5715ch1 - 1 Wound Rotor Induction Generators(WRIGs Steady...

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1 -1 1 Wound Rotor Induction Generators (WRIGs): Steady State 1.1 Introduction . ...................................................................... 1 -1 1.2 Construction Elements . ..................................................... 1 -4 Magnetic Cores Windings and Their mmfs Slip-Rings and Brushes 1.3 Steady-State Equations. ...................................................... 1 -9 1.4 Equivalent Circuit . ........................................................... 1 -11 1.5 Phasor Diagrams . ............................................................. 1 -13 1.6 Operation at the Power Grid. .......................................... 1 -18 Stator Power vs. Power Angle Rotor Power vs. Power Angle Operation at Zero Slip (S = 0) 1.7 Autonomous Operation of WRIG. ................................. 1 -22 1.8 Operation of WRIG in the Brushless Exciter Mode. ..... 1 -28 1.9 Losses and Efficiency of WRIG. ...................................... 1 -33 1.10 Summary. .......................................................................... 1 -34 References. ................................................................................... 1 -36 1.1 Introduction Wound rotor induction generators (WRIGs) are provided with three phase windings on the rotor and on the stator. They may be supplied with energy at both rotor and stator terminals. This is why they are called doubly fed induction generators (DFIGs) or double output induction generators (DOIGs). Both motoring and generating operation modes are feasible, provided the power electronics converter that supplies the rotor circuits via slip-rings and brushes is capable of handling power in both directions. As a generator, the WRIG provides constant (or controlled) voltage V s and frequency f 1 power through the stator, while the rotor is supplied through a static power converter at variable voltage V r and frequency f 2 . The rotor circuit may absorb or deliver electric power. As the number of poles of both stator and rotor windings is the same, at steady state, according to the frequency theorem, the speed ω m is as follows: (1.1)± where p 1 is the number of pole pairs R is the mechanical rotor speed ωωωω mm R p =⋅ 12 1 ; © 2006 by Taylor & Francis Group, LLC
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1 -2 Variable Speed Generators The sign is positive ( + ) in Equation 1.1 when the phase sequence in the rotor is the same as in the stator and ω m < 1 , that is, subsynchronous operation. The negative ( ) sign in Equation 1.1 corresponds to an inverse phase sequence in the rotor when m > 1 , that is, supersynchronous operation. For constant frequency output, the rotor frequency 2 has to be modified in step with the speed variation. This way, variable speed at constant frequency (and voltage) may be maintained by controlling the voltage, frequency, and phase sequence in the rotor circuit. It may be argued that the WRIG works as a synchronous generator (SG) with three-phase alternating current (AC) excitation at slip (rotor) frequency 2 = 1 m . However, as 1 ω m , the stator induces voltages in the rotor circuits even at steady state, which is not the case in conventional SGs. Additional power components thus occur. The main operational modes of WRIG are depicted in Figure 1.1a through Figure 1.1d (basic config- uration shown in Figure 1.1a). The first two modes (Figure 1.1b and Figure 1.1c) refer to the already defined subsynchronous and supersynchronous generations. For motoring, the reverse is true for the rotor circuit; also, the stator absorbs active power for motoring. The slip S is defined as follows: (1.2) FIGURE 1.1 Wound rotor induction generator (WRIG) main operation modes: (a) basic configuration, (b) subsynchro- nous generating ( r < 1
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5715ch1 - 1 Wound Rotor Induction Generators(WRIGs Steady...

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