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A hydro generator uses a 60 Hz synchronous machine rated 26.5 MVA, 13.8 kV, 0.9 pf. The exciter has a rated current of 694 A, a resistance of 0.4 Ω at 75^oC, and a time constant of 250 ms. The exciter is fed from a three-phase full bridge thyristor rectifier supplied at the 575 V from a three-phase Y-Y transformer connected at the generator terminals. Figure 14 illustrates the circuit configuration.

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1. Plot the generic transfer characteristics of the rectifier, average output voltage as a function of delay angle α, for 5 equally distributed values varying from 0^o to 150^o, for the exciter load.

2. Set the delay angle α to 0^o. Plot the instantaneous waveform across the field winding for five ac AC cycles. Indicate maximum and minimum values. Plot the approximate harmonic spectrum of the field voltage. Plot the average value field current waveform once the steady-state value is reached. Estimate the magnitude of the current ripple.

3. For the same delay angle α, plot the ac current waveform at the secondary windings of the transformer for five ac AC cycles once the steady-state current is reached. Plot and record the rms value. Indicate all instantaneous values. Plot the harmonic spectrum of the line current using ScopeView. Plot and record the power factor.

4. The delay angle is set to the value corresponding to rated current, computed in Question 1. Repeat questions (2) and (3).

5. Plot the voltage and current across each switch for the rated delay angle α for five fundamental cycles.

6. The transformer output voltage is changed so the maximum rectifier output voltage is 300 V. Using the transformer secondary voltage and delay angle α computed in Section I plot and record field current, power factor, harmonic spectrum of the field voltage and harmonic spectrum of the line current.

7. Set the delay angle α back to 0^o. Change the transformer parameters as follows: R = 0.008 p.u. and L = 0.08 p.u. Plot the secondary voltages of the transformer.

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