Consider the variable-frequency VSC system of Figure 10.2 to control a 1.68 MW doubly-fed asynchronous machine with the parameters given in Table 10.1. The machine stator is directly interfaced with an AC system whose frequency is wo = 377 rad/s. The AC system is represented by a Thevenin equivalent circuit with a 2300-V (line-to-line, rms) voltage source and a series inductance of Lg = 750 µ.H. The inductance can be regarded as the leakage inductance of a 1.6 MVA interface transformer, corresponding to a leakage reactance of 0.09 per-unit (pu). In the VSC system of Figure 10.2, the machine rotor is controlled by a two- level VSC. Figure 10.18 illustrates that a three-phase switch connects the rotor terminals to the corresponding AC-side terminals of the VSC. At the start-up, the switch is open and will not be closed until the machine accelerates (by an external mechanical torque) and w, lies within a narrow range about wo. The need for this mechanism can be explained as follows. Since the stator is supplied from the AC system, A, is fairly constant at its nominal value and das/dt = 0. Now assume that during the start-up ird and irg are both zero and w, is small (zero under a standstill condition). Then, based on (10.77), w = wo - w, has a large value (equal to wo for the standstill condition). Thus, based on (10.101) and (10.102), Vrd = 0 whereas Vrg has a large value. Consequently, Vrabe has a large amplitude, and a large DC-bus voltage is required for the VSC to properly operate. However, if w, is close to do the amplitude of Vrabc is considerably smaller and a significantly lower DC-bus voltage is allowed. In the system of Figure 10.2, the DC-bus voltage of the VSC is 1500 V. The VSC adopts the third-harmonic injected PWM strategy with a switching frequency of fs = 1620 Hz. The on-state resistance of each switch cell is about 0.9 m2, which is considerably smaller than R, and can be ignored in the control design process. Assuming that a time constant of t; = 3.0 ms is desired for the current-control loops, based on (10.97) and (10.98) we find kp = 17.97 and ki = 333 s-1. Figure 10.18 also shows that a three-phase star-connected series RLC fil- ter is connected in parallel with the machine stator terminals. This filter is to mitigate harmonic distortion of Vsabe, which is measured for flux observation (Fig. 10.15). Without the filter, Vsabe includes switching voltage notches that may adversely impact the flux observation process. The reason is that, once the switch is closed and the VSC is activated, the rotor terminal voltage becomes a switched waveform. On the other hand, the rotor and stator windings behave analogous to the windings of a transformer. Consequently, the rotor voltage is transferred to the stator circuit and, due to the AC system inductance, switching notches will appear at the stator terminals. The filter parameters are Rƒ = 20 ms2, Lf = 90 µH, and Cf = 100 µF.

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Consider the variable-frequency VSC system of Figure 10.2 to control a 1.68 MW doubly-fed asynchronous machine with the parameters given in Table 10.1. The machine stator is directly interfaced with an AC system whose frequency is wo = 377 rad/s. The AC system is represented by a Thevenin equivalent circuit with a 2300