EEET2389 Power Electronics Converter
Task:
Naturally Sampled Moduation.
Create a PSIM simulation of the three phase VSI topology shown in Fig. 1, with circuit parameters given in Table I. Your simulation should include standard elements such as:
• A Simulation Control Panel, used to set the simulation run-time, resolution, etc.
• A File containing all major parameters for the inverter.
• An earth node to ensure numerical stability.
Use IGBT devices for the inverter switches, and you should include sufficient current and voltage probes so that you can measure the following:
• All switched inverter voltages with respect to the inverter negative rail [i.e. Va(t),
Vb(t) and Vc(t)].
• All switched inverter line to line voltages [i.e. Vab(t), Vbc(t) and Vca(t)].
• All output inverter line currents [i.e. ia (t), ib (t) and ic (t)].
Construct a three phase, naturally sampled, sine-triangle pulse width modulator. Set the frequency of the triangular carrier voltage sources to achieve a carrier frequency of 2kHz.
The sine wave sources should be set for a fundamental frequency of 50Hz, and the amplitudes set to achieve a modulation depth of 0.8. Include all parameters in the File. It is recommended to include probes to measure all modulator signals, including the three AC references and the carrier waveform for de-bugging purposes.
Set the simulation parameters to show four fundamental AC cycles. Set the resolution of the simulation such that you have at least 500 points per half-carrier interval. This is important in order to obtain meaningful FFT results.
Run the simulation to show steady state inverter responses, and show on four separate axes:
(i) the triangular carrier and the three PWM reference waveforms,
(ii) the switched phase voltage with respect to the negative rail of the DC bus,
(iii) the switched line to line voltage, and
(iv) the three line currents.
Observe the switched output. This situation should correspond to the linear modulation range, and you should observe a two-level phase voltage and a three-level line to line voltage with no evidence of pulse-dropping.
Identify the modulation depth at which pulse-dropping occurs. Do this by increasing the modulation depth, and re-running the simulation to observe the switched outputs, until pulse-dropping is just observed. How does this value compare with theory?
At this critical modulation depth, generate the spectra for the following:
(i) the switched phase voltage with respect to the negative rail of the DC bus, and
(ii) the switched line to line voltage.
Show the spectra for frequencies up to 5kHz, and use logarithmic scales for the vertical axes showing four decades. Using cursors, measure the magnitude of the fundamental component for both the phase and line to line voltages. Does this agree with the theoretical prediction. [Hint : What is the maximum output voltage an inverter can produce without pulse-dropping?]
Using cursors, measure the frequencies for the dominant line to line voltage harmonics in the first and second carrier sideband group. How do these frequencies compare with the predictions from theory? [Hint : What does the Double Fourier series predict?]
