PROJECT TITLE: CASCADED MULTILEVEL INVERTER EMPLOYING NEAREST LEVEL MODULATION (NLM) AND CONVENTIONAL MULTICARRIER PWM SWITCHING TECHNIQUE
Since the last entry, some of the progress I have developed up to week 13 is I managed to design the control circuit of in phase Multicarrier PWM. Some basic simulation of switching technique was carried out and more reading materials was found and used as guide for my project. The references are taken from trusted sources like IEEE Xplore and IET website. One of the journals, "A New Multicarrier SPWM Technique for Five Level Cascaded H-Bridge Inverter" by Abhispek Paikray and Banaja Mohanty is one of the most vital references that gives a guide in order to implement Multicarrier PWM switching technique for the operation of my five level cascaded multilevel inverter (H-bridge).
Figure 1: The Multicarrier PWM control circuit Figure 2: In phase Triangular Carrier
Figure 3: The output waveform of the Multicarrier PWM Control Circuit
Next, I managed to obtain the Multicarrier PWM control signal waveform from the control circuit. The result from the control circuit which was the PWM waveform was presented above. I also managed to combine the Multicarrier PWM control circuit with cascaded multilevel inverter power circuit in order to obtain multistages voltage. The output waveform of the multilevel inverter employing multicarrier PWM switching technique was shown below as well as its frequency spectrum. The obtained total harmonic distortion (THD) from the inverter waveform is 27.57% without filter.
Figure 4: The H-Bridge 5-Level Inverter Employing Multicarrier PWM
Figure 5: The output waveform of the inverter
Figure 6: The frequency spectrum of the inverter output waveform
Besides that, some basic comparison between Nearest Level Modulation (NLM) switching technique and Multicarrier PWM switching technique also had been done by me. The total harmonic distortion (THD) for the NLM is much lower compared to Multicarrier PWM which is 17.08% over 27.57%. Next, the simplicity of NLM control circuit can be observed since it does not require carrier frequency compared to Multicarrier PWM. Both switching techniques managed to obtain 5 level of voltage stages with same voltage amplitude 30V, 15V, 0V, -15V and -30V.
Result Explanation
- Figure 1 shows the design of Multicarrier PWM control circuit which include the sine wave and triangular wave generator. The carrier frequency of triangular wave generator was set to 2000 Hz while the sine wave generator was set to 50 Hz.
- Figure 2 shows the in phase Triangular Carrier that was obtained from the equation blocks in control circuit.
- Figure 3 shows the generated PWM signals as a result from the comparison between the sine wave and the triangular carrier wave.
- Figure 4 shows the combination of Multicarrier PWM control circuit with the power circuit of the multilevel inverter. The control circuit designed is used to operate the switches of the inverter.
- Figure 5 shows the output waveform of the multilevel inverter in which there are 5 stages of voltage, 30V, 15V, 0V, -15V and -30V. These voltages were determined according to both separate DC sources in power circuit which is 15V each.
- Figure 6 shows the frequency spectrum and THD of the output waveform of multilevel inverter employing Multicarrier PWM. The maximum frequency was set to 3000 Hz to observe the spectrum correctly. The scale is too small so the other small harmonics also are visible in the frequency spectrum. The THD obtained is 27.57% without using any filter.
Future Improvement
Some improvement should be taken based on the result for the future work are:
- The scale of frequency for FFT analysis should be increase so that the correct frequency spectrum of the output waveform can be obtained without small harmonics.
- Any changes on variables of control circuit should be configured to obtain the lowest possible total harmonic distortion (THD) for the Multicarrier PWM on 5 level inverter.
- Thorough comparison should be made between NLM and Multicarrier PWM switching technique on multilevel inverter.