Control of a conventional converter station which uses a three-phase controlled bridge for converting AC to DC and vice-versa is achieved by fast-acting control of the valve firing angles. Tap-changers which are fitted on the converter transformer provide a near constant AC voltage input to the bridge to correct relatively slow changes of AC voltage. This paper describe a modification of the conventional bridge circuit, using by-pass valves connected to tapping points on the secondary winding of the transformer. This scheme permits a wide range of voltage control with a reduction in both the harmonic generation and the reactive voltampere absorption, and possibly eliminates the need for an on-load tap-changer on the converter transformer. Experimental results on a laboratory model compare well with the predicted values.

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<p class="Abstract">The conventional three-phase alternating current (AC) to direct current (DC) converter can be modified using two isolated-gate bipolar transistor (IGBT) as by-pass switches connected to tapping points on the secondary side of the transformer. This scheme yields a reduction in both harmonic contents and reactive volt-ampere absorption. This modified converter possibly eliminates the need for an on-load tap-changer on the converter transformer. The modified AC/DC converter is fully analyzed and implemented under balanced conditions using MATLAB-Simulink. The expressions of the output DC voltage are derived for different cases. The supply current harmonic contents, the reactive power absorption and the power factor have been compared for three schemes; the conventional bridge, the modified bridge using one by-pass IGBT valve and the modified bridge with two by-pass IGBT valves. </p>

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