SimPowerSystems 5.2

AC-DC-AC PWM Converter

This example of AC-DC-AC converter illustrates use of Universal Bridge, Multimeter, and Powergui blocks, as well as discrete control blocks of the Extras library.

G. Sybille (Hydro-Quebec)

Contents

Circuit Description

A 60 Hz, voltage source feeds a 50 Hz, 50 kW load through an AC-DC-AC converter. The 600V, 60 Hz voltage obtained at secondary of the Wye/Delta transformer is first rectified by a six pulse diode bridge. The filtered DC voltage is applied to an IGBT two-level inverter generating 50 Hz. The IGBT inverter uses Pulse Width Modulation (PWM) at a 2 kHz carrier frequency. The circuit is discretized at a sample time of 2 us

The load voltage is regulated at 1 pu (380 V rms) by a PI voltage regulator using abc_to_dq and dq_to_abc transfomations. The first output of the voltage regulator is a vector containing the three modulating signals used by the PMW Generator to generate the 6 IGBT pulses. The second output returns the modulation index.

The Discrete 3-Phase PWM Pulse Generator is available in the Extras/Discrete Control Blocks library. The voltage regulator has been built from blocks of the Extras/Measurements and Extras/ Discrete Control libraries

The Multimeter block is used to observe diode and IGBT currents. In order to allow further signal processing, signals displayed on Scope1 block (sampled at simulation sampling rate of 2us ) are stored in a variable named 'psbbridges_str' (structure with time) .

Demonstration

Start the simulation. After a transient period of approximately 50 ms, the system reaches a steady state. Observe voltage waveforms at DC bus, inverter output and load on Scope1. The harmonics generated by the inverter around multiples of 2 kHz are filtered by the LC filter.

As expected the peak value of the load voltage is 537 V (380 Vrms). In steady state, the mean value of the modulation index is m = 0.80 and the mean value of the DC voltage is 778 V. The fundamental component of 50 Hz voltage buried in the chopped inverter voltage is therefore: Vab = 778 V * 0.612 * 0.80 = 381 V rms

Once simulation is completed, open the Powergui and select 'FFT Analysis' to display the 0 - 7000 Hz frequency spectrum of signals saved in the 'psbbridges_str' structure. The FFT will be performed on a 2-cycle window starting at t=0.1-2/50 (last 2 cycles of recording). Select input labeled 'Vab Load' . Click on Display and observe the frequency spectrum of last 2 cycles. Notice harmonics around multiples of the 2 kHz carrier frequency. Maximum harmonic is 1.4 % of fundamental and THD is 2%.

Observe diode currents on trace 1 of Scope2, showing commutation from diode 1 to diode 3. Also observe on trace 2 currents in switches 1 and 2 of the IGBT/Diode bridge (upper and lower switches connected to phase A). These two currents are complementary. A positive current indicates a current flowing in the IGBT, whereas a negative current indicates a current flowing in the antiparallel diode.