Slip Power Dissipation

30.2.3.1 Slip Power Dissipation

Figure 30.17 shows a system in which the power delivered by the rotor is dissipated in a resistor.

The variable resistor can be substituted by the power con-

b verter in Fig. 30.21. The power converter controls the power GB c delivered to the resistor using an uncontrolled rectifier and a

parallel DC–DC chopper. This design has the disadvantage of

Short-circuit bar

Variable

the current having a higher harmonic content. This is caused

resistor

by the rectangular rotor current waveform in the case of a three phase uncontrolled rectifier. This disadvantage can be avoided using a six IGBT’s controlled rectifier, however this topology

FIGURE 30.17 Slip power dissipation in a resistor.

increases the cost significantly.

30 Wind Turbine Applications 805

Rotor main GB winding

Transformer

Rotor auxiliary

winding

ROTOR

GB Variable resistor

a sf 1 f 1 a’

FIGURE 30.22 Slip power dissipation in an external resistor using

b AC b’

brushless machine.

c AC c’

FIGURE 30.20 Brushless doubly fed induction machine. Stator winding

Rotor CONTROL

a winding CIRCUIT

b GB

FIGURE 30.21 Variable Variable resistor using a power converter.

resistor

FIRING CONTROL UNIT

CIRCUIT OPTICAL

The variation of the rotor resistance is not a recommended COUPLING technique due to the high copper losses in the regulation resis- FIGURE 30.23 Slip power dissipation in an internal resistor using

ROTOR

tance and so, the generator system efficiency is lower. It only brushless machine. can be efficient within a very narrow range of the rotor speed. Another disadvantage is that this technique is applicable only to wound-rotor machines and so, slip rings and brushes are controls the slip power. In some cases a transformer is used needed. In order to solve this problem some brushless schemes due to public grid voltages which can be higher than rotor are proposed. A solution is to use a rotor auxiliary winding voltages. which couples the power to an external variable resistor. The

Disregarding losses, the simplified scheme of Fig. 30.24 scheme can be observed in Fig. 30.22.

shows real power flux in all different connection points of Another solution is to dissipate the energy within a resistor the diagram. In this figure, the electrical power in the stator

placed in the rotor as it is shown in Fig. 30.23. This method machine P S , the mechanical power (1 − s) · P S , and the slip is currently used in generators for wind conversion systems, power and power converter s ·P S are represented. but as the efficiency of the system decreases with increasing

In generation mode, the power is positive when the arrow the slip, the speed control is limited to a narrow margin. This direction shown in Fig. 30.24 is considered. The power handled

scheme includes the power converter and the resistors in the on the power converter depends on the sign of the machine rotor. Trigger signals to the power switches are accomplished slip. When this slip is positive, i.e. subsynchronous mode of by optical coupling.

operation, the slip power goes through the converter from the grid to the rotor of the machine. On the other hand, when the slip is negative, i.e. hypersynchronous mode of operation,

the slip power comes out of the rotor to the power converter. In Fig. 30.18 the connection scheme shows that slip power Since the slip power is the real power through the converter, is injected into the public grid by a power converter and a this power is determined directly by the maximum slip or by transformer. The power converter changes the frequency and the speed range of the machine. For instance, if the speed range

30.2.3.2 Single Doubly Fed Induction Machine

806 J. M. Carrasco et al. Since the feeding frequency of the rotor is much lower than

the grid’s, a cycloconverter, as shown in Fig. 30.26, can be used. In this case the controllability of the system is greatly

(1 −s)P s

improved. The cycloconverter is an AC–AC converter based on the use of two three-phase thyristor bridges connected in

s.P s

parallel, one for each phase. This scheme allows working with speed above and below the synchronous speed.

The two schemes mentioned before (Figs. 30.25 and (1 −s).P s

Fig. 30.26) are based on line switched converters. A disad- vantage of this scheme is that voltages in the rotor decrease when the machine is working in frequencies close to the syn-

chronous frequency. This fact makes the line-commutated s.P s

POWER

CONVERTER

converter not to commute satisfactorily, and we need to use a forced-commutated converter. Also, when a forced- switched converter is used, quality of the voltage and current injected into the public grid is improved. The forced-switched power converter scheme is shown in Fig. 30.27. The converter

FIGURE 30.24 Simplified scheme of a single doubly fed induction includes two three-phase AC–DC converters linked by a DC machine.

capacitor battery. This scheme allows, on one hand, a vec- tor control of the active and reactive power of the machine, and on the other hand, a decrease by a high percentage of

used is 20% of the synchronous speed, the power rating of the the harmonic content injected into the grid by the power converter is 20% of the main power.

converter.