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In recent years Operational Transconductance Amplifier based high frequency integrated circuits, filters and systems have been widely investigated. The usefulness of OTAs over conventional OP-Amps in the design of both first order and second order active filters are well documented. This paper proposes two first order voltage controlled all pass filters with phase tuning capability. Simulation results are obtained in TINA® and verified by experimental results.

Smelting requires extremely large DC currents, often in excess of 320 kA, to facilitate the production of commercial quantities of aluminium using the reduction process. The DC load consists of one or more potlines, which in turn are made up of many series-connected reduction cells, often in groups of 250 or more. Each cell has a nominal operating voltage which is controlled to around 4.0-4.5 V. The reduction cell is constructed with a movable carbon anode, electrolyte, aluminium cathode, cathode carbon and steel collector as shown in Figure 10.2. Figure 10.2 Simplified schematic of an aluminium reduction cell.

IntroductionAs the rating and acceptability of high power self-commutating switches improve, the boundaries between the HVDC and FACTS technologies are gradually becoming blurred. HVDC has already started using the new devices (thus improving its control flexibility) and FACTS controllers are increasing their power range (which may finally result in the control of the total power transfer of an asynchronous interconnection). Therefore, modern back-to-back (BTB) conversion could be considered as part of the HVDC and FACTS technologies.Bidirectional BTB asynchronous interconnections with ratings of up to 1500 MW are already being used for the interconnection of networks of different frequencies, like the recent Brazil-Argentina scheme [1], and for trading reserves [2] or shifting peak energy loading times between networks with different time zones, such as between Finland and Russia [3].The distinguishing feature of FACTS with respect to traditional HVDC has been its greater control flexibility. However, several fully flexible PWM-based HVDC voltage source conversion schemes in the 300 MW region are already operating successfully and the power rating capability of the PWM-VSC technology is on the increase.Although the large power BTB interconnections under consideration are still based on conventional thyristor conversion, the self-commutating multilevel technology is likely to be a viable alternative in the near future. However, under conventional control the self-commutating multilevel BTB interconnection lacks independence of reactive power controllability between the two ends of the link, an important shortcoming.The MLCR concept described in Chapter 4 and used in Chapter 8 for the series-connected double converter groups, is used here for the firing control of the parallel-connected BTB converter configuration to provide four-quadrant controllability. The parallel MLCR configuration has a simpler structure and is the preferred option for the large BTB asynchronous application, which will normally consist of groups of two paralleled bridges connected in series.

As explained in the first chapter, both the transistor and thyristor types of self-commutating power switches can now be combined in series to form reliable high-voltage valves. The seriesconnected power switches can be fired synchronously or asynchronously (with switch voltage clamping assistance). Synchronous control, used in two-level VSC, causes static and dynamic voltage sharing problems as well as high dv=dt.Possible alternatives to two-level conversion for high-voltage applications are the multipulse and multilevel topologies. Increasing the pulse number has been traditionally achieved in current source conversion by the series or parallel connection of bridges, their respective voltage waveforms being phase-shifted with respect to each other by appropriate connections of the interface transformers. Applying the multibridge concept to self-commutating voltage source conversion improves the converter output waveforms without the assistance of highfrequency switching. However, for high-pulse conversion the increased number of converter transformers required makes this solution unattractive.A more effective alternative for high-voltage application is the multilevel concept with asynchronous firing control; this improves the dynamic voltage balancing of the valves, while the steady state voltage sharing is achieved by means of clamping devices.Like in the case of PWM, multilevel converters can vary the phase position of the converter fundamental frequency voltage with respect to the AC system voltage waveform; however, their effect on the fundamental frequency voltage magnitude is very different. In the PWM solution the magnitude of this voltage can be varied independently from the DC voltage, whereas in the multilevel alternative the voltage magnitude is fixed by the DC voltage. The main object of multilevel conversion is to generate a good high-voltage waveform by stepping through several intermediate voltage levels, i.e. the series-connected devices are switched sequentially producing an output waveform in steps. This eliminates the low-order harmonics and reduces the dv=dt rating of the valves by forcing them to switch against a fraction of the DC voltage.