Abstract-This paper gives an overview of the development of the induction machine as a generator for wind turbines from the middle of the 20th century up to now. After a short history chapter the different energy conversion concepts are presented. At first a squirrel cage machine is direct coupled to the grid (Danish concept). To adjust the machine speed the second step is a wound rotor machine with collector rings to change rotor resistance. This concept allows the first adaption of the operating point of the wind turbine and the induction machine. Another concept is to connect the stator of a squirrel cage machine with a full inline voltage source converter to the grid to get the flexibility in rotational speed and to control the reactive power. The doubly fed induction machine is the latest development. This concept uses a bidirectional voltage source converter in the rotor circuit with a rated power of only 30% of the rated generator power. Now it is possible to change the rotational speed and the reactive power independently and in a wide range. A generalized control scheme of a modern doubly fed induction machine (DFIG) is shown in the last part to explain the variability and the range of application. Today over 70% of the wind turbines are build up with DFIG.

The use of proportional resonant (PR) current controller in grid side wind power converters instead of the traditional proportional integral (PI) controller has gained a large popularity. Particularly its capability for compensating harmonics in the current waveforms is an essential feature. Due to the replacing of the generator side converter by a simple diode rectifier, harmonics with variable frequencies into dependence of the generator frequency appear in the current waveforms. To reach the IEEE standard for the total harmonic distortion (THD) a frequency adaptive PR-controller for compensating these harmonics is required.

No abstract

No abstract

This paper presents how to operate a double fed induction generator (DFIG) with a rated power of approximately 30kW with unmodified low cost standard converters. The stator of the machine is directly connected to the grid and the rotor is fed with variable voltage and frequency from two standard low cost voltage source converters. Actually such machines are used in wind generators with a rated power of more than 500 kW with special build converters. In the presented solution the rotor of the DFIG is fed with two DC-Link connected standard voltage source converters. The original software is designed to calculate speed and phase of the rotor by receiving a speed signal from the rotor shaft. Therefore it is necessary to add two small application modules to modify the firmware externally to realize the calculation of slip frequency and phase in real time. The equivalent network parameters of the DFIG are determined by the converter itself by using an autotune function. With this parameters it is possible to develop an analytical model of the system to set up a closed loop active and reactive power control. The test system provides the ability to control both power types decoupled and with an approximately linear characteristic.