Three-Phase 6-Pulse Thyristor Converter

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 \(10\,\mathrm{A}\) and then steps it up to \(25\,\mathrm{A}\).

../../_images/schematic7.svg — Fig. 1 Feedback-controlled three-phase grid-connected thyristor (SCR) rectifier

Model

Power circuit

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 Single-Phase 2-Pulse Thyristor Converter.

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:

\[\begin{aligned} V_{\mathrm{out}} = \frac{3 \cdot \sqrt{3} \cdot V_{\mathrm{peak}}}{\mathrm{\pi \cdot cos(\alpha)}} \end{aligned}\]

Where \(V_{\mathrm{peak}}\) is the amplitude of the AC input phase (to neutral) voltage and \(\mathrm{\alpha}\) 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 Single-Phase Diode Rectifier with PFC.

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 \(t = 10\,\mathrm{ms}\), the reference current is ramped up to \(10\,\mathrm{A}\), and subsequently stepped to \(25\,\mathrm{A}\) at \(t = 60\,\mathrm{ms}\). Run the simulation and observe the DC current waveform in the scope. Notice the low-frequency AC-component of the current.