Transformerless series dip/sag compensation with a multilevel cascaded inverter

[摘要] ENGLISH ABSTRACT: This thesis covers the development of a multilevel cascaded inverter for the purpose of costeffectivetransformerless series dip compensation. Of all known power quality problems, voltagedips are the greatest reason for concern. Dips/sags occur more frequently than outages and thereforetend to be more costly for industry as modem technical equipment becomes all the more sensitive tothe quality and reliability of supply. A number of devices already exist to compensate for thisproblem, but the cost of most of these systems does not always justify the financial losses theycompensate for. All of these systems are using transformers and/or large filter components thatcontribute to the size, price and losses to quite a large extent. Series injection dip compensatorsoffer the advantage of only having to compensate for the decrease in supply voltage during a dip.This results in a significant reduction in the converter ratings and energy storage requirementscompared to conventional uninterruptible power supplies or shunt injection power quality devices.Existing inverter topologies, including multilevel inverters, were therefore studied and compared aspossible solutions for cost-effective transformerless series dip compensation. On the basis of theseconsiderations the multilevel cascaded inverter seems to be the most cost-effective option. Therelatively low harmonic content of its unfiltered output also eliminates the need for a large outputfilter. A single-phase dip compensator, with this topology, was designed and built according tospecifications stated by Eskom, the main utility in South Africa. Batteries as energy storage andautomotive MOSFETs as switching components, proved to be most cost-effective options for thespecified power ratings. Control algorithms for dip compensation with the multilevel inverter werealso developed. Some of these algorithms are based on existing techniques, but two new algorithmswere also developed to implement force commutation of the thyristors and to share the powerdissipation in the dip compensator. Simulations indicated that these algorithms could be suitableand sufficient for their application. This dip compensator with its control algorithms was tested witha dip generator, developed at the University of Stellenbosch, for different types of loads. Theexperimental results confirmed the simulations and showed a very good performance for thespecified conditions. An optimised design of this dip compensator will make it a cost-effectivesolution for dip compensation.

[发布机构] Stellenbosch University