Soft-Switched Hybrid-Modulation for an Isolated DC-Link-Capacitor-Less Pulsating-DC-Link Inverter
thesisposted on 2017-07-22, 00:00 authored by Arash Rahnamaee
The main goal of this dissertation is to increase the performance of the capacitor-less high-frequency pulsating dc-link (HFPDCL) inverters suitable for renewable energy sources. The HFPDCL inverters are compact, small foot-printed, modular, and reliable since they do not employ large dc-link capacitors and line-frequency transformers. However, they require additional power conversion stages which increases the losses of the converter. The dc-link capacitor of the conventional fixed dc-link inverters provides a buffer stages for the inverter and it decouples the power conversion stages of the converter. Therefore, Power conversion stages of the HFPDCL inverters are not decoupled. As a result, they need a modulation scheme to synchronize the operation of the conversion stages and reduce the switching losses of the converter. Auxiliary circuits conventionally are used to provide a soft-switching condition for power converters to reduce the switching losses. This increases the efficiency and performance of the converter. However, they require additional passive and active components. The modulation-based soft-switching schemes are a better solution since they do not require any additional components. Not only can they provide soft-switching for the converter, but they can also reduce the switching requirement of the converter. They use the pulsating dc-link feature of the HFPDCL inverters to provide soft-switching condition. The modulation scheme also needs to synchronize the operation of the different conversion stages of the HFPDCL inverters. This dissertation presents modulation-based, soft-switching schemes to increase the performance of the HFPDCL inverters without using any auxiliary circuits. It reduces the switching requirement and provides a soft-switching condition to reduce the switching losses of the HFPDCL inverters to increase the overall efficiency. They also generate high-quality output waveforms with low total harmonic distortion (THD). The presented soft-switching modulation scheme operates based on the pulsating nature of the dc-link of the HFPDCL inverters. The three presented modulation schemes are as follows: 1) a discontinuous modulation scheme that synthesizes the fixed-width pulses on the pulsating-dc link (PDCL) waveform, 2) soft-switched hybrid modulation that reduces the switching requirement of the output inverter of the HFPDCL inverter and provides a soft-switching condition for it, and 3) a pulse-density pulse-width hybrid modulation (discontinuous modulation) to reduce the switching requirement of the conversion stages of the HFPDCL inverter and provide a soft-switching condition for it. Finally, two experimental setups are implemented to validate the performance of the presented soft-switched modulation schemes: 1) a discrete design that uses discrete silicon switching devices, and 2) a compact and modular design that uses power modules to increase the power density of the implemented HFPDCL inverter. The experimental results validate the performance of the presented soft-switched modulation schemes.