Voltage Source Inverter with Pre-Charge

Overview

This demonstration shows a closed-loop controlled 3-phase voltage source inverter with a DC-link pre-charge.

Fig. 1 Feedback-controlled three-phase grid-connected thyristor (SCR) rectifier

Model

Power circuit

A stiff three-phase voltage source with line inductance is connected to the AC-side of 2-level IGBT converter. This model extends the Voltage Source Inverter demo model by including pre-charging resistors connected to the three-phase source. These resistors are used to limit the inrush current during the initial charging of the DC-link capacitor.

Control

The inverter is controlled with an outer voltage control loop and an inner current control loop. The DC-link voltage is measured and compared against a voltage setpoint. The error signal is converted to a d-axis current setpoint via a PI regulator. The circuit is controlled to achieve unity power factor by setting the d-axis current setpoint to zero. The dq-current setpoints are then fed to a current controller.

The three-phase currents are measured and transformed to the dq-axis. A PI regulator with a feedforward term is used to convert the current error signal into a corresponding modulation signal. The modulation signal is scaled with the DC-link voltage and fed into a PWM modulator to generate the gating signals for the IGBT converter.

Simulation

Initially all the controls are disabled and the 2-level IGBT converter acts as a passive rectifier. The rectified three-phase voltage initially charges the DC-link capacitor to the rectified three-phase voltage and the charge current is limited by the pre-charge resistors. After \(t = 30\,\mathrm{ms}\) the pre-charge resistors are switched out of the power line. This causes a rise in the input current. At \(t = 50\,\mathrm{ms}\) the controls are enabled and the capacitor voltage is increased to the desired \(700\,\mathrm{VDC}\) operating point. newline

In the PLECS scope labelled with “i_d, v_d, i_vsi”, save the trace and label it “Inrush limited”. Simulate the model again, without using the “pre-charge resistors”, by setting the “Step time” of the “Charge” step function to \(t = 0\,\mathrm{s}\). Compare the diode current waveform to observe the effect of pre-charging on this circuit.

Fig. 2 Simulation result