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VOLTAGE ORIENTED CONTROL – SIMULATION

3.9.3

Simulation results

SIMULATION 1 AND 2 The following Table 3.7 present a summary of the simulation parameters. All the parameters

for this “Simulation1” are presented in APPENDIX F, Table 0.3.

The simulation is performed with approximation :

¬ We assume that the

voltage is equal to the grid voltage

(Figure 3.48). That is only

true if the grid impedance is small enough compared to impedance RL. ¬ We assume that we know exactly the converter parameters, that is :

GRID AND CONVERTER

Grid

(V), line to line RMS

(), grid resistance (H), grid inductance () (H) (F)

(V) STEP (V) ()

VALUE 400 1e 3 1e 5 0.05 3e 3 2200e 6 700 30 150

TIME STEP II: SW1 closing at … (s) STEP III: IGBTs ON at … (s)

step at … (s)

CONTROLLER , current controller , current controller , voltage controller , voltage controller

Sample freq.

(kHz)

VALUE 0.005 0.01 0.025

VALUE 37.6991 628.3185 0.0028 0.01 20

PWM Triangular freq. Mode

(kHz)

VALUE 10 Asymmetric

Table 3.7 : Summary of simulation1 parameters

The basic waveforms are plotted in Figure 3.50. All voltages are phase to ground voltages.

For this simulation1, we selected a small grid impedance and we observe a voltage

without

distortions, and we can say that

and the grid voltage

are equal (stiff grid). We will see the

influence of the grid impedance in the simulation3.

On the current plot (iabc

), when the breaker SW1 is closing at time 0.005s, no current is

flowing into the rectifier. Then, at time 0.01s, we activate the IGBTs, we close SW2 and the system is starting. We observe a small ripple on the current due to the switching. This ripple increases if the switching frequency decreases or if the line inductance decreases. Finally, at time 0.025s, the step on is starting. The current is reaching the saturation limit of ±15A and the DC link voltage is increasing linearly from 700V (initial condition) to 730V. Another

Simulation have been done with a bigger step of 100V on

with saturation limit in the current

controller about ±50A (see APPENDIX F, Figure 0.27).

Furthermore, we observe a small overshoot on the DC link voltage

. This overshoot may

increase if the switching frequency decrease. On Simulation2 Figure 3.51a., we plot the DC link voltage for a switching frequency selected at 3kHz. We observe oscillations. These oscillations can be remove by adjusting the PI controller coefficients. The calculation for the PI coefficients depends on

the bandwidth

and

selected as follwow :

and

.

58

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