Reduced semiconductor energy conversion systems.
Date of Issue2013
School of Electrical and Electronic Engineering
Power converters are extensively used in energy conversion systems for converting electrical energy from one form to another. With the development of advanced semiconductor devices, modem power converters are also usually constructed with fully controllable switches, making them suitable for driving a wide range of loads. Some example applications of power converters are Uninterruptible Power Supplies (UPSs) for supporting critical loads during voltage outages, Universal Power Quality Conditioners (UPQCs) for power quality enhancement, renewable energy interfacing converters for green energy delivery and Dynamic Voltage Restorers (DVRs) for regulating load voltages. Presently, most applications use a few types of proven traditional converter topologies. These converters have long historical records, and are therefore more trusted by the industry. However, relying on the traditional converters only does not guarantee better efficiency, lower cost and innovativeness. That prompts many researchers to propose new converter topologies usually with lower component counts. Lesser components are however almost always accompanied by some performance tradeoffs. A few commonly quoted tradeoffs are loss of independency between multiple driven loads, limited amplitude and phase-shift, and much higher stresses experienced by the remaining components. These tradeoffs can be expensive at times even though components are saved. It is therefore important to note that not all reduced component topologies are rewarding. Even for those proven useful, they cannot be generalized as suitable for all applications. A detailed application study needs to be conducted before a sound judgment can be made for the considered topology especially with reduced components. The same principle applies to the nine-switch converter recently proposed for replacing the more generalized twelve-switch back-to-back converter found in many ac-ac energy conversion systems. As their names implied, the saving expected is three semiconductor switches or 25% in percentage term. This surely is an attractive saving if no severe limitation in performance is accompanied. Unfortunately, the nine-switch converter is presently burdened by high de-link voltage and heavily limited phase-shift between its terminal outputs even though it has been proven to work in motor drives and UPSs. These limitations are however not always severe. They are applicationrelated even though it has presently not been clarified in the literature. It is therefore the intention now to study the nine-switch converter in greater details, believing that it can bring sizable advantages if controlled, designed and applied properly. The investigation planned for the thesis is thus to revisit the nine-switch modulation principles and its existing ac-ac converter applications with an intermediate de-link, The intention is to identify areas where modulation can be improved and quantify limitations faced by the nine-switch topology. Understanding those enables new modulation schemes to be proposed for the nine-switch converter before trying them with two suitable energy conversion systems. The first system is an UPQC chosen to represent an example series-shunt ac-ac system. The term "series-shunt" is used here for representing any system with two sets of three-phase ac terminals. One set of terminals is connected in series with the grid, while the other is connected in shunt. The analysis shows that with UPQC or most series-shunt systems, limitations faced by the nine-switch converter can greatly be reduced without affecting terminal performances. The saving of 25% semiconductor is thus less burdensome, and hence more attractive. The second system investigated is an integrated renewable energy conversion system that can either be single or three-phase. It uses the same concepts as the UPQC, but is not confined to only ac sources and loads. It is in fact the first attempt to merge different ac and de sources, storages and loads with a single integrated converter rather than multiple independent converters usually with more switches. The latter of course allows each converter to be controlled as per it is operating individually without interfering with the others. That is certainly an advantage, but because of the intermittent nature of most renewable sources, having multiple individual converters might not be a cost effective solution since most ofthem will not operate continuously. Using a single integrated system might therefore be more attractive especially when performance analysis proves that the saving in switches is not accompanied by burdensome limitations. Although the two studied energy systems have demonstrated improvements, they nonetheless use the same nine-switch converter as other existing ac-ac applications mostly in ac motor drives. To gain further improvements, modifications to the basic converter topology must be done, where one possible area is to generate more than two switching levels per phase. The reduced switch concept is thus modularized, multiplied and then cascaded to form a new reduced switch multilevel converter. The proposed converter has been tried as an online UPS and an interline DVR, which so far have been proven to function well with no or minimized limitations. Together, the four presented energy systems form an enriching guide for designer's reference. They help to clarify the real application scopes, advantages and disadvantages of the reduced semiconductor topologies. This information can then be weighed against the 25% saving in semiconductor, before making a sound decision on whether to pursue it. Practicality wise, all modulation schemes, converter topologies and energy systems presented have already been tested in the laboratory.
DRNTU::Engineering::Electrical and electronic engineering