
Wind Power Systems – Wind TurbineGenerators
•Slip of an induction machine is the difference ofrotating speed between magnetic field and rotor
when it is in the motor mode, the rotor rotates alittle slower than magnetic field whereas in thegenerator mode, the rotor rotates faster
•Example 6.8 on page 333 of the textbook: youshould master it!
Wind Power Systems – Speed Controlfor Maximum Power
•The energy conversion efficiency of a wind turbine isa function of TSR, i.e., rotor speed should followwind speed that is random in nature
•It is needed to control forces on the rotor and thus tolimit the power in very high winds in order to avoiddamage to the wind turbine
Wind Power Systems – Speed Controlfor Maximum Power
•Turbine blades should change their speed as thewind speed changes for maximum efficiency
Wind Power Systems – Speed Controlfor Maximum Power
•The impact of rotational speed adjustment has ondelivered power
Wind Power Systems – Speed Controlfor Maximum Power
•The generator of a wind turbine may need to spin ata fixed speed in order to deliver current and voltagein phase with the grid
•For grid-connected turbines, the challenge is toaccommodate variable rotor speed and fixedgenerator speed
•How can this challenge be solved?
Wind Power Systems – Speed Controlfor Maximum Power
•Pole-changing induction generators
–Induction generators spin at a frequency that is largelycontrolled by the number of poles
–If the number of poles could be changed, the wind turbinecould have several operating speeds, approximating multi-step rotational speed adjustment
–It is a kind of discrete control
Wind Power Systems – Speed Controlfor Maximum Power
•Multiple gearboxes
–Some wind turbines have two gearboxes with separategenerators attached to each, one high-wind-speed gearratio and one low-speed gear ratio
–It is a kind of discrete control and rare in practical use
Wind Power Systems – Speed Controlfor Maximum Power
•Variable-slip induction generators
–The slip of induction generators is a function of the dc resistance in therotor conductors, and by adding variable resistance to the rotor, theamount of slip can range up to around 10%
–One way is to have adjustable resistors external to the generator
•Use a wound rotor with slip rings and brushes similar to what a synchronousgenerator has, and more maintenance will be required.
–Another way is to mount the resistors and the electronics needed tocontrol them on the rotor
•Send signals to the rotor telling how much slip to provide, e.g., OptiSlip®
–It is a kind of limited continuous control
Wind Power Systems – Speed Controlfor Maximum Power
•Indirect grid connection systems
–The wind turbine is allowed to spin at whatever speed needed todeliver the maximum power
–When attached to a synchronous or induction generator, the electricaloutput will have variable frequency, and the generator cannot bedirectly connected to the utility grid, which is of fixed frequency (50-or 60-Hz)
–Variable-speed wind turbines have advantages of higher annualenergy production and greatly minimizing the wear and tear on thesystem caused by rapidly wind speeds
–It is a kind of continuous control
Wind Power Systems – Speed Controlfor Maximum Power
•Power control: all wind turbines are designed withsome sort of power control
–The simplest, most robust and cheapest control method isthe stall control, where the blades are bolted onto the hubat a fixed angle
•The rotor stalls when the wind speed exceeds a certain level
•It is a slow aerodynamic power regulation causing less powerfluctuations than a fast-pitch power regulation
•Drawbacks of the method are lower efficiency at low wind speeds,no assisted startup and variations in the maximum steady –statepower due to variations in air density and grid frequencies
Wind Power Systems – Speed Controlfor Maximum Power
•Power control: all wind turbines are designed withsome sort of power control
–The simplest, most robust and cheapest control method isthe stall control, where the blades are bolted onto the hubat a fixed angle
•The rotor stalls when the wind speed exceeds a certain level
•It is a slow aerodynamic power regulation causing less powerfluctuations than a fast-pitch power regulation
•Drawbacks of the method are lower efficiency at low wind speeds,no assisted startup and variations in the maximum steady –statepower due to variations in air density and grid frequencies
Wind Power Systems – Speed Controlfor Maximum Power
–Another type of control is pitch control, where the bladescan be turned out or into the wind as the power outputbecomes too high or too low , respectively
•Advantages of this type of control are good power control (at highwind speeds the mean value of the power output is kept close tothe rated power of the generator), assisted startup and emergencystop
•Some disadvantages are the extra complexity arising from thepitch mechanism and the higher power fluctuations at high windspeeds
Wind Power Systems – Speed Controlfor Maximum Power
–The third control strategy is the active stall control, wherethe stall of the blade is actively controlled by pitching theblades
•At low speeds the blades are pitched similar to a pitch-controlledwind turbine to achieve maximum efficiency and at high windspeeds the blades go into a deeper stall by being pitched slightlyinto the direction opposite to that of a pitch-controlled turbine
Wind Power Systems – Speed Controlfor Maximum Power
•The most commonly applied wind turbine configurations canbe classified both by their ability to control speed and thetype of power control they use
Note: The grey zones indicate combinations that are not in use in the wind turbine industry today
Wind Power Systems – Speed Controlfor Maximum Power
•Type A: fixed speed
–This configuration denotes the fixed-speed wind turbine with ansquirrel cage induction generator (SCIG) directly connected to the gridvia a transformer
–Since the SCIG always draws reactive power from the grid, thisconfiguration uses a capacitor bank for reactive power compensation
–A smoother grid connection is achieved by using a soft-starter
–The main drawbacks of this concept are that it does not support anyspeed control , it requires a stiff grid and its mechanical constructionmust be able to tolerate high mechanical stress
Wind Power Systems – Speed Controlfor Maximum Power
•Type B: limited variable speed
–This configuration corresponds to the limited variable speed windturbine with variable generator rotor resistance
–It uses a wound rotor induction generator (WRIG) and the generator isdirectly connected to the grid
–A capacitor bank performs the reactive power compensation and asmoother grid connection is achieved by using a soft-starter
–OptiSlip® is such a design with a variable additional rotor resistance,which can be changed by an optically controlled converter mountedon the rotor shaft and the optical coupling eliminates the need forcostly slip rings and brushes
Wind Power Systems – Speed Controlfor Maximum Power
•Type C: variable speed with partial scale frequency converter
–This configuration , known as the doubly fed induction generator(DFIG) , corresponds to the limited variable speed wind turbine with awound rotor induction generator (WRIG) and partial scale frequencyconverter (rated at approximately 30% of its nominal generator power)on the rotor circuit
–The frequency converter performs the reactive power compensationand the smoother grid connection
–It has a wider range of dynamic speed control compared with the TypeB
Wind Power Systems – Speed Controlfor Maximum Power
•Type D: variable speed with full scale frequency converter
–This configuration corresponds to the full variable speed wind turbine,with the generator connected to the grid through a full-scalefrequency converter
–The frequency converter performs the reactive power compensationand the smoother grid connection
–The generator can be excited electrically [wound rotor synchronousgenerator (WRSG) or WRIG] or by permanent magnet [permanentmagnet synchronous generator (PMSG)]
–Some full variable-speed wind turbine systems have no gearbox and adirect driven multipole generator with a large diameter is used
