A converter transformer is a main component in the HVDC transmission system. The converter transformer failure is the major cause of worry of electric power utilities. It is the top portions of the secondary windings of the converter transformers that fail leaving the primary winding unaffected. In this article, a practical HVDC system has been modelled to find the the causes for these failures. The MATLAB/SIMULINK environment has been used as the modelling environment. The possible solutions that emerged include providing passive filters on the secondary windings in the converter transformer, R-C snubbers across the secondary windings, connecting a parallel capacitor
on the dc side of the converter etc. The merits and demerits of the solutions have also been explained.
There has been tremendous progress in the flexible ac transmission systems (FACTS) devices and as a result, the HVAC is emerging as a tough competitor of the power carrier to HVDC. But the devices such as the IGBTs, IGCTs, GTOs, MOSFETs etc still gives HVDC an edge. The converter transformes is a vital device in the HVDC system which serves as the isolating device between the power transformer and the dc page link and steps
down the voltage as required by the thyristor converters. The onload tap changers on the primary side helps in keeping the constant voltage ac voltage supplied to the thyristor converters. Reports show that out of 22 failures in the last few years, 14 failures were secondary winding failures. The reasons of such failures can be :
-corrosive oil-forming copper sulphide sediments
-voltage transients arising during the commutation process
-temperature rise

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