Series operation of thyristors in HVDC converters

High voltage direct current. (hvdc) transmission of electrical paver has been made possible with the recent advent of multi-anode mercury arc valves with grading electrodes. However, mercury are valves do suffer from random phenomena such as arcback, which requires special bracing for the converter transformer. In addition, mercury arc valves need considerable auxiliaries, including cooling and vacuum plants. Research and development directed towards the improvement in the voltage and current capabilities of thyristors are yielding encouraging results. In contrast to mercury arc valves, thyristors do not suffer from random phenomena and require minim auxiliaries. Considering these advantages, thyristors offer a viable alternative to mercury arc valves for hvdc converters. The comparatively low ratings of thyristors necessitate series and parallel connection of thyristors in order to construct a valve of sufficient rating for hvdc operation. The design of thyristor valve should ensure even distribution of voltage and current among all thyristors of the valve, during all operating conditions. To achieve this objective transmission of the firing signal to the gate of each thyristors should have good coherence. The thesis examines design aspects, and considers the different methods of firing both theoretically and practically. Finally, the design and subsequent experimental results of the prototype thyristors valve rated at 10kv and 10A are also included