Double-fed asynchronous machine

The double fed asynchronous machine ( English Double fed induction generator (DFIG), English Double fed induction machine (DFIM) or English Doubly-fed wound rotor asynchronous machine (DASM)) describes a system of slip ring rotor - asynchronous machine with rotor-side frequency converter to control the speed and the reactive power .

Basics

DASM (left) in the traction power converter plant in Karlsruhe. Output 25 MW

In slip-ring machines, the slip power from the rotor circuit can be fed back into the network via a converter (over-synchronous generator operation or under-synchronous motor operation), or power is fed to the rotor (under-synchronous generator operation or over-synchronous motor operation). This method is used for large drives with a limited speed range, such as wind turbines , boiler feed pumps , pump turbines for storage power plants or traction current converters .

Layout and function

Basic system representation

The asynchronous machine is connected directly to the power supply on the stand . The frequency converter in the rotor circuit is now usually built using IGBT converter technology from 2 or 3-point bridges. With the converter on the machine side, sinusoidal voltages and currents of variable frequency and amplitude are switched from the voltage intermediate circuit into the rotor circuit. An "Active Front End" is connected to the intermediate circuit, a self-commutated, actively regulated line inverter. This means that energy can be fed back into the grid from the rotor circuit . In the past, a diode bridge for rectifying and was on the machine side network side a thyristor in the inverter mode used. This arrangement is referred to as a subsynchronous converter cascade (USK). However, a USK can only be operated as a motor below the synchronous speed. The area of ​​application is best suited for driving mechanical loads such as pumps and fans.

properties

Main active power flow in generator mode, subsynchronous

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

${\displaystyle n={\frac {f_{1}-f_{2}}{p}}}$

The network or stator frequency, the frequency of the rotor currents and p denotes the number of pole pairs . A negative frequency can be understood as a reversal of the rotating field; this occurs at oversynchronous speeds. The relative difference between the mechanical and the synchronous angular velocity, the slip  s, also has an effect on the active power flow . This is described by the law on the distribution of the air gap power : ${\displaystyle f_{1}}$ ${\displaystyle f_{2}}$ ${\displaystyle P_{\delta }}$

${\displaystyle P_{1}=P_{mech}+\underbrace {s\cdot P_{1}} _{P_{2}}}$

Assuming lossless transmission, the power in the rotor ,, is proportional to the slip and the active stator power . With synchronous speed, the active power flow in the rotor also reverses depending on the operating mode. ${\displaystyle P_{2}}$ ${\displaystyle P_{1}}$

business

Nameplate of a double-fed asynchronous machine

The speed can be set by impressing a desired rotor current frequency. If only a limited speed setting range is required, as is the case with wind turbines , for example , the converter can be dimensioned much smaller because only s-times the active stator power has to be passed through the converter. The stator reactive currents can be reduced by increasing the amplitude of the rotor current, since the magnetizing reactive power no longer has to be covered exclusively by the stator , as is the case with the squirrel cage rotor . The double-fed asynchronous machine can therefore even emit inductive reactive power, i.e. act as a capacitor (for example as a dynamic phase shifter in the Goldisthal pumped storage plant ).

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

application

Converter performance
comparison for wind farms with different generator types

The main area of ​​application of the double-fed asynchronous machine is the generation of electricity from wind energy . Although the Growian wind turbine already had such a technology, this technology is still currently (2013) used in many new wind turbines. Although the smaller converter has an advantageous effect on offshore wind turbines, the slip rings require maintenance. In this respect, it is not yet possible to predict which technology will prevail in the long term.

In 2004, the largest such cascade drives in Europe went into operation with an installed capacity of 340 MVA (325 MW in motor mode / 265 MW in generator mode) and a speed adjustment of (−10… +4%). These currently most powerful variable-speed asynchronous machines in Germany are operated in the pumped storage power plant in Goldisthal, Thuringia. The two motor-generators of the plant have a nominal output of 265 MW, with outputs of more than 300 MW being achieved with a full upper basin and thus maximum head.

From 1985 to 1990, 5 double-fed asynchronous machines were used in the Neuhof substation to exchange energy between the East German and West German power grids.

Advantages and disadvantages

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

advantages

• variable-speed operation
• separate regulation of reactive power and active power
• Low losses compared to the solution with a full converter

disadvantage

• Slip rings
• Sensitive to network disturbances

Statutory provisions and other regulations

• EN 60 034 Part 1 General provisions for rotating electrical machines
• EN 60 034 part 8 Terminal designations and direction of rotation for electrical machines
• DIN IEC 34 Part 7 Types of rotating electrical machines
• EN 60034-5 Degrees of protection of rotating electrical machines
• EN 60034-6 Types of cooling, rotating electrical machines
• DIN IEC / TS 60034-25 (VDE 0530 Part 25) Guidelines for the design and operating behavior of induction motors that are specially dimensioned for converter operation

literature

• Siegfried Heier : Wind power plants, system design, grid integration and control. 4th edition, BG Teubner, Stuttgart 2005, ISBN 3-519-36171-X .
• Alberto Ortega Fraile: The double-fed asynchronous machine: Investigation as a generator for a wind turbine. Vdm Verlag Dr. Müller (August 2008), ISBN 978-3-639-03244-4 .

See also

• Asynchronous generator
• Cascade machine

Web links

• Double fed asynchronous generator for WKAs under control aspects

Individual evidence

1. ↑ JB Ekanayake, L. Holdsworth, N. Jenkins: Comparison of 5th order and 3rd order machine models for doubly fed induction generator (DFIG) wind turbines . In: Electric Power Systems Research . tape 67 , no. 3 , December 1, 2003, ISSN  0378-7796 , p. 207–215 , doi : 10.1016 / S0378-7796 (03) 00109-3 ( sciencedirect.com [accessed June 8, 2020]).
2. ↑ Christoph Nicolet, Oliver Braun, Nicolas Ruchonnet, Antoine Beguin, Johann Hell, Francois Avellan: Full Size Frequency Converter for Fast Francis Pump-Turbine Operating Mode Transition . S. 2 (English, powervision-eng.ch [PDF]).
3. ^ ^{A b} Daniel Schäfer, Jean-Jacques Simond: Adjustable speed Asynchronous Machine in Hydro Power Plants and its Advantages for the Electric Grid Stability . 1998, p. 1 .