6-pulse thyristor controlled rectifier doesn't work in continuous conduction mode

I made a simulink model of a 6-pulse thyristor controlled rectifier and when I apply low currents it works fine (discontinuous conduction mode), but at higher currents (continuous conduction mode) the waveforms that I am getting doesn't make sense.

The simulink model is the following:

• Input: Three phase AC 400V voltage source (without impedance) that goes into a transformer (400:230), then into some inductance filters (0.18 mH each) and finally into the rectifier (Universal Bridge, 3 bridge arms and the rest of the parameters left as default).
• Output: The output of the rectifier goes into a LC filter (2 inductances of 0.5 mH each, one for the positive branch and another for the negative branch, and a 45 mF capacitor). After the filter it goes into a diode, and then into a resistive load, which is calculated with Vc/Iref
• Control: The control is achieved with the "Thyristor Rectifier Voltage Controller (Three-Phase)", which includes PLL and PID for the angle given the input voltage (defined to be the voltage just before the rectifier, "Vin"), the output voltage (Vout) and the desired output voltage (Vc). In the "Thyristor Rectifier Voltage Controlled" I configured the Kp and Ki, left the PLL parameters as default, defined the Thyristor pulse width with 120*pi/180 and configured the pulse ordering as "Natural order of commutation"

The following is the simulink circuit

And the following are some configurations

Control block configuration Universal Bridge configuration Capacitor configuration (initial voltage is set to Vc)

When I execute I get the following results:

• 20 A load (discontinuous current mode): These results are expected

• 50 A load (continuous current mode): These results are NOT expected. Voltage output seems to be fine but the rectifier output voltage and the inductance voltage doesn't look good and the same with the inductor current.

I will leave a download for the simulink model and a matlab script to setup some parameter values.

Answer 1

[Shivam Gothi]

Hello @JORDI RUHI FERNANDEZ,

As per my understanding, you are trying to maintain a constant DC bus voltage using closed loop voltage control. You are using the block "Thyristor Rectifier Voltage Controller (three phase)" for this purpose.

Upon investigating the simulink model attached by you, I found the model to be correct, but the PI controller performs well, only when you operate at discontinuous conduction mode. At continuous conduction mode, the performance of your PI controller is not good and therefore, you need to re-tune the parameters of PI controller. To demonstrate this, I provided a constant firing angle of () to the thyristor bridge. This roughly corresponds to the DC bus voltage of 243.7 V.

I have used the below formulla to find this firing angle.

Below given are the waveforms captured on scope:

You can see that the results are as expected. (Note: the "I_out_ref" variable was set to value of 50 in the script).

As a next step, I simulated the system by re-tuning the gains of PI controller through some number of trial and errors.

Below given are the waveforms captured on scope.

You can see that the response is much improved as compared with the previous case and the waveforms are much closer to the expectations . Low frequency oscillation is witnessed in the inductor current, which is due to improper tuning of PI controller parameters. But, you can tune them in a proper way to achieve a better response.

I am attaching the modified "rectifier_setup.m" file with this answer.

I hope this answers your question !