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Thyristor Converter 6-Pulse, 3-Phase

Overview

This example demonstrates a feedback-controlled three-phase grid-connected thyristor (SCR) rectifier. The control scheme first ramps up the output DC current from 0 to 10 A and then steps it up to 25 A.

Electrical model

This three-phase, 6-thyristor AC-DC converter rectifies a three-phase grid source on the AC-side to supply a desired current on the DC-side. A detailed description on the behavior of thyristors can be found in the demo model Thyristor Converter 2-Pulse, 1-Phase.

A full-wave rectifier utilizes both the positive and negative pulses of the sinusoidal input waveform to produce a DC output. In reality the output will still contain a certain amount of ripple, but filtering can achieve a specific target of reduced high frequency distortion.

The output voltage of a three-phase rectifier can be estimated with this equation:

Vout = (3 *  3  Vpeak) pi cos(α), where Vpeak is the amplitude of the AC input phase (to neutral) voltage and α is the firing angle for the thyristors.

The thyristor firing angle is synchronized with the input voltage to achieve a maximum power factor. A demonstration of a single-phase full-wave diode rectifier with power factor correction is given in the demo model Diode Rectifier with PFC, 1-Phase.

Control

A 6-Pulse Generator component is used to control the firing of the thyristors. A Phase-Locked Loop (PLL) detects the phase angle of the three-phase supply voltage. The low-frequency DC-side current is measured and fed into a current controller. This current is compared with a reference DC current and the error is fed into a PI controller, which generates a firing-angle setpoint. The 6-Pulse generator produces the the switching signals for the 6 thyristors based on the firing angle setpoint from the current controller and the phase angle information from the PLL.

Simulation

The DC-side reference current is initially set to zero. At t=10 ms, the reference current is ramped up to 10 A, and subsequently stepped to 25 A at t=60 ms. Run the simulation and observe the DC current waveform in the scope. Notice the low-frequency AC-component of the current.