Recent developments in high-voltage direct current transmission technology and the plans for multi-terminals motivate research efforts on the fast DC breaker. A novel concept to interrupt current up to several kiloamperes within a few milliseconds is presently under development, using a vacuum interrupter together with power electronics to create an artificial zero-crossing in the current through the switch. To achieve this objective, one critical component in the concept is an ultra-fast linear mechanical actuator. A prototype has been developed based on a Thomson coil. The experimental results are presented and show how a contact gap in a vacuum interrupter was effectively created in a controlled manner. Current of several kiloamperes was successfully interrupted within 3ms, thanks to the high-speed operation of the linear actuator.

This thesis describes an effort to investigate the use of gate-commutated thyristors(GCTs) in cascaded converters. Cascaded converters, such as modularmultilevel converters (M2Cs) and cascaded H-bridge converters (CHBs), haveproved to be especially suitable in high-voltage, high-power applications. Allof the most important advantages of cascaded converters, e.g. redundancy andscalability, can be attributed to the modular structure. Of special interestregarding the choice of semiconductor power devices is the reduced requirementon the switching frequency of individual devices. This brings a shift in thetrade-off between switching and conduction losses, where the latter has moreimportance in cascaded converters than in other topologies. This shift favorsthyristor-type devices like the GCT, which can achieve very low conductionlosses.To quantify the potential gain in the application of GCTs in cascadedconverters the losses have been calculated and a comparison between differentsubmodule implementations has been presented. The comparison has shownthat GCTs can provide 20-30% lower losses compared to insulated-gate bipolartransistors (IGBTs) in a typical HVDC application. In order to verify the lowlosses of GCT-based submodules, extensive work has been put into buildingand testing full-scale submodules employing GCTs. A resonant test circuithas been developed in which the submodules can be tested in steady-stateoperation which allows calorimetric measurements of the losses. The calorimetricmeasurements have verified that the loss calculation was reasonableand not lacking any important components.A drawback of GCTs is that the gate-drive units require much more powerthan gate-drive units for comparable IGBTs. In order to employ GCTs inhigh-voltage cascaded converters some means of supplying this power in thesubmodule must be provided. One option is to take this power from thesubmodule dc-link, but this requires a dc-dc converter capable of high inputvoltages. A tapped-inductor buck converter with a novel, autonomous highsidevalve was developed for this application. The autonomous operation of thehigh-side valve allows reliable operation without galvanic isolation components.A converter with a high-side valve with series-connected MOSFETs capable ofan input voltage of 3 kV has been presented.

A high-voltage tapped-inductor buck converter utilizing a novel autonomous high-voltage valve is presented in this paper. Its application as auxiliary power supply for high-power cascaded-converter submodules is discussed, followed by a presentation of the most challenging problems in designing a low-power high-voltage step-down converter. The TI-buck converter topology is analyzed, which together with the novel autonomous high-voltage valve overcomes most of the problems. Analytical expressions describing the operation as well as circuit simulations are presented. Finally, an experimental evaluation of a 3-kV 70-W prototype converter utilizing the proposed autonomous high-voltage valve with series-connected super-junction MOSFETs is presented.

With a steady increase in electricity consumption in both developed and developing regions combined with demand for sustainable energy infrastructure, trends towards more renewable energy sources and more open electricity markets are becoming more prominent. High Voltage DC (HVDC) connections and Static Synchronous Compensators (STATCOM) are two technologies that play a part in fulfilling this increasing demand. HVDC is beneficial in cases where production and consumption are geographically separated or for sub-sea cable transmissions. STATCOM helps to increase the capacity of AC transmission in addition to enhancing ac-grid voltage quality. A key component in both HVDC and STATCOM converters are the semiconductor switching devices. Switching devices have a fundamental impact on performance levels that can be obtained in terms of efficiency, reliability and functionality. This paper serves two purposes. The first is to give a historical overview of switching devices employed in HVDC transmission systems and STATCOMs. This starts with the use of mercury arc valves some 100 years ago and it continues with the semiconductor switching devices that are currently being employed in HVDC and STATCOM applications. A second purpose of this paper is to indicate developments in switching technology that are of interest for HVDC and STATCOM. In order to do this in a structured manner, the technologies are compared in terms of efficiency, reliability and functionality. Developments that are discussed in this paper are the emergence of Silicon Carbide (SiC) devices and the improvement of Insulated Gate Bipolar Transistor (IGBT) and Integrated Gate Commutated Thyristor (IGCT) devices in silicon. Currently, applications are based on silicon based thyristors or IGBT technology. Line commutated converters based on thyristors are cost-effective and efficient; however, due to lack of controlled turn-off capability, functionality is limited. Using voltage source converter technology with semiconductors having turn-off capability, such as the IGBT, increased functionality is obtained. The IGCT, a gate controlled thyristor with turn-off capability, has lower conduction losses compared to an IGBT with the same active area. In case a modular multilevel converter is used, the switching frequency of the individual switches can be reduced for the same performance. This leads to a shift towards the importance of having low conduction losses as opposed to low switching losses. A further shift can be achieved by using soft-switching techniques. Regarding SiC devices, different maturity is reached for unipolar and bipolar devices. Unipolar devices in SiC have been marketed successfully at low voltages (≤ 1700V). An example is SiC Schottky diodes for power factor correctors. For the voltages typically applied in HVDC and STATCOM, the drift region resistance would impose serious limitations on efficiency (if high current densities are used). Bipolar devices such as SiC IGBTs and IGCTs have been demonstrated in laboratory setups and results have been published; however, reliable operation is currently impeded due to the propagation of crystal lattice defects which causes rapid degradation of such devices.

This paper presents the design and implementation of cascaded-converters cells based on integrated gate-commutated thyristor (IGCT) half-bridges. The cell design is presented together with a discussion of means to supply gate drive power and as well as the cooling of the cell components. Also, the operation of the cells in a high-power resonant test circuit is presented.

The tapped-inductor buck (TI-buck) converter has shown to be a suitable solution for auxiliary power supply for modular multilevel converter submodules. Such application features a large step-down voltage conversion, made at relatively low output power. This converter operates in discontinuous conduction mode with zero voltage switching of the high-voltage valve. This paper treats the dynamic behaviour of the aforementioned converter. First, an average output current model of the converter is developed and a small signal model is obtained. Then, a closed-loop output voltage control, which uses the switching frequency as control variable, is designed and implemented using a microcontroller. Measurements on a down-scaled prototype shows that the control system provides a well-controlled average output voltage, which is stable under significant load variation. Finally, a solution for implementing the start-up of the converter is presented and tested.

This paper describes the use of the Auxiliary Reso-nant Commutated Pole (ARCP) converter topology for the cellsof a Modular Multilevel Converter. The workings of the ARCPtopology are explained as well as the calculation method usedto compare the efficiency to a conventional solution. Comparisonis done for an HVDC as well as for a STATCOM application.In both cases, use of the ARCP topology can lead to significantreductions in switching losses. The implications of these findingsare dealt with as well. These include a higher possible switchingfrequency and a higher practically feasible cell voltage.

This paper presents a test circuit and methodology suitable for testing high-power half-bridge submodules such as would be used in cascaded converters for high-voltage dc converters. The circuit utilizes a series resonant tank and two half-bridge submodules with voltage and impedance cancellation in order to minimize hardware requirements.

Tapped-inductor buck converters can provide large step-down ratios at high efficiency and are well suited in auxiliary power supplies for modular multilevel converter cells supplying gate drive units etc. In this paper the design and testing of three low-leakage tapped inductors for use in a 3kV, 100W buck converter is described.

This paper firstly presents a discussion of the requirements for an auxiliary power supply for high-power modular multilevel converter submodules. Next, some of the most challenging problems in designing a low power high voltage step-down converter are presented. Further, a suitable topology, the tapped-inductor buck converter, that overcomes most of the problems is analyzed. Both analytical expressions describing the operation and circuit simulations are presented. Finally, an experimental evaluation of a 3 kV, 100W prototype converter utilizing an autonomous high-voltage switch is presented.