This dissertation presents control, analysis, and design of silicon carbide (SiC)-based critical conduction mode (CRM) high-frequency soft-switching three-phase ac-dc converters (inverter and rectifier). The soft-switching technique with SiC devices grounded in CRM makes the operation of the ac-dc converter at hundreds of kHz possible while maintaining high efficiency with high power density. This is beneficial for rapidly growing fields such as electric vehicle charging, photovoltaic (PV) systems, and uninterruptable power supplies, etc. However, for the soft-switching technique to be practically adopted to real products in the markets, there are a lot of challenges to overcome. In this dissertation, four types of the challenges are carefully studied and discussed to address them.
First, the grid-tied inverters used for distributed energy resources, such as PV systems, must continue operating to deliver power to the grid, when it faces flawed grid conditions such as voltage drop and voltage rise. During abnormal grid conditions, delivering constant active power from the inverter to the grid is essential to avoid large voltage ripples on the dc side because it could trigger over-voltage protection or harm the circuitries, eventually shutting down the inverter. Hence, in such cases, unbalanced ac currents need to be injected into the grid. When the grid voltages and the ac currents are not balanced, there is a chance for the CRM soft-switching inverter to lose its soft-switching capability. Continuous conduction mode operation emerges, causing hard-switching where discontinuous conduction mode (DCM) operation is expected. This leads to huge turn-on loss and high dv/dt noise at the active switch's turn-on moment. To eradicate the hard-switching problem, two improved modulation schemes are developed; one with off-time extension in the CRM phase, the other by skipping switching pulses in the DCM phase. The DCM pulse skipping is applied for a variety of grid imbalance cases, and it is proven that it can be a generalized solution for any kinds of unbalanced grid conditions.
Second, the CRM soft-switching scheme with 2-channel interleaving achieves high efficiency at heavy load. Nevertheless, the efficiency plunges as the output load is reduced. This is not suitable for PV inverters, which take account of light load efficiency in terms of "weighted efficiency". Small inductor currents at light load cause the switching frequency to soar because of its CRM-based operation characteristic, causing large switching loss. To increase the inductor current dealt with by the first channel, a phase shedding control is proposed. Gate signals for the second channel are not excited, increasing the first channel's inductor current, thus cutting down the first channel's switching frequency. To prevent the unwanted circulating current formed by shared zero-sequence voltage in the paralleled structure, only two phases in the second channel working in high frequency are shed. The proposed phase shedding control achieves a 0.5 to 3.9 % efficiency improvement with light loads.
Third, due to the usage of SiC devices, high dv/dt generated at switching nodes over the system parasitic capacitance causes substantial common mode (CM) noise compared to that with Si devices. In this case, a balance technique with PCB winding inductors can effectively reduce the CM noise. First, winding interleaving structure is selected to minimize the eddy current loss in the windings. But the interwinding capacitance caused by the winding interleaving structure aggravates the CM noise. Impact of the interwinding capacitance on the CM noise is analyzed with a new inductor model containing the interwinding capacitance. Then, finally, a novel inductor structure is proposed to remove the interwinding capacitance and to improve the CM noise reduction performance. The soft-switching ac-dc converter built with the final PCB magnetics features almost similar efficiency compared to that with litz-wire inductor and 14 to 18 dB CM noise reduction up to 15 MHz.
Lastly, the soft-switching technique is extended to inverters in standalone mode. To meet tight ac voltage total harmonic distortion requirements, a current control in dq-frame is introduced. As for the ac voltage regulation at no-load, on top of the improved phase shedding control, a frequency limiting with fixed frequency DCM method is applied to prevent excessive increase in the switching frequency. Then, how to deal with short-circuit at the output load is investigated. Since the soft-switching modulation violates inductor voltage-second balance during the short-circuit, the modulation method is switched to a conventional sinusoidal PWM at fixed frequency. It is concluded that all the additional requirements for the standalone inverters can be satisfied by the introduced control strategies. / Doctor of Philosophy / The world is facing an unprecedented weather crisis. Global warming is getting more severe because of excessive amount of carbon emission. In an effort to overcome this crisis, paradigm of energy and lifestyle of people have changed. Penetration of distributed energy resources (DERs) such as wind turbines, and photovoltaic systems has been dramatically increased. Instead of internal combustion engine vehicles (EVs), electric vehicles hit the mainstream. In these changes, power electronics plays a critical role as the key element of the systems. Especially, three-phase inverter/rectifiers are essential parts in such applications.
Most important aspects of the three-phase inverter/rectifier are efficiency and power density. In the past decades, Silicon (Si) power devices were mostly used for the systems and the technology based on Si has almost reached to its physical limits. The switching frequency of Si-based inverter/rectifier is limited below 20 – 30 kHz to reduce switching loss. This impedes high power density due to bulky passive components such as inductors and capacitors.
Nowadays, the advent of wideband gap such as Silicon Carbide (SiC) and Gallium Nitride (GaN) power devices gives us a great opportunity to improve the efficiency and the power density with its high switching speed capability, low switching energy and low on-resistance. The SiC power devices are more suitable for DERs and EVs due to higher voltage rating. Using SiC power devices allows to increase inverter/rectifier' switching frequency about five times to have similar efficiency with those based on Si power devices, making the power density high. However, there is still room to push the switching frequency even higher to hundreds of kHz with soft-switching.
In this sense, studies on soft-switching techniques for three-phase inverter/rectifier have been intensively conducted. Particularly, soft-switching techniques based on critical conduction mode (CRM) are regarded as the most promising solutions because it does not have any additional circuits to achieve the soft-switching, keeping the system as straightforward as possible. However, most of the studies for the CRM-based soft-switching three-phase inverter/rectifier mainly focus on limited occasions such as ideal operation conditions. For this technique to be widely used and adopted in industry, more practical cases for the systems need to be studied.
In this dissertation, the soft-switching three-phase inverter/rectifier under diverse situations are investigated in depth. First, behavior of the soft-switching inverter/rectifier under unbalanced grid conditions are analyzed and control methods are developed to maintain its soft-switching capability. Second, how to improve light load efficiency is explored. Circulating current issue for the light load efficiency improvement is analyzed and a control method is proposed to eliminate the circulating current. Third, a design methodology and considerations of inductors based on PCB magnetics are discussed to reduce electromagnetic noise and improve system efficiency. Lastly, the soft-switching technique is extended to standalone mode applications dealing with strict voltage regulation, no-load operation, and output short-circuit.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/112329 |
| Date | 01 November 2022 |
| Creators | Son, Gibong |
| Contributors | Electrical Engineering, Li, Qiang, Burgos, Rolando, Lee, Fred C., Southward, Steve C., Centeno, Virgilio A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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