High-voltage thyristor strings for inverter applications

The application of thyristors to inverters is, in general, difficult. Rapid change of voltage and current requires high component tolerance. Thyristor turn-off time is most important. High forward current and reverse voltage provide considerable component stress. Static inverters are enjoying great popularity as frequency changers, motor speed controls, standby systems, phase changers, "direct voltage transformers," and in direct voltage transmission systems. A noticeable trend in these applications is the increasing power capability of the inverter. The use of higher operating potentials provides a means of handling higher power without using components having extremely large effective areas. Some applications, notably direct power transmission, are feasible only at operating potentials in the kilovolt range. Above about 400 volts, series strings are necessary. Combining a difficult application and cascaded components results in a design problem for both the component and the system designer. In addition to the normal inverter application troubles is the necessity for dynamic voltage balance. Such mundane problems as gate drive isolation and insulation prove to be formidable. The application of series thyristor strings in an oil-cooled inverter operating at 4000 direct volts is described. System design solutions to component limitations are also discussed.