A multi-level thyristor rectifier with improved power factor

The thyristor bridge rectifier is a reliable and cost effective topology commonly used in medium voltage applications. Low and load varying input displacement factor (IDF) and poor input harmonic current profile are the main drawbacks of this topology and are gaining more and more importance. This paper proposes modification of a conventional (18 pulse) thyristor rectifier to achieve higher power factor. The proposed topology employs one GTO device across each upper bridge to short them out when necessary, hence lowering the voltage level, decreasing the delay angle and improving the power factor. In medium voltage (4160 V) AC drives, usually 3 devices are connected in series to withstand the voltage, therefore the 18 pulse configuration has the same number of SCRs as compared to a 6 pulse bridge. The paper includes analysis, simulation, and a comparison of the proposed topology with the conventional 6, 12 and 18 pulse thyristor bridges. Experimental results from a 10 kVA 12 pulse system verify the feasibility of the proposed topology.