NPC Inverter and Psim Simulation

1. 3-Level NPC Type Inverter

Fig 1. 3-Level NPC Type Inverter

Fig 2, Switching

 Fig 1 is 3-Level NPC Type Inverter and Fig 2 indicates how it works.  Fig 2 indicates how each switch is activated. There are 3 switching states which are called P, O, N. In each state, the output voltage becomes +V_dc/2, 0, -V_dc/2.

Fig 3. Switching Mode

 Left in Fig 3 indicates circuits when positive voltage and the right one is for negative voltage. Let's check how it works! Considering P switching state in positive voltage, S1 and S2 are on, and S3 and S4 are off. So we can ignore the circuit below. As a result, the output voltage is +V_dc/2. In the same way, you can see how the rest works.

2. Control

Fig 4, SPWM

 In Fig 4, we can see how SPWM works. The offset voltage is applied to control the circuit. The common value for offset voltage is 0. It makes the reference polarity voltage and the reference phase voltage same.

3. Circuit

Fig 5. Circuit for Psim Simulation 1

Fig 6. Circuit for Psim Simulation 2

 The circuit in Fig 5 and 6 is for simulating NPC inverter in Psim program. I also used DLL(Dynamic link library to give lots of waveforms and control SPWM. You can check how SPWM works if you check the code.

4. Simulation

4.1 Waveform Graph

Fig 7. Simulation Result 1

Above are the graphs of phase voltage, line voltage and polarity voltage. We can see that phase voltage has 3 voltage levels and line voltage has 5 voltage levels. Moreover, we can find sinusoidal waveform in Vas(Polarity voltage) graph.

4.2 Low Pass Filter

Fig 8. Simulation Result 2

 The waveform of polarity voltage(Van) does not look like the sine wave we familiar with. The reason why it looks like is that the wave includes harmonics which is made during conversion from DC to AC. We can get rid of those unnecessary harmonics by using a low pass filter. Harmonics have frequency several times the fundamental frequency. So if we use a low pass filter. we can remove signals with frequencies higher than the frequency we set. I used a filter with a cut-off frequency of 1kHz. As a result, I got the waveform looking more like a sine wave finally. Fig 8 demonstrates it.

4.3 Power

Fig 9. Simulation Result 3

 Fig 9 shows how the inverter circuit is efficient. It compares power in DC and AC. Red line means AC power which is measured before node S and Blue line means DC power which is measured in input DC supply. It shows that both have stability after around 0.08 seconds and they converge at the same value.