Page Contents

I. Preliminary Theoretical Calculations

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.

Open

Figure 14: Three-phase full bridge thyristor rectifier.

Steady-state Operation of the Rectifier

1. Draw the generic transfer characteristics of the rectifier, average output voltage as a function of delay angle α, for values varying from 0^o to 150^o, for passive and active loads. For the exciter load, indicate the maximum and minimum voltages and corresponding angles. Indicate the delay angle α for rated exciter current.

2. The delay angle α is set to 0^o. Draw the voltage waveform across the field winding for one ac cycle. Indicate maximum and minimum values. Draw the approximate harmonic spectrum of the field voltage. Compute the average value of the field current. Draw the field current waveform. Estimate the magnitude of the current ripple.

3. For the same delay angle, draw the ac current waveform at the secondary windings of the transformer for one ac cycle. Indicate all instantaneous values. Compute the rms value. Draw the approximate harmonic spectrum of the line current. Compute 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. The transformer output voltage is changed so the maximum rectifier output voltage is 300 V. Compute the transformer secondary voltage and delay angle for the rated current.

Transient Operation of the Rectifier

1. The field current regulator reference is subjected to a step change from 0A to rated current. Draw the trajectories, in the time domain, of the average field current and field voltage, and the corresponding delay angle, assumed to vary instantaneously (fast acting controller). The voltage is allowed to reach the maximum value.

2. The field current regulator reference is subjected to a step change from rated current to 0 A. Draw the trajectories, in the time domain, of the average field current and field voltage, and the delay angle α. It is allowed to reach the limit of 150 ^o.

II. Simulation Procedure

Steady-State Operation of the Rectifier

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 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 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.

Transient Operation of the Rectifier

1. The field current regulator reference is subjected to a step change from 0 A to rated current. Set the thyristor control to closed loop, select Scenario 1 and apply the trigger. Plot the trajectories, in the time domain, of the average field current and field voltage, and the corresponding delay angle α.

2. The field current regulator reference is subjected to a step change from rated current to 0 A. Set the thyristor control to closed loop, select Scenario 2 and apply the trigger. Draw the trajectories, in the time domain, of the average field current and field voltage, and the delay angle α. The delay angle α is allowed to reach the limit of 150^o.

III. Questions

1. Tabulate the theoretical results calculated in Section I and the simulated results obtained in Section II for both steady-state and transient operation of the controlled rectifier. Compare the results and comment on the differences.

2. When the transformer output voltage is changed so the maximum rectifier output voltage is 300 V, explain the impact of this new voltage on rated operating conditions of the thyristor rectifier (hint: rated current, power factor, harmonic spectrum).

3. When changing the ideal transformer parameters to those indicated in Question 7 in Section II, explain the impact of the AC side inductance on thyristor rectifier input AC voltage.