1、 1 Power Electronic Concepts Power electronics is a rapidly developing technology. Components are tting higher current and voltage ratings, the power losses decrease and the devices become more reliable. The devices are also very easy tocontrol with a mega scale power amplification. The prices are s
2、till going down pr. kVA and power converters are becoming attractive as a mean to improve the performance of a wind turbine. This chapter will discuss the standard power converter topologies from the simplest converters for starting up the turbine to advanced power converter topologies, where the wh
3、ole power is flowing through the converter. Further, different park solutions using power electronics arealso discussed. 1.1 Criteria for concept evaluation The most common topologies are selected and discussed in respect to advantages and drawbacks. Very advanced power converters, where many extra
4、devices are necessary in order to get a proper operation, are omitted. 1.2 Power converters Many different power converters can be used in wind turbine applications. In the case of using an induction generator, the power converter has to convert from a fixed voltage and frequency to a variable volta
5、ge and frequency. This may be implemented in many different ways, as it will be seen in the next section. Other generator types can demand other complex protection. However, the most used topology so far is a soft-starter, which is used during start up in order to limit the in-rush current and there
6、by reduce the disturbances to the grid. 1.2.1 Soft starter The soft starter is a power converter, which has been introduced to fixed speed wind turbines to reduce the transient current during connection or disconnection of the generator to the grid. When the generator speed exceeds the synchronous s
7、peed, the soft-starter is connected. Using firing angle control of the thyristors in the soft starter the generator is smoothly connected to the grid over a predefined number of grid periods. An example of connection diagram for the softstarter with a generator is presented in Figure1. Figure 1. Con
8、nection diagram of soft starter with generators. The commutating devices are two thyristors for each phase. These are connected in anti-parallel. The relationship between the firing angle ( ) and the resulting amplification of the soft starter is non-linear and depends additionally on the power fact
9、or of the connected element. In the case of a resistive load, may vary between 0 (full on) and 90 (full off) degrees, in the case of a purely inductive load between 90 (full on) and 180 (full off) degrees. For any power factor between 0 and 90 degrees, will be somewhere between the limits sketched i
10、n Figure 2. Figure 2. Control characteristic for a fully controlled soft starter. When the generator is completely connected to the grid a contactor (Kbyp) bypass the soft-starter in order to reduce the losses during normal operation. The soft-starter is very cheap and it is a standard converter in
11、many wind turbines. 1.2.2 Capacitor bank For the power factor compensation of the reactive power in the generator, AC capacitor banks are used, as shown in Figure 3. The generators are normally compensated into whole power range. The switching of capacitors is done as a function of the average value of measured reactive power during a certain period. Figure 3. Capacitor bank configuration for power factor compensation in
