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Views | March 2013

Doubly-fed induction generators in wind power generation

Doubly-fed electric machines are electric generators that have windings on stationary and rotating parts, where both windings transfer significant power between shaft and electrical system. Rohini G writes about the technical know-how of the advance mechanism being used in wind power generation.

With industrial and population growth, energy consumption has increased significantly over the last three decades. The serious issue of depletion of resources like coal, gas and petroleum at a very fast rate has motivated countries around the globe to think about alternative natural resources which are inexhaustible, sustainable and environmentally friendly. Among non-conventional resources for electricity, wind power has attracted great interest in the past few decades and has undoubtedly been the most rapidly growing renewable energy source. Though the wind industry is young from a power generation system point of view, it has benefitted significantly from the steady advances in technology made in the components dealing with grid integration, the electrical machine, power converters and control capability. The days of the simple squirrel cage induction machine are long gone. We are now able to control the real and reactive power of the machine, limit power output and control voltage and speed.

Meanwhile, doubly-fed electric machines have entered into common use only recently with the advancement of wind power technologies for power generation. They are variable speed three-phase wound-rotor induction machines with advantages over other types of generators when used in wind turbines. Doubly-fed electric machines are electric generators that have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and electrical system.

Principle

The principle of the Doubly-Fed Induction Generator (referred to as DFIG) is that rotor windings are connected to the grid via slip rings and back-to-back voltage source converters that control both the rotor and the grid currents. Thus, rotor frequency can freely differ from the grid frequency (50 or 60 Hz). By using the converter to control the rotor currents, it is possible to adjust the active and reactive power fed to the grid from the stator and this is independent of the rotating speed of the generator. In a conventional three-phase synchronous generator, with an external mechanical source i.e., prime mover rotating the generator rotor, the magnetic field in the generator rotor created by the dc current fed into the rotor winding rotates at the same speed as the rotor. As a result, a continually changing magnetic flux passes through the stator windings inducing an alternating voltage across the stator windings. Thus, the mechanical power applied to the generator shaft by the prime mover is converted to electrical power in the stator windings.

In conventional induction generators, the relation between the rotor speed and the frequency of the ac voltages induced across the stator windings of the generator is expressed using the following equation.

fstator =Nrotor x Np(1)/120

Where fstator is the frequency of the ac voltages induced across the stator windings in hertz (Hz)Nrotor is the speed of the rotor, in rotations per rotor minute (r/min) and Np is the number of poles in the generator per phase.

It is possible to determine with the above equation (1) that, when the speed Nrotor of the generator rotor is equal to the rotor synchronous speed Ns, the frequency fstator of the ac voltages induced across the stator windings of the generator is equal to the frequency fnetwork of the ac power network. The operating principle applicable in a DFIG is the same as seen in a conventional induction generator. The only difference is that the magnetic field which is created in the rotor is not static (as it is created using three-phase ac current instead of dc current), but rather rotates at a speed Nac proportional to the frequency of the ac currents fed into the rotor windings. This means that the magnetic field passing through the generator stator windings not only rotates due to the rotation of the generator rotor, but also due to the rotational effect produced by the ac currents fed into the rotor windings. Therefore, in a DFIG, both the rotational speed Nrotor of the rotor and the frequency fac of the ac currents fed into the rotor windings determine the speed Nstator of the rotating magnetic field passing through stator the stator windings, and thus the frequency fstator of the alternating voltage induced across the stator windings. When the magnetic field of the rotor rotates in the same direction as the generator rotor, the rotor speed Nrotor and the speed Nac of the rotor magnetic field add up.

The frequency fstator of the voltages induced across the stator windings of the generator can be calculated using the following equation:

fstator =Nrotor x Np+fac(2)/120

Where fac is the frequency of the ac currents fed into the DFIG rotor windings, in hertz (Hz) On the contrary, when the magnetic field at the rotor rotates in the direction opposite to that of the generator rotor, the rotor speed Nrotor and the speed Nac of the rotor magnetic field subtract from each other. The frequency fstator of the voltages induced across the stator windings of the generator can thus be calculated using the following equation:

fstator =Nrotor x Np-fac(3)/120

The main reason for using a DFIG is generally to produce three-phase voltage whose frequency fstator is constant, equal to the frequency fnetwork of the ac power network to which the generator is connected, despite variations in the generator rotor speed Nrotor. To achieve this, there must be a continuous adjustment made in the frequency fac of the ac currents fed into the rotor windings of the DFIG generator to counter any variation in the rotor speed Nrotor caused by fluctuations of the mechanical power provided by the prime mover.

The frequency fac of the ac currents that need to be fed into the DFIG rotor windings to maintain the generator output frequency fstator at the same value as the frequency fnetwork of the ac network depends on the rotational speed of the generator rotor Nrotor and can be calculated using the following:

fac = f network -Nrotor x Np(4)/120

For example, consider a DFIG with four magnetic poles supplying power to a 50 Hz network. If an external source makes the generator rotate at a speed of 1680 r/min, the frequency of the ac currents that need to be fed into the generator rotor windings can be calculated so:

The frequency fac of the ac currents to be fed into the generator rotor windings so that the frequency fstator of the generator output voltage is equal to the frequency fnetwork of the ac power network is 4 Hz. The negative polarity of the frequency fac indicates that the magnetic field created in the rotor windings must rotate in the direction opposite to the direction of the rotor. So far, it is seen how any deviation of the rotor speed Nrotor from synchronous speed Ns is compensated by adjusting the frequency of the ac currents fed into the rotor windings. In addition, to maintain the voltage produced equal to the network voltage, the magnitude of the magnetic flux must be maintained in the stator windings. This can be achieved by applying a voltage that is proportional to the frequency of the voltages applied to the rotor windings which maintains the V/f ratio constant and ensures a constant magnetic flux value in the machine.

Figure 1: Power Balance under different speed conditions.

Differences in Generators Used with Wind Turbines

The difference between a regular asynchronous generator and a DFIG is that the prior is rotor excited whereas the latter is externally excited with an ac-dc-ac convertor. The 3 phase ac input current causes the flux field of the rotor to have an apparent rotation with respect to the rotor. The frequency and phase sequence of the rotor current is controlled by this external converter such that the sum of the rotor speed and the rotor magnetic flux (relative to the rotor) equals synchronous speed. Even though in construction the DFIG is similar to a wound rotor induction generator machine, its principle of operation is equal to a variable speed synchronous generator.

Where the rotor speed is greater than the electrical synchronous speed of the stator, the slip is negative and the rotor delivers electric energy to the electric grid, due to the super-synchronous generating mode. This mode is a very efficient generating mode. In contrary, if the slip is positive, the rotor receives electric energy from the grid, but the stator keeps on delivering electric energy to the grid, this is sub-synchronous generating mode. Thus, the DFIG produces line frequency output (typically 50 -60 Hz) while having the capability of rotating over a wide range of speed, typically plus minus 33 per cent of synchronous speed. A DFIG can either source or sink reactive power. By adjusting the magnitude and angular position of the rotor flux wave using the external converter as control, the real and reactive power output of the DFIG can be precisely regulated. As the rotor circuit is controlled by a power electronics converter, the DFIG is able to both import and export reactive power. This is important for power system stability and allows the machine to support the grid during severe voltage disturbances (low voltage ride through). Second, the control of the rotor voltages and currents enables the induction machine to remain synchronised with the grid while the wind turbine speed varies. A variable speed wind turbine utilises the available wind resource more efficiently than a fixed speed wind turbine, especially during light wind conditions. Third, the cost of the converter is low when compared with other variable speed solutions because only a fraction of the mechanical power, typically 25-30 per cent, is fed to the grid through the converter, the rest being fed to grid directly from the stator. The efficiency of the DFIG is very good for the same reason.

It would be possible to obtain similar results in variable speed wind turbines using a three-phase synchronous generator and power electronics. The ac currents produced by the generator are converted into dc current by an AC/DC converter, and then converted by another AC/DC converter back to ac currents that are synchronous with the ac power network. It is therefore necessary for the power electronics devices used in such a circuit to have the size and capacity to process 100 per cent of the generator output power. The DFIG rotors are typically wound with 2-3 times the number of turns of the stator. This means that the rotor voltages will be higher and currents respectively lower. Thus, in the typical ¦ 30 per cent operational speed range around the synchronous speed, the rated current of the converter is accordingly lower. Further, the voltage transients due to the grid disturbances will also be magnified. In order to prevent high rotor voltages - and high currents resulting from these voltages-from destroying the IGBTs and Diodes of the converter, a protection circuit called crowbar is used.

The crowbar will short-circuit the rotor windings through a small resistance when excessive currents or voltages are detected. In order to be able to continue the operation as quickly as possible an active crowbar has to be used. The active crowbar can remove the rotor short in a controlled way and thus the rotor side converter can be started after~20-60 ms from the start of the grid disturbance. Thus, it is possible to generate reactive current to the grid during the rest of the voltage dip and help the grid to recover from the fault. It is important to know a little about large-size wind turbines to understand the advantages of using DFIG in wind turbines better. Large-size wind turbines are basically divided into two types: fixed-speed wind turbines and variable-speed wind turbines. In fixed-speed wind turbines, three phase asynchronous generators are generally used. As the generator output is tied directly to the grid, the speed of the generator rotor is fixed; in reality it can vary a little as the slip is allowed to vary over a range of 1 - 2 per cent typically. The mechanical power at the wind turbine rotor varies naturally with fluctuation in the wind speed. As the rotational speed is fixed, this causes the torque at the wind turbine rotor to vary. During wind gusts, the torque at the wind turbine rotor increases significantly while the rotor speed varies little. Resultantly, every wind gust stresses the mechanical components (specifically the gear box) in the wind turbine and causes a sudden increase in rotor torque, as well as in the power at the wind turbine generator output. This fluctuation in the output power of a wind turbine generator is a source of instability in the power network to which it is connected.

In variable-speed wind turbines, the rotational speed of the wind turbine rotor is allowed to vary as the wind speed varies. This prevents the use of asynchronous generators in such wind turbines as the rotational speed of the generator is quasi-constant when its output is tied directly to the grid. The same is true for synchronous generators which operate at a strictly constant speed when tied directly to the grid. This is where DFIGs come into play, as they allow the generator output voltage and frequency to be maintained at constant values, no matter the generator rotor speed and no matter the wind speed.

Summary

Using a DFIG instead of an asynchronous generator in wind turbines offers the following advantages:

1. The amplitude and frequency of the voltages remain constant while operating at variable speed.
2. The amount of power generated as a function of the available wind power is optimised.
3. Sudden variations in the rotor torque and generator output power is virtually eliminated.
4. Electrical power generation happens even at lower wind speeds/
5. Will be able to control the power factor (e.g., in order to maintain the power factor at unity).

Development of 1.6 MW DFIG

The process is on in GE for the development of 1.6 MW DFIG locally for the 50 Hz market. The product requirement specification was provided to the supplier to come up with the DFIG design to meet all the performance requirements stated. The first stage design work has been completed after passing through the conceptual design phase and the detailed design phase.

The prototype unit has been manufactured and subjected to factory testing. The results from the prototype testing give an indication that the concept of DFIG has been understood and captured in the design. This is seen in the design meeting the performance parameters like the loading specification of electrical power (kw) versus the speed (rpm), the rotor voltage being within the limit under no load and load condition, the harmonics in the stator current being well within the limits on reviewing the test results. These are some of the critical and important parameters for the DFIG design. The functional requirements of the product are all demonstrated. Some of the performance specifications i.e. efficiency, temperature rise limits and bearing vibrations after over speed run need to be met by the design. The design is being evaluated for short circuit current which is another key parameter for the DFIG.

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