Distributed energy resources like solar power (PV Panels) are usually connected to the AC grid through a single phase voltage source inverter (VSI). The major drawback associated with single phase grid tie inverters is the double frequency component of the grid that appears on the DC bus link. Large electrolytic capacitors are generally employed in the inverters to eliminate the ripple component. However, their bulkiness and relatively short lifetime are motivational factors to replace them with small film capacitors. This paper presents a synchronous boost/buck based active power decoupling circuit in parallel with the dc-bus link capacitor and discusses the different types of control strategies implemented. Simulation results are presented for each control technique and it is shown that the ripple on the DC bus link is largely reduced due to inclusion of this circuit along with an expected extension of the lifetime due to the reduction in the amount of dc-bus capacitance used. / text

abstract: Two major challenges in the transformer-less, single-phase PV string inverters are common mode leakage currents and double-line-frequency power decoupling. In the proposed doubly-grounded inverter topology with innovative active-power-decoupling approach, both of these issues are simultaneously addressed. The topology allows the PV negative terminal to be directly connected to the neutral, thereby eliminating the common-mode ground-currents. The decoupling capacitance requirement is minimized by a dynamically-variable dc-link with large voltage swing, allowing an all-film-capacitor implementation. Furthermore, the use of wide-bandgap devices enables the converter operation at higher switching frequency, resulting in smaller magnetic components. The operating principles, design and optimization, and control methods are explained in detail, and compared with other transformer-less, active-decoupling topologies. A 3 kVA, 100 kHz single-phase hardware prototype at 400 V dc nominal input and 240 V ac output has been developed using SiC MOSFETs with only 45 μF/1100 V dc-link capacitance. The proposed doubly-grounded topology is then extended for split-phase PV inverter application which results in significant reduction in both the peak and RMS values of the boost stage inductor current and allows for easy design of zero voltage transition. A topological enhancement involving T-type dc-ac stage is also developed which takes advantage of the three-level switching states with reduced voltage stress on the main switches, lower switching loss and almost halved inductor current ripple. In addition, this thesis also proposed two new schemes to improve the efficiency of conventional H-bridge inverter topology. The first scheme is to add an auxiliary zero-voltage-transition (ZVT) circuit to realize zero-voltage-switching (ZVS) for all the main switches and inherent zero-current-switching (ZCS) for the auxiliary switches. The advantages include the provision to implement zero state modulation schemes to decrease the inductor current THD, naturally adaptive auxiliary inductor current and elimination of need for large balancing capacitors. The second proposed scheme improves the system efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency within a line frequency cycle. This scheme aims at minimizing the combined switching loss and inductor core loss by including different characteristics of the losses relative to the instantaneous switching frequency in the optimization process. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

abstract: A new photovoltaic (PV) array power converter circuit is presented. The salient features of this inverter are: transformerless topology, grounded PV array, and only film capacitors. The motivations are to reduce cost, eliminate leakage ground currents, and improve reliability. The use of Silicon Carbide (SiC) transistors is the key enabling technology for this particular circuit to attain good efficiency. Traditionally, grid connected PV inverters required a transformer for isolation and safety. The disadvantage of high frequency transformer based inverters is complexity and cost. Transformerless inverters have become more popular recently, although they can be challenging to implement because of possible high frequency currents through the PV array's stay capacitance to earth ground. Conventional PV inverters also typically utilize electrolytic capacitors for bulk power buffering. However such capacitors can be prone to decreased reliability. The solution proposed here to solve these problems is a bi directional buck boost converter combined with half bridge inverters. This configuration enables grounding of the array's negative terminal and passive power decoupling with only film capacitors. Several aspects of the proposed converter are discussed. First a literature review is presented on the issues to be addressed. The proposed circuit is then presented and examined in detail. This includes theory of operation, component selection, and control systems. An efficiency analysis is also conducted. Simulation results are then presented that show correct functionality. A hardware prototype is built and experiment results also prove the concept. Finally some further developments are mentioned. As a summary of the research a new topology and control technique were developed. The resultant circuit is a high performance transformerless PV inverter with upwards of 97% efficiency. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2014

abstract: The dissertation encompasses the transformer-less single phase PV inverters for both the string and microinverter applications. Two of the major challenge with such inverters include the presence of high-frequency common mode leakage current and double line frequency power decoupling with reliable capacitors without compromising converter power density. Two solutions are presented in this dissertation: half-bridge voltage swing (HBVS) and dynamic dc link (DDCL) inverters both of which completely eliminates the ground current through topological improvement. In addition, through active power decoupling technique, the capacitance requirement is reduced for both, thus achieving an all film-capacitor based solution with higher reliability. Also both the approaches are capable of supporting a wide range of power factor. Moreover, wide band-gap devices (both SiC and GaN) are used for implementing their hardware prototypes. It enables the switching frequency to be high without compromising on the converter efficiency. Also it allows a reduced magnetic component size, further enabling a high power density solution, with power density far beyond the state-of-the art solutions. Additionally, for the transformer-less microinverter application, another challenge is to achieve a very high gain DC-DC stage with a simultaneous high conversion efficiency. An extended duty ratio (EDR) boost converter which is a hybrid of switched capacitors and interleaved inductor technique, has been implemented for this purpose. It offers higher converter efficiency as most of the switches encounter lower voltage stress directly impacting switching loss; the input current being shared among all the interleaved converters (inherent sharing only in a limited duty ratio), the inductor conduction loss is reduced by a factor of the number of phases. Further, the EDR boost converter has been studied for both discontinuous conduction mode (DCM) operations and operations with wide input/output voltage range in continuous conduction mode (CCM). A current sharing between its interleaved input phases is studied in detail to show that inherent sharing is possible for only in a limited duty ratio span, and modification of the duty ratio scheme is proposed to ensure equal current sharing over all the operating range for 3 phase EDR boost. All the analysis are validated with experimental results. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

The DC-side floating capacitors in the floating power modules of power converters are subject to high voltage fluctuation, due to the presence of reactive harmonic components. Utilizing passive capacitors, as done in traditional methods, helps reduce the DC-bus voltage ripple but makes the system bulky. An active capacitor can be integrated with the floating H-bridge power modules to remove the effect of the ripple powers on the DC bus. The auxiliary circuit, which is much smaller in size compared to an equivalent passive capacitor, helps increase the power density of the system. This work focuses on the analysis of power components, and the extension of the active capacitor to the Perturbation Injection Unit (PIU), in which the DC side is highly distorted by multiple harmonic components. A control scheme is proposed to compensate for these multiple harmonics and balance the DC-link voltage in the active capacitor. Also, an equivalent DC-bus impedance model is introduced, which is more accurate than that in existing works. Simulation studies and evaluation of the design have verified the effectiveness of the active capacitor solution. / Single-phase power converters have been widely used in many applications such as electric vehicles, photovoltaic (PV) systems, and grid integration. Due to their popular application, there is a need to reduce the sizes and volumes while still maintaining good performances of the systems. One of the most effective methods, which is a subject in many research works, is to replace the bulky passive capacitor bank in a system by an active capacitor. The active capacitor is designed to absorb the ripple components in the DC side of the converters, which results in a constant DC-link voltage. In comparison to the passive capacitor solution, the active capacitor is much smaller in size but can give a better DC-bus ripple performance. Therefore, the active capacitor has become an attractive solution for the single-phase converters. The active capacitor for the traditional rectifier, where the DC side is directly connected to a load, has been intensively investigated in the past decade. However, there is limited research regarding the active capacitor for rectifiers with floating H-bridge power modules. This work extends the application of the active capacitor to the Perturbation Injection Unit (PIU), which is a grid-connected single-phase rectifier with floating H-bridge power modules. The selection of a suitable active capacitor for the PIU is based on the evaluation of various active capacitor banks. Limits in existing control schemes, which prevent the extension of the active capacitor to the PIU, are thoroughly studied. An effective voltage-mode control scheme is then proposed for the selected active capacitor, which makes it an attractive solution for the PIU. Moreover, limits of the DC-bus impedance analysis using traditional assumptions in existing works are investigated, and an improved DC-bus impedance model is proposed. Based on the operation conditions of the PIU and the proposed impedance model, the active capacitor's components can be properly designed, and improved configurations in terms of the equivalent impedance can be analyzed. Simulation results, as well as the design and evaluation of the active capacitor, demonstrate great improvements in terms of volume and weight over the traditional passive capacitor bank.

No description available.