In Germany, the first large-scale variable-speed wind turbine (3 MW, with a rotor diameter of 10 meters) was built in 1980, but the project was not very successful due to mechanical problems. At that time, the cost of electrical components was quite high, so wind turbines often used doubly-fed asynchronous generators with small slip to save on converter costs.
The first commercially viable 750kW variable-speed wind turbines, and even larger ones, were developed in Denmark and Germany in 1995.
Dennis Equipment initially designed constant-speed wind turbines. This was mainly because wind speeds in Denmark are more stable compared to Germany. However, for wind turbines larger than 1 MW, they began to adopt variable-speed systems due to gearbox design issues. Initially, however, Dennis Equipment still used a small slip ratio, making the doubly-fed system more economical.
In 1988, the first 50 kW wind power unit used a synchronous generator and a six-pulse thyristor converter. Later, a 12-pulse converter was adopted, but this technology was discontinued due to harmonic distortion. The 750 kW doubly-fed asynchronous system, which began in 1993, suffered from persistent problems with its slip ring. The generator, manufactured by a large Australian company, underwent five modifications, but its operational lifespan was still less than two months. A year later, they abandoned the doubly-fed asynchronous system and replaced the asynchronous generator with a synchronous generator. We found that because the structure of the synchronous system was simpler, the cost of the synchronous system converter was not higher than that of the doubly-fed asynchronous system.
In Figure 1, we can see the circuit of the system: a synchronous generator, a diode rectifier bridge, a boost converter connected to the DC bus, and an IGBT converter.
In Figure 2, we can see: an asynchronous generator with slip rings, an IGBT rectifier bridge, a DC bus, and an IGBT converter. Several companies use this system.
Another advantage of grid-side converters is their performance when the grid voltage changes very rapidly.
Doubly fed induction generator (DFIG) systems function well when the grid voltage and frequency are stable. However, when using a DFIG system, if the grid voltage suddenly drops from 100% to 60%, the IGBT current on the generator side will increase to four times the rated current, and the torque on the generator shaft will also increase to four times the rated torque, potentially damaging the gearbox. If you don't want to use very large IGBTs but still want to prevent the entire system from being damaged by overvoltage, you must use a circuit breaker on the generator side. In wind power applications, we must consider the lifespan of the IGBTs. To improve the reliability of wind turbine drives, the IGBTs must have high load cycle capability. In this application, SKIIP (copper-less, press-fit technology) technology is very popular, and Semikron dominates the wind power industry.
In the table below, we compare the two systems:
