Power Generation from Renewable Energy Sources Fall 2012 Instructor: Xiaodong Chu ： Office Tel.:

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 or Betz’ law)

Flashbacks of Last Lecture Example 6.7 on page 327 of the textbook: you should master it!

Flashbacks of Last Lecture Grid-connected wind turbines use ac generators including synchronous generators and asynchronous induction generators Synchronous generators spin at a precise rotational speed determined by the number of poles and the frequency needed for the power grid – An exciter, slip rings and brushes are needed, which adds to the maintenance Most of current world’s wind turbines use induction generators, which do not turn at a fixed speed – The key advantage of induction generators is that their rotors do not require the exciter, slip rings and brushes, so they are less complicated and less expensive

Wind Power Systems – Wind Turbine Generators Classification of AC generators used in wind turbines – 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 of PMSG and WRIG for wind power?

Wind Power Systems – Wind Turbine Generators Squirrel cage induction generators

Wind Power Systems – Wind Turbine Generators Slip of an induction machine is the difference of rotating speed between magnetic field and rotor when it is in the motor mode, the rotor rotates a little slower than magnetic field whereas in the generator mode, the rotor rotates faster Example 6.8 on page 333 of the textbook: you should master it!

Wind Power Systems – Speed Control for Maximum Power The energy conversion efficiency of a wind turbine is a function of TSR, i.e., rotor speed should follow wind speed that is random in nature It is needed to control forces on the rotor and thus to limit the power in very high winds in order to avoid damage to the wind turbine

Wind Power Systems – Speed Control for Maximum Power Turbine blades should change their speed as the wind speed changes for maximum efficiency

Wind Power Systems – Speed Control for Maximum Power The impact of rotational speed adjustment has on delivered power

Wind Power Systems – Speed Control for Maximum Power The generator of a wind turbine may need to spin at a fixed speed in order to deliver current and voltage in phase with the grid For grid-connected turbines, the challenge is to accommodate variable rotor speed and fixed generator speed How can this challenge be solved?

Wind Power Systems – Speed Control for Maximum Power Pole-changing induction generators – Induction generators spin at a frequency that is largely controlled by the number of poles – If the number of poles could be changed, the wind turbine could have several operating speeds, approximating multi- step rotational speed adjustment – It is a kind of discrete control

Wind Power Systems – Speed Control for Maximum Power Multiple gearboxes – Some wind turbines have two gearboxes with separate generators attached to each, one high-wind-speed gear ratio and one low-speed gear ratio – It is a kind of discrete control and rare in practical use

Wind Power Systems – Speed Control for Maximum Power Variable-slip induction generators – The slip of induction generators is a function of the dc resistance in the rotor conductors, and by adding variable resistance to the rotor, the amount 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 synchronous generator has, and more maintenance will be required. – Another way is to mount the resistors and the electronics needed to control 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 Control for Maximum Power Indirect grid connection systems – The wind turbine is allowed to spin at whatever speed needed to deliver the maximum power – When attached to a synchronous or induction generator, the electrical output will have variable frequency, and the generator cannot be directly connected to the utility grid, which is of fixed frequency (50- or 60-Hz) – Variable-speed wind turbines have advantages of higher annual energy production and greatly minimizing the wear and tear on the system caused by rapidly wind speeds – It is a kind of continuous control

Wind Power Systems – Speed Control for Maximum Power Power control: all wind turbines are designed with some sort of power control – The simplest, most robust and cheapest control method is the stall control, where the blades are bolted onto the hub at a fixed angle The rotor stalls when the wind speed exceeds a certain level It is a slow aerodynamic power regulation causing less power fluctuations 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 –state power due to variations in air density and grid frequencies

Wind Power Systems – Speed Control for Maximum Power Power control: all wind turbines are designed with some sort of power control – The simplest, most robust and cheapest control method is the stall control, where the blades are bolted onto the hub at a fixed angle The rotor stalls when the wind speed exceeds a certain level It is a slow aerodynamic power regulation causing less power fluctuations 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 –state power due to variations in air density and grid frequencies

Wind Power Systems – Speed Control for Maximum Power – Another type of control is pitch control, where the blades can be turned out or into the wind as the power output becomes too high or too low, respectively Advantages of this type of control are good power control (at high wind speeds the mean value of the power output is kept close to the rated power of the generator), assisted startup and emergency stop Some disadvantages are the extra complexity arising from the pitch mechanism and the higher power fluctuations at high wind speeds

Wind Power Systems – Speed Control for Maximum Power – The third control strategy is the active stall control, where the stall of the blade is actively controlled by pitching the blades At low speeds the blades are pitched similar to a pitch-controlled wind turbine to achieve maximum efficiency and at high wind speeds the blades go into a deeper stall by being pitched slightly into the direction opposite to that of a pitch-controlled turbine

Wind Power Systems – Speed Control for Maximum Power The most commonly applied wind turbine configurations can be classified both by their ability to control speed and the type 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 Control for Maximum Power Type A: fixed speed – This configuration denotes the fixed-speed wind turbine with an squirrel cage induction generator (SCIG) directly connected to the grid via a transformer – Since the SCIG always draws reactive power from the grid, this configuration 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 any speed control, it requires a stiff grid and its mechanical construction must be able to tolerate high mechanical stress

Wind Power Systems – Speed Control for Maximum Power Type B: limited variable speed – This configuration corresponds to the limited variable speed wind turbine with variable generator rotor resistance – It uses a wound rotor induction generator (WRIG) and the generator is directly connected to the grid – A capacitor bank performs the reactive power compensation and a smoother 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 mounted on the rotor shaft and the optical coupling eliminates the need for costly slip rings and brushes

Wind Power Systems – Speed Control for 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 a wound rotor induction generator (WRIG) and partial scale frequency converter (rated at approximately 30% of its nominal generator power) on the rotor circuit – The frequency converter performs the reactive power compensation and the smoother grid connection – It has a wider range of dynamic speed control compared with the Type B

Wind Power Systems – Speed Control for 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-scale frequency converter – The frequency converter performs the reactive power compensation and the smoother grid connection – The generator can be excited electrically [wound rotor synchronous generator (WRSG) or WRIG] or by permanent magnet [permanent magnet synchronous generator (PMSG)] – Some full variable-speed wind turbine systems have no gearbox and a direct driven multipole generator with a large diameter is used