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Efficiency enhancements to a linear AC voltage regulator: Multi-transformer and multi-winding designs

Utility companies are expected to distribute electricity that conforms with international or national standards related to RMS voltage fluctuations. However, RMS voltage variations sometimes exceed acceptable limits and this demands consumer-end voltage regulators. Commercial AC regulators such as (i) servo-driven variacs, (ii) ferro-resonant, (iii) transformer tap changers, (iv) solid-state types, have their own advantages and limitations. Common issues include (a) slow response time, (b) low efficiency and flattened-output waveform, (c) tap dancing and related switching issues, (d) harmonics and RFI/EMI at the output, respectively. The linear RMS AC voltage regulator is a patented technique developed in the late 1980s to address most of the above issues. It is a solid-state, single-phase system that employs a line-frequency transformer with its primary connected in series with an AC-operable variable impedance based on power transistors. The secondary winding is placed in series with the varying AC input, so that the induced secondary voltage acts as a correction signal to maintain the output at the desired value. A feedback circuit varies the effective impedance of the transistor-array, thus, manipulate the induced secondary voltage to regulate the output. This is a linear technique that allows seamless transition between boost to buck mode. However, this comes at the cost of lower efficiency at high AC input, particularly when line voltage goes beyond the nominal value. Commercial partner, Thor Technologies, Australia, wanted the team to modify their servo-driven AC regulator (PS-10), based on our technique with efficiency in the range of 90-95% and the response time improved by about 10 times. During development of a commercial prototype, the author implemented two potential solutions: (i) multi-transformer and (ii) multi-winding transformer based techniques to overcome the reduced efficiency during buck-mode operation, an issue inherent to the basic linear AC technique. Thesis was aimed at improving the power stage of the linear technique while maintaining the already developed analog control loop with true RMS regulation capability. thesis presents two alternative transformer solutions to reach efficiencies in the range of $90-95\%$ on a laboratory prototype of 500VA. Experimental results were validated by constructing a theoretical model for the two transformer configurations, supported by an equivalent circuit analysis using MATLAB and LTSpice software.
Type of thesis
Wijesooriya, P. N. (2020). Efficiency enhancements to a linear AC voltage regulator: Multi-transformer and multi-winding designs (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14026
The University of Waikato
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