Doubly fed electric machine#Double fed induction generator

The double-fed asynchronous machine (English Double fed induction machine, DFIG ) are a system of wound rotor induction machine with läuferseitigem frequency converter for speed control and the reactive power.

Basics

For wound-rotor machine the slip power can be fed back from the rotor circuit via an inverter to the grid (via synchronous generator operation or synchronous motor operation), or it will power the rotor supplied ( sub-synchronous generator operation or synchronous motor operation). This method is used for large drives with limited speed range, such as wind turbines, boiler feed pumps or traction power converters.

Design and function

The induction machine is connected to the stator directly to the mains. The frequency converter in the rotor circuit is now mostly in IGBT inverter technology 2 - or 3-point constructed bridges. With the machine side converter pulsed sinusoidal voltages and currents of variable frequency and amplitude can be set to the rotor circuit of the voltage intermediate circuit. At the intermediate circuit " Active Front End " is connected, a self -guided, actively controlled power inverters. Thus it can be fed back into the grid from the rotor circuit energy. In the past, a diode bridge for rectifying the mains side and a thyristor bridge was used in the machine side inverter operation. This arrangement is referred to as under synchronous converter cascade ( USK ). However, a PSP can be operated motor only below the synchronous speed. The best location is the range of use for the drive mechanical loads such as pumps and fans.

Properties

Due to the additive superposition of stator and rotor field, the speed is directly influenced by the frequency of the rotor currents.

The mains or stator frequency, the frequency of the rotor currents and p is the number of pole pairs. A negative frequency may be understood as the reverse of the rotating field, this occurs in over-synchronous speeds. The relative difference between the synchronous and mechanical angular velocity, the slip S has an effect on the active power flow. This is described by the law on the distribution of air-gap power:

Assuming a lossless transmission from the power in the rotor is proportional to the slip and the stator active power. In the synchronous speed, therefore also rotates depending on the mode of the active power flow around the rotor.

Operation

By impressing a desired rotor current frequency, the speed can be adjusted. If one needs only a limited speed range, such as in wind turbines, the inverter can be sized much smaller because only the s- fold stand active power must be routed via the inverter. About the increase in the amplitude of the rotor current, the stator reactive currents can be reduced, since the magnetization reactive power is no longer, as in the squirrel cage, exclusively from the stand forth must be covered. The double-fed asynchronous machine therefore can not make even inductive reactive power, thus act as a capacitor (for example, as a dynamic phase shift in the pumped storage plant Goldisthal ).

At a current control in the rotor circuit, as is usually the case, the double-fed asynchronous machine would stationary exactly behave like a variable speed synchronous machine. In practice, the principle of field orientation is used for the dynamic real and reactive power control. The slip ring apparatus is a considerable disadvantage compared to drives or asynchronous squirrel-cage motors with double-fed induction machines.

Application

The main application of the double-fed asynchronous machine is the generation of electricity from wind energy. Although already had the wind turbine Growian about such a technique, this technique is also currently still (2013 ) used in many new wind turbines. For offshore wind turbines, the smaller inverter does indeed noticeably advantageous, the slip rings are, however, maintenance- prone. To that extent is not yet clear which technology will prevail in the long term.

In 2004, the largest such cascade drives went to Europe with an installed capacity of 340 MVA ( 325 MW in motor mode / 265 MW in generator mode ) and a speed adjustment of (-10 ... 4 %) in operation. This most powerful variable-speed induction machines in Germany are operated in pumped storage power plant in Thuringia Goldisthal. The two motor - generators, the plant has a rated capacity of 265 MW, are being achieved at full upper basin and thus maximum fall height ratings of more than 300 MW.

From 1985 to 1990 served five double-fed asynchronous machines in Neuhof substation to the energy exchange between the East and West German electricity grid.

Pros and Cons

The double-fed asynchronous machine combines the advantages of the asynchronous and synchronous machine.

  • Variable-speed operation
  • Separate control of reactive power and active power
  • Low losses compared to the solution with full converter
  • Slip rings
  • Sensitive to power disturbances

Legal regulations and other rules

  • EN 60 034 Part 1 General requirements for rotating electrical machines
  • EN 60 034 Part 8 Terminal markings and direction of rotation for electrical machines
  • DIN IEC 34 Part 7 types rotating electrical machines
  • EN 60034-5 protection of rotating electrical machines
  • EN 60034-6 types of cooling for rotating electrical machines
  • DIN IEC / TS 60034-25 (VDE 0530 Part 25 ) Guide for the design and performance of induction motors, which are specifically designed for converter supply
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