To extend the application of the self-contained converter to future trunk power systems, it is important to develop a stable operating scheme, as well as to realize a substantial reduction of costs through a coordinated system and a control design. Inverter overcurrent suppression controls and continuous overvoltage in various system failure conditions are essential to ensure stable operation and cost reduction of HVDC systems with auto-switched type voltage converters. Control and protection schemes of converters including such suppression controls have been developed using the CRIEPI power / ac system simulation test and EMTP analysis.
Continuous operation has been achieved by adding a new overcurrent suppression scheme to the converter control. In the event of a DC line ground fault, selection of the ground circuit constant and the adoption of a high-speed converter control practically ensure the reduction of DC overvoltage while being suppressed the converter overcurrent. The converter block and the restart sequence after a DC fault, which is coordinated with the operation of the DC circuit breaker, allows a stable recovery of the HVDC transmission as fast as the usual line switched HVDC system.